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Antiferromagnetic exchange bias of a ferromagnetic semiconductor by a ferromagnetic metal

Description: We demonstrate an exchange bias in (Ga,Mn)As induced by antiferromagnetic coupling to a thin overlayer of Fe. Bias fields of up to 240 Oe are observed. Using element-specific x-ray magnetic circular dichroism measurements, we distinguish an interface layer that is strongly pinned antiferromagnetically to the Fe. The interface layer remains polarized at room temperature.
Date: November 5, 2009
Creator: Olejnik, K.; Wadley, P.; Haigh, J.; Edmonds, K. W.; Campion, R. P.; Rushforth, A. W. et al.
Partner: UNT Libraries Government Documents Department

Localized ferromagnetic resonance force microscopy of permalloy-cobalt films

Description: We report the Ferromagnetic Resonance Force Microscopy (FMRFM) experiments on a combined permalloy-cobalt continuous film. Our studies demonstrate the capability of FMRFM to perform local spectroscopy of different ferromagnetic materials. Theoretical analysis of the uniform resonance mode at the edge of the film provides good quantitative agreement with the experimental data. Our experiments demonstrate the micron scale lateral resolution and allow to extract local magnetic properties in continuous ferromagnetic samples.
Date: January 1, 2008
Creator: Nazaretski, Evgueni; Movshovich, Roman; Martin, Ivar; Cha, Kitty V; Akhadov, Elshan A; Obukhov, Yu et al.
Partner: UNT Libraries Government Documents Department

Spin--wave spectrum of an amorphous ferromagnet

Description: The spin-wave spectruin of an amorphous Heisenberg ferromagnet is calculated by a diagrammatic expansion making use of a transformation due to Taylor and Wu Phys. Rev., B2: 1752 (1970). The upper limit of the spectrum is found to occur at frequencies below that of the corresponding crystalline system, while the low-frequency part of the spectrum is enhanced. Internal van Hove singularities are absent in the spin-wave spectrum of the amorphous ferromagnet. (auth)
Date: January 1, 1973
Creator: Gubernatis, J.E. & Taylor, P.L.
Partner: UNT Libraries Government Documents Department


Description: Progress is briefly reported on studies in dielectric spectros copy, magnetic resonance, high-field-strength research, ferroelectrics and ferromagnetics, semiconductors, single crystals, and ceramics. (T.R.R.)
Date: June 1, 1958
Partner: UNT Libraries Government Documents Department

Spin polarization and magnetic dichroism in core-level photoemission from ferromagnets

Description: In this thesis we present a theoretical investigation of angle- and spin-resolved core-level photoemission from ferromagnetic Fe and Ni. We also consider magneto-dichroic effects due to reversal of the photon helicity or reversal of the sample magnetization direction. In chapter 1, we provide a brief outline of the history of photoemission, and show how it has played an important role in the development of modern physics. We then review the basic elements of the theory of core-level photoemission, and discuss the validity of the some of the commonly-used approximations. In chapter 2, we present a one-electron theory to calculate spin- and angle-resolved photoemission spectra for an arbitrary photon polarization. The Hamiltonian includes both spin-orbit and exchange interactions. As test cases for the theory, we calculate the spin polarization and magnetic dichroism for the Fe 2p core level, and find that agreement with experiment is very good.
Date: May 1, 1997
Creator: Menchero, J.G.
Partner: UNT Libraries Government Documents Department

Ferromagnetic resonance imaging of Co films using magnetic resonance force microscopy

Description: Magnetic resonance force microscope (MRFM) technique has been applied to the study of spatial imaging in thin Co ferromagnetic film. A novel approach is proposesd to improve spatial resolution in MRFM, which is limited by the broad width of Co ferromagnetic resonance (FMR) line. The authors introduce a selective local field with a small yittrium iron garnet (YIG) grain. They have performed MRFM detected FMR on a sample consisting of two sections of Co films laterally separated by {approximately}20 {micro}m. The experimental results demonstrate the scanning imaging capabilities of MRFM. The results can be understood qualitatively by means of the calculated magnetic field and field gradient profiles generated by the YIG shere.
Date: July 1, 1998
Creator: Suh, B.J.; Hammel, P.C.; Zhang, Z.; Midzor, M.M.; Roukes, M.L. & Childress, J.R.
Partner: UNT Libraries Government Documents Department

TORO II simulations of induction heating in ferromagnetic materials

Description: TORO II is a finite element computer program that is used in the simulation of electric and magnetic fields. This code, which was developed at Sandia National Laboratories, has been coupled with a finite element thermal code, COYOTE II, to predict temperature profiles in inductively heated parts. The development of an effective technique to account for the nonlinear behavior of the magnetic permeability in ferromagnetic parts is one of the more difficult aspects of solving induction heating problems. In the TORO II code, nonlinear, spatially varying magnetic permeability is approximated by an effective permeability on an element-by-element basis that effectively provides the same energy deposition that is produced when the true permeability is used. This approximation has been found to give an accurate estimate of the volumetric heating distribution in the part, and predicted temperature distributions have been experimentally verified using a medium carbon steel and a 10kW industrial induction heating unit. Work on the model was funded through a Cooperative Research and Development Agreement (CRADA) between the Department of Energy and General Motors` Delphi Saginaw Steering Systems.
Date: September 1, 1997
Creator: Adkins, D.R.; Gartling, D.K.; Kelley, J.B. & Kahle, P.M.
Partner: UNT Libraries Government Documents Department

Magnetic Particle Process Improvement

Description: The magnetic particle testing process is performed to find linear, surface and near surface discontinuities in ferromagnetic test materials. A wet fluorescent method is used at Honeywell Federal Manufacturing & Technologies (FM&T). This method employs a liquid carrier mixed with iron oxide particles in suspension, and the particles used in the method are coated with a fluorescent dye to make them visible under a black light. The process in its current state employs the use of a tank of liquid solution of a mineral oil carrier with iron oxide particles in suspension. The change to the use of an aerosol delivery system with the same material reduces the amount of waste involved in the process while preserving the sensitivity of the testing, shortens the flowtime for the test, and saves labor and material costs.
Date: August 13, 2002
Creator: Hubert, R.R.
Partner: UNT Libraries Government Documents Department

Linear and circular dichroism in angle resolved Fe 3p photomission. Revision 1

Description: Using a recently developed spin-polarized, fully relativistic, multiple scattering approach based on the layer KKR Green function method, we have reproduced the Fe 3p angle-resolved soft x-ray photoemission spectra and analyzed the associated large magnetic dichroism effects for excitation with both linearly and circularly polarized light. Comparison between theory and experiment yields a spin-orbit splitting of 1.0--1.2 eV and an exchange splitting of 0.9-- 1.0 eV for Fe 3p. These values are 50--100% larger than those hitherto obtained experimentally.
Date: December 7, 1994
Creator: Tamura, E.; Waddill, G.D.; Tobin, J.G. & Sterne, P.A.
Partner: UNT Libraries Government Documents Department

A Boltzmann-equation approach to the magnetoresistance of ferromagnetic-normal metallic multilayers

Description: The Boltzmann equation is solved for a system consisting of a ferromagnetic - normal - ferromagnetic metallic trilayer. The in- plane conductance of the film is calculated for two configurations: the ferromagnetic layers aligned (i) parallel and (ii) antiparallel to each other. The results explain the giant negative magnetoresistance encountered in these systems when an initial antiparallel arrangement is changed into a parallel configuration by application of an external magnetic field. A large negative magnetoresistance requires, in general, considerable asymmetry in the interface scattering for the two spin orientations.
Date: June 1, 1992
Creator: Falicov, L. M. & Hood, R. Q.
Partner: UNT Libraries Government Documents Department

Asymmetric Ferromagnet-Superconductor-Ferromagnet Switch

Description: In layered ferromagnet-superconductor-ferromagnet F{sub 1} /S/F{sub 2} structures, the critical temperature T{sub c} of the superconductors depends on the magnetic orientation of the ferromagnetic layers F{sub 1} and F{sub 2} relative to each other. So far, the experimentally observed magnitude of change in T{sub c} for structures utilizing weak ferromagnets has been 2 orders of magnitude smaller than is expected from calculations. We theoretically show that such a discrepancy can result from the asymmetry of F/S boundaries, and we test this possibility by performing experiments on structures where F{sub 1} and F{sub 2} are independently varied. Our experimental results indicate that asymmetric boundaries are not the source of the discrepancy. If boundary asymmetry is causing the suppressed magnitude of T{sub c} changes, it may only be possible to detect in structures with thinner ferromagnetic layers.
Date: November 4, 2011
Creator: Cadden-Zimansky, P.; Bazaliy, Ya.B.; Litvak, L.M.; Jiang, J.S.; Pearson, J.; Gu, J.Y. et al.
Partner: UNT Libraries Government Documents Department


Description: The magnetic and galvanomagnetic properties of US, including resistivity, magnetoresistance, and Hall effect, were studied. The resistivity was found to be approximately a hundred times higher in US than in the common ferromagnetic metals, but the temperature dependence of resistivity was similar, showing an abnormal decrease below the Curie point. The magnetoresistance of US was found to be negative with a sharp minimum at the Curie point. The study of the Hall effect revealed that two Hall coefficients could be identified. The extraordinary Hall coefficient, which is related to the magnetization of the material, was found to be proportional to the second power of the resistivity. The ordinary Hall coefficient R/sub o/, related to the applied field, was found to be positive and temperature dependent, with a maximum near the Curie point. The temperature dependence of R/sub o/ constituted the biggest dissimilarity between US and other ferromagnetic substances. The effective carrier concentration evaluated from this coefficient was 0.45 hole per atom at absolute zero. The study of the magnetic properties yielded values of the saturation magnetization at various temperatures which indicated that at absolute zero the atomic moment is 1.05 plus or minus 0.03 Bohr magnetons. These results are correlated with a band picture based on overlapping 7s and 5f bands. (auth)
Date: June 1, 1963
Creator: Kzamierowicz, C.W.
Partner: UNT Libraries Government Documents Department


Description: Ferromagnetic nanocomposites, or ''exchange spring'' magnets, possess a nanoscaled microstructure that allows intergrain magnetic exchange forces to couple the constituent grains and alter the system's effective magnetic anisotropies. While the effects of the anisotropy alterations are clearly seen in macroscopic magnetic measurement, it is extremely difficult to determine the detailed effects of the system's exchange coupling, such as the interphase exchange length, the inherent domain wall widths or the effective anisotropies of the system. Clarification of these materials parameters may be obtained from the ''micromagnetic'' phenomenological model, where the assumption of magnetic reversal initiating in the magnetically-soft regions of the exchange-spring maqet is explicitly included. This approach differs from that typically applied by other researchers and allows a quantitative estimate of the effective anisotropies of an exchange spring system. Hysteresis loops measured on well-characterized nanocomposite alloys based on the composition Nd{sub 2}Fe{sub 14}B + {alpha}-Fe at temperatures above the spin reorientation temperature were analyzed within the framework of the micromagnetic phenomenological model. Preliminary results indicate that the effective anisotropy constant in the material is intermediate to that of bulk {alpha}-Fe and bulk Nd{sub 2}Fe{sub 14}B and increases with decreasing temperature. These results strongly support the idea that magnetic reversal in nanocomposite systems initiates in the lower-anisotropy regions of the system, and that the soft-phase regions become exchange-hardened by virtue of their proximity to the magnetically-hard regions.
Date: August 18, 2002
Creator: LEWIS,L.H. & HARLAND,C.L.
Partner: UNT Libraries Government Documents Department

Polarization dependence of the magnetic fluctuations in the weak itinerant ferromagnet MnSi below {Tc}

Description: The dispersion of the spin-flip and non-spin-flip excitations in the weak itinerant ferromagnet MnSi have been measured in the ferromagnetic phase using inelastic polarized neutron scattering. Because of the steep dispersion curve, the excitation have been determined by means of constant energy scans between 0 and 7 meV. The decrease of the intensity of the spin-flip excitations with increasing energy can be explained by the interaction of the spin waves with the Stoner excitations. The interpretation of the non-spin-flip scattering is a more difficult task because of the low intensity. The non-spin-flip excitations are clearly quasielastic at small E-transfer similar as in a localized ferromagnet. Their spectral weight is significant near {Tc}.
Date: September 1, 1997
Creator: Tixier, S.; Boeni, P.; Endoh, Y.; Roessli, B. & Shirane, G.
Partner: UNT Libraries Government Documents Department

Theoretical studies of magnetic systems. Final report, August 1, 1994--November 30, 1997

Description: During the grant period the authors have studied five areas of research: (1) low dimensional ferrimagnets; (2) lattice effects in the mixed valence problem; (3) spin compensation in the one dimensional Kondo lattice; (4) the interaction of quasi particles in short coherence length superconductors; and (5) novel effects in angle resolved photoemission spectra from nearly antiferromagnetic materials. Progress in each area is summarized.
Date: December 31, 1997
Creator: Gor`kov, L.P.; Novotny, M.A. & Schrieffer, J.R.
Partner: UNT Libraries Government Documents Department

History Dependent Phenomena in the Transverse Ising Ferro-Glass: the Free Energy Landscape

Description: In this paper the authors investigate the relationship between glassy and ferromagnetic phases in disordered Ising ferromagnets in the presence of transverse magnetic fields, {Lambda}. Iterative mean field simulations probe the free energy landscape and suggest the existence of a glass transition line in the {Lambda}, temperature T plane well within the ferromagnetic phase. New experimental field-cooled and zero-field-cooled data on LiHo{sub x} Y{sub 1{minus}x}F{sub 4} provide support for our theoretical picture.
Date: November 8, 2000
Partner: UNT Libraries Government Documents Department

First-principles exchange interactions between ferro and antiferromagnetic films: Co on NiMn, a case study

Description: Heisenberg exchange parameters at the interface of antiferromagnetic NiMn with ferromagnetic Co are calculated from first-principles. The authors use a layer version of the Korringa-Kohn-Rostocker multiple scattering approach and an expression, which is based on the expansion of the band energy, to calculate the exchange parameters from the underlying electronic structure. For bulk systems, the parameter sets yield Curie temperatures that are in good agreement with experimental values. In the interface region, the inter-layer interactions in NiMn change significantly compared to the bulk while the intra-layer interactions are almost unchanged.
Date: December 31, 1997
Creator: Schulthess, T.C. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Electrical and Magnetic Properties of Holmium Single Crystals

Description: Magnetic moment measurements are made on holmium single crystals (hcp) from 1.3 to 300 deg K in magnetic fields from 250 to 18,000 Oe, with the field applied along the c axis, the a axis, and a (1010) direction. The STA1010! direction is the direction of easy magnetization with an extrapolated effective moment per atom at saturation of 10.34 Bohr magnetons. Basal plane measurements show antiferromagnetism below the Neel temperature of 132 deg K with basal plane anisotropy occurring below 80 deg K. Below the Neel point, an anomalous type of transition to ferromagnetic behavior upon application of sufficient field is observed. Magnetization curves for the c axis are linear down to 60 deg K, while measurements below 20 deg K show an initial magnetization of approximately 1.7 Bohr magnetons followed by nearly linear magnetization curves. Electrical resistivity measurements are made from 4.2 to 300 deg K. For the a axis, the resistivity changes slope slightly at 20 deg K. A larger change in slope occurs at the Neel temperature of 132 deg K. For the c axis, the resistivity changes slope slightly at 20 deg K, reaches a peak at 120 deg K, goes through a minimum at 132 deg K, and remains constant until approximately 150 deg K, after which it increases linearly with increasing temperature. (auth)
Date: September 15, 1962
Creator: Strandburg, D. L.; Legvold, S. & Spedding, F. H.
Partner: UNT Libraries Government Documents Department