Chairs

J. Michael Coey 
Univ of Dublin 
Trinity College 
Dept of Physics 
Dublin, 2 IRELAND 
35-3-16081470

Laura Lewis
Dept. of Applied Science
Brookhaven National Lab
Bldg 480
Upton, NY 11973-5000
516-344-2861

Bao-Min Ma 
Rhodia Corp 
CN 7500 
Cranbury, NJ 08512 
609-860-4550

Thomas Schrefl
Inst of Applied & Technical Physics
Vienna Univ of Technology
Vienna, A-1040 AUSTRIA
43-1-58801-5619 x13729

Ludwig Schultz
Inst for Metallic Materials
IFW Dresden
Dresden, D-01171 GERMANY
49-351-4659321

Proceedings published as Volume 577
of the Materials Research Society
Symposium Proceedings Series.

* Invited paper

TUTORIAL
STH: CONCEPTS AND EXPERIMENTAL METHODS IN MICROMAGNETISM
Monday, April 5, 2:00 - 5:00 p.m.
Salon 6 (Marriott Hotel)

in recent years in both permanent magnets and magnetic recording have emphasized the importance of understanding magnetism on a mesoscopic length scale. This tutorial session will review basic concepts in micromagnetism with reference to the fundamental length scales (exchange length, domain wall width, single domain particle size, radius for coherent rotation, superparamagnetic blocking radius). It will also present computer simulation methods for studying magnetization reversal and review the range of experimental techniques available to investigate magnetic domain structures.

Instructors:
George Hadjipanayis, University of Delaware
Ralph Skomski, University of Nebraska
 

SESSION H1: PERMANENT MAGNET PROCESSING
Chair: C. H. Sellers
Tuesday Morning, April 6, 1999
Salon 6 (M)

8:30 AM *H1.1
HYDROGENATION DISPROPORTIONATION DESORPTION RECOMBINATION PROCESSES APPLIED TO NdFeB-, SmFe- AND SmCo-TYPE ALLOYS. O. Gutfleisch , M. Kubis, A. Handstein, K.H. Muller and L. Schultz, Institute of Solid State and Materials Research Dresden, Dresden, GERMANY.

Recent aspects of hydrogenation disproportionation desorption and recombination (HDDR) phenomena in NdFeB-, SmFe- and SmCo-type alloys are reviewed in this paper. The effects of additives on hydrogen sorption and magnetic properties are discussed. Differences in the HDDR processing of Nd₂Fe₁₄B- and Sm₂Fe₁₇-type alloys such as the effect of pre-milling on the magnetic properties are investigated. The possibilities of producing magnetically anisotropic NdFeB HDDR magnets by hot deformation or alternatively prealigning and compacting anisotropic powders are described. Current models for the inducement of magnetic anisotropy in NdFeB HDDR powder via compositional changes or a modified processing route are summarised. The application of extreme hydrogenation conditions, namely high hydrogen pressure or milling in a hydrogen atmosphere at enhanced temperatures, allows the disproportionation of thermodynamically very stable compounds such as Sm₂Fe_17-xGa_x (x1), SmCo₅ or Sm₂Co₁₇. Reactive milling and subsequent recombination in a vacuum leads to structures, of both the disproportionated and the recombined type, which are on a remarkably finer scale than those commonly observed for standard HDDR procedures. Exchange coupling between the nanoscaled grains can result in magnetically single phase behavior despite a multiphase microstructure and in particular for the Sm₂Co₁₇ alloy, a remanence enhancement was observed after recombination at temperatures 700°C. The potential of this modified HDDR procedure regarding the tuning of one- or multi-phase microstructures, possibly consisting of normally metastable phases, suitable for an effective magnetic exchange coupling is assessed.

9:00 AM *H1.2
ISOTROPIC AND ANISOTROPIC Nd-Fe-B MAGNET DEVELOPMENTS. V. Panchanathan , Magnequench International Inc., Anderson, IN.

The rapidly solidified Nd-Fe-B materials form the entire basis of bonded magnet industry. The rapid solidification is carried out by melt spinning and the microstructure of melt spun ribbons affect the magnetic properties. In an alloy of compositions (wt. ) Nd 28, Co 5, B 0.9, Fe Balance, a uniform microstructure across the thickness of the ribbons resulted in high magnetic properties (Br > 8.4 kG). What is significant is the improved intrinsic induction value compared to those in which the microstructure across the thickness is nonuniform. The characteristics of this material are discussed. Rapid solidification is also used to make high energy product magnets by hot deformation. Development work was carried out to improve the energy product of radially oriented rings in the Nd-Fe-Co-Ga-B systems. It is possible to extrude rings over a wide range of composition and BHmax of > 40MGOe is obtained in radially oriented rings. The relationships between Nd, B and magnetic properties of rings are also discussed.

9:30 AM *H1.3
COMPARISON OF PRAEODYMIUM- AND NEODYMIUM-BASED NANOCRYSTALLINE HARD MAGNETIC ALLOYS. H.A. Davies , C.L. Harland, J.I. Betancourt R., Univ of Sheffield, Dept of Engineering Materials, Sheffield, UNITED KINGDOM; G. Mendoza, CINVESTAV, Saltillo, MEXICO.

Most of the research on nanophase RE-Fe-B hard magnetic alloys thus far has focused on Nd-based alloys, partly because these have been commercialised and are widely used in microcrystalline form. They have been shown to undergo progressive enhancement of remanence Jr above the Stoner-Wohlfarth value with decreasing grain size of the 2/14/1 hard magnetic phase dg, for dg below about 40 nm, due to increasingly significant intercrystalline ferromagnetic exchange coupling. At least for dg down to a critical value, the maximum energy product is also enhanced. However,a significant disadvantage of the Jr enhancement is that the intrinsic coercivity iHc undergoes a parallel reduction and, for dg below about 20 nm, this can become unacceptably low, particularly for sub-stoichiometric concentrations of Nd having a nanocomposite structure. Recent initial studies at Sheffield and elsewhere have demonstrated that PrFeB nanophase alloys offer significant advantages over their Nd-based counterparts in having substantially higher iHc, for a given degree of Jr enhancement. This results largely from the higher anisotropy constant of the Pr-based 2/14/1 hard magnetic phase. The results of the recent research at Sheffield on the nanocrystalline PrFeB-based alloys will be briefly reviewed and compared with those for corresponding Nd-based alloys. The influence of dg, Fe:RE ratio, Co substitution for Fe and of Nd/Pr mixtures will be discussed and assessed.

10:30 AM H1.4
SURFACE COATING OF RETM - HDDR PROCESSED AND MECHANICALLY ALLOYED POWDERS. Spomenka Kobe , Sasa Novak, Irena Skulj, Paul J. McGuiness, Jozef Stefan Institute, Ljubljana, SLOVENIA.

Abstract. Chemical surface modification can be used as a method for the corrosion protection of sensitive powders based on intermetallic alloys of rare earths and transition metals. The surface coating is used to prevent hydrolysis of fine powders, based on Nd₂Fe₁₄B, Sm₂Fe_17-xTa_x and Sm₂Fe_17-xTa_xN prepared by HDDR processing and mechanically alloying. Powders coated by a chemisorbed organic substance, after exposure to a humid atmosphere, do not show any chemical or physical change. Different coating agents were used and the necessary amount of various materials was optimised with the emphasis on minimising their quantity. A simple experiment shows that the surfactant is successfully chemisorbed onto the powder surface and that the coated powders are hydrophobic over a long period. The magnetic properties of the samples were measured after exposure to the same corrosion tests. Measurements on coated and bonded samples were compared with the measurements on non-coated samples. By using Auger electron spectroscopy the thickness of the coating was controlled. In order to distinguish the nature of the bonding between the powder surface and the surface-active substance FT-IR spectroscopy in absorbance and diffuse reflection modes was used. The protection of the fine particles is based on the formation of a covalent bond between the hydroxyl groups at the particle surface and the surface-active substance. The monomolecular layer of organic substance does not damage the magnetic properties of the powder, but successfully protects the powder against humidity.

10:45 AM H1.5
MICROSTRUCTURAL EVALUATION DURING CASTING AND HDDR OF ZIRCONIUM-MODIFIED NdFeB ALLOYS. D.N. Brown , I.R. Harris and M. Strangwood, The Univ of Birmingham, School of Metallurgy and Materials, Birmingham, UNITED KINGDOM.

Small additions of zirconium (0.1 - 0.5 at.%) are known to be very effective in producing anisotropy in NdFeB-based HDDR powder, but the precise mechanism for this effect is not clear. In this work a number of alloys (including near stoichiometric compositions) have been produced with varying zirconium contents, and their microstructures have been characterised by optical and SEM investigations. Sintered, fully dense magnets as well as HDDR powder have been prepared from these compositions and their magnetic properties have been correlated with the processing conditions and consequential microstructures. A possible role for zirconium has been proposed as a result of these observations.

11:00 AM H1.6
EFFECTS OF QUENCHING ENVIRONMENT ON THE STRUCTURE OF MELT-SPUN Nd₂Fe₁₄B. M.J. Kramer , Yali Tang, K.W. Dennis, R.W. McCallum, Ames Laboratory, Iowa State University, Ames, IA.

Melt-spun Nd₂Fe₁₄B ribbons were produced under active vacuum and different partial pressures of inert gases. Microstructure and thermal analyses were performed to understand the microstructural evolution and glass formation ability (GFA) of the ribbons. He atmosphere enhances the quenchability of the ribbons over Ar and vacuum. Ribbons made under 250 Torr He have more uniform microstructure and smoother surfaces than those under 760 Torr He. The higher quenchability induced by He, which increases the interfacial heat transfer coefficient between the melt and rotating wheel during melt spinning, is due to its higher thermal conductivity compared to Ar. The lower pressure stabilizes the turbulence between the melt-pool and Cu wheel, hence enhances the heat transfer resulting in a more uniform quench. As a result, a more uniform ribbon microstructure can be obtained at relatively low wheel speeds.

11:15 AM H1.7
PROCESSING Sm-Fe(Ta)-N HARD MAGNETIC MATERIALS. Kristina Éuuek, Paul J. McGuiness , Spomenka Kobe, Institute Jouef Stefan, Ljubljana, SLOVENIA.

SmFe based alloys interstitially modified with nitrogen are potential candidates for high energy permanent magnets. In order to obtain the optimum properties a thorough understanding of the starting material and processing parameters is required. The microstructures of two cast alloys of composition Sm_13.8Fe_82.2Ta_4.0 and Sm_13.7Fe_86.3 were carefully examined with a SEM equipped with EDX and the exact stoichiometries of the phases were determined. The SmFeTa material was found to contain significant amounts of TaFe₂ as well as the Sm₂Fe₁₇, SmFe₂, SmFe₃ phases observed in the SmFe material but without the -iron dendrites which are characteristic of the latter material. XRD was used to monitor the disproportionation of the material over a range of temperatures. The optimum conditions necessary to provide the highest coercivities using the HDDR process, and for HDDR process combined with pre-milling were investigated. The coercivities obtained after using the HDDR process and subsequent nitriding were 680 kA/m for the SmFeTaN and 360 kA/m for the SmFeN samples. Significantly higher coercivites of 1000 kA/m for SmFeN and 1275 kA/m for SmFeTaN were achieved by reducing the particle size with milling prior to the HDDR process. Magnetic results were compared with the phase composition, determined using scanning electron microscopy (SEM) and x-ray diffraction (XRD). The better coercivities obtained with the Ta containing sample were found to be due to the presence of a much smaller amount of a. The milling prior to the HDDR treatment improves the magnetic properties because of the small particle size which prevents the grains growing too large, with their consequent very negative effect on the coercivity.

11:30 AM H1.8
MICROSTRUCTURAL AND MAGNETIC IMPROVEMENTS IN TUBE CAST Pr-Fe-B-Cu ALLOYS VIA HEAT TREATMENTS AND RAPID UPSET FORGING. Gareth Hatch , Dexter Magnetic Technologies, Elk Grove Village, IL; Andrew Williams, Rex Harris, School of Metallurgy and Materials, University of Birmingham, Birmingham, UNITED KINGDOM.

Alloys of Pr-Fe-B-Cu were cast into copper and stainless steel tubes. It could be seen that a preferred crystallographic orientation was obtained after cooling. Long columnar grains led to fine platelets of Pr₂Fe₁₄B matrix phase surrounded by various grain boundary phases, and there was a significant reduction in the amount of free Fe present, in comparison to conventional slab cast alloys. In order to improve the magnetic properties in the as-cast state, two alternative routes were used. The first involved a series of two step heat treatments to develop an improved microstructure. After an optimum heat treatment of 1000C for 24 hours + 500C for 3 hours, significantly improved magnetic properties were obtained for a 17.5 % Pr alloy; B_r = 752 mT, H_ci = 613 kAm^-1 and BH_max = 96 kJm^-3.
The second route involved a rapid upset forging [RUF] process, with a strain rate of 11.5 s^-1, to hot deform the alloys. Following post-forging heat treatments, properties of B_r = 966 mT, H_ci = 780 kAm^-1 and BH_max = 160 kJm^-3 were obtained for a 17.5% Pr alloy. That nature of the improvements in properties as a result of heat treatments and RUF were investigated and are discussed in the present work.

11:45 AM H1.9
HIGH PERFORMANCE, LIGHTWEIGHT THERMOPLASTIC/RARE EARTH ALLOY MAGNETS. Jun Xiao ¹ and Joshua Otaigbe^1,2, Dept. of Materials Science and Engineering¹ and Dept. of Chemical Engineering², Iowa State University, Ames, IA.

Due to their superior mechanical damping characteristics, corrosion resistance, and facile processing, thermoplastic polymer-bonded rare earth alloy magnets are used extensively in electromechanical devices such as motors and actuators. In these applications, injection-moldable thermoplastic/Nd-Fe-B alloy magnets offer massive opportunities for component design of parts with intricate and complex shapes. However, the high injection molding temperature (350  50C) required by the high-performance thermoplastic polymers causes thermal oxidation of the Nd-Fe-B alloys, unless the alloys are coated with an impervious coating or passivation layer prior to incorporating them into the thermoplastic polymer. In this paper, we report progress on our exploratory research on surface modification of magnetic Nd-Fe-B fillers, characterization of suitable magnetic rare earth alloy powders and high-performance polymer matrices, processability, and properties of the thermoplastic/Nd-Fe-B magnets. The results suggest that blending liquid crystal polymer (LCP) with a high-thermoplastic polymer such as poly (phenylene sulfide) (PPS) provides the required balance of properties. These properties include superior magneto-mechanical performance, minimal melt viscosity at optimal Nd-Fe-B volume loading, enhanced thermal stability, high stiffness, high strength, improved dimensional stability, and excellent chemical resistance; making the thermoplastics magnets suitable for use in high temperature and aggressive environments where commercial polymer-bonded magnets are not useable.
 

SESSION H2: INTRINSIC PROPERTIES OF PERMANENT MAGNETIC MATERIALS
Chair: Gary J. Long
Tuesday Afternoon, April 6, 1999
Salon 6 (M)

1:30 PM *H2.1
NEUTRON DIFFRACTION CHARACTERIZATION OF PERMANENT MAGNET PHASES. William B. Yelon , University of Missouri Research Reactor, Columbia, MO.

In order to understand the properties of magnetic materials it is necessary to carefully follow the changes in properties as the structures change. These changes can be due to the substitution of one element by another, such as Fe substitution by Al, Si or Ga in the Re₂Fe₁₇ phase, or by interstitial insertion. Neutron diffraction is the ideal probe for such analysis since it is sensitive to light atoms, and can distinguish nearby elements in the periodic chart such as Fe and Co. Powder diffraction eliminates the need for difficult sample preparation. At the University of Missouri Research Reactor we have developed a high performance powder diffractometer that allows measurement on 1 gm samples in a few hours. We have used this instrument to develop a comprehensive atlas of the location and effect on bonding of a wide variety of substituents in CaCu₅ derived phases (2:17, 1:12) as well as determining the location, population and effect on bonding of various interstitial atoms introduced from the melt or from the gas phase. The neutron data is frequently used as input for theoretical modeling and as constraints on Mossbauer fitting. This talk will review some of the major findings of studies over the last several years.

2:00 PM *H2.2
MAGNETIC CHARACTERISTICS OF RCo_7-xZr_x ALLOYS (R=Sm,Gd,Pr,Er,Y and La). M.Q. Huang ^a,b, W.E. Wallace^a, M. McHenry^a and S.G. Sankar^b, ^aCarnegie Mellon Univ., Department and Materials Science and Engineering, Pittsburgh, PA, ^bAdvanced Materials Corporation, Pittsburgh, PA; Qun Chen and B.M. Ma, Rhodia Corporation, Cranbury, NJ.

In the 1980's, a Sm₂Co₁₇ phase with TbCu₇ structure was observed in the polycomponent Sm(Co,Fe,Cu,Zr)_7+x alloys and ribbon samples. This phase plays an important role in developing the high coercivity in 2:17 type magnets. In 1982, Strnat et al. observed that the Sm(Co,Fe,Cu,Zr)_7+x magnets with x=0, exhibited better temperature dependence of coercivity than that of magnets with x>0. Currently, there is an intense search to find new magnets or to improve the magnetic properties of existing magnets for high temperature applications. A systematic study of the RCo₇ system will be of use in this regard. A brief review is presented of recent work dealing with the magnetic properties of RCo_7-xZr_x alloys (R=Sm,Gd,Pr,Er,Y and La). The experimental results obtained show that Zr atoms partly replace a dumbbell of Co atom pair and play an important role in stabilizing the TbCu₇ structure while significantly increasing the anisotropy field, H_a. For example, when R=Sm, Ha increases from 90 kOe for x=0 to 180 kOe for x=0.5 at 300 K, and from 140 kOe for x=0 to 300 kOe for x=0.5 at 10 K. In the case of R=Y, H_a (which is only contributed by the Co sublattice) increases from 50 kOe for x=O to 75 kOe for x=0.4 at 300K and from 60 kOe for x=0 to 80 kOe for x=0.4 at 10 K. In particular, we note that, for Sm(Co,Fe,Cu,Zr)₇ alloys, after some heat treatment, the aligned powders (38 m) show a large increase of coercivity, H_c, from 2.7 kOe to over 20 kOe at 300 K and from 14.8 kOe to over 50 kOe at 10 K. Other magnetic properties and the phase transition phennomenon between CaCu₅, TbCu₇, Th₂Zn₁₇, and Ce₂Ni₇ at different heat treatment conditions will also be discussed.

2:30 PM H2.3
A LOW TEMPERATURE MÖSSBAUER SPECTRAL STUDY OF THE SPIN-REORIENTATION IN Nd₆Fe₁₃Si. D. Hautot and Gary J. Long, Department of Chemistry, University of Missouri, Rolla, MO; F. Grandjean, Institute of Physics, University of Liége, Sart-Tilman, BELGIUM and C.H. de Groot and K.H.J. Buschow, Van der Waals-Zeeman Institute, University of Amsterdam, Amsterdam, THE NETHERLANDS.

The Mössbauer spectra of Nd₆Fe₁₃Si have been measured between 4.2 and 135 K and fit with the same model that was used in our previous study¹ between 80 and 295 K. The spectra reveal a spin reorientation such that at 110 K there is a 50:50 mixture of axial and basal magnetic phases. Above 155 K the magnetization is axial, whereas between 80 and 4.2 K, an essentially constant approximately 25:75 mixture, respectively, of the axial and basal phases is observed. The hyperfine fields and isomer shifts vary smoothly with temperature and the temperature dependence of the latter yields an effective Mössbauer temperature of 270 K. The quadrupole shifts are essentially independent of temperature indicating a rather constant electronic structure and coordination environment at the iron sites.
¹D. Hautot, G. J. Long, F. Grandjean, C. H. de Groot, and K. H. J. Buschow,  83, 1554 (1998).

2:45 PM H2.4
A MÖSSBAUER SPECTRAL STUDY OF THE MAGNETIC PROPERTIES OF Ce₂Fe₁₇H_x AND Pr₂Fe₁₇H_x. F. Grandjean , Institute of Physics, University of Liége, Sart-Tilman, BELGIUM; Gary J. Long and D. Hautot, Department of Chemistry, University of Missouri-Rolla, Rolla, MO and O. Isnard and S. Miraglia, CNRS, Laboratoire de Cristallographie, Grenoble, FRANCE.

The Mössbauer spectra of Ce₂Fe₁₇H_x and Pr₂Fe₁₇H_x, where x is 0, 1, 2, 3, 4, and 5, have been measured between 4.2 and 295 K and reveal different magnetic structures with differing hydrogen content. In the case of Ce, the introduction of as little as one hydrogen changes the low temperature magnetization from helical to basal. In Pr₂Fe₁₇H_x, for x < 3 the magnetization is basal at all temperatures, whereas for x > 3 it is axial. Pr₂Fe₁₇H₃ has a basal magnetization above 120 K, and an axial magnetization below 80 K. In order to account for the observed Mössbauer spectral components associated with the iron 18h site in Pr₂Fe₁₇H₅, a model is proposed in which the hydrogen jumps between the partially occupied tetrahedral 18g hydride sites, jumps which cease below ca. 100 K. For Pr₂Fe₁₇H₄ the iron 18h spectral components do not exhibit a similar behavior upon cooling and the hydrogen atom is still jumping between the six 18g sites even at 4.2 K.

3:30 PM *H2.5
FIRST-PRINCIPLE STUDIES OF PERMANENT-MAGNET MATERIALS. S.S. Jaswal and R.F. Sabiryanov, University of Nebraska, Lincoln, NE.

First-principle electronic structure studies are being carried out to complement the experimental research in our group on hard-magnet materials. Since the discovery of Nd₂Fe₁₄B in 1984, the research in this area has been concentrated on T(Fe,Co)-rich rare-earth (R) compounds such as RT₁₂ and R₂T₁₇ and exchange coupled hard/soft phases. The large magnetovolume effect in Fe-rich compounds is exploited to improve their magnetic properties such as Curie temperature and magnetization by interstitial and substitutional modifications. The structurally matched soft and hard phases are studied for strong exchange coupling needed to improve the hard-magnet properties. Self-consistent spin-polarized electronic structure calculations are used to derive the magnetic properties of Fe-rich compounds and the results are compared with the available experimental data.^1-3 Computer simulations are carried out for promising exchange-coupled systems.^4-5 These results will be reviewed in this paper.
Supported by NSF, DOE, AFOSR, DARPA and National Computational Alliance.
1. R.F. Sabiryanov and S.S. Jaswal, J. Appl. Phys. 81, 5615 (1997).
2. R.F. Sabiryanov and S.S. Jaswal, Phys. Rev. Lett. 79, 155 (1997).
3. R.F. Sabiryanov and S.S. Jaswal, Phys. Rev. B 57, 7767 (1998).
4. R.F. Sabiryanov and S.S. Jaswal, J. Magn. Magn. Mater. 177-181, 989 (1998).
5. R.F. Sabiryanov and S.S. Jaswal, Phys. Rev. B (accepted)

4:00 PM H2.6
AB-INITIO STUDY OF MAGNETIC PROPERTIES OF Nd₅Fe₁₇. R.F. Sabiryanov and S.S. Jaswal, University of Nebraska, Center for Materials Research and Analysis, Lincoln, NE.

R₅Fe₁₇ compound has attracted some attention lately as a possible permanent-magnet material. Recent neutron-diffraction studies of this crystallographically complicated compound (264 sites per unit cell with 14 and 7 different Fe and Nd sites respectively!) determined the coordinates of its atomic positions.^1,2 Some estimates of the magnetic moments are available. The results depend strongly on the particular model used to group atoms. Fe atoms with a small number of Fe neighbors have very large magnetic moments. Using tight-binding linear-muffin-tin-orbital method, the first-principles spin-polarized studies of Nd₅Fe₁₇ compound are being carried out in order to determine the electronic structure, magnetic moments, and the Curie temperature. The results will be analyzed in terms of effect of the local environment on the magnetic properties of individual Fe sites.
1. C. Lin et al., J. Magn. Magn. Mater 186, 129 (1998).
2. W. B. Yelon et al., MMM'98 Conf. in Miami, Florida.
Supported by NSF, DOE, AFOSR, DARPA and National Computer Alliance.

4:15 PM H2.7
EVIDENCE FOR SPIN REORIENTATION TRANSITION IN RFe₁₀MB/Nd₂Fe₁₄B-TYPE NANOCOMPOSITE PERMANENT MAGNETS BASED ON Nd-Fe-Cr-Co-B. Satoshi Hirosawa , Hirokazu Kanekiyo and Yasutaka Shigemoto, Sumitomo Special Metals Co., Ltd., R&D Division, Osaka, JAPAN.

Recent progresses of materials development and their utilization of Cr-and-Co-doped Fe₃B/Nd₂Fe₁₄B-type nanocomposite permanent magnets are reported. A practical balance between enhanced intrinsic coercivity and temperature stability of magnetic properties has been realized by simultaneous additions of Cr and Co. This type of magnets are less susceptible to oxidation so that even fine powders (e. g., under 38 micrometer sieve) are non flammable in contrast to the conventional Nd₂Fe₁₄B-type melt-spun materials due to the less rare-earth content. The stability in terms of the structural losses is also superior to that of the Nd₂Fe₁₄B-type melt-spun materials. Utilization of the Cr- and-Co-doped Fe₃B/Nd₂Fe₁₄B-type nanocomposite permanent magnets as a hard magnetic component of injection-molded resin-bonded magnets is promising because of the excellent stability of the magnetic properties of fine powders. Direct processing of a melt into the nanocomposite structure has become possible recently in addition to the conventional processing route of crystallization of an amorphous precursor, opening up the possibility of less-expensive production of the material.

9:00 AM *H3.2/I6.2
EUROPEAN MAGNETS WITH ENHANCED REMANENCE FOR GREATER EFFICIENCY (EMERGE). V. Archambault , Rhodia, Centre de Recherches, Aubervilliers, FRANCE.

EMERGE is an European research program whose objective is to develop a low Rare Earth containing material with improved coercivity ranging from iHc = 0.4 MA/m with Jr = 1.2 T (target 1) to iHc = 0.6 MA/m with Jr = 1.0 T (target 2). A coarse selection is performed in phase 1 by selecting the compositions to study through statistical set up, by making and optimizing rapidly quenched material, by performing advanced analysis by modeling the coercivity and by making bonded magnets. Results of phase 1 are the starting point of phase 2, which consists of a much finer selection around promising compositions found in phase 1. The investigations have yielded two families of alloys, each of them being a viable candidate for one of the project targets. The most promising family with respect to target 1 is formed by alloys around the nominal composition Nd_3.25Tb₁Fe_72.75Co₅B₁₈. Tb and Co contents have been found to be crucial in these alloys for achieving high coercivities, up to 0.42 MA/m, which are about 0.1 MA/m higher than the previously reported values for this type of alloys. A typical value for the remanence is 1.05 T. The best results have been found when using fairly high heating rates in the anneal treatment. The amount of Tb could be reduced which yields a lower coercivity but a higher remanence. In order to reach the high coercivity of target 2, 5.5 at  of RE and at least 5 at  of Co are required. With respect to this target second family, with a nominal composition of Nd_4.5Tb₁Fe_71.5Cr₅B₁₈, has shown good results. Coercivities up to 0.74 MA/m have been found, however, still combined with rather low remanences or typically 0.8 T.

9:30 AM *H3.3/I6.3
MICROMAGNETICS OF NANOCRYSTALLINE PERMANENT MAGNETS. Thomas Schrefl , Josef Fidler, Institute of Applied and Technical Physics, Vienna University of Technology, Wiedner Hauptstr, Vienna, AUSTRIA.

Micromagnetic finite element calculations provide the theoretical limits for the remanence, the coercive field, and the coercive squareness of nanocomposite Nd₂Fe₁₄/(-Fe,Fe₃B,Fe₂₃B) magnets. The influence of the intrinsic magnetic properties and the microstructure were investigated using an energy minimization technique. The coercive field reaches a maximum as a function of the average grain size at about 15 nm - 20 nm. An increase of the hard phase anisotropy by 10% enhances the coercive field by more the 100 kA/m in two-phase Nd₂Fe₁₄/-Fe and by 70 kA/m in two-phase Nd₂Fe₁₄/Fe₃B magnets. This effect is most pronounced for an average grain size of 20 nm. The reduction of the magnetization and the exchange constant in the soft magnetic Fe₃B phase by 20% improves the coercive field without a significant loss in the remanence. A smaller magnetization results in a lower demagnetizing field which in turn increases of the coercive from 600 kA/m to 800 kA/m keeping a remanence of 1.1 T. A reduction of the exchange constant within Fe₃B deteriorates the loop shape but increases the coercive field. The magnetizability of a Nd₂Fe₁₄/(-Fe/Fe₃B magnet improves with increasing -Fe content. An applied field of 960 kA/m leads to 85% saturation in an two-phase Nd₂Fe₁₄/-Fe, resulting in a remanence of 1.2 T and a coercive field of 340 kA/m. The numerical integration of the Gilbert equation of motion shows how reversed domains nucleate and expand in a realistic system consisting of 343 polyhedral grains. Dynamic simulations for magnetostatically interacting particles of a bonded magnet show a slight influence of the particle arrangement on magnetization reversal. The interaction field in the range of 100 kA/m to 300 kA/m rapidly decreases with the distance from the particle. This work has been supported by the Austrian Science Foundation FWF (Grant No. P10511-NAW) and the EC-BRITE/EURAM project BRPR-CT95-0097.

10:30 AM *H3.4/I6.4
COERCIVITY IN LEAN-RARE EARTH NANOCOMPOSITE HARD MAGNETIC MATERIALS. Stephane David , Kenneth Mackay, Marlio Bonfim, Alain Fontaine and Dominique Givord, CNRS-Laboratoire Louis Neel, Grenoble, FRANCE.

A series of RE-TM-B (3RE5.5 at.) nanocomposite magnets has been prepared using the classical method of rapid quenching followed by appropriate annealing. The composition of these exchange-spring type materials was varied with a view to the optimization of their technical magnetic properties -mainly coercivity. Elemental additions and substitutions that allowed significant improvement are reviewed and discussed. From the fundamental point of view, the analysis of the hysteresis cycles and static susceptibility measurements revealed that magnetization reversal in these materials may be separated into two distinct contributions arising from the different phases as predicted by Kneller and Hawig: reversal is essentially reversible at low fields, while the irreversible process starts at higher field values. This trend is found to be common to all the investigated samples. To complement this study, we have investigated the magnetization reversal in a Nd-based nanocomposite hard/soft system, by using element-selective X-ray Magnetic Circular Dichroism (XMCD)-magnetometry. A static external field was applied to the sample, while the helicity of the incident X-rays was tuned by flipping a diamond Quarter Wave Plate between two angular positions before and after Bragg reflection . By selecting the Nd L₂ absorption edge, it has been possible to record the changes in the dichroism related to the orientation of the Neodymium magnetic moments in the hard phase as a function of the external field. This procedure has allowed the hysteresis cycle of both hard and soft phases to be separated from the total cycle, thus providing unique insights into the characteristic magnetization reversal mechanisms occurring in exchange-spring magnets. The magnetization reversal processes will be discussed in the light of the above complementary results. E.F. Kneller and R. Hawig, IEEE Trans. Magnetics, 27 (1991) 3588-3600.

11:00 AM *H3.5/I6.5
AMORPHOUS-PHASE REMAINING Fe/NdFeB NANOCOMPOSITE HARD MAGNETS. Masaaki Hamano , Minoru Yamasaki, Hirotaka Mizuguchi, Toda Kogyo Corp, AMC Div, Hiroshima, JAPAN; Takayuki Kobayashi, Dept of Physics, Shiga Univ of Medical Science, Shiga, JAPAN; Hiroshi Yamamoto, Dept of Electrical Engineering, Meiji Univ, Kawasaki, JAPAN; Akihisa Inoue, Inst for Materials Research, Tohoku Univ, Sendai, JAPAN.

Amorphous-phase remaining Fe/NdFeB nanocomposite melt spun and heat treated ribbons with various additional elements can be obtained when Nd content is less than 8 at. The optimum surface velocity of the single roller apparatus for better coercive force (H_cJ) was 10-15 m/s. By adding Nb, H_cJ increases proportionally. This may be attributed to the fine-grained structure, according to the micromagnetic theory. For example, nearly homogeneous crystal size distribution of around 5 nm is observed in a sample of Nd₈Fe₇₆Co₈Nb₂B₆. When Nd=8 at, maximum H_cJ is as large as 575 kA/m (7.22 kOe) at Nb=2.5 at, and the best (BH)_max studied is 154 kJ/m³ (19.4 MGOe). By high-resolution transmission electron microscopy, remaining amorphous phase is observed in all ribbons with Nd=8 at, which shows soft magnetic properties and is thought to act as a crystal growth inhibitor. To estimate the existing ratio of the amorphous phase and crystallized phases, the Meffect has been studied. The ratio of the Nd₂Fe₁₄B₁ phase seems to increase with increasing Nb content. Epoxy-resin bonded magnets were prepared by the compression molding. It was found that, comparing to the MQP magnet using MQP-B powder, (BH)_max is in the same order of 72 kJ/m³ (9 MGOe), and magnetizability at lower magnetic field than 800 kA/m (10 kOe) is superior. Initial flux loss at 100C for 1.8 ks, however, is in the range of 3-8  depending on Nb contents.

11:30 AM *H3.6/I6.6
LOCAL MAGNETIC STRUCTURE AND MAGNETIC PROPERTIES OF BULK GLASSY Nd-Fe-Al: LOW FIELD AC-SUSCEPTIBILITY AND HIGH FIELD dc-MAGNETIC STUDIES. K.V. Rao , R. Ortega, Royal Inst. of Technology, Stockholm, SWEDEN; Josep Nogues, J.S. Munoz, Autonoma Univ. de Bellaterra, Barcelona, SPAIN; and A. Inoue, Inst. for Metal Resaerch, Tohoku Univ, Sendai, JAPAN.

A metallic system consisting of low dimensional network of entities can be frozen directly into a glassy bulk material by slow quenching from the supercooled liquid. This has been recently demonstrated by Inoue(1) in a number of metallic systems containing La, Al, Mg, Zr etc. Of particular interest to this presentation are Nd-rich Nd-Fe-Al alloys which, when rapidly quenched form the liquid state by melt spinning, are found to be soft ferromagnets at room temperature with almost negligible magnetic coercivity. In contrast, on slow cooling at rates as low as 1K/s, the same melt is frozen into a hard magnetic bulk glassy material with diameters which can be as large as 7mm! Room temperature coercivities of over 1 Tesla is observed, while the magnetization does not saturate in external fields up to 30 Tesla. From Low temperature magnetic hysteretic loop measurements Hc(T) is found to exhibit a maximum (at a value of 3.5 Telsa ) around 50K. Low field ac susceptibility as well as dc hysteretic loop investigations of both the melt-spun, as well as slow-cooled bulk alloy in their glassy states will be discussed. The hard magnetic properties for the bulk glassy state appears to arise from the ferrromagnetic nanoscale magnetically granular entities embedded in an antiferromagnetic Nd-rich matrix. (1) see for example: Akihisa Inoue, Akira Takeuchi, and Tao Zhang, Metallurgical and Materials Transactions 29A, 1779 (1998)
 
 
 

SESSION H4/I7: JOINT SESSION:
NANOSCALE HARD MAGNETISM II
Chair: David C. Crew
Wednesday Afternoon, April 7, 1999
Salon 5/6 (M)

1:30 PM *H4.1/I7.1
HARD MAGNETIC NANOPARTICLES AND NANOCOMPOSITES. Sara A. Majetich , Yan Jin, Anit Giri, Krishna Chowdary, Carnegie Mellon Univ, Physics Dept, Pittsburgh, PA.

There are many nanostructured hard magnetic materials, such as Sm₂Co₁₇ and Alnico. Unfortunately itís difficult to extrapolate from the behavior of isolated particles to that of nanocrystalline solids with coupled grains that may be of different phases. To bridge this gap, we investigate the magnetic behavior of magnetic nanoparticles and nanocomposites on a nanometer scale. The magnetization reversal, or coercivity, of nanoparticles and nanocomposites is investigated on a nanometer scale using the Foucault method of Lorentz microscopy. First nanoparticles with extremely large values of the magnetocrystalline anisotropy will be studied to extend the existing models of coercivity. In high anisotropy ball milled SmCo₅ nanoparticles the emphasis will be on understanding the roles of strain, grain boundaries, and particle size in order to maximize the switching field. Both nanocrystalline SmCo₅ and nanocomposite SmCo₅/FeCo exchange spring magnets will be made by compacting the nanoparticles. Different compaction methods are used to determine the method which best overcomes the high frictional forces between nanoparticles to yield the highest density, yet does so without significant grain growth. Lorentz microscopy will be used to observe magnetization reversal on a submicron length scale. These results, in combination with standard methods to analyze the microstructure and chemical composition, will identify the weak links which reduce the coercivity of a permanent magnet.

2:00 PM *H4.2/I7.2
A STUDY ON THE PHASE TRANSFORMATION AND EXCHANGE COUPLING OF (Nd_0.95La_0.059.5Fe_balCo₅Nb₂B_10.5) NANOCOMPOSITES. Q. Chen and B.M. Ma, Rhodia Inc., Rare Earths and Gallium, Cranbury, NJ; B. Lu, M.Q. Huang, and D.E. Laughlin, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA.

The phase transformation and the exchange coupling of the (Nd_0.95La_0.05)_9.5Fe_balCo₅Nb₂B_10.5 have been investigated. Nanocomposites were obtained by treating amorphous precursors at temperatures ranging from 650 to 850C for l0 minutes. The magnetic properties were characterized via the vibrating sample magnetometer (VSM). X-ray diffraction (XRD), thermomagnetic analysis (TMA), and transmission electron microscopy (TEM) were used to perforrn phase identification, measure grain size, and analyze phase distribution. The strength of the exchange coupling between the magnetically hard and soft phases in the corresponding nanocomposite was analyzed via the Henkel plot. It was found that the remanence (B_r), coercivity (H_ci)₅ and maximum energy product (BH_max) obtained were affected by the magnetic phases present as well as the grain size of constituent phases and their distribution. Although the Henkel plot successfully interpreted the effect of the exchange coupling on the Br_r, H_ci, and BH_max obtained for most of the samples, it became inadequate for samples treated above 750C. The B_r, H_ci and BH_max degraded severely when the thermal treatment temperature increased from 750 to 850C. This degradation may be attributed to the grain growth of the main phases, from 45 to 68nm, and the development of precipitates, which grew from 5nm at 750C to 12nm at 850C. Moreover, the amount of the precipitates was found to increase with the thermal treatment temperatures. The precipitates, presumably borides, may cause a decrease in the amount of the -Fe and Fe₃B and result in a redistribution of the Co in the nanocomposites. The increase of the Co content in the Nd₂(Fe Co)₁₄B may explain the increase of its Curie temperature with the thermal treatment temperatures. In this paper, we examine the impacts of these factors on the exchange coupling of (Nd_0.95La_0.05)_9.5Fe_bal Co₅Nb₂B_10.5 nanocomposite.

2:30 PM H4.3/I7.3
MAGNETIC PROPERTIES OF POLYMER BONDED EXCHANGE-SPRING NdFeB MAGNETS. B. Mokal , N.A. Smith, A.J. Williams and I.R. Harris, The Univ of Birmingham, School of Metallurgy and Materials, Birmingham, UNITED KINGDOM.

The production of remanence enhanced melt spun ribbons, using a method of mechanical alloying of free iron to the starting alloy, has been studied. The final composition is Nd_8.1Fe_77.1Co_10.9B_3.9 and both hard and soft materials possess an initial fine grain size; approximately 30nm for the melt spun ribbon and 1mm for the a-Fe. The magnetic properties of PTFE bonded magnets fabricated using such material have been measured and found to exhibit exchange-spring behaviour. The experiments performed in this work are described with respect to the volume fraction of the soft phase addition and subsequent recrystallisation temperature of the amorphous as-milled material. Remanence enhancement has been observed and this can be attributed to the exchange interactions between the ferromagnetic soft and hard phases. For 30% by volume addition of a-Fe, typical values of remanence were found to be 945mT with a coercivity of 320 kA/m. The reversible and irreversible components of the magnetisation have been determined, in order to illustrate the exchange coupled and coercivity behaviours respectively; the magnitude of the soft phase grain diameter and its dependence on the former has also been depicted.
 

SESSION H5/I8:
JOINT IN-ROOM POSTER SESSION:
NANOSCALE HARD MAGNETISM III
Chair: Qun Chen
Wednesday Afternoon, April 7, 1999
4:00 P.M.
Salon 5/6 (M)

H5.1/I8.1
MAGNETO-OPTICAL INDICATOR FILM INVESTIGATION OF THE REMAGNETIZATION BEHAVIOR OF EXCHANGE-SPRING MAGNETS. J.Z. Hilt, Dept. of Physics, Miami University, Oxford, OH; A.J. Shapiro, R.D. Shull , National Institute of Standards and Technology, Gaithersburg, MD; V.I. Nikitenko, V.S. Gornakov, Institute of Solid State Physics, RAS, Chernogolovka, RUSSIA; J.S. Jiang, A. Inomata, C.H. Sowers, S.D. Bader, Argonne National Laboratory, Argonne, IL.

Epitaxial Sm-Co (350 A)/Fe(500 A) bilayer films were grown on Cr (200 A) buffered MgO (110) substrate by sputtering. Magnetic hysteresis loops measured in a vibrating sample magnetometer (VSM) showed two distinct coercivities and saturation values indicating the sample was a composite of both a soft and hard ferromagnet. For the first time in such a material, the remagnetization process was observed using the magneto-optic indicator film (MOIF) technique. In order to investigate the magnetic spin rotation process inside the bilayer during remagnetization, a 0.3 mm hole was made in the sample, and the magnetostatic field (Hms) around the hole was visualized through the intensity changes of the double Faraday effect in a transparent indicator film with in-plane anisotropy. Black and white contrast on opposite sides of the microhole was observed, indicating the direction of magnetization in the sample around the hole. We followed the line of contrast antisymmetry and analyzed the spin rotation process in the soft ferromagnetic component during remagnetization. This was compared to the macroscopic magnetization as determined by the VSM. Features of magnetization reversal of the soft ferromagnetic layer in both easy and hard directions are discussed.

H5.2/I8.2
MAGNETIZATION AND INTERACTIONS OF SINGLE DOMAIN Sm₂Co₁₇ PARTICLES EMBEDDED IN CaO MATRIX. Wenge Liu and Paul G. McCormick , Special Research Centre for Advanced Mineral and Materials Processing, The University of Western Australia, Perth, AUSTRALIA.

A system composed of  8 vol% isolated Sm₂Co₁₇ nano-sized particles (9-90nm in size) embedded in a CaO matrix was prepared by mechanical milling and subsequent heat treatment of a mixture of Sm₂O₃, CoO and Ca with a suitable amount of CaO as diluent. The hysteresis curves of the composite system were measured at temperatures between 5 and 350K. At 5 K the Sm₂Co₁₇ phase exhibited a coercivity of 24 kOe. A nearly linear temperature dependence of the coercivity was obtained up to 350 K. Isothermal remanent magnetization (IRM) and dc demagnetization (DCD) data were collected on the composite system at room temperature. The magnetization processes of the single domain particles are discussed based on the initial magnetization and demagnetization measurements. Good agreement of the experimental data with the Wohlfarth remanence relation was obtained, suggesting that the Sm₂Co₁₇ nanoparticles do not interact with each other due to the separation of the non-magnetic CaO matrix.

H5.3/I8.3
MAGNETIC PROPERTIES OF Fe/NdFeB NANOCOMPOSITE ALLOYS PREPARED BY TWO-STEP RAPID SOLIDIFICATION. Minoru Yamasaki , Hirotaka Mizuguchi, Hirohumi Morioka, Masaaki Hamano, Toda Kogyo Corp, AMC Div, Hiroshima, JAPAN; Akihisa Inoue, Inst for Materials Research, Tohoku Univ, Sendai, JAPAN.

Nanocomposite Fe/NdFeB alloys of the composition of Nd_xFe_balCo₈Nb_yB_z, where x:8.0-8.5, y:1.5-2.0, z:6.0-6.5 in at, have been prepared using the two-step rapid solidification apparatus, which is composed of a gas atomizing system at the upper side and a collision-and-solidificating rotor system at the lower side. The quenching rate of the melt can be controlled by experimental conditions such as temperatures of the melt, the diameter of the melt injecting nozzle, the diameter of atomizing gas orifice, pressures of atomizing Ar gas, rotating speeds of the rotor and the distance from the nozzle to the rotor. The coercivity (H_cJ) of flat powders obtained depends on powder sizes. The best H_cJ in each sieved powder is 525 kA/m for a size more than 500 m, 449 kA/m for 150-500 m, and 391 kA/m for 50-150 m. These values are not so high as those measured for ribbons prepared using a single roller apparatus. This should be attributed to the powder size distribution which causes fluctuation of the quenching rate. Thickness of the flat powder is found to be controllable up to 70 m, which is useful to get compression molded bonded magnets with a high density and a high remanence (Br). A bonded magnet with (BH)_max:61.8 kJ/m³, Br:0.732 T, H_cJ:427 kA/m, and density of 6.20 Mg/m³ is obtained. Further investigation to improve magnetic properties are now in progress.

H5.4/I8.4
MICROSTRUCTURAL CHARACTERIZATION OF MELT SPUN Nd₂Fe₁₄B MAGNETS. J.A. Horton , L. Heatherly, M.K. Miller, E.D. Specht, Oak Ridge National Laboratory, Oak Ridge, TN and V. Panchanathan, Magnequench International, Inc., Anderson, IN.

Several microstructural characterization techniques were applied to melt spun Magnequench B+ powder and to hot pressed MQ2 Nd₂Fe₁₄B magnets in order to better describe the microstructure, especially grain boundary phases and how these relate to the magnetic and mechanical properties. Auger electron spectroscopy, atom probe field ion microscopy, and transmission and scanning electron microscopy were used. Cleavage plane orientations in Nd₂Fe₁₄B were identified by X-ray diffraction and found to be rather random. Cleavage steps were not found by atom force microscopy. The small grain sizes of less than 100 nm in Magnequench MQ material preclude an easy assessment of the fracture mode by scanning electron microscopy. Auger electron spectroscopy, with its simple sample preparation, showed that much of the surface is covered with a 1 nm thick layer of a neodymium-rich phase, presumably the 70Nd-30Fe eutectic phase, suggesting that the hard Nd₂Fe₁₄B grains do not cleave but instead failure is at or in the grain boundary phase. Preliminary atom probe analysis shows excellent quantification. Further analysis of the atom probe data will be presented. Research sponsored by the U.S. Department of Energy, Office of Energy Research by the Laboratory Technology Research Program and by the Division of Materials Sciences, through the Center of Excellence for Synthesis and Processing of Advanced Materials under contract DE-AC05 96OR22464 with Oak Ridge National Laboratory managed by Lockheed Martin Energy Research Corp.

H5.5/I8.5
MAGNETIC DOMAIN OBSERVATION ON MELT-SPUN Nd-Fe-B RIBBONS USING MAGNETIC FORCE MICROSCOPY. A. Gavrin , Indiana Univ., Purdue Univ. Indianapolis, Dept. of Physics, Indianapolis, IN; C. Sellers, Magnequench International, Inc., Anderson, IN; S.H. Liou, Behlen Laboratory of Physics, University of Nebraska, Lincoln, NE.

We have used Magnetic Force Microscopy (MFM) to study the magnetic domain structures of melt-spun Nd-Fe-B ribbons. The ribbons are commercial products (Magnequench International, Inc. B and B+) with a thickness of approximately 20 microns. These materials have identical composition, Nd_12.18B_5.36Fe_76.99Co_5.46, but differ in quenching conditions. In order to study the distribution of domain sizes through the ribbon thickness, we have prepared cross-sectional samples in epoxy mounts. In order to avoid artifacts due to tip-sample interactions, we have used high coercivity CoPt coated MFM tips. Our studies show domain sizes typically ranging from 50-200 nm in diameter. This is in agreement with studies of similar materials in which domains were investigated in the plane of the ribbon. However, we also find that these products differ substantially in the uniformity of the domain sizes as measured across the ribbon. While the B+ material shows nearly uniform domain sizes throughout the cross section, the B material shows considerably larger domains on one surface, presumably due to the differing quench conditions. This region varies in thickness, disappearing in some areas, and reaching a maximum thickness of 2.75 mm in others. The domain size within this region is approximately 1.5 times larger in radius than the remainder of the sample. We will also describe bulk magnetic measurements, and suggest that the uniformity of the B+ domain structure is responsible for its superior magnetic performance.

H5.6/I8.6
DISORDER IN THE L1₀ PHASE OF FePd VERSUS COMPOSITION. Andrew Janssen, Luke S.J. Peng, Gary S. Collins , Washington State Univ, Dept of Physics, Pullman, WA.

Many magnets with high uniaxial anisotropy are single-phase intermetallic compounds having a substantial homogeneity range around some stoichiometric composition. The variable composition is achieved by introduction of structural point defects, for example antisite atoms formed from excess atoms of one element. However, when the ordering energy is not very high, thermally activated defect combinations such as antisite atom pairs may also be present, with possible effects on magnetic properties. In the present work, Mössbauer spectroscopy of 57Fe was used to resolve signals due to point defects in FePd and to determine their concentrations. FePd has the tetragonal L1₀ (CuAu) structure between 50 and 62 at.% Pd. Measurements were made on samples having 50, 54 and 58 at.% Pd. Samples were prepared by arc-melting and annealing for 1 hour at 600 C. Spectra were analyzed using a local environment model that fits hyperfine field shifts due to defects in the closest 3 or 4 shells. It was assumed that defects were either Pd_Fe or Fe_Pd antisite atoms, with Pd_Fe present as structural defects in the Pd-rich samples. Superpositions were included in the fits of as many as 50 spectral components that correspond to different numbers of defects in the shells, with intensities constrained by assuming random distributions of the defects. In addition to the structural Pd_Fe defects known to be present, the fits yielded fractional concentrations of 6.4, 1.9 and 0.4% of antisite atom pairs, respectively, in the 50, 54 and 54% Pd samples. This strong dependence of the concentration of thermally activated defects on composition will be shown to be explained by a unique, composition-independent activation enthalpy of an antisite atom pair, equal here to 0.4 eV. The composition dependence of thermally activated defects in other magnets will be discussed. - This work was supported in part by the National Science Foundation under grant DMR 96-12306 (Metals Program).

H5.7/I8.7
Nd RICH NdFeB TAILORED FOR MAXIMUM COERCIVITY. Er. Girt , Department of Material Science and Mineral Engineering, University of California, Berkeley, CA; Material Science Division, LBNL, Berkeley, CA; Kannan M. Krishnan, Material Science Division, LBNL, Berkeley, CA; G. Thomas, Department of Material Science and Mineral Engineering, University of California, Berkeley, CA; Material Science Division, LBNL, Berkeley, CA; Z. Altounian, Centre for the Physics of Materials, Department of Physics, McGill University, Montreal, Quebec, CANADA.

We obtained the largest reported value of the coercivity in rapidly quenched NdFeB samples by tailoring the composition and microstructure of the ribbons. NdFeB ribbons were prepared using the rapid quenched technique, with the following compositions: Nd_12.7Fe_81.2B_6.1, close to the Nd₂Fe₁₄B composition and Nd₃₀Fe₆₄B₆, Nd₃₀Fe₆₅B₅ and Nd₃₀Fe_65.5B_4.5, the Nd-rich compositions with the varied ratio between Fe and B. The Nd-rich NdFeB ribbons were prepared with the aim to obtain the magnetic Nd₂Fe₁₄B particles imbedded in the non-magnetic Nd matrix. As quenched and annealed samples are analyzed using X-ray diffraction and TGA , thermo-gravitometric analyzer. Results show that the as-quenched samples with the Nd concentration equal to 30 percent are not amorphous, consisting of small crystals of alpha-Nd, gamma-Nd and Nd₂Fe₁₄B. The TGA measurements show the existence of only one magnetic phase, Nd₂Fe₁₄B, in all the samples. The SQUID measurements are done on the annealed samples at room temperature. For Nd_12.7Fe_81.2B_6.1 the largest coercivity, 1.25 kOe, was obtained for the ribbons annealed at 650 C for 4 min. For the Nd-rich NdFeB ribbons the results show an increase of the coercivity with the increase in Fe:B. The largest coercivity, 21 kOe, was obtained in the Nd₃₀Fe_65.5B_4.5 samples (Fe:B=14.5:1). The excess of Nd and Fe in this sample forms the Nd-rich NdFe phase, with a composition close to the eutectic in the binary Nd-Fe phase diagram, which improves the coercivity of the samples.
 

SESSION H6: PERMANENT MAGNET APPLICATIONS
Chair: V. Panchanathan
Thursday Morning, April 8, 1999
Salon 6 (M)

8:30 AM *H6.1
NOVEL PERMANENT MAGNETS AND THEIR USES. Steve Constantinides , The Arnold Engineering Company, Marengo, IL.

Recent developments in permanent magnets are reviewed and applications discussed. Factors driving innovation include higher temperature capability, improved corrosion resistance, lower cost materials and greater design flexibility. Improvements include high coercivity, corrosion resistant neodymium-iron-boron, hybrid bonded magnets, bonded samarium cobalt and newly commercialized samarium-iron-nitride. In addition to new materials, considerable effort has been expended in improving application of existing materials, especially with regard to minimizing product cost.

9:00 AM H6.2
APPLICATIONS OF RARE-EARTH PERMANENT MAGNET STRUCTURES TO ELECTRICAL MACHINERY. Herbert A. Leupold , Anu Tilak, US Army Research Laboratory, Sensors Directorate, Adelphi, MD; David LaGraffe, Richard Marchand, US Military Academy, West Point, NY.

A variety of promising permanent magnet structures based upon the magic cylinder and magic mangle concepts are investigated with regard to their suitability for application to electrical generators and motors. Such structures have been shown to generate unusually great magnetic fields in relatively large volumes with the use of very little magnetic material. Other advantages are minimal stray flux and net magnetic moment. The former quality affords close packing in systems with nearly-filled, sensitive components, while lack of a magnetic moment in some of these configurations allows torque-free rotations against adverse magnetic fields. Approximations to magic cylinders known as magic mangles afford considerable reduction in moments of inertia of rotating parts, thereby resulting in improved rotational responsiveness. Magic mangles also offer greater ease of manufacture at the expense of some loss of magnetic field. Generators and motors based on the various structures are compared with each other and with conventional existing configurations with regard to electrical output, expense and difficulty of manufacture and structural mass and bulk. For example, a hand-operated magic mangle generator is calculated to have one quarter the mass and bulk of an existing conventional generator of similar electrical output and structural complexity. Similar advantages exist for the other cylindrical and mangle type embodiments, indicating potential advantageous use, especially in space and airborne vehicles.

9:15 AM H6.3
SMALL BRUSHLESS DC-MOTOR WITH SENSORLESS CONTROL - A HOLLOW SHAFT PROTOTYPE. R.E. Hanitsch , B. Frenzel, University of Technology Berlin, Electrical Engineering Department, Berlin, GERMANY.

The brushless dc-motor has continued to grow in both performance and overall market share. The development of neodymium-iron-boron magnets has impacted brushless dc-motor design more than any other motor topology. The trend towards high-speed, high-flux-density drives is motivated by the quest for the most efficient use of energy. For implantable medical devices, such as left ventricular heart assist systems a brushless dc-motor is a good drive choice. A left ventricular assist device pumps the blood from the left apex into the aorta and could be helpful in 80 of cases where a heart transplantation is necessary. A transmitting fluid is used to activate the bloodpump and the envisaged brushless dc-motor should incorporate a centrifugal pump in the hollow shaft to move the fluid. The requirements for the motor are: high reliability, low level of noise emission, restricted overtemperature, and still high efficiency. In order to come close to the envisaged design guidelines a two pole, two phase motor with an airgap winding was designed, built and tested. To utilize the copper volume of the winding best the motor control is of bidirectional type. For the prototype design sintered Nd-Fe-B magnet blocks were glued on the rotor. The dimensions of the active parts of 9W motor are:
outer stator diameter 22mm and 40mm length specific torque and specific power were calculated using the total active volume:
296Nm/m³ and 592kW/m³. The major restriction was the maximum overtemperature of 2K for the housing of the motor. A thermal model of the motor had to be developed and the current density had to be adjusted to avoid a significant overtemperature although the torque requirements had to be met. The detection of the rotor position is sensorless and is based on the back EMF detection of the two phase arrangement by dividing each phase into two winding sections. Focus will be on the magnetic field and torque calculation. An overview of the electronic control will be provided.

9:30 AM *H6.4 RARE EARTH-COBALT MAGNET MATERIALS FOR APPLICATIONS AT TEMPERATURES UP TO 500C. Christina H. Chen, Marlin S. Walmer and Michael H. Walmer, Electron Energy Corp., Landisville, PA; Sam Liu and G. Edward Kuhl, University of Dayton, Dayton, OH.

Electron Energy Corporation and the University of Dayton are currently engaged in a joint project to develop permanent magnets capable of operating at or above 400C. The work is sponsored jointly by the US Air Force and the DARPA. This paper reports the progress and results achieved to date. RE₂TM₁₇ magnets with high (BH)_max and _IH_C > 12 kOe at 400C have been produced. These magnets have a low temperature coefficient of _IH_C and a straight line B vs. H demagnetization curve up to 500C. A straight-line extrinsic demagnetization curve provides greater design flexibility and facilitates reduced size and weight of magnetic circuits. Straight-line demagnetization curves are almost a necessary magnetic feature of magnets used in dynamic systems. Another characteristic of these new magnets is low irreversible loss after exposure to high temperature. Magnetic properties of the new magnets will be shown in comparison to those of the best commercial magnets to illustrate the improvements that have been made. This paper also reviews the magnetic properties of different rare earth-cobalt magnet materials, which includes magnets with either high energy products, high coercive force, high thermal stability, or magnets with various temperature coefficients of B_r ranging from +0.08%/C through zero to -0.04%/C. This extensive range of magnetic properties provides designers with a large selection for use in meeting diverse magnetic circuit requirements.
 
 
 

SESSION H7: MICROSTRUCTURE AND MICROMAGNETICS
Chair: Jeffrey E. Shield
Thursday Morning, April 8, 1999
Salon 6 (M)

10:30 AM *H7.1
HIGH PERFORMANCE MAGNETS - MICROSTRUCTURE AND COERCIVITY. J. Fidler , S. Sasaki and E. Estevez-Rams, Institute of Applied and Technical Physics, Vienna University of Technology, AUSTRIA.

The importance of newly developed permanent magnetic materials in many electro-, magnetomechanical and electronic applications can be attributed to the drastic improvement of the magnetic energy product and coercive field of (Nd,Dy)-(Fe,Co)-B:(M1,M2) based materials. This enables the invention of many new applications of permanent magnets. Advanced SmCo₅/Sm₂Co₁₇- and Nd₂Fe₁₄B-based permanent magnets exhibit a complex, multiphase microstructure. These magnets exhibit the highest values of iHc and (B.H)max, obtained so far. The hysteresis properties are governed by a combination of the intrinsic properties of the material, such as saturation polarisation, exchange and magnetocrystalline anisotropy. The other important factors are the microstructural parameters, such as grain size, the orientation of the easy axes of the grains and the distribution of phases. The grain size of the magnets and the alignment of the grains strongly depend on the processing parameters. The formation and distribution of the phases is determined by the composition of the magnets and the annealing treatment. The intergranular structure between the grains plays a significant role determining the magnetic properties. The coercivity is determined by the long range dipolar interaction and short range exchange coupling between neighbouring grains. The doping of elements changes the phase relation and favours the formation of new phases. Additional secondary non-magnetic intergranular phases decrease the remanence and interrupt the magnetic interactions between the grains, thereby improving the coercivity of large grained sintered magnets. The influence of oxygen on the hard magnetic properties is more complex. Special emphasis will be laid on the study of the role of oxygen, grain alignment and abnormal grain growth on the coercive field and the energy product of advanced, high remanence sintered Nd-Fe-B magnets. In comparison to sintered magnets the relationships between microstructure and coercivity in melt-quenched magnets will be shown. This work is supported by the project FWF P13433.

11:00 AM *H7.2
Fe EPITAXY WITH Nd₂Fe₁₄B IN THE Nd-Fe-B SYSTEM DURING SOLIDIFICATION. R.W. McCallum , C.P. Li, K.W. Dennis and M.J. Kramer, Ames Laboratory, Iowa State University, Ames, IA.

While there have been numerous studies of melt-spun Nd₂Fe₁₄B (2-14-1) permanent magnet material, there is a limited understanding of the microstructure development in melt-spun 2-14-1 alloys. Two primary deficiencies of current models are the lack of consideration of peritectic reactions and the failure to consider the effects of the heat of crystallization, i.e. recalescence. The transition from amorphous to nanophased and subsequent increasing grain size with decreasing wheel speed can be explained using a solidification model which takes into account both the degree of undercooling and recalesence. In addition, this model can be used to explain the development of the textured 2-14-1 in the underquenched condition in regions of high thermal gradients. The model correctly predicts the direction of alignment of the 2-14-1 grains and explains the phase selection as a function of undercooling.

11:30 AM *H7.3
HYSTERESIS LOOPS AND COERCIVITY MECHANISMS IN SINTERED AND NANOCRYSTALLINE PERMANENT MAGNETS. Helmut Kronmüller , Max-Planck-Institut für Metallforschung, Stuttgart, GERMANY.

The hysteresis loops of nanocrystalline permanent magnets (pms) produced by the melt-spin technique have been investigated for compositions based on the intermetallic compounds Nd₂(Fe,Co)₁₄B, Pr₂Fe₁₄B and the carbides Sm₂Fe_17-xGa_xC_y. The following three types of pms have been studied: 1) High-coercivity pms with exchange decoupled grains. 2) High-remanence exchange-spring pms. 3) High-coercive-high-remanence composite pms with exchange coupled soft and hard magnetic grains. The temperature dependence of the coercive field  for all three types of pms obeys a relation for a modified nucleation field,  (K₁ = first anisotropy constant, M_S = spontaneous magnetization).
For an analysis of the characteristic differences between the microstructural parameters  and N_eff as obtained for the three types of pms, computational micromagnetism on the basis of the Finite Element Technique is applied. This powerful method allows a quantitative analysis of the role of grain size, grain boundaries, texture of easy directions and of soft magnetic phases in composite materials. In order to obtain satisfatory results, a self-adapting algorithm has been developed where the mesh size is adapted to the gradients of the direction cosines of the spontaneous magnetization.
It turns out that excellent magnetic properties of composite pms can only be obtained if the grain boundaries are as ideal as possible. Remanence and coercive field are found to decrease linearly with a corresponding reduction of the crystal anisotropy or of the exchange constant within the grain boundaries. In composite pms the diameters of the soft magnetic grains should be smaller than twice the domain wall width, , of the hard magnetic phase. For larger diameters H_C decreases according to a -law with  = 1.75, 0.762, 0.701 for multilayers, cubic and spherical grains. From these model calculations general rules for the development of optimized sintered as well as nanocrystalline pms with large remanences and large coercivities are derived.
 

SESSION H8: MICROSTURCTURE AND MICROMAGNETICS (continued)
Chair: Jeffrey E. Shield
Thursday Afternoon, April 8, 1999
Salon 6 (M)

1:30 PM H8.1
MICROSTRUCTURAL INVESTIGATION OF RE₃(Fe,V)₂₉ (RE = Nd, Tb) MAGNETIC MATERIALS. Johannes Bernardi , Manfred Noner, Josef Fidler, Institut für Angewandte und Technische Physik, Vienna University of Technology, AUSTRIA; Xiu-Feng Han, Fu-Ming Yang, Institute of Physics, Chinese Academy of Science, P.R. CHINA.

Rare-earth (RE), iron-rich RE₃(Fe,M)₂₉ alloys with M=Ti, V, Cr, Mn and Mo have gained wide interest during the last years due to their potential as a permanent magnetic material. This novel intermetallic ferromagnetic compounds increase significantly their Curie temperature, saturation magnetization or anisotropy field upon interstitial modification with nitrogen or carbon.
In the present investigation the microstructure of V stabilized RE₃(Fe,V)₂₉ (RE=Nd,Tb) has been investigated by transmission electron microscopy (TEM). The investigated samples were prepared by arc melting and subsequent annealing above 910C for one to five days. X-ray diffraction confirms that the samples can be indexed based on a monoclinic Nd₃(Fe,Ti)₂₉-type structure (3:29) with A_2/m space group¹. Nd₃(Fe,V)₂₉ shows a microstructure with grains of several microns that contain typically a high density of planar defects. Electron diffraction confirms that the grains have usually the mono-clinic 3:29 structure. In addition, several grains are found that can only be indexed according to the rhombohedral Th₂Zn₁₇ structure. The close relationship between the monoclinic 3:29 structure, rhombohedral Th₂Zn₁₇ and tetragonal ThMn₁₂ makes it often difficult to unambiguously interprete diffraction patterns. Tb₃(Fe,V)₂₉ contains a microstructure with a similar grain size of several microns. Here we find regularly grains with a twinned crystal structur. While the rhombohedral 2:17 structure is also found in the Tb-containing samples, there is no clear indication for the formation of the tetragonal 1:12 phase.
¹ Xiu-Feng Han; Yang, F.M.; Pan, H.G.; Wang, Y.G.; Wang, J.L.; Liu, H.L.; Tang, N.; Zhao, R.W.; Li, H.S., Journal of Applied Physics, (81), 7450, (1997).

1:45 PM H8.2
MAGNETIZATION REVERSAL IN MELT-QUENCHED NdFeB. D.C. Crew , L.H. Lewis, Materials Science Division, Department of Applied Science, Brookhaven National Laboratory, Upton, NY; P.G. McCormick, R. Street, The University of Western Australia, Nedlands, WA, AUSTRALIA; V. Panchanathan, Magnequench International, IN.

Melt-quenched NdFeB is an important modern permanent magnet material. However there still remains doubt as to the magnetization reversal mechanism which controls coercivity in material prepared by this processing route. To investigate this problem a new technique based on measurements of reversible magnetization along recoil curves has been used. This technique is able to determine if free domain walls are present during magnetization reversal. For this study samples of isotropic (MQI), hot pressed (MQII) and die upset (MQIII) melt-quenched NdFeB were examined. The results obtained are consistent with magnetization reversal for grains with sizes below the single domain limit being a single domain process, probably a form of incoherent rotation. For grains above the single domain limit in size, the reversal process is one in which domain walls are present during reversal and are able to contribute to the reversible magnetization through domain wall bowing. It is known from previous work that the proportion of multi-domain grains increases during hot pressing and die upsetting and it is hypothesized that the differences in the reversible magnetization behavior may be explained by the differing fractions of multidomain grains in the examined materials.

2:00 PM H8.3
CHEMICAL ORDERING AND MICROSTRUCTURAL INFLUENCES ON DOMAIN WALL MOTION IN Sm₂Fe₁₇ AND Sm₂Fe₁₇N_x. Brian E. Meacham and Jeffrey E. Shield, University of Utah, Department of Materials Science and Engineering, Salt Lake City, UT.

Rapidly solidified Sm-Fe alloys form a chemically disordered Sm₂Fe₁₇ structure. In this structure, the Fe dumbbell substitution on the Sm₂Fe₁₇ lattice occurs more or less randomly and the structure resembles the TbCu₇ prototype structure. The corresponding microstructure consists of dense, randomly oriented antiphase boundaries. Increased chemical ordering can be achieved by heat treating, resulting in a well-ordered Sm₂Fe₁₇ structure. The antiphase domain structure is also altered during the heat treatment. The density of antiphase boundaries decreases, and the boundaries become normal to the c-axis. The amount of chemical order and the structure of the antiphase domains affects the magnetic behavior. These crystalline defects can inhibit Bloch wall motion, altering the magnetization and de-magnetization processes. In this paper, the effect of chemical order and antiphase boundary structure on the Bloch wall movement in Sm₂Fe₁₇ and Sm₂Fe₁₇N_x was investigated. The chemical order and antiphase domain structure was controlled by heat treatment of melt-spun ribbon at temperatures from 500 to 900C. The chemical order was characterized by x-ray and electron diffraction, while the antiphase boundary structure was characterized by transmission electron microscopy. Magnetic susceptibility measurements revealed the effect of defect density and structure on the magnetization process.
 

SESSION H9/I10/L8: JOINT SESSION:
THIN FILM PERMANENT MAGNETS
Chair: Sara A. Majetich
Thursday Afternoon, April 8, 1999
Salon 1-3 (M)

2:45 PM *H9.1/I10.1/L8.1
PERMANENT MAGNETISM IN EXCHANGE-COUPLED NANOCOMPOSITES. D.J. Sellmyer , J.P. Liu and R. Skomski, University of Nebraska, Behlen Laboratory of Physics and Center for Materials Research and Analysis, Lincoln, NE.

Magnetic nanocomposites are of increasing importance in permanent magnetism and related areas such as magnetic recording, because they make it possible to improve the materials' performances beyond that of the pure substances. High energy products in real two-phase permanent magnets mean a trade-off between remanent magnetization and coercivity, which is optimized by a suitable nanostructure [1]. The key requirement is to incorporate a large amount of fine-grained soft phase into a hard-magnetic matrix, but there are also important secondary factors such as grain, geometry, grain-size distribution, and hard-soft interface exchange. Special processing methods and magnetic hardening have been investigated in nanostructured Fe-Pt [2], Pr-Co [3], and Sm-Co [4] films consisting of hard and soft regions. The nearly ideally structured Fe-Pt films, consisting of hard FePt and soft iron-rich regions, exhibit room-temperature coercivities of up to 20 kOe and energy- products higher than 50 MGOe. The Pr-Co and Sm-Co films contain hard and soft magnetic grains with dimensions about 10 nm, and develop coercivities up to 40 kOe in the latter case. The observed hysteresis loops are described in terms of a novel model which incorporates reduced exchange at grain boundaries.[5].
Research supported by US DOE, AFOSR and DARPA through ONR.
1. R. Skomski and J.M.D. Coey, Phys. Rev. B 48, 15812 (1993). 2. J.P. Liu, C.P. Luo, Y. Liu and D.J. Sellmyer, Appl. Phys. Lett. 22, 483 (1998). 3. J.P. Liu, Y. Liu and D.J. Sellmyer, J. Appl. Phys. 83, 6608 (1998). 4. Y. Liu, R.A. Thomas, S.S. Malhotra, Z.S. Shan, S.H. Liou and D.J. Sellmyer, J. Appl. Phys. 83, 6244 (1998), and to be published. 5. R. Skomski, J.P. Liu, J.M. Meldrim and D.J. Sellmyer, Proc. Tenth Int. Symp. Mag. Anisot. and Coercivity in Rare Earth-Trans. Metal Alloys, (Workstaff-Informationsgesell-schaft, Dresden, 1998), p. 277.

3:15 PM H9.2/I10.2/L8.2
ON THE RELATIONSHIP OF HIGH COERCIVITY AND L1₀ ORDERED PHASE IN CoPt AND FePt THIN FILMS. Roger A. Ristau , Katy Barmak, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA; Laura H. Lewis, Materials Science Division, Department of Applied Science, Brookhaven National Laboratory, Upton, NY; Kevin R. Coffey, J. Kent Howard, IBM Storage Systems Division, San Jose, CA.

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM), and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered structure to the L1₀ ordered structure occurred during post-deposition annealing and was characterized using digital analysis of dark field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the volume fraction transformed was quantified for numerous intermediate steps between the disordered and ordered phases. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in our samples was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed. An increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase is proposed as a mechanism for the high coercivity in these films.

3:30 PM H9.3/I10.3/L8.3
MAGNETIC EXCHANGE-COUPLING IN CoPt/Co BILAYER THIN FILMS. J. Kim , K. Barmak, Dept. of Materials Science and Eng., Lehigh University, Bethlehem, PA; L.H. Lewis and D.O. Welch, Materials Science Division, Dept. of Applied Sciences, Brookhaven National Laboratory, Upton, NY.

Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase hard/soft composite magnets. CoPt films, with a thickness of 25 nm, were sputter-deposited from a nearly equiatomic alloy target onto oxidized Si wafers. The films were subsequently annealed at 700C and fully transformed from the FCC phase to the magnetically hard, ordered L1₀ phase. The coercivity of the films increased rapidly with annealing time until it reached a plateau at approximately 9.5 kOe. Fully-ordered CoPt films were then used as substrates for deposition of Co layers, with thicknesses in the range of 2.8-225 nm, in order to produce the hard/soft composite bilayers. As predicted by theory, the exchange coupling between the soft Co phase and the hard, ordered CoPt phase decreased as the thickness of the soft phase increased. This decrease in coupling was clearly seen in the magnetic hysteresis loops of the bilayers. At small thicknesses of Co (a few nanometers), the shape of the loop was one of a uniform material showing no indication of the presence of two phases with extremely different coercivities. At larger Co thicknesses, constricted loops, i.e., ones showing the presence of a mixture of two ferromagnetic phases of different hardnesses, were obtained. In addition to the magnetic exchange behavior, the relationship of this behavior to the microstructure of the bilayer films will be discussed.
** Research performed under the auspices of the U.S. Dept. of Energy, Division of Materials Science, Office of Basic Energy Sciences under contract No. DE-AC02-98CH10886.

3:45 PM H9.4/I10.4/L8.4
STUDY OF EPITAXIAL CO-ALLOY/NiAl THIN FILM GROWTH ON Si SUBSTRATES. Heng Gong , Wei Yang, David E. Laughlin, David N. Lambeth, Carnegie Mellon University, Data Storage Systems Center, Pittsburgh, PA.

Co-alloy thin films are currently the most popular magnetic recording media for hard disks. Compared to the widely used Cr underlayers, NiAl thin films can exhibit smaller and more uniform sized grains, which in turn can induce smaller Co grain sizes. Hence, NiAl underlayers can be used to improve the media signal to noise ratio. Besides the grain size, control of the thin film texture is an important microstructural property for achieving high media performance. Textures that induce in-plane Co easy axes are usually desired in order to achieve high coercivity.
For polycrystalline Co/NiAl thin films sputtered on amorphous substrates, the NiAl layers usually show both (110) and (112) textures, while the Co layers exhibit a (10-10) orientation. Because of the observation of the two underlayer textures and the weak signal obtained from X-ray diffraction peaks for the high angle (112) plane and very thin films, uncertainty has existed in the understanding of the orientational relationship between the Co and the NiAl layers. In this study, by using the Ag templates grown on single crystal Si substrates, we have prepared epitaxial Co/NiAl thin films in order to study the orientation relationships. These epitaxial layers each show well defined orientation, hence the epitaxial relationships are clearly determined. Specifically, we report, for the first time, the epitaxial growth of NiAl(110) films by sputter deposition on Ag/Si(111) templates. Likewise, NiAl (112) textured films were also successfully grown on Ag/Si(110) templates.
The epitaxial orientational relationship, determined by X-ray pole figure phi-scan measurements, were found to be Co(10-11)  NiAl(110)  Ag(111)  Si(111), in which the Ag layer contains two twin-related variants, and the NiAl layer consists of three orientational variants. On the other hand, on Si(110) substrates the relationship is Co(10-10)  NiAl(112)  Ag(110)  Si(110).
From these observations it is inferred that the (10-10) Co texture in polycrystalline films is induced by the (112) NiAl texture and not by the (110) NiAl texture.

4:00 PM H9.5/I10.5/L8.5
COERCIVITY AND MICROSTRUCTURE ANALYSES OF SPUTTERED Nd(Dy) FeB FILMS. J.L. Tsai and T.S. Chin, Department of Materials Science & Engineering, National Tsing Hua University, Hsinchu, TAIWAN, R.O.C.

Nanocrystalline Nd(Dy)FeB films were prepared by DC magnetron sputtering on Si(111) with W as the underlayer. The Nd(Dy)FeB films were deposited at the substrate temperature (Ts) 600-700C for 10-18 minutes and with a thickness of 321-1068 nm. XRD was used to identify the phases in Nd(Dy)FeB films. It was found that [105] texture of the 2:14:1 phase exists at Ts 650-700C. The [105] peak slightly deviates from [006] with an angle of about 11.3 degree and it is the dominating orientation of almost all grains. Room temperature magnetic properties of Nd(Dy)FeB films was measured by VSM with a maximum field 2T. Coercivity and remanence ratio of 5.11-11.7 kOe, 0.68-0.90 perpendicular to the films surface, respectively were obtained. The microstructure of Nd(Dy)FeB films were observed by HRTEM. It was found that Nd₂Fe₁₄B grains have an average grain size about 50nm surrounded by Nd-rich phase at the grain boundary when Ts was 650C. The composition of the Nd-rich phase was NdFe₂ as checked by EDX with a beam size less than 10 nm.
To analyze the temperature dependence of coercivity field of a film (Ts=650C, 935nm thick), nucleus expansion model (phenomenological model) was adopted [1.2]. The model takes into account the geometrical effect of nucleated domains, the effect of the disturbance of domain-wall energy  and the thermal activation S_v. The parameters = and N were obtained by plotting (H₀/Ms) vs. /(M_s²v^1/3). The value of activation volume is about 4.71x10^-20 cm³ at 100 K via magnetic viscosity measurements. From the coercivity analyses based on phenomenological model, it was found that reversed domain preferentially occurs on the area with small domain-wall energy which is influenced by the activation volume v. N assists the nucleation of reversed domain and thermal activation S_v slightly affects the magnetization reversal process.
1. X.C. Kou, H. Kronmuller, D. Givord and M.F. Rossignol, Phys. Rev. B 3849 (1994)
2. M. Becher, M. Seeger, J. Bauer, D. Goll and H. Kronmuller, Fifteenth International Workshop on Rare-Earth Magnets and Their Applications.

4:15 PM H9.6/I10.6/L8.6
INCREASING THE IN-PLANE COERCIVITY OF CoCrPt PERMANENT MAGNETS DEPOSITED ON NiMn. Paul E. Anderson , Steven P. Bozeman, Seagate Technology, Minneapolis, MN.

CoCrPt alloys are frequently used to provide the horizontal stabilization for magnetoresistive and giant magnetoresistive sensors in an abutted junction configuration. It is well known that the in-plane coercivity of CoCrPt films is strongly dependent on the seed layer, which is typically Cr. In the case of a bottom spin valve, CoCrPt and any seed layer are deposited on the antiferromagnetic pinning layer (NiMn), which results in a reduced coercivity as compared with deposition on Al₂O₃ or Si. We present CoCrPt crystallographic texture and grain size measurements in order to explain the drop in coercivity, and discuss various means of restoring the in-plane coercivity.

4:30 PM H9.7/I10.7/L8.7
THE INFLUENCE OF MICROSTRUCTURE ON THE TECHNICAL MAGNETIC PROPERTIES IN NANOSTRUCTURED CoPt L1o HIGH ANISOTROPY RECORDING MEDIA. Sangki Jeong , D.E. Laughlin and M.E. McHenry, Data Storage Systems Center & Dept. of Materials Science, Carnegie Mellon University, Pittsburgh, PA.

Recently, CoPt and FePt thin films have received attention for potential applications in extremely high density recording (EHDR)¹ because of their large magnetocrystalline anisotropy. In recording media an acceptable writing coercivity, in-plane texture, small and exchange decoupled grains are required for increasing the recording density. It is well known that the magnetic properties of CoPt alloys strongly depends on annealing temperature, time and composition due to atomic ordering and the fraction of the anisotropic ordered tetragonal phase^2,3. Here we calculate magnetocrystalline anisotropy using Neels phenomenological model and focus on its functional dependence on the long range order parameter and composition. Based on our analysis, we predict that the magnitude of the magnetocrystalline anisotropy will be very sensitive to atomic order and composition. Importantly, there is a possibility to control the anisotropy by controlling composition and degree of order. We have produced Co₅₀Pt₅₀ thin films and Co₄₆Pt₅₄ films encapsulated in SiO₂ using conventional RF sputtering. Our experimental data indicates slow ordering kinetics in thin films (below 20 nm thickness) encapsulated in SiO₂. Coercivities are observed to be strongly dependent on the film thickness. In our nanostructured films kinetic considerations are very important since the films we obtain are generally in a non-equilibrium and strained state⁴. Previous results for CoPt and FePt alloy films² showed (111) texture. SiO₂ encapsulation in our films results in nearly random orientation of the magnetic easy axes. We have modeled the effect of texture on hysteresis, using the Stoner-Wohlfarth model. In this analysis, it was found that the magnetic properties of a (111) textured non-interactiong assembly was similar to that of a 3-D random distribution. Further investigation of reversal mechanism and exchange coupling was carried out using micromagnetic calculations. Numerical simulation of the magnetization reversal process were developed within a 2-dimensional nanostructured array model of interacting single domain assembly, varying the ordered fraction and exchange coupling constant between the particles. From the simulations, literature results, and our experimental data, it is concluded that the reversal process is strongly affected by the exchange coupling between hard and soft phases (like exchange spring magnets). SiO₂ encapsulation leads to poor exchange coupling between grains.
1. D.J. Sellmyer, M. Yu and R.D. Kirby, Nano-98 (1998).
2. Kevin R. Coffey, Michael A. Parker and J. Kent Howard, IEEE Trans. Mag-31, 2737, (1995). 3. G. Hadjipanayis and P. Gaunt, J. Appl. Phys. 50(3), March (1979).
4. Bing Zhang, Univ. of Pittsburgh, Ph. D Thesis, (1991).

4:45 PM H9.8/I10.8/L8.8 Nd-Fe-B THIN FILMS SPUTTERED ON SILICON. H. Bartsch de Torres , H. Wurmus, Technical University Ilmenau, Dept of Materials and Materials Science, Ilmenau, GERMANY.

Hard magnetic NdFeB - thin films were sputtered by a triple-source-dc-magnetron at 450C. The film composition was varied by adjusting the power of the sources. The maximum sputtering rate was 2 m/h and the film thickness about 2 m. We achieved both, perpendicular and parallel orientation of the easy-axis. The films were evaporated on Si-substrates directly and using metallic and dielectric interlayers. A consideration of various materials were given with regard to the diffusion behavior. The best composition of all investigated alloys was Nd_8.8Fe_78.6B_10.9. With this composition we obtained a maximum product of energy of 100kJ/m³, maximum B, of 1T and maximum H_c of 500kA/m.