Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc) in thin film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric(FE) and ferromagnetic oxides as the background, we are now exploring such materials, as epitaxial thin films as well as nanostructures. Specifically, we are studying the role of thin film growth, heteroepitaxy and processing on the basic properties as well as magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling.
What is the importance of this work ? Antiferromagnets(AFM) are pervasive in the recording industry. They are used as exchange biasing layers in MTJ&’s etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed at room temperature. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet( and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility.

Domain engineered states of Pb(Mg1/3Nb2/3)O3 single crystals have extremely large piezoelectric and electromechanical coupling coefficients. This high performance is due to the presence of a sequence of phase transformations. This sequence is notably dependent upon crystallographic orientation along which fields are applied. Of particular interests, large reversible field-induced strains are found along the (110) that are nearly hysteretic, and which can be cycled without notable fatigue. These findings point to a particular transformational pathway that has near-complete stress accommodation between nanometer scale polar domains: this allows for the transformation characteristics to reversibly contribute to the effective linear piezoelectric ones.

Interfaces and domain boundaries are not necessarily a simple structural juxtaposition of adjacent bulk materials, but contain novel structural elements which do not exist in the bulk. Examples are superconducting twin boundaries in WO3 and ferroelectricity in paraelectric CaTiO3. Such functional interfaces are at the core of research in the emerging field of ‘domain boundary engineering&’ where polar, conducting, chiral and other interfaces and twin boundaries have been discovered. We show that the effect of functional interfaces can be optimized if the number of twin boundaries is increased in densely twinned materials. Such materials can be produced by shear in the ferroelastic phase rather than by rapid quench from the paraelastic phase. In a combination of experimental work and large scale computer simulation of simple toy models the conditions for the nucleation of domain pattern with high junction densities are discussed.

Among various materials exhibiting reversible phase transformations structures with low crystal symmetry, so-called modulated phases, exhibit the best functional properties. This is observed e.g. in ferroelectrics, magnetic shape memory alloys and magnetocaloric materials. There are two competing concepts to explain the origin of these modulation and their extraordinary functional properties: 1) The adaptive concept describes modulations as twin boundaries at the nanoscale, which is possible for stiff materials with a low twin boundary energy [1]. 2) Electronic instability and Fermi surface nesting.
Both concepts differ fundamentally with respect to causality. 1) As adaptivity is a continuum concept extrapolated to atomic scale, modulations are a consequence of the macroscopic elastic constrains. 2) An electronic instability allows for modulations as deviations from a crystal structure and the macroscopic martensitic microstructure is a consequence of this modulated structure. In a century where solid state physics becomes more and more dominated by quantum mechanics instead of continuum mechanics the adaptive concept obviously becomes under stress.
By analysing previous experiments we will show that modulated martensite under mechanical stress provides direct evidence for the adaptive concept. It gives a straight forward explanation for several anomalies observed during pseudoelastic and plastic deformation of modulated martensite: Intermartensitic transformations [2], pseudoplastic anisotropy [3] and asymmetry between tensile and compressive experiments [4].
[1] A. G. Khachaturyan, S. M. Shapiro, and S. Semenovskaya, Phys. Rev. B 43 (1991) 10832
[2] V. V. Martynov, and V. V. Kokorin, J. Phys. III France 2 (1992) 739
[3] V. A. Chernenko, J. Pons, E. Cesari, K. Ishikawa, Acta Mat. 53 (2005) 5071
[4] E. Panchenko, Y. Chumlyakov, H. J. Maier, E. Timofeeva, I. Karaman, Intermetallics 18, (2010) 2458

The renaissance of the field of multiferroics, which started around the beginning of the last decade, has to date yielded a much deeper understanding of the subject of single phase and composite multiferroics. In this talk, I will cover self-organized organic exciton based multiferroics. The macroscopic ferromagnetic and magnetoelectric responses correspond semi-quantitatively to the features of their nanostructures. Such an excitonic room temperature magnetoelectrics are based on organic photovoltaic materials, which has great potential in optospintronics using the effect to control magnetization with an electric field, light illumination and strain, or conversely, magnetic field controlled photovoltaics. An electric field of 5 MVm-1 switches the room temperature saturation magnetization on and off resulting in a magnetoelectric coupling coefficient of 40 mV/cm Oe at 100 Oe. Illumination of the composite with 615 nm light increases the room temperature magnetization about two fold.

We discuss a general concept for the giant low-hysteretic response caused by an isothermal diffusionless phase transition across the congruent boundary line separating the phases with differing symmetry and ferroic properties. The isothermal Across Boundary Transition (ABT) is induced by application of the external field that thermodynamically favors the ferroic phase on the opposite side of the phase boundary line. The field-induced ABT changes the symmetry and ferroic properties of the initial compositionally homogeneous single phase state without changing its compositional homogeneity. A response to the applied field caused by such a phase transition is giant with respect to the conventional intrinsic responses not generating a phase transformation. The main principles guiding to a choice of such systems are formulated and specific examples of using this approach to obtaining ferroic properties that have not be observed before are presented. Experimental results will be presented from the selected system exhibiting the ABT. Physical properties from a diverse group of material systems will be compared to evaluate the influence of ABT.

Metal hydrides present a feasible means of energy storage and hydrogen sensing but have several performance criteria that must be addressed, including the hysteresis effect during loading and unloading. We will present the results of a theoretical and experimental study which demonstrates the possibility to control or eliminate hysteresis during metal-hydride transformation in epitaxial Pd thin films. Theoretical analysis predicts stabilization of two-phase metal-hydride state in film due to its elastic interaction with the substrate. It is shown, by atomic force and scanning electron microscopy, that transformation in 50nm and 100nm thick epitaxial Pd films on Al2O3 substrate proceeds by the formation of transversely modulated two-phase nanostructure. Morphology and crystallographic orientation of the metal-hydride interface corresponds to the theoretically predicted orientation for coherent phases. This nanostructure is changed by altering temperature or hydrogen pressure. The possibility to engineer metal-hydride transformation in other epitaxial films, particularly MgH, will be discussed.

Theoretical misfit-temperature diagrams as well as experimental observations of monoclinic phases in epitaxial ferroelectric films, particularly, BiFeO3 films show that the phase transformation under mechanical constraint decreasing symmetry of an initial phase results in formation of low-symmetry phases which are unstable without the constraint. Misfit-temperature diagrams constructed on the basis of Landau-Devonshire potential renormalized by elastic interactions with a substrate allow one to estimate the relative stability of different low- symmetry single-phase states. However, during the phase transformation under constraint the low- symmetry single phase can be less stable than heterophase polydomain structure of phases with higher symmetry.Termodynamic analysis of a low- symmetry “constraint” phase vs heterophase polydomains and their functional properties is a subject of this presentation.
The elastic domain theory shows that the stability of two- phase equilibrium is determined by competition of phase-substrate elastic interaction which initiates mixture formation and phase-phase elastic interaction which opposes phase mixture. If phase-substrate interaction is dominant, formation of polydomain heterophase structure with nonconvex dependence of its free energy on phase (domain) fraction. If phase-phase interaction dominates, the two phase state is unstable and direct and reversible transformations proceed with hysteresis.
For stable two phase state, changing temperature or external electrical or mechanical fields leads to domain evolution which results in enhanced extrinsic properties, including large electrical susceptibility, piezo effect, or elastic compliance. This enhancement is more pronounced for mixture of incompatible phases than for mixture of compatible phases or twins because non-convex free energy lowers the stability of the two-phase state and makes it more sensitive to influence of external field. Besides that, the elastic distortion of coherent phases decreases the interface energy and the Peierls&’ barrier for interface movement
The presented thermodynamic concept on stability and evolution of heterophase polydomain structure is applied for analysis of recent experimental observations of nanostructures and their field induced response in epitaxial films Extension of these ideas to constrained nanoparticles and nanowires is discussed.

Shape memory effect is closely related to the reversible martenstic phase transformation, which is diffusionless and involves shear deformation. The fully recoverable transformation between the two phases with different crystalline symmetry results in reversible changes in physical properties such as electrical conductivity, magnetization, and elasticity. Accompany the transformation is the change of entropy. Fascinating applications are developed based on these changes. In this paper, geometric nonlinear theory of martensite and supporting experiments will be briefly presented; applications related to energy conversion such as power generation and refrigeration as well as the recent development will be reviewed.

FeSi, FeAl, FeCo and particularly FeGa, form a family of rare earth free ferromagnetic materials featuring a large processing dependent magnetostriction, lambda;, who also display a very small magnetocrystalline anisotropy, K, and therefore large chi;&’s. However, the shared origin of this technologically attractive property combination, high chi; and large lambda;, has been overlooked. Here, we highlight the extended linear range of chi; (Permendur effect) common to all family members and we report that cubic Fe82Ga18 and Fe35Co65 display a uniaxial magnetic anisotropy at low magnetic fields. The Permedur effect is consistent with the unusual field independent Barkhausen noise which we also report whereas the uniaxial anisotropy is seemingly inconsistent with the intrinsic four-fold symmetric magnetocrystalline anisotropy of the cubic alloys. In this presentation we will illustrate how all stated common properties of the Fe-(Si, Al, Co, Ga) family are compatible with the idea of a composite of a ferromagnetic cubic matrix and exchange coupled nano-precipitates of lower-than-cubic symmetry. This insight is general and can serve as a guiding principle for the search of better functional ferroic materials.

The elastocaloric effect that occurs during the stress-induced martensitic transformation in shape memory alloys is a promising mechanism in view of solid state cooling applications. It also allows for downscaling to feature sizes in the µm range, thus being attractive for micro-cooling applications using thin film materials. Thin films have further advantages: their geometry expedites heat transfer to and from the surroundings, consequently allowing for higher cycling frequencies which compensate for the smaller transformation volume. Also, since sputtered films can be structured using optical lithography and wet chemical etching, their geometry can be easily optimized in respect to the requirements of the repeated martensitic transformation in elastocaloric cooling applications, potentially improving the films fatigue behavior. Additional work steps such as laser cutting are redundant and do not impair the fatigue life of the films, e.g. through the formation of TiC.
In this study, we present the superelastic and elastocaloric properties of sputtered TiNi and TiNiCu films, their fatigue behavior and the relation between elastocaloric effect size and functional fatigue. The temperature change in TiNi films degrades by a factor of two within 150 cycles, which reflects the strong mircostructural changes in the binary alloy, high remanent strains and the significant change in superelastic stress-strain curves. TiNiCu films, which can exhibit stress hysteresis of less than 100 MPa, reveal highly stable superelastic properties and no significant elastocaloric fatigue was found in TiNiCu up to more than 1500 cycles [1].
[1] C. Bechtold, C. Chluba, R. Lima de Miranda, and E. Quandt, “High cyclic stability of the elastocaloric effect in sputtered TiNiCu shape memory films,” Appl. Phys. Lett., 101 (2012) 9, 091903.

The evolution of temperature profiles is investigated during reverse transformation in pseudoelastic NiTi films in order to evaluate the potential for elastocaloric microcooling. The caloric effects associated with the first order transformation in shape memory alloys are of special interest for cooling applications due to the absorption of a large quantity of latent heat. Ferroelastic NiTi alloys, for instance, exhibit a unique combination of large latent heat in the order of 24 J/g, large transformation strain of 8 % and good scalability on miniaturization. Compared to bulk, film materials are expected to enable faster heat transfer and higher cycling frequencies.
Ni50.4Ti49.6 films of 22 µm thickness are fabricated by DC magnetron sputtering and subsequent rapid thermal annealing. The start and finish temperatures of martensitic and reverse transformation Ms/Mf and As/Af are determined by differential scanning calorimetry at a rate of 20K/min to be -31/-47 °C and 0/26°C, respectively. Tensile specimens are prepared by optical lithography and sacrificial layer etching. A series of tensile loading and unloading experiments are performed in strain control mode at room temperature. Corresponding temperature profiles are monitored by infrared thermography with a spatial resolution of 10 µm. In addition, strain profiles are determined in situ using the digital image correlation technique.
The experiments are complemented by coupled finite element simulations using a two-phase phenomenological model that accounts for the effects of local absorption of latent heat and heat transfer. For the time-dependent thermal simulation, a linearized expression is used to describe the change of the martensite phase fraction within each time step in order to calculate the change of temperature due to the absorption of latent heat.
The simulations show that adiabatic conditions are reached for a strain rate larger than 0.24 s-1. Upon loading, the formation of martensite is hindered by the self-heating effect causing incomplete formation of martensite depending on strain rate and maximum strain, which results in a reduced release of latent heat. After ambient cooling to room temperature, unloading favours the formation of austenite due to the release of stress. Thus, absorption of latent heat is enhanced. In optimum case, the simulations show a maximum decrease of temperature by 35 K. So far, experimental results have been obtained at the maximum strain rate of 10-1 s-1. In this case, heat transfer cannot be neglected as the corresponding time constants are in the order of milliseconds. Consequently, only a reduced maximum temperature decrease by 8 K is observed. The coefficient of performance R given by the ratio “total heat absorption / mechanical work” reaches values up to 10 at strain levels > 1.5 %. Therefore, the elastocaloric properties of NiTi films are of considerable interest to solid state cooling and heat pumping in miniature dimensions.

The structural, electronic and magnetic properties of novel functional Pd-Pt-Ni-Mn-(Ga, Sn) alloys are studied by first-principles and Monte Carlo tools. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from the competition of ferro- and antiferromagnetic interactions with increasing number of Mn and Pt atoms in the unit cell. We show that the resulting complex magnetic ordering is the driving mechanism of structural transformations and the related multifunctional properties of the Heusler alloys such as magnetic shape-memory and magnetocaloric effects. The thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. Entropy and specific heat changes associated with the magnetic changes and emergence of microstructure across the magnetostructural transition are pointed out. We show how to optimize the functional properties by tuning the compositional changes, for example, a magnetic shape-memory effect of more than 14% can be achieved in Pt-Ni-Mn-Ga alloys. The theoretical studies are accompanied by experimental investigations.

The Lorentz observation mode of transmission electron microscopy (TEM) allows for the direct imaging of magnetic domain walls and domain configurations. Using phase reconstruction methods, in particular the Transport-of-Intensity Equation, one can obtain a quantitative measure of the product of the local sample thickness and the component of magnetic induction normal to the electron beam. By repeating this measurement as a function of sample tilt, one can then apply principles of vector field electron tomography to reconstruct the magnetic vector potential in three dimensions. We will present examples of phase reconstructions for a number of ferromagnetic shape memory alloys in the Ni-Mn-Ga and Fe-Pd systems, as well as examples of 3D reconstructions of the magnetization. There is a distinct difference between domain wall configurations in samples with relatively widely spaced twin variant boundaries (several hundred nanometers and above) and samples with a twin boundary spacing that is similar to the magnetic domain wall width. In the latter case, we have observed the formation of both hexagonal and square lattices of magnetic vortices that are pinned on the twin boundaries. We will present evidence for these regular arrays of vortices using phase reconstructed Lorentz TEM as well as vector field electron tomography. In addition, we will present preliminary results of in-situ observations of domain wall motion under an applied magnetic field.

Nanostructures that undergo well-defined shape changes when exposed to light could have applications as actuators or as components of nanoscale machines. One way to drive photo-induced shape changes is to use photochemistry to induce structural changes within the molecules that compose a molecular crystal. The newly formed product molecules comprise a new crystal phase, and the ensuing phase transformation can then be used to drive dynamic shape changes of the crystal. If the photochemical reaction is reversible, the overall shape changes can also be reversible. The goal is to have small structures whose photomechanical response is intrinsic and does not depend on factors like light intensity, incident angle, or exposure time. We have investigated several classes of molecular photoreactions, including photodimerization and trans-cis isomerization, in order to characterize how molecular-scale chemical changes can drive micro-scale mechanical motions and deformations in shaped molecular crystals. Using a variety of methods, we prepare different classes of single crystal shapes, including nanowires, micro-ribbons, and micro-needles, all composed of photoreactive molecules. Depending on the nature of the photochemical reaction, the shape of the crystal, and the orientation of the molecules within the shaped crystal, different types of motions can be induced by photoexcitation, including expansion, bending, twisting, and curling. In many cases, the energy that drives complex shape changes like twisting arises from interfacial strain between the reactant and product crystal phases. It is possible to design crystals where the production of both phases is an intrinsic property of the crystal and does not depend sensitively on the amount of light exposure. Photoreactive molecular crystals comprise a large class of photomechanical materials whose properties can be tuned using the tools of organic chemistry and crystal engineering.

Metal-organic framework (MOF) materials are crystalline phases containing both inorganic and organic structural elements. They may be dense or nanoporous, and span the full range of dimensionalities from 0-D (molecular) to 3-D. We particularly are interested in dense MOF materials since they have eye-catching properties in areas that have traditionally been dominated by inorganics. In this presentation, I will discuss the unique mechanical properties of multiferroic metal-formate framework perovskites. Firstly, I will present an unusual multiferroic manganese-formate framework perovskite, [(CH2)3NH2][Mn(HCOO)3], which exhibits a weakly first order ferroelastic phase transition at ~272 K (from orthorhombic Pnma to monoclinic P21/n) and a further antiferromagnetic transition at ~8.5 K. The main structural changes, through the ferroelastic phase transition, are orientational ordering of the azetidium groups and associated changes in hydrogen bonding. In marked contrast to conventional improper ferroelastic oxide perovskites, the driving mechanism is associated with the X-point of the cubic Brillouin zone rather than being driven by R- and M-point octahedral tilting. A total ferroelastic shear strain of up to ~5% is substantially greater than found for typical oxide perovskites, and highlights the potential of the flexible framework to undergo large relaxations in response to local structural changes. Measurements of elastic and anelastic properties by resonant ultrasound spectroscopy show some of the characteristic features of ferroelastic materials. Secondly, I will present a systematic study of how the different hydrogen-bonding can affect the mechanical properties of two structurally similar MOF perovskites, [(CH2)3NH2][Mn(HCOO)3] and [(NH2)3C][Mn(HCOO)3]. The attaching N-Hhellip;O bonds, formed between the azetidium and the [Mn(HCOO)3]- framework in the former, are much weaker than those bridging N-Hhellip;O bonds formed between the guanidinium and [Mn(HCOO)3]- in the later, which leads to substantial difference of Young&’s moduli, hardness and stress-strain properties of these two perovskites. This study indeed underlines the promising possibility of tuning properties of MOFs via an approach of crystal engineering. To finish, I will present the rare negative linear compressibility (NLC) of a zinc-formate framework, [NH4][Mn(HCOO)3], which crystallizes in a hexagonal system with an acs network topology. This framework material shows a high degree of mechanical anisotropy and NLC along its c axis. High-pressure single-crystal X-ray diffraction studies and DFT calculations indicate that contraction of the Zn-O bonds and tilting of the formate ligands with increasing pressure induce changes in structure that result in shrinkage of the a and b axes and the NLC effect along c. This work opens up an exciting new area for MOF materials with respect to rare functionality known only in classical inorganic and organic materials.

Using aberration-corrected (scanning) transmission electron microscopy it is now possible to obtain picometer precission on atomic positions surrounding lattice defects. Also, precise chemical occupation at or even concentration of individual lattice sites can be obtained. In the present lecture details of atomic shifts near a single ferroelastic twin domain boundary wall leading to ferrielectricity in orthorhombic CaTiO3 will be discussed as well as lattice site occupation and nano-precipitate formation and their effects on hysteresis in the ternary shape memory Ni-Ti-Nb system.
In CaTiO3 the twin domain wall is formed by a symmetry-breaking transformation from a high-temperature tetragonal to a low-temperature orthorhombic phase. The analysis clearly indicates shifts in the Ti atomic positions whereas any shifts in the Ca atomic positions are too small to be identified. In the direction perpendicular to the wall, systematic deviations for Ti of 3.1 pm in the second closest layers pointing toward the twin wall are found. A larger displacement is measured in the direction parallel to the wall in the layers adjacent to the twin wall. The averaged displacement in these layers is 6.1 pm. In all layers further away from the twin wall, no systematic deviations in any direction are observed. These displacements yield a spontaneous polarization of the wall.
In Ni-Ti-Nb Nb-rich nano-precipitates form upon quenching in the matrix while the eutectic contains large Nb-rich particles. The cube-on-cube orientation with the Ni-Ti-rich matrix yields periodic interface dislocations which can be visualized by FFT-based strain analysis. In situ TEM cooling shows that the martensitic transformation as well as the reversed transformation to austenite are hampered by the presence of these precipitates, which could explain part of the increase in hysteresis when compared with binary material. HAADF-STEM reveals Nb atoms on the Ti atom sites of the matrix affecting the Ni/Ti ratio and thus also the hysteresis, possibly via the middle eigenvalue = 1 concept. In this material these investigations are supplemented by 3D work using FB/SEM slice-and-view on samples with different heat-treatments to allow growth of the precipitates. Also the Ti2Ni-based precipitates formed upon annealing are investigated bt precession electron diffraction and structure refinement which confirms the preference of a Ti-site for the soluble Nb- atoms.

Liquid crystal elastomers and glasses are rubbery and glassy materials that possess liquid crystal order. These are made by cross-linking long chain polymers with liquid crystalline ligands either a part of or pendant from the main chain. Elastomers are lightly cross-linked while glasses are heavily cross-linked. They both undergo a spontaneous deformation as they undergo a symmetry-breaking isotropic to nematic transition in response to light and temperature changes. The talk will describe novel ways that this transition can be exploited in thin films for a diverse range of thermo-mechanical, opto-mechanical and photonic applications.

β-eucryptite, a lithium aluminum silicate with chemical composition LiAlSiO₄, has attracted a lot of attention owing to its near-zero (slightly negative) coefficient of thermal expansion and exceptional thermal stability. This anomalous thermal behavior makes β-eucryptite applicable for a variety of applications including heat exchangers, ring laser gyroscope and telescope mirror blanks. Recent in-situx-ray diffraction and spectroscopic studies have revealed that β-eucryptite transforms reversibly to a newly discovered polymorph, ε-eucryptite, under moderate pressure ~1 GPa. Such a structural transition has potential applications in toughening ceramic composites (by dispersing particles of β-eucryptite in a matrix such as zirconia) provided it can be controlled. It is therefore, essential to gain a fundamental understanding of response of β-eucryptite to applied pressure. Although the β to ε transformation has been observed empirically, the crystallographic details of the high-pressure ε phase and the atomic scale mechanisms governing this transition are still unknown. In the present work, we have employed ab-initio metadynamics to explore the phase transition in β-eucryptite at room temperature close to the experimental transition pressure. Using the simulation box edges as collective variables, the atomic structure of ε-eucryptite was determined. The atomic scale pathways for the β to ε transition, as well as the reverse transformation were identified. The results have been discussed in the context of developing toughened ceramic composites based on β-eucryptite.

Magnetic shape memory alloys are being widely explored for many technological applications. Typical representatives of this class of materials are intermetallic Ni-Mn-X (X = Ga, In, Sn, Sb) Heusler alloys. The Heusler alloys have been experimentally studied quite extensively for their magnetocaloric and magnetomechanical properties and for their applications [1-3]. These compounds crystallize in the cubic structure (Fm3m, space group no. 225) with Cu2MnAl (L21) as prototype. The Ni2 atoms form a primitive cubic sublattice and adjacent cubes of this Ni2 sublattice are filled alternating by Mn and X. The primitive cell of L21 structure contains four atoms that from the base of the face-centered cubic primitive cell. The result is a lattice with the Fm3m symmetry were the Wyckoff positions 4a (0, 0, 0), 4b (1/2, 1/2, 1/2), and 8c (1/4, 1/4, 1/4) are occupied by X, Mn, and Ni2 atoms, respectively. From theoretical point of view, the stoichiometric Ni2MnX and off-stoichiometric Ni2+xMn1-xX or Ni2Mn1+xX1-x have been investigated in several papers with the use of ab initio calculations [4-6]. For those alloys the exchange magnetic couplings were calculated with the primitive cell of austenite and martensite. All calculations have shown strong competitive behavior between the ferromagnetic and antiferromagnetic order.
In this work we study the influence of supercell scaling on magnetic properties in Ni-Mn-X alloys by means of ab initio calculations and the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) code based on DFT approximation [7, 8]. The exchange magnetic parameters and total magnetic moments have been calculated for the supercell structures which included 16 and 32 positions per cell. For example, in the first (second) case for stoichiometric Ni2MnX the eight (sixteen) of them are occupied by Ni atoms and four (eight) positions by Mn and X atoms, respectively. The calculated parameters have been compared with the ones obtained using primitive cell with four atoms. We have found that the increase of atoms in the supercell of Heulser alloys leads to increasing the ferromagnetic and antiferromagnetic interactions between Mn and Ni atoms.
This work was supported by RFBR (grants 11-02-00601 and 12-02-31129), and RF President grant MK-6278.2012.2.
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3. V.D. Buchelnikov, and V.V. Sokolovskiy, Phys. Met. Metallogr. 112, 633 (2011).
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5. V.D. Buchelnikov, V.V. Sokolovskiy, H.C. Herper et al., Phys. Rev. B 81, 094411 (2011).
6. V.V. Sokolovskiy, V.D. Buchelnikov, M.A. Zagrebin et al., Phys. Rev. B 86, 134418 (2012).
7. H. Ebert, et. al. Rep. Prog. Phys. 74, 096501 (2011).
8. H. Ebert, SPR-KKR package Version 5.4 on http://ebert.cup.unimuenchen.de.

Multifunctional materials are the key for future technological devices. Crystal structure of these materials plays an important role in determining their multifunctional properties. BiFeO3 _ one of the multifunctional materials possessing simultaneously different ferroic orders such as ferroelectricity, ferromagnetism. It is already well established that crystal structure of BiFeO3 particles can easily be tuned by doping subsequently their multifunctional properties. Un-doped BiFeO3 has a rhombohedrally distorted perovskite structure (ABO3) with space group R3c having Curie&’s temperature sim; 830oC, and Neel&’s temperature ~ 370oC with a spatially modulated spiral spin structure. And doping of suitable rare earth ions may lead to structural transformation from rhombohedral (R3c) to orthorhombic (Pnma) or tetragonal (Pmna).
Here we report the synthesis of rare earth metal ions (Gd3+, Tb3+, and Dy3+) doped BiFeO3 nanoparticles by sol-gel method, and effect of doping on structural and magnetic properties has also been studied. Morphological study has been carried out using transmission electron microscopy. X-ray diffraction study has been utilized for the crystallographic analysis and structure evaluation. The magnetic study reveals that for the synthesized nanoparticles, possessing ferromagnetic character, the saturation magnetization increases with increase in doping concentration.
The present study reveals that the rare earth doped BiFeO3 nanoparticles are likely to be the best multifunctional futuristic material that could prove to be useful to explore their applications in nanoelectronic and spintronic devices.

Electrocaloric materials, which gain increasing interest for novel solid state cooling devices, show a diffusionless phase transition, which is induced by electric fields. However, the interplay between the microstructural changes and the electrocaloric effect at such phase transitions is not completely understood so far. Epitaxial films offer the opportunity to study these correlations in more detail. Therefore, epitaxial (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT) were grown on different single crystalline substrates using off-axis pulsed laser deposition. The influence of different deposition parameters was investigated in order to realize PMN-PT films with a pure perovskite structure. The structure of the grown film was characterized in detail to probe, if the microstructure might be described with an adaptive concept similar to magnetocaloric Heusler alloys. Finally, electric polarization measurements were used to determine the temperature change by the electrocaloric effect indirectly. First attempt to correlate the microstructure with the caloric effect will be discussed.
This work is supported by DFG under SPP 1599 (www.FerroicCooling.de)

Giant electrocaloric effects have been reported in ferroelectric thin films [1,2], as thin films can support large driving fields. However, measurements of electrically driven heat Q and temperature change ΔT are typically indirect as the direct measurement of films is challenging, and electrocaloric strengths Q/ΔE and ΔT/ΔE tend to be relatively small as electrocaloric effects in films are disproportionately small with respect to the large driving fields. Here we address both of these issues via direct measurements of both Q and ΔT in single-crystal BaTiO₃ near the very sharp first order ferroelectric phase transition. We find giant-strength electrocaloric effects that are reversible at sufficiently high temperatures.
[1] A. S. Mischenko, Q. Zhang, J. F. Scott, R. W. Whatmore and N. D. Mathur, Science 311, 1270 (2006).
[2] B. Neese, B. Chu, S.-G. Lu, Y. Wang, E. Furman and Q. M. Zhang, Science 321, 821 (2008).

The Heusler Ni-Mn-Ga alloys are interesting for practical applications because of the numerous unusual effects such as a shape memory effect in ferromagnetic state, a giant magnetocaloric effect and etc. The strong magnetoelastic interaction between the magnetic and structural subsystems is one of the reason of these effects. The magnetic field-induced strain in Ni-Mn-Ga alloys reaches 10% [1,2]. The recent theoretical results show that the adding Pt to Ni-Mn-Ga alloys, a higher the magnetic field-induced strain of 14% can be achieved [3,4]. In our study we investigate the structural and magnetic properties of Ni_2-xPt_xMnGa alloys with the help ab initio calculations [5-7]. Equilibrium value of lattice constant of Ni_2-xPt_xMnGa is calculated with the help of QUANTUM ESPRESSO simulation package [5]. Received values are the same as theoretically calculated in [3]. These values we used for further calculations of magnetic properties for different concentrations of Ni_2-xPt_xMnGa. These calculations carried out with the help of Spin-Polarized Relativistic Korringa-Kohn-Rostoker (SPR-KKR) simulation package [6,7]. Using SPR-KKR package we have calculated exchange parameters and magnetic moment of Ni_2-xPt_xMnGa for concentration range 0 le; x le; 2.0. Calculated results shown that the exchange parameter of Ni-Mn interaction is increased until 4,81 meV at x=0.6 and then decreased with increasing of Pt excess. The exchange parameter of Mn-Mn interaction is changed from ferramagnetic (at x=0) to antiferromagnetic (at x=1).
This work was supported by RFBR (grants 11-02-00601 and 12-02-31129), and RF President grant MK-6278.2012.2.
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3. M. Siewert, M. E. Gruner, A. Dannenberg, Appl. Phys. Lett. 99, 191904 (2011).
4. 3. M. Siewert, M.E. Gruner, A. Hucht et al., Advanced Engineering Materials 14, 530 (2012).
5. P. Giannozzi, S. Baroni, N. Bonini et al, J. Phys.: Condens. Matter. 21, 395502 (2009).
6. H. Ebert, D. Ködderitzsch and J. Minár, Reports on Progress in Physics 74, 096501 (2011).
7. H. Ebert, SPR-KKR package Version 5.4 on http://ebert.cup.unimuenchen.de.

In the last time a large amount of works has been devoted to investigation of the properties of Heusler alloys. These alloys have attracted much attention considering its promising applications in view of their unique properties such as the shape memory effect, the magnetoresistance, the magnetocaloric effect etc [1-3]. The interest of study of the properties of the Fe2+xMn1-xAl Heusler alloys was also promoted because of it was recently experimentally revealed that in Fe-Mn-Al compounds there are anomalous behaviors of optical, magnetic and transport properties due to the competing ferromagnetic - antiferromagnetic coupling interaction between the Fe and Mn atoms at the order-disorder structural transition [4, 5]. Recently it was found that addition of the Ni atoms in Fe-Mn-Al system has led to new "superelastic alloys" in which the revert back to their original shape after deformation is occurred in temperatures ranging from -196 to 240 degrees Celsius [6].
In this work we theoretical investigate the magnetic properties of non-stoichiometric Fe2+xMn1-xAl alloys with the help of ab initio DFT calculations. To perform this study we used the spin-polarized relativistic Korringa-Kohn-Rostoker code [7, 8]. The atomic disorder at specific site is implemented using the coherent potential approximation. The composition dependences of the exchange couplings, total magnetic moments and DOS curves at Fermi level were obtained for different phases. Our simulations have shown that the interaction between the Fe atoms is larger than other interactions and this interaction exhibits a ferromagnetic behavior. The magnetic properties depend very strongly on the crystalline structure, on the phase composition, and on the degree of order. The ab initio exchange integrals are used for calculation of Curie temperatures by means of mean-field approximation and Heisenberg model with Monte Carlo method. Calculated Curie temperatures in non-stoichimetric Fe-Mn-Al alloys are in a good agreement with experimental data [2] and with existed theoretical results [6, 7].
This work was supported by RFBR (grants 11-02-00601 and 12-02-31129), and RF President grant MK-6278.2012.2.
1. Vasil'ev A.N., Buchel'nikov V.D., Takagi T., et al. Phys. Uspekhi 46, 559 (2003).
2. Planes A., Manosa L., and Acet M., J. Phys.: Condens. Matter. 21, 233201 (2009).
3. P. Entel, M. E. Gruner, A. Hucht, et al. in: Disorder and Strain-Induced Complexity in Functional Materials, Springer Series in Materials Science 148, p. 19-47 (2012).
4. E.I. Shreder, A.D. Svyazhin, and K.A. Fomina, Phys. Met. Metallorg. 113, 146 (2011).
5. Z. Liu, X. Ma, F. Meng, and G. Wu, J. Alloys and Compounds 509, 3219 (2011).
6. T. Omori, K. Ando, M. Okano et al., Science 333, 68 (2011).
7. H. Ebert, et. al. Rep. Prog. Phys. 74, 096501 (2011).
8. H. Ebert, SPR-KKR package Version 5.4 on http://ebert.cup.unimuenchen.de.

With the rapid development in science and technology, the demand of high performance devices governing multifunctional properties has been increasing significantly. Magnetic Shape Memory Alloys (MSMAs) have been found as most promising due to their exceptional properties viz. high frequency response and high magnetic-field-induced strain. NiMnGa MSMAs has been received tremendous interest due to excellent two-way shape memory effect, superior super-elasticity and high thermal/ magnetic field induced recoverable stresses. Ni-Mn-Ga thin films show higher ductility and amenable for bending, which enables the feasibility for MEMS applications.
In the present paper, NiMnGa films have been deposited onto Silicon (100) substrates using magnetron sputtering of single alloy target Ni50Mn30Ga20. Two different target powers of 30 W and 70 W at working pressure of 1.5 X 10-3 mbar have been selected. The film thickness has been measured by creating a step onto the substrate and measuring the step height using a surface profilometer. The thickness of the films deposited has been found to ~ 500 nm, which is supported by cross-sectional SEM studies. EDS area scan has been carried out at different locations to ensure the uniformity in the composition and is found to be same with ±0.5 % variation. Films deposited at applied target power of 70 W have been found to have a better stoechiometry when compared to those deposited with 30 W. The influence of deposition temperature on film composition has also studied. Magnetic investigations have been performed onto two samples viz. films deposited at 300 0C in as-deposited condition and post-annealed at 5000C, by using SQUID for B-H & M-T plot. As deposited films have shown an open hysteresis along with undefined Curie temperature, whereas a distinct close hysteresis and well defined Curie temperature have been observed at 335 K, in the case of post-annealed films.

Abstract:
NiTi shape memory alloy (SMA) thin films have been considered as the most promising actuation materials for Micro Electro Mechanical Systems (MEMS) based devices due to their unique properties viz. shape memory effect (SME), pseudo-elasticity (PE) and tunable mechanical properties. The data presented in this paper covers the effect of post deposition annealing on the changes in the various phases in the films and their impact on their mechanical properties.
Ni-rich NiTi films have been deposited on Silicon (100) substrates maintained at 300o C and post annealed at 500OC and 600OC for four hours in high vacuum. These films have been deposited by using DC Magnetron Sputtering of an equi-atomic NiTi alloy target. The composition of films determined by Energy dispersive X-ray Spectroscopy (EDS) have observed to be Ni / Ti composition in the atomic ratios of 51.8 / 48.2. The effect of post-annealing temperature on the structural and mechanical properties has been investigated by using XRD and Nanoindenter. The appearance of Austenite phase as a dominant phase along with Martensite and R-phase has been observed. The thicknesses of sputter deposited films are around 2 mu;m. Nanoindentation testing has been carried out at a constant penetration depth of 150 nm, which is less than one tenth of film thickness. The hardness and modulus values are evaluated by using Oliver and Pharr method and found to be increasing with increasing annealing temperature. The micro-hardness and modulus values of the films deposited at 300O C and post annealed at 500 and 600O C varied between 2.52 to 6.50 GPa and 43 to 143 GPa respectively.
Keywords: Shape memory Alloys, Magnetron Sputtering, Nanoindentation, Atomic Force Microscope (AFM).

Fulvalene diruthenium which was initially synthesized in the 1990s, is attracting renewed attention due to its potential for solar energy storage [1]. Upon absorption of a photon, geometry of the molecule changes from its stable conformation to a higher energy, meta-stable isomer. The molecule retains the stored energy until it is recovered with an external stimulus such as heat. Such isomeric transitions in molecules are similar to phase transformations in crystals.
Fulvalene diruthenium consists of two cyclopentadiene rings connected with a carbon-carbon bond. Each ring has one Ru atom located at the axis of the ring. In the stable conformation Ru atoms are lined up nearly parallel to each other pointing in the same direction. When the molecule absorbs light, one of the rings rotate around the C-C bond so that Ru atoms point in opposite directions. This conformation is the meta-stable isomer.
The objective of this research is to investigate whether anti-ferromagnetic interaction between the two halves of the fulvalene molecule can be created by replacing Ru with transition metals like chromium (Cr), vanadium (V) or titanium (Ti). Magnetic interaction might be helpful to increase the energy difference between the stable and meta-stable conformations which relates to the amount of energy that can be stored. Also the molecular transformation can be controlled by an external magnetic field if magnetic interaction exists.
Properties of the fulvalene molecule with the transition metals are calculated using the DFT method with B3LYP hybrid functional and Pople type basis sets augmented with polarization functions. Atomic models consist of the same molecular geometry as given in [1] without the carbonyl ligands. Ru is replaced by one of the mentioned transition metals to create fulvalene dichromium (FvCr2), fulvalene divanadium (FvV2) and fulvalene dititanium (FvTi2).
Fulvalene diruthenium serves as a test case for the calculations since experimental values are available in [1]. Calculated energy difference between the stable and meta-stable conformations is 1.4 eV, which compares favorably with the experimental value of 1.3 eV.
Calculated energy differences between the stable and meta-stable conformations of FvCr2, FvV2 and FvTi2 are 2.8 eV, 0.9 eV, and 2.9 eV, respectively. FvCr2 and FvTi2 show an increase of stored energy as compared to the reference case. Calculated spin values of Cr or Ti indicate that they have opposing spins, hence antiferromagnetic in the meta-stable conformation. Spin values are two orders of magnitude smaller in the stable conformation, showing weaker magnetic interaction in that state.
[1] “Photochemistry of Fulvalene Tetracarbonyl Diruthenium and Its Derivatives: Efficient Light Energy Storage Devices,” R. Boese, et al., J. Am. Chem. Soc., 1997, 119 (29), pp 6757-6773

Due to the technologically important properties of superelasticity and shape-memory, shape memory alloys have been widely used in Micro- and nanoelectromechanical systems. It is of both scientific and technological interest whether and why the stress-induced martensitic transformation (SIMT) of SMAs exhibits size dependence. Previous approaches on the size dependence of superelasticity in shape memory alloys have already observed that the plateau strain in the stress-strain curve, decrease significantly with the decrease of the grain size, thin film thickness and pillars diameter. However, the physical mechanism underlying this phenomenon is not clear so far. In the present work, through in situ TEM compression on the single crystal Cu-Al-Ni pillars, we find that the onset of the plateau in the stress-strain curve is actually not the critical stress for the nucleation of SIMT; it is indeed the critical stress for the strongly correlated nucleation. The nucleation of SIMT always occurs at a stress lower than the plateau stress. The origin of size dependence of plateau strain arises from the size dependence of probability to generate strongly correlated nucleation prior to SIMT. The decrease of the pillars diameter leads to the absence of potential nucleation sites, thus reduces the probability of strongly correlated nucleation in SIMT. As a result, the plateau strain decreases with the pillars diameter.

I will describe our work on the design and discovery of new classes of ferroelectric and antiferroelectric materials using a combined crystallographic database / first principles approach. For ferroelectrics, using the design principle that any polar structure type can have ferroelectric representatives if the barrier to switching is lowered by appropriate chemical variation, we have recently identified a new family of ferroelectrics in the intermetallic LiGaGe structure type. For antiferroelectrics, we used a design principle based on the close relationship between ferroelectrics and antiferroelectrics to identify a previously unrecognized class of antiferroelectrics, related to the LiGaGe-type ferroelectrics, in the MgSrSi structure type. The discovery of new classes of antiferroelectrics is expected to open the way to increased recognition and application of antiferroelectrics as functional materials.

Piezoelectric materials are among the most widely-used functional materials for their ability of energy conversion between electrical energy and mechanical energy. After the half-century dominance of PZT family materials in piezoelectric-industry, more and more concerns on environment protection and human health appeal for substitutes of PZT, urging the exploration of Pb-free piezoelectric materials. In this study, based on the recently proposed MPB mechanism (PRL.103.257602), a new Pb-free ceramic material Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3 is designed and exhibits the piezoelectric coefficient d33 of ~550 pC/N for its optimal composition, comparable to that of the backbone piezoelectric material PZT-5H. Besides, the measured transitional thermal hysteresis around triple point indicates a first order phase transition, and it proves the non-isotropicity around triple point experimentally, which is lack of consideration in previous literature. Our work may also shed light on exploration of other functional ferroic materials based on MPB mechanism.

BCZT represents a wide range of materials including classical ferroelectrics as well as ferroelectrics exhibiting a diffuse phase transition or relaxor behavior. Above Curie temperature, the electrocaloric (EC) effect is determined by both the temperature and field dependencies of the dielectric permittivity. Simple analytic formulas are derived to demonstrate that a key parameter in achieving large adiabatic temperature changes is the applied electric field. Thus, the performance of EC cooling devices is limited by their electric breakdown strength rather than by their intrinsic material properties. Consequently, there is no need in using lead-containing EC materials exhibiting special properties. However, a main challenge in designing BaTiO₃-based EC cooling devices is the manufacture of high resistive (less oxygen-deficient) materials stable against electric breakdown. Intrinsic acceptor formation and doping of BCZT by acceptors are discussed.
The cooling power of an EC device is determined by its heat capacity, the temperature change during heat absorption, the thermal resistance at the interfaces and the frequency of the cooling cycle. Describing temperature oscillations as a diffusion wave with equal real and imaginary parts of the wavenumber, heat transfer occurs over a distance of half a wavelength. This defines an optimum frequency for applying and removing the external electric field to EC layers of a given thickness. A maximum cooling power is achieved when the thermal relaxation time determined by the product of thermal capacitance and thermal resistance of the EC layer is lower than its heat transition time. This ensures complete heat absorption and rejection and may be satisfied only by a sufficiently low interface thermal resistance.
Operational parameters of EC devices consisting of multiple stacks (to increase thermal mass) of thin (thickness limited by the heat diffusion distance) BCZT layers will be estimated.

Concomitant magnetic and structural phase transitions are important in many phenomena such as magnetostriction, magnetoresistance and magnetocalorics [1]. However, structural phase transitions arise in only a few classes of magnetic material. Here [2] we report extrinsic magnetostructural transitions in ferromagnetic La_0.7Ca_0.3MnO₃ manganite films due to strain from structural phase transitions in BaTiO₃ substrates. Macroscopic magnetometry data reveal that magnetically driving these transitions results in film entropy changes that are comparable with the best magnetocaloric effects in bulk materials [3]. Photoemission electron microscopy (PEEM) with x-ray magnetic circular dichroism (XMCD) contrast, and ferromagnetic resonance (FMR), reveal that our extrinsic magnetocaloric effects arise due to a change in the fraction of the magnetically polarized phase.
[1] Magnetism and Structure in Functional Materials, edited by A. Planes, Ll. Manosa and A. Saxena, Materials Science Series, Vol. 79 (Springer-Verlag, Berlin, 2005).
[2] X. Moya, L. E. Hueso, F. Maccherozzi, A. I. Tovstolytkin, D. I. Podyalovskii, C. Ducati, L. C. Phillips, M. Ghidini, O. Hovorka, A. Berger, M. E. Vickers, E. Defay, S. S. Dhesi and N. D. Mathur, Nature Materials (DOI 10.1038/NMAT3463).
[3] K. A. Gschneider, Jr. and V. K. Pecharsky, Annu. Rev. Mater. Sci. 30, 387-429 (2000).

The first symmetry-lowering phase transition to occur after the Big Bang -- the so-called Grand Unification Transition -- is believed to have yielded cosmic strings, formed as topologically protected defects when symmetry-lowered regions of different phase intersected. It is extraordinarly difficult, however, to study cosmic string formation directly because of issues associated with replaying the Big Bang in the laboratory. Here I will show that the multiferroic hexagonal manganites -- with their coexisting magnetic, ferroelectric and structural phase transitions -- generate topologically protected defects at their domain intersections which are the crystallographic equivalent of cosmic strings. The ferroic phase transitions are readily accessible in the laboratory, and the resulting domains straightforward to characterize, so the hexagonal manganites can be used to test the so-called "Kibble-Zurek" scaling laws proposed for defect-formation in the early universe. I will describe how electronic structure calculations and symmetry analysis allowed identification and quantification of the important features of the hexagonal manganites, and present experimental results of the first unambiguous
demonstration of Kibble-Zurek scaling.

Twin boundaries play a critical role in imparting superior strength, ductility and stability to nanotwinned metals. Here, we investigate the mechanisms that govern the kinetic properties of twin boundaries based on thermal fluctuations calculated by way of molecular dynamics. Interpreting our simulations within the context of the recent work by Karma et al, our results reveal that the twin boundary migration is coupled to shear deformation up to 0.9 homologous temperature. Unlike high angle grain boundaries, coherent twin boundaries do not exhibit any roughening transition from shear coupling to pure sliding governed by capillary fluctuations.

The cofactor conditions [1,3] are conditions of compatibility between phases, which allow for perfect interfaces between austenite and martensite, as well as solutions of the crystallographic theory with an arbitrary volume fraction of the twin variants. We present the first example of an alloy whose composition has been tuned to satisfy these conditions [1]. This alloy is an off-stoichiometric alloy based on the Zn2CuAu Heusler system. Despite having transformation strain of more than 10%, the tuned alloy exhibits repeatable hysteresis of less than 1 C measured by Differential Scanning Calorimetry (DSC) in bulk samples. This represents approximately one order-of-magnitude smaller hysteresis than any other bulk alloy with comparable (>5%) transformation strain. The alloy also exhibits exceptional reversibility under cycling in the DSC machine, as quantified by the migration of transformation temperature. The microstructure of martensite observed in this alloy looks completely different from any other martensites, but is consistent with a theoretical study of the cofactor conditions [2]. Overall, the example indicates that the cofactor conditions represent a promising strategy to achieve exceptional reversibility of phase transformations for shape memory, ferromagnetic shape memory, energy conversion, magnetocaloric, microelectronic and photonic applications.
References
[1] X. Chen, V. Dabade and R. D. James, Exceptionally low hysteresis and novel microstructures in Zn2CuAu alloys with satisfaction of the Cofactor Conditions, in preparation.
[2] X. Chen, V. Srivastava, V. Dabade and R. D. James, Study of the Cofactor Conditions: conditions of supercompatibility between phases, in preparation.
[3] R. D. James and Z. Zhang, Vol. 79, edited by A. Planes, L. Manosa, and A. Saxena (Springer, New York, 2005) p. 159.

Photonics Integrated Circuits (PICs) are being applied by the telecommunications industry as transceivers for fibre optic networks. The core component of a typical PIC is the laser array and these devices can have relatively low operating temperatures (15°C - 25°C) with a tight operating range (±0.1K). To accommodate such a specification, a thermal control system is required that can change the cooling rate through feedback. The power density of next-generation PICs is at such a level to demand liquid-based cooling architectures, in which control is achieved by varying the rate of coolant. Consequently, micro-valve structures are required, ideally involving passive actuation to meet stringent reliability standards. One solution to this challenge is to exploit the phase-change driven shape memory effect of the NiTi Shape Memory Alloy (SMA). A micro valve could be developed from the NiTi SMA, thermally coupled to the laser array component in order to work passively to regulate the flow of coolant in a micro channel. Such a valve would have to be intrinsically reliable, and the goal of this paper is to investigate the conditions that will affect this reliability. The objective of the work is to investigate the mechanical properties relevant to the design of a passive NiTi SMA micro valve, with a focus on the plastic softening and hysteresis that will occur after the formation of stress-induced martensite. Temperature controlled tensile tests were performed over a range of 25°C - 80°C on specimens of 0.5mm diameter NiTi wire, and strain measurements were performed with the use of Digital Image Correlation (DIC). Samples were put through a small number of loading-unloading cycles for a range of temperatures to investigate plastic softening. Samples from various stages of the testing were mounted in an epoxy resin and cross sectioned. The microstructure of these samples was then analysed with the use of Electron Back Scatter Diffusion (EBSD) in a SEM. The paper provides valuable insight into the conditions that will affect the reliability of a passive NiTi SMA valve subjected to a sufficient pressure to form stress-induced martensite.

NiTiCu Shape Memory alloy thin films have huge potential making various MEMS based devices because of their excellent shape recovery properties. The shape recovery is because of martensitic transformation which is based on reversible twinning. The martensitic transformation and hence the shape recovery gets affected by irreversible plastic deformation. Most of the MEMS devices will be exposed to mechanical contacts. These stresses by these contacts will induce plastic deformation. Hence it is very important to understand the transformation behavior of NiTiCu thin films when subjected to contact stresses.
Systematic investigations were carried to understand the behavior of the NiTiCu films when subjected to indentation, mimicking nano-mechanical contacts. The films were deposited by sputtering at room temperature. Ti-rich TiNiCu films were deposited by in-house fabricated three target Magnetron Co-Sputtering system at a working pressure of 1.5x10-2 mbar for 30 min at room temperature. The composition of the films obtained from EDS was found to be 60:33:7 (Ti:Ni:Cu at%). The films were post annealed at 500°C for four hours in order to ensure the crystallanity of the films for observing the shape memory effect. Nano-mechanical contacts were simulated using a nanoindenter with various localized contact stresses. Various contact stresses were applied by using loads varying from 0.25 mN to 25 mN. The films were heated above the transformation temperature to 250°C and cooled down to room temperature in high vacuum, enabling the shape recovery of the indents by martensitic transformation. Morphology and topography of the indents were studied by SEM and AFM imaging before and after heating the sample. Detailed subsurface deformation of the nano-indents was studied by sequentially making the sections using Focused Ion Beam (FIB). The cracks induced by the contact stresses have influenced the shape and amount of recovery of the NiTiCu films.

Within the class of reversible transition that goes through first order transition shape memory alloy enjoys widespread applications in the field of defense, medical industry and national security. Since last decade, multiferroic alloys, that has more than one ferroic properties (ferroelasticity, ferroelectric and ferromagnetic) have been considered for future material. We conjectured that by using two of the ferroic properties, these alloys can be exploited to use as alternate energy. Strategically, guided by theory[1], known as ‘Geometrically non linear theory of martensite&’, the series of low hysteresis alloy is discovered. The general strategy to achieve low hysteresis in big first order phase transformations will also be discussed. We discuss a new method for the direct conversion of heat to electricity using the recently discovered multiferroic alloy, Ni45Co5Mn40Sn10[2,3]. This alloy undergoes a low hysteresis, reversible martensitic phase transformation from a nonmagnetic martensite phase to a strongly ferromagnetic austenite phase upon heating. When biased by a suitably placed permanent magnet, heating through the phase transformation causes a sudden increase of the magnetic moment to a large value. As a consequence of Faraday&’s law of induction, this drives a current in a surrounding circuit. Other new methods for the direct conversion of heat to electricity involving magnetism and phase transformations suggested by the underlying theory will also be discussed. Role of hysteresis in magnetocaloric material, elastocaloric material and electrocaloric material will also be discussed.
[1] Z. Zhang , R. D. James , S. Müller , Acta Mater. 2009 , 57 , 4332 . Invited overview.
[2] V. Srivastava , X. Chen , R. D. James , Appl. Phys. Lett. 2010 , 97 ,014101.
[3] V. Srivastava, Y. Song, K. Bhatti and R. D. James. The Direct Conversion of Heat to Electricity Using Multiferroic Alloys. Advanced Energy Materials (invited, Inaugural Issue),1 (2011), 97-104.

Magnetic cooling could be a radically different energy solution substituting conventional vapor compression refrigeration in the future. For the largest cooling effects of most potential refrigerants we need to fully exploit the different degrees of freedom such as magnetism and crystal structure. We report now for Heusler-type Ni-Mn-In-(Co) magnetic shape-memory alloys, the adiabatic temperature change delta T(ad)= -8K under a moderate field of 2 T. Here it is the structural transition that plays the dominant role towards the net cooling effect. A phenomenological model is established that reveals the parameters essential for such a large delta T(ad). We also demonstrate that obstacles to the application of Heusler alloys, namely the usually large hysteresis and limited operating temperature window, can be overcome by using the multi-response to different external stimuli and/or fine-tuning the lattice parameters, and by stacking a series of alloys with tailored magnetostructural transitions.

We present a thermodynamic model of multiferroic materials undergoing first order phase transformation that can be used in a new energy conversion method from heat to electricity. The new method was recently demonstrated on the alloy Ni45Co5Mn40Sn10. This method of converting heat to electricity provides a possible route to generate electricity without significant greenhouse gas emission, potentially applicable to energy conversion using the waste heat from power plants, automobile exhaust systems, and computers, as well as natural sources such as solar- and geothermal conversion. The general idea makes use of the fact that magnetoelectronic properties such as magnetization and polarization are sensitive to changes of lattice parameters, which usually occur at structural phase transformations. Using standard methods of electromagnetic conversion, such as induction and charge separation, the abrupt change of a suitable magnetoelectronic property can be converted into electricity. An attractive feature of this method is the elimination of the generator, the heat is converted directly to electricity. Previous works on the reversibility of martensitic phase transformations enables us to reduce the hysteresis to near zero by tuning the compositions of alloys or oxides, which guarantees the high durability of energy conversion devices utilizing such materials. What the potential efficiency and power output are, and how best to design the device rest on thermodynamic arguments. The thermodynamic model for this new energy conversion method presented in this work is based on the Gibbs free energy, which can be explicitly evaluated for a material from calorimetry and magnetometry measurements. Using this model, we predict the effect of magnetic (or electric) field on transformation temperature, via the Clausius-Clapeyron relation. Field and temperature induced phase transformations can also be predicted, and they play the central roles in analyzing the new energy conversion method. Such Clausius-Clapeyron relations depend on various material properties, so the implications of our thermodynamic model for future material development are also discussed. This work was supported by MURI W911NF-07-1-0410, NSF-PIRE (OISE-0967140), DOE (DE-FG02-05ER25706), MURI FA9550-12-1-0458, and the Initiative for Renewable Energy and the Environment at the University of Minnesota. Parts of this work were carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which receives partial support from NSF through the NNIN program. CL&’s contribution (and part of KB&’s) was specifically supported by DOE award DE-FG02-06ER46275.

A simplification of the coupled differential equations describing energy conservation in the solid and fluid of an AMR leads to analytic solutions for idealized conditions [1]. The resulting expression for cooling power is a function of two parameters related to material properties and operating conditions [2]. Experimental measurements of AMR performance with single materials are found to follow the trends predicted by the analytic cooling power expression where appropriate average properties are used. By fitting experimental curves of temperature span as a function of cooling power to the analytic cooling power expression, the preferred functional dependence of magnetocaloric effect as a function of temperature is determined. Results are used to identify preferred material properties and how different materials impact layering strategies in a multi-material AMR.
[1] Rowe, A., “Thermodynamics of Active Magnetic Regenerators: Part I,” Cryogenics, 52, 2012.
[2] Rowe, A., “Thermodynamics of Active Magnetic Regenerators: Part II,” Cryogenics, 52, 2012.

The Heusler-derived multiferroic alloy system Ni_50-xCo_xMn_25+ySn_25-y near x = 6 and y = 15 has recently been shown to exhibit, at just above room temperature, a highly reversible martensitic phase transformation with an unusually large magnetization change. This system is in fact prototypical of a newly developed family of alloys with outstanding potential for sensing, actuation, magnetic shape memory, magnetocaloric, and energy conversion applications, enabled via hysteresis control. In this work we provide a thorough study of the martensitic transformation, hysteresis, short- and long-range magnetic ordering, electronic behavior, and magnetic inhomogeneity in bulk Ni_50-xCo_xMn_25+ySn_25-y, creating a detailed picture of the phase behavior around the critical region around y = 25, using x-ray diffraction, thermodynamic, magnetometry, electronic transport, and neutron scattering methods. In addition to clarifying the fundamentals of the interplay between the structural phase transformation and the magnetic/electronic properties, and the extent to which the behavior is universal across other alloy systems, we focus in detail on the issue of magnetic inhomogeneity. The formation of nanoscopic ferromagnetic spin clusters in globally non-ferromagnetic phases, which has been observed in numerous studies of off-stoichiometric Heusler alloys, is studied directly for the first time via small-angle neutron scattering, combined with magnetometry and transport. We discuss in detail the form and origin of these spin clusters, their physical size, their inter-cluster interactions, the nature of the ground state magnetic ordering in the martensitic phase, and the broader implications for our understanding of such alloy systems.
Work supported by UMN IREE, MURI, and NSF/PIRE. Neutron scattering work supported by DOE.

The magnetic shape memory alloy Ni-Mn-Ga belongs to a class of active materials that show high prospects for the application in microsystems. The actuation mechanism is based on the magnetically induced reorientation of martensitic variants which is realized by twin boundary motion. With its high achievable strains of up to 10 % [1] and high cycling frequencies of up to 250 Hz [2], Ni-Mn-Ga films are an alternative to conventional piezoelectric materials. Because of the high achievable strains, there is often no need for the integration of additional levers in the actuator design. The application of Ni-Mn-Ga as active material in microactuators requires the preparation of freestanding Ni-Mn-Ga films with a defined twinning configuration and low twinning stress. We present Ni-Mn-Ga films that reveal a-c-twin boundaries of 14M martensite. These twin boundaries are highly mobile and can be moved by an external magnetic field in bulk single crystals [1]. However, up to now, the twinning stress of a-c-twin boundaries in freestanding Ni-Mn-Ga films is in the range of 25 MPa [3] and thus exceeds the maximum of the magnetostress of 2 MPa. Furthermore, elastic incompatible interfaces between areas with differently oriented twin boundaries can hinder any change in film extension.
We present results of the first training experiments on freestanding Ni-Mn-Ga films that aimed at defining the orientation of martensitic variants during the reversible austenite-martensite transition. We thus developed a training device that can be used for thermo-magneto-mechanical treatment of the freestanding films and allows us to monitor the austenite-martensite transformation in-situ. We could observe a macroscopic strain of about 1.6 % which can be ascribed to the thermal shape memory effect. For thermo-mechanical training, a tensile of about 0.6 MPa was applied along the long film axis in order to align the long a-axis of the 14M martensite along the tensile direction. For thermo-magneto-mechanical treatment, we additionally applied an external magnetic field of 1 T in the film plane and perpendicular to the tensile load to fully define the orientation of 14M variants during austenite-martensite transformation. The influence of the thermal, thermo-mechanical and thermo-magneto-mechanical training, respectively, on the film structure and microstructure was analyzed ex-situ after 25 training cycles. The fixing points represent inactive areas where twin boundary motion is hindered. Beyond the area of fixation we observed a coarsening of the initial twin variants by the different training cycles. Structural investigations revealed that the out-of-plane orientation of the short and magnetic easy c-axis of 14M martensite is favored by all training methods.
This work was funded by DFG via SPP 1239.
[1] A. Sozinov et al., Appl. Phys. Lett. 80, 2002, 1746.
[2] I. Aaltio et al., Smart Mater. Struct. 19, 2010, 075014.
[3] A. Backen et al., Acta Mater. 58, 2010, 3415.

The complex microstructure present in modulated shape memory alloys is beneficial for magnetic field induced reorientation or reversible field induced phase transition [1]. For instance, the big difference in mobility between type I and type II twin boundaries highlights the importance of a full monoclinic description of the lattice [2]. Hence it is important to understand the microstructure from the atomic up to the macroscale.
As a model system we analyze epitaxial Ni-Mn-Ga films exhibiting 14M martensite and performed X-ray diffraction in 2-circle and 4-circle geometry. The film substrate serves as a reference frame, which allows to probe absolut orientations of variants.
The resulting complex diffraction patterns and pole figures are explained using a scale-bridging model based on the Wechsler-Lieberman-Read theory. We incorporate the concept of adaptive martensite where modulated martensite is described as a nanotwinned tetragonal martensite [3, 4] and nano- and mesoscopic variants of tetragonal martensite are connected by branching [5]. In this model, the c/a-ratio of the tetragonal martensite is the only parameter necessary to describe all reflections in the pole figures by calculating the habit planes and twin variant orientations of non-modulated and modulated martensite. We observe the functional c-a-twin boundaries of 14M, but also b-c and a-b twin boundaries are present. The latter can also be considered as stacking faults of the nanotwinned lattice.
Using kinematic diffraction simulation on such a faulted nanotwinned lattice, we show that stacking faults may lead to superstructure reflections that overlap with the reflex positions of the fundamental tetragonal lattice. This confirms TEM and SEM observations these samples, which are dominated by faulted superlattices.
[1] R. Niemann, U. K. Rossler, M. E. Gruner, O. Heczko, L. Schultz, S. Fahler, Adv. Eng. Mat. 14, 2012, 562
[2] L. Straka, O. Heczko, H. Seiner, N. Lanska, J. Drahokoupil, A. Soroka, S. Fahler, H. Hanninen, A. Sozinov, Acta Mat. 59, 2011, 7450
[3] A. G. Khachaturyan, S. M. Shapiro, S. Semenovskaya, Phys. Rev. B 43, 1991, 10832-10843
[4] S. Kaufmann, U. K. Rossler, O. Heczko, M. Wuttig, J. Buschbeck, L. Schultz, S. Fahler, Phys. Rev. Lett. 14, 2009, 145702
[5] S. Kaufmann, R. Niemann, T. Thersleff, U. K. Rossler, O. Heczko, J. Buschbeck, B. Holzapfel, L. Schultz, S. Fahler, New J. Phys. 13, 2011, 053029

Technical application imposes fundamental constraints on intrinsic properties, as transformation temperatures, saturation magnetization or magnetocrystalline anisotropy of ferromagnetic shape memory alloys (FSMAs). Our aim is to improve the intrinsic properties of Fe70Pd30 by systematic alloying with third elements. To gain a comprehensive understanding of the heuristic trends, however, a large experimental data basis used in combination with predictive theoretical tools is essential. Due to the large number of samples required, conventional fabrication and characterization methods can not provide sufficient results on a reasonable time scale. Combinatorial materials science offers a route for a comprehensive understanding by fabricating and investigating full ternary materials libraries. Elements added into the Fe-Pd system are Mn and Cu. All system form single phases within different compositional regions. When Mn is alloyed into Fe-Pd, the martensitic transformation is shifted to higher values originated by an anomalous increase of the thermal expansion coefficient [1,2,3]. Magnetic properties are only slightly lowered. In combination with ab initio modelling the addition of Cu into Fe-Pd allows to increase transformation temperatures (Tmax = 358 K) without degrading the magnetic properties. Further a stabilization of the metastable transforming phase is observed as well as an increase of the anomalous Invar-type reduction of thermal expansion [4]. To gain further insight into intrinsic properties, Fe70Pd30-xCux thin films with different Cu contents (x = 0, 3, 7) were epitaxially grown [5]. We investigated the variation of magnetic properties in dependence of the unit cell strain as previously reported for binary Fe70Pd30 [6,7]. The results indicate that Cu improves the epitaxial growth allowing a much better film quality. Furthermore a substantially increase of the magnetocrystalline anisotropy constant K1 as a key intrinsic magnetic property of FSMAs by 40 % is observed. The values obtained for the fct structure exceed those reported for the Ni-Mn-Ga system, which makes the Fe-Pd-Cu system of particular interest for microsystems with a high energy density. The presented results indicate that combinatorial screening methods accompanied by extended investigations and simulations provide a comprehensive tool in the search for new FSMAs.
This work is supported by the DFG through SPP1239.
[1] S. Kauffmann-Weiss et al., Adv. Eng. Mater. 14, No. 8, 724-749 (2012).
[2] S. Hamann et al., Actuator 2008 Conference Proceedings, Bremen, 271-274 (2008).
[3] S. Thienhaus et al., Sci. Technol. Adv. Mat. 12, 054206 (2011).
[4] S. Hamann et al., Acta Mater. 58, 5949-5961 (2010).
[5] S. Kauffmann-Weiss et al., Acta Mater. 60, 6920-6930 (2012).
[6] J. Buschbeck et al., Phys. Rev. Lett. 103, 216101 (2009).
[7] S. Kauffmann-Weiss et al., Phys. Rev. Lett. 107, 206105 (2011).

FePd based alloys have attracted tremendous interest during the past 30 years - initially due to their Invar as well as magnetic properties, and, more recently, owing to their magnetic shape memory (MSM) behavior with theoretical strains up to approximately 5%. Fe7Pd3 exhibits four different metastable phases: the austenite fcc phase and three martensite phases (fct, bct, and bcc). Our work aims at exploring the influence of generalized internal stresses due to (i) point defects and (ii) deviations from equilibrium short-range order. While the former are stresses in a strict mechanical sense, the latter are the conjugate variable of the short-range order parameter. In a given sample, manipulation of both, (i) and (ii) can conveniently be achieved by means of irradiation with energetic ions. 500 nm thick single crystalline Fe7Pd3 films were deposited at 850°C on MgO (001) single crystalline substrates. The unirradiated samples exhibit prevalently the austenite fcc phase. These thin films were ion-irradiated with 1.8 MeV Kr+ -ions. Fluency dependant T2T-measurements show that ion-irradiation-assisted phase selection along the whole transformation path ranging from fcc->bcc is possible.
Fluency dependant pole figure measurements describe the fcc->fct and the transformation into the bcc phase in detail. An orientation relationship according to Nishiyama-Wassermann for the fcc->bcc transformation is observed. An ion-induced surface twinning structure is observed by AFM and SEM measurements. Ion irradiation thus provides a way to sustainably manipulate and fine-tune the Fe7Pd3 phase post deposition - for use in application and further fundamental studies on the nature of the transformation.
This project is funded by the Leipzig Graduate School of Natural Sciences through the German Science Foundation, the European Union and the Free State of Saxony as well as the German Federal Ministry of Education and Research (BMBF, PtJ-BIO, 0315883).
[1] Ion-irradiation-assisted phase selection in single crystalline Fe7Pd3 ferromagnetic shape memory alloy thin films: From fcc to bcc along the Nishiyama-Wassermann path, A. Arabi-Hashemi and S. G. Mayr, Phys. Rev. Lett., Accepted Wednesday Aug 29, 2012

Many similar phenomena, such as caloric effects and those resulting
from disorder, are ubiquitous in all four primary ferroic materials.
Ferroic materials possess two or more orientation states (domains)
that can be switched by an external field and show hysteresis. Typical
examples include ferromagnets, ferroelectrics and ferroelastics which
occur as a result of a reversible phase transition with the onset of
spontaneous magnetization (M), polarization (P) and strain (e),
respectively. The fourth class of ferroic materials called
ferrotoroidics (with an ordering of magnetic vortices) has been
recently found. Phase transitions result from symmetry breaking:
Broken rotational symmetry in a crystal leads to ferroelasticity, broken
inversion symmetry leads to ferroelectricity and broken time reversal
symmetry results in ferromagnetism. Simultaneous spatial inversion and
time reveral symmetry breaking leads to ferrotoroidoc behavior. I will emphasize the role of long-range, anisotropic forces such as those
arising from either the elastic compatibility constraints or the (polar
and magnetic) dipolar interactions (or toroidal quadrupolar interactions)
in determining the microstructure. I will attempt to demonstrate that In the presence of disorder all ferroic materials are expected to exhibit tweed precursors and glassy behavior. Similarly, all ferroics are likely to exhibit caloric effects. Finally, I will comment on
bioferroics: ferroic phenomena in biological systems.

Magnetic properties of polycrystalline (at.%) Ni_50+xMn_37-xSn₁₃ and Ni_50+yMn_39-ySn₁₁ alloys have been studied in high magnetic fields up to 56 T in a temperature range 4.2 - 300 K. Field dependencies of magnetization M(H) measured at liquid helium temperature for Ni₅₂Mn₃₅Sn₁₃ and Ni₅₃Mn₃₆Sn₁₁ indicate that the Sn content has a profound effect on the magnetism of Ni-Mn-Sn alloys. Magnetization saturation determined by extrapolation of the magnetization curves to H = 0 T appeared to be almost the same (20 emu/g and 18 emu/g in Ni₅₂Mn₃₅Sn₁₃ and Ni₅₃Mn₃₆Sn₁₁, respectively). However, susceptibility of paraprocess in Ni₅₃Mn₃₆Sn₁₁ (chi;_p asymp; 2.6×10^-5 emu/gOe) is two times larger than that in Ni₅₂Mn₃₅Sn₁₃ (chi;_p asymp; 1.2×10^-5 emu/gOe). These data indicate that antiferromagnetic interaction is enhanced in the alloy with the lower Sn content.
Pronounced metamagnetic anomalies have been observed in Ni₅₁Mn₃₆Sn₁₃ at T = 275 K and in Ni₅₂Mn₃₅Sn₁₃ at T = 300 K, i.e. below their martensitic transition temperatures A_s asymp; 302 K and A_s asymp; 327 K, respectively. Origin of the anomalies is a magnetic field induced transition from paramagnetic low-temperature martensitic phase to ferromagnetic high-temperature austenitic phase.

Ni-Mn-based Heusler alloys undergo martensitic transformations with a strong interplay between structural and magnetic degrees of freedom, leading to several interesting properties such as magnetic shape memory, magnetic superelasticity and inverse magnetocaloric effect. Beside the magnetic field induced properties, the magnetic interactions are investigated around the martensitic transformation [1, 2]. Ferromagnetic resonance (FMR) is one of the techniques to understand magnetic behaviour in the austenite and martensite states. We report on the temperature dependence of FMR to discuss magnetic interactions in Sn-doped Ni50Mn34In16 alloy and the effects of these interactions on the inverse magnetocaloric effect.
This work was supported by Deutsche Forschungsgemeinschaft (No. SPP1239).
[1] S. Aksoy et al., Phys. Rev. B., 79 (2009) 212401.
[2] S. Aksoy et al., J. Phys. Conf. Ser., 200 (2010) 092001.

“The material is the machine” is a new paradigm in mechanical engineering relying on multifunctional materials such as multiferroics. Thermal, magnetic, electrical, and strain hysteresis characterize the response of multiferroics to external fields. The thermal hysteresis was recently modeled [1]. Also in the case of ferromagnetic shape memory alloys substantial progress was made elucidating the complex interactions of ferroelasticity, ferromagnetism and magnetocaloric effects [2]. Chemical composition of shape memory alloys can strongly impact their phase stability and therefore the functional properties; chemical compositions can be carefully selected tailoring lattice parameters to achieve almost perfect reversibility and a small hysteresis. At a given chemical composition, crystal size at the nanoscale can significantly impact the formation of thermoelastic martensite [3]. The present paper reports on recent results in the field of ultrafine grained ferromagnetic Ni54-Mn25-Ga21 shape memory alloys (mean grain size 140 nm) and nanocrystalline Ti50-Ni40.75-Pd9.25 shape memory alloys (mean grain size 60 nm); in their coarse grained state Ti50Ni50-xPdx alloys show a minimum of the hysteresis at x=9.25. In both materials, the small grains tend to supress the formation of martensite and to decrease its thermal stability upon cooling and heating, respectively. Caused by the geometrical constraints of the grain boundaries, self-accommodation at a mesoscale is strongly inhibited. Therefore in the case of Ni-Mn-Ga, contrary to coarse grains containing non-modulated tetragonal martensite, accommodation of transformation strains of martensite confined to small grains favours the formation of a metastable adaptive 14M structure. In the case of the low-hysteresis Ti-Ni-Pd alloy, results obtained by X-ray diffraction, in-situ synchrotron experiments, and transmission electron microscopy show the formation of orthorhombic B19 martensite both in coarse grains and in nanograins. In the case of the coarse grains, martensite does not contain twins since special lattice parameters yield a transformation stretch matrix with a middle eigenvalue almost identical to 1 [4]. This facilitates compatibility of the austenite with a single variant of the martensite [1]. Contrary to that, in the nanograins martensite contains fine twins. The twinning might be forced by minimization of the overall transformation strain energy required in small grains [3]. Finally, it should be noted that crystal size at the nanoscale increases the thermal hysteresis (measured values were 7°C and 36°C in the coarse grained and nanocrystalline material, respectively).
[1] Z. Zhang, R.D. James, S. Müller, Acta Mater. 57 (2009) 4332.
[2] Adv. Eng. Mater. 14, Vol. 8, 2012 (special issue edited by. S. Fähler; Wiley-VCH, Weinheim, Germany).
[3] T. Waitz, K. Tsuchiya, T. Antretter, F.D. Fischer, MRS Bulletin 34 (2009) 814.
[4] R. Delville et al., Phil. Mag. 90, (2010) 177.

The vast family of Ni-Mn based Heusler alloys provides an extended playground of physical properties. The interplay between a reversible martensitic transformation (MT) and magnetically ordered states gives rise to a series of functional properties, such as giant magneto- and baro-caloric effects, giant magnetoresistance, magnetic shape memory and magnetic superelasticity, that can be exploited for developing innovative devices. The path towards the technological application of these materials relies on the enhancement of the sensitivity of the MT to external forces, such as magnetic field (dT_M/dH) or pressure (dT_M/dp), which lead to the actuation of the reversible transformation itself. Also, the possibility to exploit these materials in an easy-to-obtain polycrystalline form, rather than as single crystals, can extend their application possibilities.
We have shown that by proper Co-doping Mn-rich Ni₂MnGa alloys it is possible to revert the order of the structural and magnetic transitions, giving rise to a reverse MT between a paramagnetic low temperature phase and a ferromagnetic high temperature one [1]. The reverse transformation increases the magnetization jump between the two well beyond the maximum values showed by the ternary alloy NiMnGa. The corresponding magnetocaloric effects are greatly enhanced [2].
By partially replacing Ga with a few at.% of In, it is possible to tune independently the magnetic and structural critical temperatures, enlarging the compositional range where the reverse MT is realized. Magnetic measurements under pressure highlight that Co- and In-doping increases also the sensitivity of the MT to the applied pressure, allowing for remarkable values of the dT_M/dp parameter. This feature is linked with the structural modifications induced by the stoichiometric changes; temperature dependent X-ray diffraction measures collected in a wide temperature range show that the volume difference between the parent and product phases is greatly enhanced by Co and In; ΔV/V values beyond 1% and higher than any other NiMnX Heusler alloy are found (X being a IIIa-Va element).
Magnetostriction measurements in extremely high magnetic fields (up to 30T) confirmed the remarkable structural (ΔV/V) and magnetic (dT_M/dH) changes related to the MT and account for the behavior of these alloys in extreme conditions; a reversible linear deformation of 0.35% in polycrystalline unstrained samples is realized. Such a value is expected to be enhanced by proper microstructure texturing: the structural characterization highlights a considerable anisotropy of the martensitic cell, the c/a parameter being of the order of 1.2.
[1] S. Fabbrici et al., Appl. Phys. Lett. 95, 022508 (2009)
[2] S. Fabbrici et al., Acta Mater 59, 412-419 (2011)

It is difficult to obtain a single crystal of materials whose production process includes solid-solid transformations. L10-type ferromagnetic alloys are ones of these materials and would have multi-variant structures due to the transformation from FCC to L10-type structure under no external field. While, it is reported that single variant structure can be obtained by applying magnetic field during heat treatment, the detailed mechanism is still unclear. Therefore, calculations of evolution of microstructures by Phase Field Modeling have been carried out to clarify this mechanism. Chemical, interface, elastic strain and magnetic energies are taken into account. Calculated results agree with experimental results, that is, multi-variant structures were obtained under no magnetic field and variant selection took place under magnetic field. As is well known, magnetic energy is smaller than other energies. As was expected, the analysis of driving force suggests that the magnitude of driving force component of magnetic energy is small and that interface and elastic strain effects drive the variant selection. Further analysis indicates that magnetic field is effective only at the very beginning of phase transformation and triggers the variant selection driven by interface and elastic strain energies. Magnetic energy drives preferential formation of the magnetically favorable variant. As a result, interface energy becomes dominant driving force of variant selection because the size of magnetically favorable variant becomes bigger. And then, magnetically favorable variant becomes dominant, which is the reason why elastic strain energy becomes dominant.

In recent years metamagnetic shape memory has been reported in (Ni,Co)-Mn-Al bulk alloys. It is noted that in these samples the magnetization of the martensite phase is considerably smaller than that of parent phase and the martensitic transition temperatures (TM) are reduced by the application of magnetic fields. However, the understanding of the effect of Co addition on magnetic properties of these alloys is still incomplete.
In this work we present detailed magnetic properties of Ni45Co5Mn32Al18 melt spun ribbon. The emphasis is to understand the origin of the shift of TM with field, using Arrot plots. In the M-T curves, as the temperature was reduced, the room temperature paramagnetic phase of the ribbon was observed to go through a ferromagnetic transition at ~200K and a typical metamagnetic behavior as reported by Kainuma et. al for bulk samples, was observed below 170K. This data has been analysed using Arrot plots. For the analysis, presence of both antiferromagnetic (L) and ferromagnetic (M) interaction in the magnetic sublattice is considered. Considering moments M and L as scalar quantities and minimizing the free energy with respect to both M and L, we can write the equation
Atilde;M+CM^(3 )- B_o+ γL^2 M=0
The Arrot plots show convex turning below 170K, at around 200K the line is almost straight and passes near the origin. In the paramagnetic phase displacement is to the right with increasing temperature. As the temperature increases from 5K to 170K the Arrot plot are displaced to the left. Due to the renormalization of the ferromagnetic properties by the antiferromagnetic order the slope of Arrot plot changes and the antiferromagnetic and ferromagnetic moment does not change significantly with field for low field strengths. The coupling constant γ is calculated. A positive value of γ shows that the ferromagnetic instability of the system is eliminated and the antiferromagnetic order is stabilized down to the lowest temperatures. The antiferromagnetic transition temperature shifts to lower temperature with increasing magnetic field, consequently, exchange bias measurements at low temperature (~2K) shows large exchange bias field of 320 Oe at 2K.