Critical behavior of noninteracting spin waves

Monte Carlo techniques are used to show that a free-spin-wave theory of the quantum Heisenberg spin-1/2 ferromagnet, formulated to accurately represent the state space of the underlying system, predicts the critical temperature of the interacting theory to within 1% and critical exponents to within a factor of two.     

Temperature dependent statistical theory of two spin cross relaxations

A temperature dependent statistical theory of cross relaxation between two different spin systems is presented. An exact evaluation is made for the magnetic resonance line shape due to the diagonal part of the dipolar interaction at arbitrary temperatures. The cross relaxation transition probability is found to be inversely proportional to the Zeeman energy imbalance when it is large compared to dipolar energy. This seems to agree with the Mims experiment on Ruby in the negative frequency range.     

Microscopic theory of dipole-exchange spin waves in magnetic multilayers

A microscopic model is employed to calculate the spectrum of dipole-exchange spin waves in multilayers in which thin ferromagnetic films are separated by non-magnetic spacers. Two configurations are considered: in one the films have magnetizations parallel to each other, in the other the magnetizations are antiparallel. The calculations extend a previous microscopic formalism that allows the calculation of the dipole-exchange spin wave spectrum in thin films. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness.Received: 26 May 2004, Published online: 12 August 2004PACS: 75.30.Ds Spin waves - 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces     

Accounting for the local surface geometry in the classical theory of spin waves

The effect of the surface energy on the formation of the spin spectrum is considered. Different aspects are examined of the effect of the surface on the energy spectrum of the system associated with structural features of the surface structure. The calculations are carried out for the spectrum of elementary excitations of a ferromagnetic system that is bounded by a surface. Calculations are conducted following the standard method of the Hamiltonian diagonalization. Relationships are obtained that completely determine the surface spectrum in the zero approximation in the spin-spin interaction.     

Low-temperature relaxation in spin glasses

We analyze the mechanism governing the long-time, low-temperature relaxation of the thermoremanent magnetization (σ_TRM) in metallic spin glasses. Ideas of quantum tunneling and “screening” of the local magnetic moments are employed to explain the existence of a hierarchy of relaxation times. This results in a time decay which is described by anenhanced power law: $$\sigma _{TRM} = \sigma _0 \exp - A[\ln (\omega t)]^y = \sigma _0 (\omega t)^{ - A[\ln (\omega t)]y - 1} $$ withy≧1. A crossover is predicted below which most parameters become temperature-independent.     

Spherical spin waves

The spin waves in a two-sublattice ferrimagnetic sphere are derived from the equation of motion for the sublattice magnetization. The pinning conditions are discussed and the results are compared with those obtained by numerical diagonalization of corresponding Heisenberg hamiltonian. For a sphere with the diameter of about 40 lattice constants the continuum approximation is fully applicable for estimation of some (long wavelength) spin wave energies and angle of the core spins. The transition probabilities seem to be considerably overestimated by this method.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.The author wishes to thank Dr. V. Kamberský for many valuable discussions which enabled him to improve the text.     

Pions as spin waves: Chiral symmetry breaking in lattice gauge theory

In hamiltonian lattice gauge theory, the fermion vacuum at lowest order in 1/g² can be determined from degenerate perturbation theory plus mean field-spin wave techniques. Using compact QED as an example, we show that: (i) chiral symmetry is spontaneously broken; and (ii) $\text{m}{}_{\mathrm{<mtext>pseudoGoldstone</mtext>}}{}^2\text{∝ m}{}_{\mathrm{<mtext>fermion</mtext>}}\text{〈}\text{ψ}\text{ψ〉}$. The pseudoscalar pseudoGoldstone particles—the “pions” of this abelian theory—correspond to antiferromagnetic spin wave excitations of the fermion vacuum.     

The problem of summation over physical states in a theory of interacting spin waves

A new representation for the spin operators is suggested and on its basis the formalism involving both the rigorousness of diagram technique for the spin operators and the simplicity of diagram technique for the Fermi- and Bose-system is developed to calculate the Green's spin functions. This method is a basis for constructing a theory of high-frequency and thermodynamic properties of the magnetic ordered crystals and a systematic summation over physical states of interacting magnons at any temperatures.     

Low-temperature nuclear spin relaxation in β-aluminas

²⁷A$\text{l}$ NMR T₁ measurements for Na-, K-, and Na_0.5K_0.5β-alumina in the temperature range ～5–180K are reported. Both Na- and Kβ-alumina exhibit a nearly linear temperature dependence of the spin-lattice relaxation rate T^?1₁ up to ～55K which is attributed to the presence of two-level system (TLS) tunneling modes. A shallow T₁ minimum at ～30K is observed in Na_0.5K_0.5β-alumina. It is suggested that the origin of this minimum is associated with the distribution of TLS parameters, and may be a low-temperature manifestation of the mixed-alkali effect commonly observed in glasses in the higher temperature ionic hopping regime.     

Nonlinear theory of the excitation of spin waves in ferrites by a longitudinal ultrahigh-frequency magnetic field

The problem of parametric excitation of spin waves in ferrites due to the action of a perturbing super high-frequency (SHF) magnetic field polarized along the direction of the constant magnetic field H₀ is considered in a nonlinear approximation. Such an examination enables one to determine the conditions for parametric excitation of spin waves and to find the amplitude and phase of the oscillations of the system's magnetization in the steady state. The frequency interval, within whose limits parametric excitation of spin waves is possible, is determined. Stability of the steady state under the conditions of parametric resonance is investigated. The results obtained are compared with existing experimental data.     

Low-temperature heat capacity of a dipole spin glass

Low-temperature behavior of the heat capacity of a dipole spin glass is studied analytically. Different contributions to the heat capacity are investigated, their comparison is performed, and their relationship with experimental data is analyzed.     

Interaction of solitary spin waves

We present the results of a computer experiment devoted to the problem of the interaction of two magnetic solitary spin waves moving in the direction perpendicular to the axis of easy magnetization in an uniaxial ferromagnet. Such waves being particular solutions of the Landau-Lifshitz equations move like a domain wall under the influence of an external magnetic field. Our computer experiment shows that the two solitary spin waves during their interaction, behave as two solitons and thus the concerned Landau-Lifshitz equations allows N-soliton solutions.     

On the spin waves in space-disordered media

A generalization of the hydrodynamic long-wavelength theory of spin waves in the space-disordered media is considered. The generalization includes all non-linear effects and the relativistic corrections.     

Weyl monopoles dance with the spin waves

In a Weyl semimetal(WSM),the conduction and valence bands cross each other near the Fermi energy,and the crossing points,called Weyl points,exhibit a monopole-like distribution of the Berry curvature.The Berry curvature is a fictitious magnetic field in the momentum-space and induces the anomalous velocity to the real-space electron motion.Therefore,Weyl monopoles play essential roles in the charge transport,for example,the anomalous Hall effect(AHE).The anomalous Hall conductivity(AHC)is nearly proportional to the distance between the Weyl point pairs with opposite chirality.The magnetic order and spin structure sensitively modify Weyl point positions and energies.     

Classical spin theory

Tensor and vector equations of motion of a classical charged particle with spin have been derived within the framework of the special theory of relativity on the basis of Frenkel's tensor. The anomalous magnetic moment of the particle is considered in a natural manner in deriving the equations. The expression for the forces acting on the particle is constructed with consideration of the effect of spin on the motion trajectory. The spin equations proved to coincide with those obtained previously by Nyborg and Good. The properties of these equations have been studied, and it has been shown that the various equations are in fact variant forms of one and the same equation. In the absence of an anomalous magnetic moment the tensor equation coincides with Frenkel's spin equation, and in the same situation the vector equation transforms to the equation obtained by Tamm. In the special case of homogeneous fields the vector equation coincides with the well-known Bargmann-Michel-Telegdi equation. In conclusion we present spin motion equations for a particle with electric and magnetic charges.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 67–76, February, 1980.