Extended spin waves in aperiodic ferromagnetic chains

In this work, a one-dimensional quantum Heisenberg ferromagnet with aperiodic exchange couplings is considered. To produce an aperiodic distribution of exchange couplings, it is used a sinusoidal function whose phase φ varies as a power-law, φ∝n^ν, where n labels the positions along the chain. By using exact diagonalization, the spin-wave participation number and the local density of states are computed. The numerical calculations indicate that for 0 < ν < 1, this ferromagnetic system displays a phase of extended spin waves in the low-energy region. For ν > 1 all spin waves are localized except for the zero energy mode. By integrating the time-dependent Schr?dinger equation, the temporal evolution of the mean-square displacement of the wave-packet was followed. Associated with the emergence of extended spin waves, it was observed that the wave-packet mean-square displacement displays a ballistic spread.     

Low-temperature properties of ferromagnetic Fibonacci superlattices

Theoretical analysis of the layered quasi-periodic Fibonacci structures (superlattices-sequence) is presented for the systems consisting of n_A and n_B ferromagnetically ordered planes within the layers with S_a and S_b spins, respectively, while the interfaces are coupled with bilinear and/or biquadratic exchange interaction, within the framework of localized spin model in the low-temperature limit. Transfer matrix method and direct diagonalization after the bosonization in Bloch's approximation resulted both in the same analytical expression for the magnon-excitation energy. The equivalence (at low-temperatures) of the transfer matrix (spin) and boson approach was discussed, as well as the role of the interlayer biquadratic coupling between different blocks constituting the Fibonacci sequences. Also, our approach allows the determination of the internal energy and calculation of the magnon contribution to the specific heat. It was clearly demonstrated that the magnon specific heat vanishes for T → 0. Our results are compared with the results of other authors.     

Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the spin components (), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches. Received 6 April 1999 and Received in final form 9 July 1999     

Brillouin scattering of light by spin waves in ferromagnetic nanorods

We report the investigations of spin wave modes of arrays of Ni and Co nanorods using Brillouin light scattering. We have revealed the significant influence of spin wave modes along the nanorod axis in contrast to infinite magnetic nanowires. Unusual optical properties featuring an inverted Stokes/anti-Stokes asymmetry of the Brillouin scattering spectra have been observed. The spectrum of spin wave modes in the nanorod array has been calculated and compared with the experiment. Experimental observations are explained in terms of a combined numerical–analytical approach taking into account both the low aspect ratio of individual magnetic nanorods and dipolar magnetic coupling between the nanorods in the array. The optical studies of spin-wave modes in nanorod metamaterials with low aspect ratio nanorods have revealed new magnetic and magneto-optical properties compared to continuous magnetic films or infinite magnetic nanowires. Such magnetic artificial materials are important class of active metamaterials needed for prospective data storage and signal processing applications.     

Dynamic properties of a symmetric spin nanocontact

This paper describes a theoretical study, at a microscopic scale, of the properties of a symmetric magnetic nanocontact. In particular, we study a symmetric nanocontact separating two waveguide groups of semi-infinite spin ordered ferromagnetic monatomic chains. The individual and total conductance of bulk magnons of the chains, scattering coherently at the nanocontact, and the localised density of spin states in the nanocontact domain, are calculated and analysed. The inter-atomic magnetic exchange is varied on the nanocontact to investigate the consequences of magnetic softening and hardening for the calculated properties. Transmission and reflection scattering cross sections are calculated from elements of a Landauer type scattering matrix. The results highlight the localized spin states on the nanocontact domain and their interactions with incident magnons. The results demonstrate also the magnetic and symmetry properties of the nanocontact domain.     

Effects of competition between the anisotropy and the spin quantum number on the magnon energy gap in a four-layer ferromagnetic superlattice

The magnon energy bands are studied for a four-layer ferromagnetic superlattice, with regard to the effects of the competition between the anisotropy and the spin quantum number. A special attention is also paid on the effects of the symmetry of the system. It is found that three modulated energy gaps exist in the magnon energy band along K_x direction perpendicular to the superlattice plane. The magnetic anisotropy affects significantly the magnon energy gaps. The zero energy gap Δω₂₃ correlates with the conditions between anisotropy constants, D₁+D₃=D₂+D₄ and D₁=D₃ (or D₂=D₄), while the disappearance of the magnon energy gaps Δω₁₂ and Δω₃₄ corresponds to a translational symmetry of x-direction in a unit cell. When the parameters of the system deviate from these conditions, the energy gaps Δω₁₂, Δω₂₃ and Δω₃₄ become larger. There is a competition effect of the anisotropy and the spin quantum number on the magnon energy gaps Δω₁₂ and Δω₂₃. When the symmetry of the system is higher, the competition can achieve a balance to cause the zero energy gap.     

On evaluation of transverse spin fluctuations in quantum magnets

A numerical method is described for evaluating transverse spin correlations in the random phase approximation. Quantum spin-fluctuation corrections to sublattice magnetization are evaluated for the antiferromagnetic ground state of the half-filled Hubbard model in two and three dimensions in the whole U/t range. Extension to the case of defects in the AF is also discussed for spin vacancies and low-U impurities. In the limit, the vacancy-induced enhancement in the spin fluctuation correction is obtained for the spin-vacancy problem in two dimensions, for vacancy concentration up to the percolation threshold. For low-U impurities, the overall spin fluctuation correction is found to be strongly suppressed, although surprisingly spin fluctuations are locally enhanced at the low-U sites. Received 27 April 1998 and Received in final form 13 August 1998     

Brillouin light scattering from spin waves in magnetic layers and multilayers

We give an overview on our experimental and theoretical investigations of Brillouin light scattering in magnetic thin films, layered magnetic structures and superlattices. For epitaxial Fe(1 10) layers on W(1 10) the in-plane and out-of-plane magnetic surface anisotropy constants are determined, and the influence of Pd overlayers on the surface anisotropies is studied. For Fe/Pd superlattices a magnetic polarization of the Pd at the interfaces is established and the interface anisotropy constant is determined. For second order Fe/Pd superlattices, formed by alternating two Fe/Pd bilayers with different repeat periods, the Brillouin spectrum is obtained and compared to calculations. In the case of magnetic/nonmagnetic multilayered structures we investigate theoretically the crossing regime between dipolar and exchange-dominated modes. For small spacer-layer thicknesses, interlayer exchange coupling shifts the spin-wave frequencies of all but the highest-frequency dipolar mode into the exchange-mode regime. In case of all-magnetic multilayered structures, such as Fe/Ni multilayers, a new type of propagating collective excitations arising from coupled exchange modes is predicted.     

Quantum fluctuations and ground state of weakly interacting helix spin chains in a magnetic field

Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J₁ develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J₂ is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit. Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997     

界面各向异性对双层铁磁薄膜自旋波性质的影响

在海森堡模型的基础上,采用界面参数化方法,将双层铁磁薄膜中自旋波本征值问题归结为联立求解能量约束方程和界面参数化方程.重点研究了界面各向异性对薄膜中自旋波本征问题的影响.结果表明:界面各向异性使对称模的波形在界面处呈现明显的钉扎现象,且界面模的能量随各向异性场增强而增大.     

Many-body Green's function theory for the magnetic reorientation of thin ferromagnetic films

The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term, an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms. The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good approximation for realistic coupling strengths. We apply the new method to monolayers with spin and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some significant differences. Received 19 June 2000 and Received in final form 2 August 2000     

Spin wave propagation in the domain state of a random field magnet

Inelastic neutron scattering with high wave-vector resolution has characterized the propagation of transverse spin wave modes near the antiferromagnetic zone center in the metastable domain state of a random field Ising magnet. A well-defined, long wavelength excitation is observed despite the absence of long-range magnetic order. Direct comparisons with the spin wave dispersion in the long-range ordered antiferromagnetic state reveal no measurable effects from the domain structure. This result recalls analogous behavior in thermally disordered anisotropic spin chains but contrasts sharply with that of the phonon modes in relaxor ferroelectrics. Received 2 November 2002 / Received in final form 4 February 2003 Published online 11 April 2003 RID="a" ID="a"leheny@pha.jhu.edu     

Order by thermal disorder in 2D planar rotator model with dipolar interactions

The energy of a square planar rotator model of spins interacting via dipolar forces is minimized by infinite inequivalent configurations corresponding to spins arranged on four interpenetrating sublattices making angles , , , with a reference axis, being arbitrary. This infinite degeneracy of the ground state is accidental in nature and one expects that it is removed by thermal fluctuations in agreement with Monte Carlo simulation. Indeed we find that the elementary excitation energies which depend on lead to a free energy which is a function of with minima at and corresponding to columnar configurations. This selection of columnar configurations out of the infinite ground state manifold is an example of order by thermal disorder. Received 4 August 1998     

Softening of spin-wave stiffness near the ferromagnetic phase transition in diluted magnetic semiconductors

Employing the self-consistent Green's function approach, we studied the temperature dependence of the spin-wave stiffness in diluted magnetic semiconductors. Note that the Green's function approach includes the spatial and temperature fluctuations simultaneously which was not possible within conventional Weiss mean-field theory. It is rather interesting that we found the stiffness becomes dramatically softened as the critical temperature is approached, which seems to explain the mysterious sharp drop of magnetization curves in samples within diffusive regime.     

Quantum spin dynamics of the bilayer ferromagnet La Sr Mn O

We construct a theory of spin wave excitations in the bilayer manganite La_1.2Sr_1.8Mn₂O₇ based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. Received 15 January 2002 Published online 6 June 2002     

Finite size corrections within the continuum limit for quantum spins: two-magnon bound states in 1D Heisenberg ferromagnet

A continuum medium approach is proposed to describe the finite size dependent effects for the 1D isotropic Heisenberg ferromagnet. The results are compared to the exact Bethe ansatz solution for the finite chain. The approach is shown to adequately account for the behaviour of the eigenfunctions and eigenenergies. The continuum is obtained by integration in Fourier space via introduction of cut-offs at the integration limits and analytical continuation from the discrete lattice to the continuous medium. It offers a new perspective on the instability of bound states, and reveals the linear behaviour of the amplitude in the critical region and other features of the model in an analytical way. We further apply this approach to investigate the long wavelength expansion of the master equation and to show the route of constructing reliable approximations valid for more complicated models. It is concluded that the approach can be useful to study mesoscopic spin systems. Received 28 May 2000 and Received in final form 6 April 2001     

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr _1-x Ca_xMnO₃ system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr _0.8 Ca_0.2MnO₃ single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr³⁺ ions. Received 21 May 2001 and Received in final form 14 December 2001     

Analytical result of the effect of spin scattering and impurity-assistance on magnetoresistance in doped magnetic tunneling junctions

An extended tunneling Hamiltonian method is proposed to study the temperature-dependent tunneling magnetoresistance (TMR) in doped magnetic tunnel junctions. It is found that for nonmagnetic dopants (Si), impurity-assisted tunneling is mainly elastic, giving rise to a weak spin polarization, thereby reduces the overall TMR, while for magnetic ions (Ni), the collective excitation of local spins in δ-doped magnetic layer contributes to the severe drop of TMR and the behavior of the variation of TMR with temperature different from that for Si-doping. The theoretical results can reproduce the main characteristic features of experiments. Received 13 January 2002 / Received in final form 30 November 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: yctao12@163.com