Jumps In Magnetic Moment Research Articles

Electronic structure and magnetic properties ofSr2CrReO6/BaTiO3hybrid heterostructures

We report on the electronic structure and magnetic properties of hybrid heterostructures of the ferromagnetic double perovskite ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$ (SCRO) and ferroelectric ${\mathrm{BaTiO}}_{3}$ (BTO) calculated in the GGA approach using the fully relativistic spin-polarized Dirac LMTO method. The electronic band structure and optical properties are studied in the BTO and SCRO oxides as well as in SCRO/BTO heterostructures with different supercells: SCRO/BTO monolayered $(1\ifmmode\times\else\texttimes\fi{}1\ifmmode\times\else\texttimes\fi{}1)$, $(2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}1),$ and $(1\ifmmode\times\else\texttimes\fi{}1\ifmmode\times\else\texttimes\fi{}2)$ heterostructures. We investigated theoretically the magnetization, spin and orbital magnetic moments as well as magnetocrystalline anisotropy energy (MAE) in the SCRO on the BTO substrate as a function of the temperature assuming that the finite temperature can be mimicked by the experimental lattice constants corresponding to this temperature. We found that the spin and orbital magnetic moments are monotonic, almost linear functions of temperature but change abruptly at the points of the BTO structural phase transitions. The magnetization $M$ also possesses ``jumps'' at the structural phase transitions in agreement with the experiment. However, the theoretically predicted jumps in the total magnetic moment are much smaller than the experimentally measured ones. We found that the Cr/Re disorder increases the jump in the magnetic moment by an order of magnitude compared to the calculated data without disorder. The microscopic origin of such huge effect is discussed. The theoretically calculated temperature dependence of the total magnetic moment with 25% of the Cr/Re disorder is in excellent agreement with the critical amplitude, obtained from the fitting of the experimentally measured magnetization $M(T)$. From the theoretically calculated MAE we found that the easy axis of magnetization for the tetragonal and orthorhombic phases is along the $\ensuremath{\langle}001\ensuremath{\rangle}$ direction in agreement with the experiment. However, it changes to the $\ensuremath{\langle}11\overline{1}\ensuremath{\rangle}$ direction in the rhombohedral phase. The intersite Cr/Re disorder reduces the MAE for the tetragonal and orthorhombic phases and increases it for the rhombohedral phase. We found that the major contribution to the MAE is due to the orbital magnetic anisotropy at the Re site. We also found that the MAE in the monolayered SCRO/BTO heterostructure is significantly increased in comparison with the MAE in SCRO on BTO substrate. The element-specific x-ray absorption spectra as well as the x-ray magnetic circular dichroism at the Ti, Ba, and Re ${L}_{2,3}$ edges in SCRO/BTO heterostructures are investigated theoretically from first principles and compared with available experimental spectra.

Magnetic moment jumps in flat and nanopatterned Nb thin-walled cylinders

Penetration of magnetic flux into hollow superconducting cylinders is investigated by magnetic moment measurements. The magnetization curves of a flat and a nanopatterned thin-walled superconducting Nb cylinders with a rectangular cross section are reported for the axial field geometry. In the nanopatterned sample, a row of micron-sized antidots (holes) was milled in the film along the cylinder axis. Magnetic moment jumps are observed for both samples at low temperatures for magnetic fields not only above Hc1, but also in fields lower than Hc1, i. e., in the vortex-free regime. The positions of the jumps are not reproducible and they change from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than Hc1. At temperatures above 0.66Tc and 0.78Tc the magnetization curves become smooth for the patterned and the as-prepared sample, respectively. The magnetization curve of a reference flat Nb film in the parallel field geometry does not exhibit jumps in the entire range of accessible temperatures.

Barkhausen noise in metallic glasses with strong local anisotropy: model and theory

.We discuss a model metallic glass in which Barkhausen noise can be studied in exquisite detail, free of thermal effects and of the rate of ramping of the magnetic field. In this model the mechanism of the jumps in magnetic moment that cause the Barkhausen noise can be fully understood as consecutive instabilities where an eigenvalue of the Hessian matrix hits zero, leading to a magnetization jump Δm which is simultaneous with a stress and energy changes Δ σ and ΔU respectively. Due to a large effect of local anisotropy, in this model Barkhausen noise is not due to movements of magnetic domain boundaries across pinning sites. There are no fractal domains, no self-organized criticality and no exact scaling behavior. We present a careful numerical analysis of the statistical properties of the phenomenon, and show that with every care taken this analysis is tricky, and easily misleading. Without a guiding theory it is almost impossible to get the right answer for the statistics of Barkhausen noise. We therefore present an analytic theory that culminates in a probability distribution function that is in excellent agreement with the simulations.

The role of the self-consistent equation in identifying the Andreev spectrum in a mesoscopic NS structure

A self-consistent equation of differential form is obtained for a dimensionless magnetic flux in an NS structure, which is responsible for the magnetic moment jumps in the system.

Spin-current-induced classical and quantum effects in the dynamics of a mesoscopic magnet

The dynamics of a high-spin quantum system with magnetic anisotropy of the easy plane type under the action of spin-polarized current permeating this system is considered. The spin-polarized current (with electron spins polarized along the hard magnetic axis of the system) induces the reorientation of the magnetic moment of the system from the easy plane to the hard magnetic axis. Analytical expressions describing characteristics of the reorientation process in the limiting cases of strong and weak dissipation are obtained. Under strong dissipation conditions, the reorientation is shown to have a threshold character with “soft” (continuous) displacement of the magnetic moment from the easy plane. Under weak dissipation, the reorientation occurs as a discrete process, that is, is accompanied by magnetic moment jumps and hysteresis as the spin current increases and decreases. At a fairly low temperature and weak damping, quantum effects arise in the system. The spin current induces excitations quasi-anionic in character, Bloch oscillations of magnetic moment precession, and tunneling between different precession quantum modes. These quantum effects, in particular, manifest themselves in the system under consideration by magnetic moment jumps and magnetic susceptibility peaks.

Conjectured chaotic nature of the ionosphere-magnetosphere coupling in a reconfigurating magnetosphere

Abstract. During substorms the magnetic field configuration changes in time; stretching of the magnetosphere during growth phase is followed by its collapse after the onset of the expansion phase. In this study the ionospheric origin oxygen ion dynamics in a time-dependent magnetosphere is analyzed. An induction electric field of several mV/m due to the reconfigurating magnetic field determines the details of the ion extraction from the auroral topside ionosphere. Two regimes of motion are discernible; a regime in regions far from the equatorial plane where the magnetic moment is conserved and a regime near the equatorial plane, in which the motion produces magnetic moment jumps and oscillations. The time spent by the ion in the regions is determined by the initial characteristics of the ion and by the field transition features. Lyapunov characteristic exponents are calculated to estimate the sensitivity of the system to initial conditions. Their values are higher for orbits with chaotic segments compared with the orbits in the static magnetic field and depend on the amplitude of the induced electric field. It follows from the study that the region of chaos usually localized far beyond 10 RE in the plasma sheet is expected to approach closer to the Earth ( r = 6 - 7 RE) during substorm associated reconfigurations of the magnetosphere, due to the auroral ionospheric ions.

Particle Dynamics Near κ = 1 and the Centrifugal Impulse Model

Charged particle motion in magnetic field reversals is well understood in two limiting regimes: the adiabatic regime where the guiding center theory and adiabatic invariance are valid, and the current sheet regime where one can separate timescales of the characteristic z-oscillation and gyromotion about the field normal to the reversal plane. Both approximations fail when the magnetic field curvature radius is on the order of the particle gyroradius, i.e. when the ratio of minimum curvature radius to maximum gyroradius, κ2, is near 1. Near κ = 1 the motion has been termed “strongly” or “globally” chaotic, and several proposed applications exploit this chaotic behavior. However, it has been shown recently that there is a characteristic “three-branched” behavior in this regime only part of which is related to nonadiabaticity and the phase dependence which leads to chaos. Moreover, a simple physical model attributing the nonadiabatic jumps in magnetic moment to an impulsive centrifugal force, is able to reproduce this behavior. We give further justification to this centrifugal effect and scrutinize the limits of applicability of this centrifugal impulse model. A phenomenological extension to higher κ values is developed, which extends the model's validity in pitch and phase angle as well.

A simple model of magnetic moment scattering in a field reversal

We examine the nonadiabatic motion of charged particles in a field reversal using a simple centrifugal impulse model. It is shown that, though this model cannot account for long term behaviors, it accurately describes the abrupt magnetic moment change experienced by the particles between two consecutive adiabatic (magnetic moment conserving) sequences. Good agreement is obtained between analytical estimates and trajectory computations. Our analysis also suggests a new pitch angle dependent parameter to characterize the jumps in magnetic moment. Application to particle motion in a cusped field geometry demonstrates that magnetic moment damping and subsequent particle escape can be adequately modeled by means of such centrifugal impulses.