257 research outputs found
Collapse of the Small‐Angle Magnon Scattering in Fe as a Function of Magnetic Field
The dependence of the spin wave energy on the magnetization M⇒ and the applied magnetic field H⇒ in Fe (and other ferromagnets) has not been very well investigated with neutrons. According to the Holstein‐Primakoff dispersion relation, the contributions of the Zeeman energy gμBHgμBH and the dipole‐dipole interactions 4π gμBM sin2θq4πgμBMsin2θq do not simply add linearly to the exchange energy Dq2. However, in order to see these contributions, one must observe the very low energy (.01 – .1 mev) spin waves. One of the predictions of this dispersion relation is that the scattering of neutrons by spin waves near the origin should disappear as the magnetic field is increased. This is a consequence of the kinematics of the scattering process. Using our double‐Si crystal technique for small angle scattering we have experimentally observed this collapse at a field of about 8 kG in Fe at room temperature as predicted by theory. We have also measured the scattering due to these very low energy spin waves at temperatures up to .7 Tc and compared the data on an absolute scale with the theoretical cross section. The agreement is reasonably good.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87920/2/1340_1.pd
Magnetic Reversal on Vicinal Surfaces
We present a theoretical study of in-plane magnetization reversal for vicinal
ultrathin films using a one-dimensional micromagnetic model with
nearest-neighbor exchange, four-fold anisotropy at all sites, and two-fold
anisotropy at step edges. A detailed "phase diagram" is presented that catalogs
the possible shapes of hysteresis loops and reversal mechanisms as a function
of step anisotropy strength and vicinal terrace length. The steps generically
nucleate magnetization reversal and pin the motion of domain walls. No sharp
transition separates the cases of reversal by coherent rotation and reversal by
depinning of a ninety degree domain wall from the steps. Comparison to
experiment is made when appropriate.Comment: 12 pages, 8 figure
First-principles calculations of magnetization relaxation in pure Fe, Co, and Ni with frozen thermal lattice disorder
The effect of the electron-phonon interaction on magnetization relaxation is
studied within the framework of first-principles scattering theory for Fe, Co,
and Ni by displacing atoms in the scattering region randomly with a thermal
distribution. This "frozen thermal lattice disorder" approach reproduces the
non-monotonic damping behaviour observed in ferromagnetic resonance
measurements and yields reasonable quantitative agreement between calculated
and experimental values. It can be readily applied to alloys and easily
extended by determining the atomic displacements from ab initio phonon spectra
Controlled switching of N\'eel caps in flux-closure magnetic dots
While magnetic hysteresis usually considers magnetic domains, the switching
of the core of magnetic vortices has recently become an active topic. We
considered Bloch domain walls, which are known to display at the surface of
thin films flux-closure features called N\'eel caps. We demonstrated the
controlled switching of these caps under a magnetic field, occurring via the
propagation of a surface vortex. For this we considered flux-closure states in
elongated micron-sized dots, so that only the central domain wall can be
addressed, while domains remain unaffected.Comment: 4 pages, 3 figure
Incommensurate Spin Density Waves in Iron Aluminides
Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al), and Fe(43Al) show n varying from 11 to 6 for →q=2π(h±1/n,k±1/n,l±1/n)/a0, where (h,k,l) and a0 characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel
M.I.T./Canadian Vestibular Experiments on the Spacelab-1 Mission. Part 1: Sensory Adaptation to Weightlessness and Readaptation to One-G: An Overview
Experiments on human spatial orientation were conducted on four crewmembers of Space Shuttle Spacelab Mission 1. The conceptual background of the project, the relationship among the experiments, and their relevance to a 'sensory reinterpretation hypothesis' are presented. Detailed experiment procedures and results are presented in the accompanying papers in this series. The overall findings are discussed as they pertain to the following aspects of hypothesized sensory reinterpretation in weightlessness: (1) utricular otolith afferent signals are reinterpreted as indicating head translation rather than tilt, (2) sensitivity of reflex responses to footward acceleration is reduced, and (3) increased weighting is given to visual and tactile cues in orientation perception and posture control. Results suggest increased weighting of visual cues and reduced weighting of graviceptor signals in weightlessness
Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands
We have investigated three-dimensional magnetization structures in numerous
mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism
combined with photoemission electron microscopy (XMCD-PEEM). The particles are
epitaxial islands with an elongated hexagonal shape with length of up to 2.5
micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal
asymmetric magnetization distributions at the surface of these particles.
Micromagnetic simulations are in excellent agreement with the observed magnetic
structures and provide information on the internal structure of the
magnetization which is not accessible in the experiment. It is shown that the
magnetization is influenced mostly by the particle size and thickness rather
than by the details of its shape. Hence, these hexagonal samples can be
regarded as model systems for the study of the magnetization in thick,
mesoscopic ferromagnets.Comment: 12 pages, 11 figure
Ferromagnetic order in U(Rh,Co)Ge
We report the variation of ferromagnetic order in the pseudo-ternary
compounds URh_{1-x}Co_{x}Ge (0 \leq x \leq 1). Magnetization and transport data
taken on polycrystalline samples show that the Curie temperature T_{C}
gradually increases with increasing Co content from a value of 9.5 K for URhGe
to a maximum value of 20 K for x = 0.6 and then steadily decreases to 3 K for
UCoGe. The magnetic interaction strength varies smoothly across the series. For
all samples the electrical resistivity for T < T_{C} follows the behaviour \rho
= \rho_{0} + AT^2. The A coefficient is dominated by scattering at spin waves
and is strongly enhanced for x = 0 and 1.Comment: 12 pages (4 figures), submitted to SS
Magnetic Susceptibility of Multiorbital Systems
Effects of orbital degeneracy on magnetic susceptibility in paramagnetic
phases are investigated within a mean-field theory. Under certain crystalline
electric fields, the magnetic moment consists of two independent moments, e.g.,
spin and orbital moments. In such a case, the magnetic susceptibility is given
by the sum of two different Curie-Weiss relations, leading to deviation from
the Curie-Weiss law. Such behavior may be observed in d- and f-electron systems
with t_{2g} and Gamma_8 ground states, respectively. As a potential application
of our theory, we attempt to explain the difference in the temperature
dependence of magnetic susceptibilities of UO_2 and NpO_2.Comment: 4 pages, 3 figure
The determination of errors in polarized neutron diffractometry
The polarized neutron method of determining the magnetic form factor of magnetic materials is examined in detail with special attention given to the way in which statistical errors are propagated. Because of the nonlinear relation between the polarizing efficiency of a crystal and the magnetic scattering length, the usual methods of linear error theory will not work. However, this difficulty can be circumvented by using the nonlinear formulas directly. The statistical error analysis is applied to the practical problem of how long one should count on a particular Bragg peak. This time will depend on the ratio of the magnetic to the nuclear scattering lengths, p/b, and on the accuracy with which the beam polarizations and flipping efficiencies have been determined.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33028/1/0000412.pd
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