It has been known that the Co-Cr film grown at elevated
temperature reveals higher coercive field such that lower
media noise can be obtained than that grown at room
temperature. It was suggested that Cr precipitates increase
the barrier for dipole rotation. In this study,
two-dimensional phase separation of Co-Cr random binary
alloy on W(110) surface was confirmed with UHV scanning
tunneling microscopy. Kinetically limited percolation
structure was observed on the 1.1 monolayer Co and 0.1
monolayer Cr film co-deposited at room temperature, an
initial random binary mixture. Upon annealing the mixture at
420 K and quenching to room temperature, we observed
separation of the Co- rich phases and the Cr- rich phases of
about 10 nm size. An evidence of compositional inhomogenity
was observed in Cr-rich phases but not in Co-rich phases. As
a kinetic pathway for the sputter grown film, this
experiment supports the model that phase separation
dynamically occurs during the growing process of Co and Cr
through the surface diffusion of adatoms.
[BC36.02] Sputtered SmCo Thin Films: Microstructural and Magnetic Properties
V. Neu, S.A. Shaheen (The Center for Materials Research and Technology (MARTECH), Florida State University, Tallahassee, FL 32306-4351)
In-plane oriented thin films of permanent magnet materials are of particular interest for magnetic field biasing in various miniature device designs. In this context we have studied the sputter deposition of Sm-Co thin films with strong in-plane texture. With a Sm-content ranging from 15 to 19 at% these samples span the transition from the TbCu_7-type structure to the CaCu_5-type structure. Rietveld analysis of x-ray data has been used to detect this transition at around 17 at% Sm. To examine the origin of the generally observed crystallographic in-plane texture emphasis has been laid on describing the influence of composition and sputter gas pressure on crystallographic and magnetic properties separately, although those parameters are coupled in the deposition process. Thus we find a strong dependence of the in-plane texture on the Sm-content, whereas for a fixed Sm-content the sputter gas pressure has no significant impact on the properties of the film.
[BC36.03] Magnetic Properties of FeMnO_3.
D. Seifu (Department of Physics, Morgan State University, Baltimore, MD 21251), A. Kebede (Department of Physics, North Carolina Aamp; T University, Greensboro, NC 27411), F.W. Oliver, E. Hoffman, E. Hammond (Department of Physics, Morgan State University, Baltimore, MD 21251), C. Wynter (Department of Chemistry, Nassau Community College, Nassau, NY 11530), A. Aning (Department of Materials Science and Engineering, Virginia Polytechnic and State University, Blacksburg, VA 27411), L. Takacs (Department of Physics, University of Maryland at Baltimore County, Baltimore, MD 21250), I-L. Siu, J.C. Walker (Department of Physics, John Hopkins University, Baltimore, MD 21250)
We present results of Mössbauer effect and magnetic measurements on FeMnO_3 produced by mechanical alloying. At room temperature we observed a quadrupole doublet in the Mössbauer spectrum which indicates that the material is nonmagnetic. In the temperature range 30 K to 50 K the spectrum developed a sextet, which is indicative of magnetic ordering. This magnetic transition is further established by magnetization measurements.
[BC36.04] Nanostructured Sm-Co Powders by Mechanical Milling
Zhongmin Chen, Michelle Corte-Real, Yong Zhang, George Hadjipanayis (Department of Physics and Astronomy, University of Delaware, Newark, DE 19716)
We have studied the structural, microstructural and magnetic
properties of nanostructured Sm-Co powders produced by
mechanical milling. Nanocrystalline SmCo5 and Sm2Co17 and
nanocomposite SmCo5/Sm2Co17 and Sm2Co17/Co were synthesized
by milling coarse as-cast powders with compositions of
SmXCo1-X and SmX(Co0.98Zr0.02)1-X. The mechanically milled
powders (for 1-96 hr) were annealed at 500-900°C for 15-30
min to obtain the desired microstructure. XRD, TEM and
magnetic measurements show that (1) Nanocrystalline (SmCo5
and Sm2Co17) and nanocomposite (SmCo5/Sm2Co17 and
Sm2Co17/Co) powders with high coercivity (up to 28 kOe) and
high Mr/Ms (up to 0.74) can be synthesized. (2) With
increase of Sm content, the coercivity increases but the
magnetization decreases, while the (BH)m shows a maximum at
around 10.5-13 atthe coercivity and the (BH)m by inhibiting grain growth
during annealing. Further work is underway to increase the
magnetization by substituting Fe for Co. These results are
very interesting and promising for the future development of
nanograin magnets with better mechanical properties.
[BC36.05] Effect of TaN Seed Layer on the Magnetic Properties of Permallloy
Zheng Gao, Kathleen O'Donell, Ned Tabat (Seagate Technology)
TaN films were deposited by sputtering from a Ta target with
nitrogen and argon gas mixture. The nitrogen composition
range was varied from 15% to 50%. The film structure,
composition and electrical resistivity were measured using
XRD, Auger spectroscopy, and four-point probe method,
respectively. The effects of the deposition power, bias
voltage, gas flow rate, and nitrogen percentage on the
nitrogen content in the TaN film were also studied. It was
found that resistivity of TaN film exponentially increases
with increasing nitrogen percentage. TaN films with varing
nitrogen concentrations were used to seed 250A thick
permalloy films (Ni81Fe19). Bilayer crystalline structure
were studied using XRD and the magnetic behavior was
measured using BH-looper. The bilayers were also
heat-treated up to 230°C to study the change of crystalline
structure and magnetic properties.
[BC36.06] New Sm2TM17 Magnets for High Temperature Applications
Jinfang Liu, Y. Zhang, W. Tang, George Hadjipanayis (Dept. of Physics and Astronomy, University of Delaware, Newark, DE 19716)
We have studied extensively the influence of Cu Zr and Fe
content and the transition metal to rare earth ratio z on
the magnetic properties and microstructure of
Sm(CobalCuxFeyZrw)z magnets with x ranging from 0.048 to
0.128, y from 0 to 0.244, w from 0 to 0.1, and z from 6.5 to
9.1. High Cu, low Fe and low z were found to contribute to
high coercivity at high temperatures. TEM studies showed
that both a low ratio z and high Cu content lead to small
cell size, which we believe is responsible for the high
coercivity at high temperatures. The density of lamella
phase does not change significantly with the ratio z. More
Cu is needed in the low ratio z magnets to get a high
coercivity, and this may be due to smaller cell size and
thus large percentage of Sm(Co1-xCux)5 cell boundary phase.
In the high Cu content magnets the coercivity is saturated.
Small temperature coefficient of intrinsic coercivity can be
achieved by reducing Fe content and ratio z. A new Sm-Co
magnet with a coercivity of 10 kOe at 773 K has been
developed. This newly developed magnet has a temperature
coefficient of coercivity ß(RT - 773 K) of -0.03is about seven times smaller than the commercial 2:17
magnets. Work supported by Air Force under Grant No. MURI
F49620-96-1-0434.
[BC36.07] Magnetic hysteresis of exchange-coupled nanocomposite films*
J.P. Liu, R. Skomski, D.J. Sellmyer (Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588)
Nanostructured composite films with magnetically hard and
soft phases have been prepared by sputtering and subsequent
heat treatment. Magnetic hysteresis loops have been measured
at the temperature range from the liquid-helium temperature
to room temperature for two systems, the Sm_2Co_7:Co
and Pr_2Co_7:Co. For optimally processed samples,
the remanence enhancement is achieved as a result of
effective intergrain exchange coupling in the composites,
which is featured by the high squareness and no-kink
behavior at the zero field points of the loops at room
temperature. However, the kink becomes more pronounced when
temperature goes down. The squareness of the loops decreases
consequently. Drastic change happens when temperature
further decreases from 50 K to 5 K. Coercivity drops by
about 70% of the room-temperature value whereas the
saturation magnetization doubles its attitude. These
phenomena are anomalous compared to the behavior observed in
the bulk materials, which may be related to a possible
magnetic phase transition in the temperature range from 50 K
to 5 K and a weak exchange coupling between the sublattices
in the rare-earth transition-metal hard phases.
[BC36.08] Rapidly Solidified Neodymium Iron Boron Magnetic Materials
Charlie Sellers, Barry Rabin (Magnequench International, Inc.), Andrew Gavrin (Indiana University, Purdue University at Indianapolis)
The development of rapid solidification as a processing route for NdFeB alloys in 1983 created a new family of high strength materials, in both magnetically isotropic and anisotropic forms. The existence of the single grain-single domain state results in unique properties which are fascinating to study, and technologically relevant. The original development of the stoichiometric material will be briefly described, including its intrinsic properties, and the entire family of related (multiphase) compositions will be discussed. Correlations between the composition, magnetic domain structure, phase structure, and magnetic properties will be illustrated with experimental results from Magnetic Force Microscopy, magnetometry, calorimetry, and other relevant techniques. The development of these characteristics during processing by various rapid solidification and consolidation routes provides an excellent opportunity for the investigation of amorphous metallic materials, illustrating the close correration between structure and properties in magnetic materials. The technological applications of these materials will be described, empasizing opportunities where this material is uniquely qualified.
[BC36.09] The Synthesis of High Coercivity Highly Textured SmCo Based Films by Pulsed Laser Deposition
R. Rani, Li Chen, T. Theodoropoulos (Queens College CUNY)
SmCo based films have been directly crystallized onto moderately heated silicon and other susbstrates, >>375 C, by high rate pulsed laser deposition, PLD. Films have been deposited to compare shadow and direct deposition. Only for shadow deposition conditions has it been possible to deposit CaCu_5-type structure films that are mirror-like, very fined grained, and with the crystallite c-axes completely aligned onto the substrate plane. For optimally deposited films, room temperature hysteresis loops, measured perpendicular to the film plane, were closed with approximately zero coercivity. For such films, hysteresis loops measured in plane with fields to 90 kOe at room temperature exhibited intrinsic coercivities of at least 22.5 kOe. Measurement fields of 19 kOe yielded only minor loops that did not enclose the origin in some cases. High coercivity samples exhibited high remanent to saturation magnetization ratios of approximately 0.9. This indicates that the films were exchange coupled. The fully textured SmCo based films in the present study were deposited using \lambda = 248 nm and a pulse energy of 1100 to 1200 mJ. The magnetic properties are a function of the laser pulse rate and this has been studied from 5 to 50 Hz repetition rates. The shadow deposition rate was approximately 9 Ås at 15 Hz.
[BC36.10] Nanocrystaline and Nanocomposite R-Fe Carbides
Naoki Hayashi, Maria Daniil, George Hadjipanayis (Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA)
Nanocomposite materials have attracted considerable
interest in recent years because of their fundamental and
technological properties. Most of the previous studies have
been focused on R-Fe-B, Sm-Co and Sm-Fe-N. We have recently
focused our efforts on the R-Fe-C system (R=Pr, Nd) based on
the 2:14:1 structure. The samples were made by melt-spinning
over a wide range of wheel speeds followed by an annealing
heat treatment to obtain a nanocrystaline/nanocomposite
structure. Samples with a composition Nd12Fe82C6 showed low
coercivities (about 2 kOe). Addition of a small amount of Zr
(1.5 atwas obtained on Nd13.5Fe80.5C6 after an aging heat treatment
at 900°C for 10 min. Optimization studies are in progress
and the results will be correlated with the microstructure
determined by electron microscopy.
[BC36.11] Magnetic Field-Induced Transitions From Spin Glass to Liquid to Long Range Order in a 3D Geometrically Frustrated Magnet.
Y.K. Tsui (Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA), C.A. Burns (Department of Physics, Western Michigan University Kalamazoo, MI 49008-5151, USA), J. Snyder, P. Schiffer (Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA)
Gadolinium gallium garnet, Gd_\small 3Ga_\small 5O_\small 12 (GGG) has an extraordinary low temperature phase diagram. including a spin glass phase near H=0T, an antiferromagnetic (AFM) phase for 0.7T\leq H\leq 1.4T and a phase without long range order at intermediate fields. We have measured the thermal properties of a high-quality single crystal of GGG as a function of field in the low temperature regime. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG suggesting that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T\sim0.18K, in analogy to the minimum in the melting curve of ^4He. This is confirmed by measurements of the latent heat of the transition below that temperature.