Dipolar interactions in ferromagnetic thin films

We introduce a discrete model describing the motion of a zigzag domain wall in a disordered ferromagnet with in-plane magnetization, driven by an external magnetic field. The main ingredients are dipolar interactions and anisotropy. We investigate the dynamic hysteresis by analyzing the effects of external field frequency on the coercive field by Monte Carlo simulations. Our results are in good agreement with experiments on Fe/GaAs films reported in literature, and we conclude that dynamic hysteresis in this case can be explained by a single propagating domain wall model without invoking domain nucleation.     

Magnetic properties and spin waves of bilayer magnets in a uniform field

The two-layer square lattice quantum antiferromagnet with spins 12 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate ground state wavefunction which consistently covers all phases by means of a unitary transformation. The excitations can be efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations throughout the whole phase diagram. Received 23 April 2001     

Anatomy of field effects on magnetization dynamics and spin transfer noise

Spin transfer-related phenomena in nanomagnets have attracted extensive studies. In this paper we shall focus on analysis of individual and combined effects of the external, anisotropy, and demagnetization fields on magnetization dynamics and spin transfer noise. It is found that individual roles of the external, anisotropy, and demagnetization fields, as well as the combined roles of external plus anisotropy fields and anisotropy plus demagnetization fields, do not change the behavior of current induced magnetization switching. Such magnetization reversal procedures are of low noise. Our dynamics and power spectral density calculations show that it is the demagnetization field that plays a major role in inducing spin transfer noise: the demagnetization field itself or in combination with the anisotropy field will result in wave-like switching; moreover, the demagnetization field, together with the external field (not too small), will lead to precession and hence the system would be in noisy states. Our modeling work for an elliptical Py alloy is qualitatively consistent with Cornell's experiment and simulation [Science 307 (2005) 228].     

On the influence of anisotropy on the magnetic small-angle neutron scattering of superparamagnetic particles

This paper presents a calculation of the magnetic small-angle neutron scattering cross-section resulting from a dilute ensemble of superparamagnetic particles exhibiting uniaxial magnetic anisotropy. We focus on the two experimentally relevant scattering geometries in which the incident neutron beam is perpendicular or parallel to an applied magnetic field, and we discuss several orientations of the anisotropy axes with respect to the field. Magnetic anisotropy has no influence on the magnetic small-angle neutron scattering when the particles are mobile, as is the case e.g. in ferrofluids, but, when the particles are embedded in a rigid non-magnetic matrix and the orientations of the anisotropy axes are fixed, significant deviations compared to the case of negligible anisotropy are expected. For the particluar situation in which the anisotropy axes are parallel to the applied field, closed-form expressions suggest that an effective anisotropy energy or anisotropy-energy distribution can be determined from experimental scattering data. Received 8 November 2001     

Switching of vortex polarization in 2D easy-plane magnets by magnetic fields

We investigate the dynamics of out-of-plane (OP) vortices, in a 2-dimensional (2D) classical Heisenberg magnet with a weak anisotropy in the coupling of z-components of spins (easy plane anisotropy), on square lattices, under the influence of a rotating in-plane (IP) magnetic field. Switching of the z-component of magnetization of the vortex is studied in computer simulations as a function of the magnetic field's amplitude and frequency. The effects of the size and the anisotropy of the system on the switching process are shown. An approximate dynamical equivalence of the system, in the bulk limit, to another system with both IP and OP static fields in the rotating reference frame is demonstrated, and qualitatively the same switching and critical behavior is obtained in computer simulations for both systems. We briefly discuss the interplay between finite size effects (image vortices) and the applied field in the dynamics of OP vortices. In the framework of a discrete reduced model of the vortex core we propose a mechanism for switching the vortex polarization, which can account qualitatively for all our results. A coupling between the IP movement (trajectories) of the vortex center and the OP core structure oscillations, due to the discreteness of the underlying lattice, is shown. A connection between this coupling and our reduced model is made clear, through an analogy with a generalized Thiele equation. Received 6 June 2002 / Received in final form 4 November 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: juan.zagorodny@uni-bayreuth.de     

Dynamics and correlations in magnetic colloidal systems studied by X-ray photon correlation spectroscopy

We have studied the static and dynamic behaviour of magnetic colloidal systems (ferrofluids) by Small-Angle X-ray Scattering (SAXS) and X-ray Photon Correlation Spectroscopy (XPCS). The main features of the novel XPCS technique will be illustrated by data taken on a model system, colloidal silica spheres. We will then present the results obtained on an optically opaque suspension of magnetic colloids (maghemite) in the wave vector range from 10^-3 to 10^-2?^-1. Translational diffusion in zero field and anisotropic diffusion under external magnetic fields will be discussed. Received 9 August 1999 and Received in final form 26 July 2000     

Hydromagnetic transverse instability of two highly viscous fluid-particle flows with finite ion Larmor radius corrections

A linear analysis of the combined effect of viscosity, finite ion Larmor radius and suspended particles on Kelvin-Helmholtz instability of two superposed incompressible fluids in the presence of a uniform magnetic field is carried out. The magnetic field is assumed to be transverse to the direction of streaming. A general dispersion relation for such a configuration has been obtained using appropriate boundary conditions. The stability analysis is discussed analytically, and the obtained results are numerically confirmed. Some special cases are recovered and corrected. The limiting cases of absence of suspended particles (or fluid velocities) and finite Larmor radius, absence of suspended particles are discussed in detail. In both cases, all other physical parameters are found to have stabilizing as well as destabilizing effects on the considered system. In the former case, the kinematic viscosity is found to has a stabilizing effect, while in the later case, the finite Larmor radius is found to has a stabilizing influence for a vortex sheet. It is shown also that both finite Larmor radius and kinematic viscosity stabilizations for interchange perturbations are similar to the stabilization effect due to a magnetic field for non-interchange perturbations. Received 13 January 2003 Published online 24 April 2003 RID="a" ID="a"Also at: Department of Mathematics, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt. e-mail: m.elsayed@uaeu.ac.ae     

One electron atoms in both a squeezed vacuum and a magnetic field via path integral methods

In the present paper we study the dynamics of one-electron atoms in the presence of both a linearly polarized squeezed vacuum and a magnetic field along the polarization vector of the photonic field. We adopt the dipole approximation and approach the problem via path integral methods. After integrating over the light variables for certain initial and final squeezed vacuum states we treat the path integral over the spatial variables via Monte-Carlo methods. As an application we calculate the survival probability of the ground state of a one-electron atom for various values of the magnetic field. Received 30 November 2000 and Received in final form 15 February 2001     

Alternating magneto-birefringence of ionic ferrofluids in crossed fields

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value H _ani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: - a rigid dipole regime (large anisotropy energy with respect to k _B T) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity H _C(H _C < H _ani), - a soft dipole regime (anisotropy energy of the order of k _B T) for maghemite nanoparticles with a relaxation time independent of the field intensity H _C(H _C > H _ani). Received 5 June 2000 and Received in final form 8 January 2001     

Effective hyperfine temperature in frustrated Gd 2Sn 2O 7: two level model and 155Gd Mössbauer measurements

Using ¹⁵⁵Gd M?ssbauer spectroscopy down to 27 mK, we show that, in the geometrically frustrated pyrochlore Gd₂Sn₂O₇, the Gd³⁺ hyperfine levels are populated out of equilibrium. From this, we deduce that the hyperfine field, and the correlated Gd³⁺ moments which produce this field, continue to fluctuate as T ↦ 0. With a model of a spin 1/2 system experiencing a magnetic field which reverses randomly in time, we obtain an analytical expression for the steady state probability distribution of the level populations. This distribution is a simple function of the ratio of the nuclear spin relaxation time to the average electronic spin-flip time. In Gd₂Sn₂O₇, we find the two time scales are of the same order of magnitude. We discuss the mechanism giving rise to the nuclear spin relaxation and the influence of the electronic spin fluctuations on the hyperfine specific heat. The corresponding low temperature measurements in Gd₂Ti₂O₇ are presented and discussed. Received 17 October 2001 Published online 6 June 2002     

Effect of sample shape on nonlinear magnetization dynamics under an external magnetic field

Effect of sample shape on the nonlinear collective dynamics of magnetic moments in the presence of oscillating and constant external magnetic fields is studied using the Landau–Lifshitz–Gilbert (LLG) approach. The uniformly magnetized sample is considered to be an ellipsoidal axially symmetric particle described by demagnetization factors and uniaxial crystallographic anisotropy formed some angle with an applied field direction. It is investigated as to how the change in particle shape affects its nonlinear magnetization dynamics. To produce a regular study, all results are presented in the form of bifurcation diagrams for all sufficient dynamics regimes of the considered system. In this paper, we show that the sample's (particle's) shape and its orientation with respect to the external field (system configuration) determine the character of magnetization dynamics: deterministic behavior and appearance of chaotic states. A simple change in the system's configuration or in the shapes of its parts can transfer it from chaotic to periodic or even static regime and back. Moreover, the effect of magnetization precession stall and magnetic moments alignment parallel or antiparallel to the external oscillating field is revealed and the way of control of such “polarized” states is found. Our results suggest that varying the particle's shape and fields’ geometry may provide a useful way of magnetization dynamics control in complex magnetic systems.     

Dipolar coupling effect in magnetic bilayer system

Equilibrium micromagnetic structures in a bilayer system composed of two thin cobalt films separated by a non magnetic spacer are systematically analysed. These 2D magnetization distributions are obtained by numerical computations according to different set of magnetic and geometric parameters. The coupling effect due to the dipolar long range interaction (or stray-field effect) between the two Co layers is studied through the evolution of the magnetic pattern in the stack with or without an applied field and compared to a continuous film of same thickness. Special attention is paid to the hysteresis process in a bilayer. Even though the general aspect of the magnetization distribution looks like a Landau-Lifshitz structure, the absence of any core in the vortex of the magnetic structure is analysed in relation to a possible disappearence of (topological) hysteresis. Received 12 December 2001     

Skyrmions on an elastic cylinder

For a spin-polarized electron gas on an elastic cylinder in an external axial magnetic field and an axial electric field we find that the corresponding Euler-Lagrange equation is the double sine-Gordon (DSG) equation with an exact 2π-skyrmion solution. The DSG skyrmion is stabilized, without Coulomb repulsion, by the curvature of the cylinder. It adopts a characteristic length ξ which is smaller than the radius of the cylinder. For an elastic cylinder this mismatch of length scales causes a deformation of the cylinder in the region of the skyrmion. Received 23 October 2001 / Received in final form 8 March 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: rossen.dandoloff@ptm.u-cergy.fr     

Evaluation of forces in magnetic materials by means of energy and co-energy methods

The evaluation of the total force of magnetic origin acting upon a body in a stationary magnetic field is often carried out using the so-called magnetic energy (or co-energy) method, which is based on the derivation of the magnetic energy (or co-energy) with respect to a virtual rigid displacement of the considered body. The application of this method is usually justified by resorting to the energy conservation principle, written in terms both of electrical and of mechanical quantities. In this paper we shall re-examine the whole matter in the context of classical thermodynamics, in order to obtain a more comprehensive and general proof of the validity of the energy (or co-energy) approach and to point out its limitations. Two typical configurations will be discussed; in the first one, the field sources are represented by conducting bodies carrying free currents, whereas in the second one a permanent magnet creates the driving field. All magnetic materials are assumed to be non-hysteretic and permanent magnets are represented by means of the well-known linear model in the second quadrant of the (B,H) plane. Received 25 July 2001 and Received in final form 5 November 2001     

Effect of positional disorder in systems of ultrafine ferromagnetic particles

A simple model for systems of dipolarly interacting single-domain ultrafine ferromagnetic particles is studied by Monte Carlo simulations of zero field cooling and field cooling as well as relaxation experiments. By investigating systems characterized by an identical moderate concentration but different types of particle positions' disorder, it is shown that the positional disorder has a crucial influence on the magnetic behavior of the system. For extreme values of positional disorder, the interplay between spatial disorder and dipolar interaction can even lead to a cooperative freezing at low temperatures. Received 28 November 2001     

Magnetoconductance of a two-dimensional metal in the presence of spin-orbit coupling

We show that in the metallic phase of a two dimensional electron gas the spin-orbit coupling due to structure inversion asymmetry leads to a characteristic anisotropy in the magnetoconductance. Within the assumption that the metallic phase can be described by a Fermi liquid, we compute the conductivity in the presence of an in-plane magnetic field. Both the spin-orbit coupling and the Zeeman coupling with the magnetic field give rise to two spin subbands, in terms of which most of the transport properties can be discussed. The strongest conductivity anisotropy occurs for Zeeman energies of the order of the Fermi energy corresponding to the depopulation of the upper spin subband. The energy scale associated with the spin-orbit coupling controls the strength of the effect. More in particular, we find that the detailed behavior and the sign of the anisotropy depends on the underlying scattering mechanism. Assuming small angle scattering to be the dominant scattering mechanism our results agree with recent measurement on Si-MOSFET's in the vicinity of the metal-insulator transition. Received 11 July 2001     

Chiral magnetism of the nucleona

We study the quark mass expansion of the magnetic moments of the nucleon in a chiral effective field theory including nucleons, pions and delta-resonances as explicit degrees of freedom. We point out that the usual power counting applied so far to this problem misses important quark mass structures generated via an intermediate isovector M1 nucleon-delta transition. We propose a modified power counting and compare the resulting chiral extrapolation function to available (quenched) lattice data. The extrapolation is found to work surprisingly well, given that the lattice data result from rather large quark masses. Our calculation raises the hope that extrapolations of lattice data utilizing chiral effective field theory might be applicable over a wider range in quark masses than previously thought, and we discuss some open questions in this context. Furthermore, we observe that within the current lattice data uncertainties the extrapolations presented here are consistent with the Padé fit ansatz introduced by the Adelaide group a few years ago. Received: 23 April 2002 / Accepted: 18 July 2002 / Published online: 17 December 2002 RID="c" ID="c"e-mail: themmert@physik.tu-muenchen.de RID="d" ID="d"e-mail: weise@ect.it Communicated by A. Sch?fer     

Two-step flux penetration in classic antiferromagnetic superconductor

The influence of antiferromagnetic order on the mixed state of a superconductor may result in creation of spin-flop domains along vortices. This may happen when an external magnetic field is strong enough to flip over magnetic moments in the vortex core from their ground state configuration. The formation of domain structure causes modification of the surface energy barrier, and creation of the new state in which magnetic flux density is independent of the applied field. The modified surface energy barrier has been calculated for parameters of the antiferromagnetic superconductor DyMo₆S₈. The prediction of two-step flux penetration process has been verified by precise magnetization measurements performed on the single crystal of DyMo₆S₈ at milikelvin temperatures. A characteristic plateau on the virgin curve B(H ₀) has been found and attributed to the modified surface energy barrier. The end of the plateau determines the critical field, which we call the second critical field for flux penetration. Received 16 August 2002 / Received in final form 22 October 2002 Published online 29 November 2002     

Effects of antisymmetric interactions in molecular iron rings

The level crossing mechanism between the ground and the first excited state of Na:Fe₆ antiferromagnetically coupled iron rings is studied by torque magnetometry down to 40 mK and in magnetic fields up to 28 T. The step width at the crossing field B_c assumes a finite value at the lowest temperatures. This fact is ascribed to the presence of level anticrossing, not expected for a ring with axial, i.e. S₆ point group, symmetry. Assuming a reduced symmetry, we revised the model Hamiltonian of such a spin system by introducing a Dzyaloshinsky-Moriya (DM) term and we show, by exact diagonalization, that DM term can account for the mixing of states with different parity. In particular, analytical as well numerical analysis show that the introduction of the DM term may contribute to the broadening of the torque step as well as for the finite energy gap at B_c observed by heat capacity in a similar ring Li:Fe₆ as previously reported [#!aclbg!#]. Received 3 September 2002 Published online 31 December 2002