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Dependence of Magnetic Hysteresis upon Wave Form

Description: Report issued by the Bureau of Standards over studies on magnetic hysteresis and its dependence upon wave form. Methods and equipment used during the studies are presented and discussed. This report includes tables, and illustrations.
Date: 1909
Creator: Lloyd, Morton G.
Partner: UNT Libraries Government Documents Department

A Summary of the results obtained in the LDRD project "Interaction of a magnetized plasma with structured surfaces- from devices to spacecraft"

Description: Our work was directed towards developing a basic understanding of a new class of phenomena: effects of surface structure on the processes in a magnetized plasma near surfaces and at some distances for them. The surface structures can be of various kinds: topographic features ranging from smooth �waviness� to a coarse �roughness,� non-uniformities of the secondary emission coefficient, dielectric impregnations into conducting surface, etc. The expected effects are strongest when the magnetic field forms a shallow angle with the surface. The current and future applications of this new branch of plasma physics include fusion devices, gas-discharge and plasma-processing devices, large spacecraft, and physical phenomena in the vicinity of celestial bodies without atmospheres. We have developed a methodology that allows one to study in a unified way effects of rough surfaces with arbitrary scales of topographic features, from sizes exceeding the ion gyro-radius, to sizes much below the electron gyro-radius, in the most interesting case of a grazing magnetic field. The results can be presented in a dimensionless form, so that they would be equally applicable to the micrometer-scale roughness of the divertor plates of fusion devices, and to 10-km-scale structures of a Lunar surface. We have identified the following new effects: 1) the plasma is absorbed by only a small fraction of the total surface, near the �mountain tops� of the bumps; 2) regions inaccessible for one or both plasma species (�shadows�) are formed behind the bumps; the size of these inaccessible domains is, generally speaking, different for the electrons and ions; 3) this latter circumstance leads to formation of fine potential structure both near the surface and in the bulk plasma, leading to enhanced plasma transport. We have investigated the processes that may lead to plasma penetration into the shadows and concluded that most probable candidates are (depending on ...
Date: February 23, 1999
Creator: Cohen, R H; Porter, G D & Ryutov, D D
Partner: UNT Libraries Government Documents Department

Sensitivity Enhancement by Exchange Mediated MagnetizationTransfer of the Xenon Biosensor Signal

Description: Hyperpolarized xenon associated with ligand derivitized cryptophane-A cages has been developed as a NMR based biosensor. To optimize the detection sensitivity we describe use of xenon exchange between the caged and bulk dissolved xenon as an effective signal amplifier. This approach, somewhat analogous to 'remote detection' described recently, uses the chemical exchange to repeatedly transfer spectroscopic information from caged to bulk xenon, effectively integrating the caged signal. After an optimized integration period, the signal is read out by observation of the bulk magnetization. The spectrum of the caged xenon is reconstructed through use of a variable evolution period before transfer and Fourier analysis of the bulk signal as a function of the evolution time.
Date: August 31, 2006
Creator: Garcia, Sandra; Chavez, Lana; Lowery, Thomas J.; Han, Song-I.; Wemmer, David E. & Pines, Alexander
Partner: UNT Libraries Government Documents Department

Probing the evolution of antiferromagnetism in multiferroics

Description: This study delineates the evolution of magnetic order in epitaxial films of the room-temperature multiferroic BiFeO3 system. Using angle- and temperature-dependent dichroic measurements and spectromicroscopy, we have observed that the antiferromagnetic order in the model multiferroic BiFeO3 evolves systematically as a function of thickness and strain. Lattice-mismatch-induced strain is found to break the easy-plane magnetic symmetry of the bulk and leads to an easy axis of magnetization which can be controlled through strain. Understanding the evolution of magnetic structure and how to manipulate the magnetism in this model multiferroic has significant implications for utilization of such magnetoelectric materials in future applications.
Date: June 9, 2010
Creator: Holcomb, M.; Martin, L.; Scholl, A.; He, Q.; Yu, P.; Yang, C.-H. et al.
Partner: UNT Libraries Government Documents Department

Direct observation of imprinted antiferromagnetic vortex state in CoO/Fe/Ag(001) disks

Description: In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the center of a confined structure. A vortex state in a thin film disk, for example, is a topological object characterized by the vortex polarity and the winding number. In ferromagnetic (FM) disks, these parameters govern many fundamental properties of the vortex such as its gyroscopic rotation, polarity reversal, core motion, and vortex pair excitation. However, in antiferromagnetic (AFM) disks, though there has been indirect evidence of the vortex state through observations of the induced FM-ordered spins in the AFM disk, they have never been observed directly in experiment. By fabricating single crystalline NiO/Fe/Ag(001) and CoO/Fe/Ag(001) disks and using X-ray Magnetic Linear Dichroism (XMLD), we show direct observation of the vortex state in an AFM disk of AFM/FM bilayer system. We observe that there are two types of AFM vortices, one of which has no analog in FM structures. Finally, we show that a frozen AFM vortex can bias a FM vortex at low temperature.
Date: December 21, 2010
Creator: Wu, J.; Carlton, D.; Park, J. S.; Meng, Y.; Arenholz, E.; Doran, A. et al.
Partner: UNT Libraries Government Documents Department

Ultrafast Enhancement of Ferromagnetism via Photoexcited Holes inGaMnAs

Description: We report on the observation of ultrafast photo-enhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100-ps time scale, after an initial sub-ps demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature {Tc} and dominates the demagnetization component when approaching {Tc}. We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20K and a transient increase in {Tc}.
Date: February 17, 2007
Creator: Wang, J.; Cotoros, I.; Dani, K.M.; Liu, X.; Furdyna, J.K. & Chemla, D.S.
Partner: UNT Libraries Government Documents Department

Characterization and Coil Test Results for a Multifilamentary NbTi Conductor Utilizing Artificial Pinning Center Technology

Description: The introduction of pinning centers via the controlled addition of a second phase, with the correct size and spacing, has been proposed as a method for producing a material with optimum flux pinning and hence a higher critical current density in practical superconductors. The demonstration of such artificial pinning center (APC) materials has been the aim of recent collaborative efforts with several U.S. manufacturers. This paper reports the coil test results for a multifilamentary NbTi conductor fabricated using an APC technique. The conductor showed improved performance compared to earlier APC conductors, and its performance is comparable to that found in conventional multifilamentary NbTi conductors. In addition to coil test results, the paper will report on the low field magnetization, SEM, and TEM studies, and the results will be compared with similar results on conventional NbTi.
Date: August 1, 1992
Creator: Scanlan, R.M.; Dietderich, D.; Ghiorso, W. & McManaman, P.
Partner: UNT Libraries Government Documents Department

Enhanced Magnetism in Epitaxial SrRuO3 Thin Films

Description: We have observed enhanced magnetization in epitaxial SrRuO{sub 3} thin films compared to previously reported bulk and thin film values of 1.1-1.6 {mu}{sub B}/Ru ion. The degree of enhancement is strongly dependent on the lattice distortions imposed on the SrRuO{sub 3} films by SrTiO{sub 3}, (LaAlO{sub 3}){sub 0.3}(SrTaO{sub 3}){sub 0.7} (LSAT), and LaAlO{sub 3} substrates. A larger enhancement of magnetization for coherently strained SrRuO{sub 3} films on SrTiO{sub 3} and LSAT compared to fully relaxed films on LaAlO{sub 3} confirms the importance of the strain state in determining the magnetic ground state of the Ru ion. In particular, SrRuO{sub 3} films on (111) SrTiO{sub 3} exhibit exhanced moments as high as 3.8 {mu}{sub B}/Ru ion, thus suggesting the stabilization of a high-spin Ru{sup 4+} state.
Date: November 20, 2009
Creator: Grutter, A.J.; Wong, F.; Arenholz, E.; Liberati, M.; Vailionis, A. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Strain Induced Magnetism in SrRuO3 Epitaxial Thin Films

Description: Epitaxial SrRuO{sub 3} thin films were grown on SrTiO{sub 3}, (LaAlO{sub 3}){sub 0.3}(SrAlO{sub 3}){sub 0.7} and LaAlO{sub 3} substrates inducing different biaxial compressive strains. Coherently strained SrRuO{sub 3} films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1-1.6 {micro}{sub B} per formula unit. A comparison of (001) and (110) SrRuO{sub 3} films on each substrate indicates that films on (110) oriented have consistently higher saturated moments than corresponding (001) films. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.
Date: January 10, 2010
Creator: Grutter, A.; Wong, F.; Arenholz, E.; Liberati, M. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Transverse field-induced nucleation pad switching modes during domain wall injection

Description: We have used magnetic transmission X-ray microscopy (M-TXM) to image in-field magnetization configurations of patterned Ni{sub 80}Fe{sub 20} domain wall 'injection pads' and attached planar nanowires. Comparison with micromagnetic simulations suggests that the evolution of magnetic domains in rectangular injection pads depends on the relative orientation of closure domains in the remanent state. The magnetization reversal pathway is also altered by the inclusion of transverse magnetic fields. These different modes explain previous results of domain wall injection into nanowires. Even more striking was the observation of domain walls injecting halfway across the width of wider (>400 nm wide) wires but over wire lengths of several micrometers. These extended Neel walls can interact with adjacent nanowires and cause a switching in the side of the wire undergoing reversal as the domain wall continues to expand.
Date: March 12, 2010
Creator: Bryan, M. T.; Fry, P. W.; Schrefl, T.; Gibbs, M. R. J.; Allwood, D. A.; Im, M.-Y. et al.
Partner: UNT Libraries Government Documents Department

Scaling Behavior of Barkhausen Avalanches along the Hysteresis loop in Nucleation-Mediated Magnetization Reversal Process

Description: We report the scaling behavior of Barkhausen avalanches for every small field step along the hysteresis loop in CoCrPt alloy film having perpendicular magnetic anisotropy. Individual Barkhausen avalanche is directly observed utilizing a high-resolution soft X-ray microscopy that provides real space images with a spatial resolution of 15 nm. Barkhausen avalanches are found to exhibit power-law scaling behavior at all field steps along the hysteresis loop, despite their different patterns for each field step. Surprisingly, the scaling exponent of the power-law distribution of Barkhausen avalanches is abruptly altered from 1 {+-} 0.04 to 1.47 {+-} 0.03 as the field step is close to the coercive field. The contribution of coupling among adjacent domains to Barkhausen avalanche process affects the sudden change of the scaling behavior observed at the coercivity-field region on the hysteresis loop of CoCrPt alloy film.
Date: October 14, 2008
Creator: Im, Mi-Young; Fischer, Peter; Kim, D.-H. & Shin, S.-C.
Partner: UNT Libraries Government Documents Department

Comment on flux creep with logarithmic U(j) dependence

Description: The numerical calculation by Wang and Dong [Phys. Rev. B 49, 698 (1994)] of flux density profiles across a slab sample exposes a possible misunderstanding of the solution given earlier by vinokur, Feigel`man and Geshkenbein, but also predicts a kink in the magnetization relaxation curve which Schnack and Griessen have already shown to be erroneous.
Date: February 1, 1994
Creator: Gilchrist, J.; Schnack, H.G. & van der Beek, C.J.
Partner: UNT Libraries Government Documents Department

Numerical simulations of magnetic reversal in layered spring magnets.

Description: This report summarizes the results of numerical investigations of magnetic reversal in layered spring magnets. A one-dimensional model is used of a film consisting of several atomic layers of soft material on top of several atomic layers of hard material. Each atomic layer is taken to be uniformly magnetized, and spatial inhomogeneities within an atomic layer are neglected. The state of such a system is described by a chain of magnetic spin vectors. Each spin vector behaves like a spinning top driven locally by the effective magnetic field and subject to damping (Landau-Lifshitz-Gilbert equation). A numerical integration scheme for the LLG equation is presented that is unconditionally stable and preserves the magnitude of the magnetization vector at all times. The results of numerical investigations for a bilayer in a rotating in-plane magnetic field show hysteresis with a basic period of 2{pi} at moderate fields and hysteresis with a basic period of {pi} (or any multiple thereof) at strong fields.
Date: January 24, 2001
Creator: Jiang, J.S.; Kaper, H.G. & Leaf, G.K.
Partner: UNT Libraries Government Documents Department

Magnetic domain structure and magnetization reversal in submicron-scale Co dots

Description: We present a magnetic force microscopy (MFM) analysis of arrays of submicron-scale Co dots fabricated by interference lithography. The dots are thin (180-300 A) and are elliptical in shape. MFM of these structures reveals that they relax into highly ordered remanent states whose symmetry and configuration are governed by their shape anisotropy. In particular, when the dots are saturated along the easy-axis, a uniformly magnetized state persists at remanence. However, when the dots are saturated in hard-axis, they relax into a single-vortex state in which the circulation can have either sign. Both remanent states are characterized by smoothly varying magnetization patterns and a high degree of uniformity across the array. We attribute the ordered behavior of these structures to the film microstructure, which allows the shape anisotropy to dominate over magnetocrystalline anisotropy. By imaging a series of minor-loop remanent states, we show that magnetization reversal in these structures occurs via the nucleation and annihilation of a single vortex. Magnetic hysteresis loop measurements are consistent with these observations and provide additional details. Furthermore, we present the results of micromagnetic simulations, which are in excellent agreement with both the MFM images and the hysteresis loop measurements.
Date: February 17, 1998
Creator: Fernandez, A.
Partner: UNT Libraries Government Documents Department

Magnetized targets for fast z-pinch implosions: a spectrum of possibilities

Description: In this brief communication, we discuss various plasma configurations that can be adiabatically compressed by an imploding liner and produce fusion-grade plasma near the liner turn around point. Our prime interest will be discussion of the ways of forming initial plasma configurations and discussion of the ways of imploding them in a 3D fashion.
Date: April 23, 1999
Creator: Ryutov, D D
Partner: UNT Libraries Government Documents Department

Direct real-space observation of nearly stochastic behavior in magnetization reversal process on a nanoscale

Description: We report a non-deterministic nature in the magnetization reversal of nanograins of CoCrPt alloy film. Magnetization reversal process of CoCrPt alloy film is investigated using high resolution soft X-ray microscopy which provides real space images with a spatial resolution of 15 nm. Domain nucleation sites mostly appear stochastically distributed within repeated hysteretic cycles, where the correlation increases as the strength of the applied magnetic field increases in the descending and ascending branches of the major hysteresis loop. In addition, domain configuration is mostly asymmetric with inversion of an applied magnetic field in the hysteretic cycle. Nanomagnetic simulation considering thermal fluctuations of the magnetic moments of the grains explains the nearly stochastic nature of the domain nucleation behavior observed in CoCrPt alloy film. With the bit size in high-density magnetic recording media approaching nanometer length scale, one of the fundamental and crucial issues is whether the domain nucleation during magnetization reversal process exhibits a deterministic behavior. Repeatability of local domain nucleation and deterministic switching behavior are basic and essential factors for achieving high performance in high-density magnetic recording [1-3]. Most experimental studies on this issue reported so far have been mainly performed by indirect probes through macroscopic hysteresis loop and Barkhausen pattern measurements, which provide the ensemble-average magnetization. Thus, they are inadequate to gain insight into the domain-nucleation behavior on a nanometer length scale during the magnetization reversal process [4-6]. Very recently, coherent X-ray speckle metrology, where the speckle pattern observed in reciprocal space acts as a fingerprint of the domain configurations, was adopted to investigate stochastic behavior in the magnetization reversal of a Co/Pt multilayer film [7,8]. However, no direct observation on the stochastic behavior of domain nucleation during magnetization reversal in real space at the nanometer scale has yet been reported. The main reason is due to limitations of ...
Date: June 1, 2007
Creator: Im, M.-Y.; Kim, D.-H.; Lee, K.-D.; Fischer, P. & Shin, S.-C.
Partner: UNT Libraries Government Documents Department

The role of hydrogen in room-temperature ferromagnetism at graphite surfaces

Description: We present a x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon {pi} states, also hydrogen-mediated electronic states exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top {approx}10 nm of the irradiated sample where the average magnetization reaches {approx_equal} 15 emu/g at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin as the one found in proton bombarded graphite. Also, our findings show that the magnetic properties of graphite surfaces, thin films or two dimensional graphene samples can be reliably studied using soft x-ray dichroism. Fundamental new insight into the magnetic properties of carbon based systems can thus be obtained.
Date: May 1, 2010
Creator: Ohldag, H.; Esquinazi, P.; Arenholz, E.; Spemann, D.; Rothermel, M.; Setzer, A. et al.
Partner: UNT Libraries Government Documents Department

Electrically Controllable Spontaneous Magnetism in Nanoscale Mixed Phase Multiferroics

Description: The emergence of enhanced spontaneous magnetic moments in self-assembled, epitaxial nanostructures of tetragonal (T-phase) and rhombohedral phases (R-phase) of the multiferroic BiFeO{sub 3} system is demonstrated. X-ray magnetic circular dichroism based photoemission electron microscopy (PEEM) was applied to investigate the local nature of this magnetism. We find that the spontaneous magnetization of the R-phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent T-phase and the epitaxial constraint. Reversible electric field control and manipulation of this magnetic moment at room temperature is shown using a combination of piezoresponse force microscopy and PEEM studies.
Date: August 2, 2010
Creator: He, Q.; Chu, Y. H.; Heron, J. T.; Yang, S. Y.; Wang, C. H.; Kuo, C. Y. et al.
Partner: UNT Libraries Government Documents Department