J. Michael Coey
Univ of Dublin
Dept of Physics
Dublin, 2 IRELAND
Dept. of Applied Science
Brookhaven National Lab
Upton, NY 11973-5000
Cranbury, NJ 08512
Inst of Applied & Technical Physics
Vienna Univ of Technology
Vienna, A-1040 AUSTRIA
Inst for Metallic Materials
Dresden, D-01171 GERMANY
Proceedings published as Volume 577
* Invited paper
of the Materials Research Society
Symposium Proceedings Series.
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.
STH: CONCEPTS AND EXPERIMENTAL METHODS IN MICROMAGNETISM
Monday, April 5, 2:00 - 5:00 p.m.
Salon 6 (Marriott Hotel)
George Hadjipanayis, University of Delaware
Ralph Skomski, University of Nebraska
SESSION H1: PERMANENT MAGNET PROCESSING
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.
Chair: C. H. Sellers
Tuesday Morning, April 6, 1999
Salon 6 (M)
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 Sm2Fe17-xGax
(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,
Sm2Fe17-xTax and Sm2Fe17-xTaxN
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
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,
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,
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 Sm13.8Fe82.2Ta4.0
and Sm13.7Fe86.3 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; Br = 752 mT, Hci
= 613 kAm-1 and BHmax = 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 Br
= 966 mT, Hci = 780 kAm-1 and BHmax
= 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 Otaigbe1,2,
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
1:30 PM *H2.1
NEUTRON DIFFRACTION CHARACTERIZATION OF PERMANENT MAGNET
PHASES. William B. Yelon , University of Missouri Research Reactor, Columbia,
Chair: Gary J. Long
Tuesday Afternoon, April 6, 1999
Salon 6 (M)
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
2:00 PM *H2.2
MAGNETIC CHARACTERISTICS OF RCo7-xZrx
ALLOYS (R=Sm,Gd,Pr,Er,Y and La). M.Q. Huang a,b,
W.E. Wallacea, M. McHenrya and S.G.
Sankarb, aCarnegie Mellon Univ., Department
and Materials Science and Engineering, Pittsburgh, PA, bAdvanced
Materials Corporation, Pittsburgh, PA; Qun Chen and B.M. Ma, Rhodia Corporation,
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 RCo7-xZrx 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, Ha. 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, Ha
(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, Hc, 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 Ce2Fe17Hx AND Pr2Fe17Hx.
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 Ce2Fe17Hx
and Pr2Fe17Hx, 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 Pr2Fe17Hx,
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
1. R.F. Sabiryanov and S.S. Jaswal, J. Appl. Phys. 81,
2. R.F. Sabiryanov and S.S. Jaswal, Phys. Rev. Lett.
3. R.F. Sabiryanov and S.S. Jaswal, Phys. Rev. B 57,
4. R.F. Sabiryanov and S.S. Jaswal, J. Magn. Magn. Mater.
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.
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,
2. W. B. Yelon et al., MMM'98 Conf. in Miami,
Supported by NSF, DOE, AFOSR, DARPA and National Computer
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,
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 Nd3.25Tb1Fe72.75Co5B18.
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 Nd4.5Tb1Fe71.5Cr5B18,
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
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 (HcJ) was 10-15 m/s. By adding Nb, HcJ
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 HcJ
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:
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.
NANOSCALE HARD MAGNETISM II
Chair: David C. Crew
Wednesday Afternoon, April 7, 1999
Salon 5/6 (M)
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
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
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 (Br), coercivity
(Hci)5 and maximum energy product (BHmax)
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
Brr, Hci, and BHmax
obtained for most of the samples, it became inadequate for samples treated
above 750C. The Br,
Hci and BHmax 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 (Nd0.95La0.05)9.5Febal
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
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 Nd8.1Fe77.1Co10.9B3.9
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.
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,
JOINT IN-ROOM POSTER SESSION:
NANOSCALE HARD MAGNETISM III
Chair: Qun Chen
Wednesday Afternoon, April 7, 1999
Salon 5/6 (M)
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.
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
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.
alloys of the composition of NdxFebalCo8NbyBz,
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 (HcJ) of flat
powders obtained depends on powder sizes. The best HcJ
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, HcJ:427 kA/m, and
density of 6.20 Mg/m3 is obtained. Further investigation to
improve magnetic properties are now in progress.
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.
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
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.
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 PdFe or FePd
antisite atoms, with PdFe 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 PdFe
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).
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: Nd12.7Fe81.2B6.1,
close to the Nd2Fe14B composition and Nd30Fe64B6,
Nd30Fe65B5 and Nd30Fe65.5B4.5,
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 Nd12.7Fe81.2B6.1
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 Nd30Fe65.5B4.5
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
8:30 AM *H6.1
NOVEL PERMANENT MAGNETS AND THEIR USES. Steve Constantinides
, The Arnold Engineering Company, Marengo, IL.
Chair: V. Panchanathan
Thursday Morning, April 8, 1999
Salon 6 (M)
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,
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 IHC
> 12 kOe at 400C have been produced.
These magnets have a low temperature coefficient of IHC
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 Br 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
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.
Chair: Jeffrey E. Shield
Thursday Morning, April 8, 1999
Salon 6 (M)
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
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 Sm2Fe17-xGaxCy.
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, MS
= spontaneous magnetization).
For an analysis of the characteristic differences between
the microstructural parameters
and Neff 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 HC 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)
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.
Chair: Jeffrey E. Shield
Thursday Afternoon, April 8, 1999
Salon 6 (M)
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 A2/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 Sm2Fe17Nx.
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
Sm2Fe17Nx 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:
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,
THIN FILM PERMANENT MAGNETS
Chair: Sara A. Majetich
Thursday Afternoon, April 8, 1999
Salon 1-3 (M)
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 . 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 , Pr-Co , and Sm-Co  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..
Research supported by US DOE, AFOSR and DARPA through
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)
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)
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  texture
of the 2:14:1 phase exists at Ts 650-700C.
The  peak slightly deviates from  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 Sv. The parameters =
and N were obtained by plotting
(H0/Ms) vs. /(Ms2v1/3).
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 Sv
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,
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 phase2,3.
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
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 Nd8.8Fe78.6B10.9.
With this composition we obtained a maximum product of energy of 100kJ/m3,
maximum B, of 1T and maximum Hc of 500kA/m.