Evidence for gapped spin-wave excitations in the frustrated Gd2Sn2O7 pyrochlore antiferromagnet from low-temperature specific heat measurements

We have measured the low-temperature specific heat of the geometrically frustrated pyrochlore Heisenberg antiferromagnet Gd2Sn2O7 in zero magnetic field. The specific heat is found to drop exponentially below approximately 350 mK. This provides evidence for a gapped spin-wave spectrum due to an anisotropy resulting from single-ion effects and long-range dipolar interactions. The data are well fitted by linear spin-wave theory, ruling out unconventional low-energy magnetic excitations in this system, and allowing a determination of the pertinent exchange interactions in this material.     

Low-temperature properties of ferromagnetic-antiferromagnetic double layer superlattices

Low-temperature magnetic properties of a Heisenberg model consisting of ferro- and antiferromagnetic layer superlattices are studied by using spin-wave theory and retarded Green's function method. The four-sublattice magnetizations and internal energy and specific heat at low temperature are calculated. The results of various physical quantities are shown for different sets of intra- and interplane coupling interactions. There is a crossover between sublattice magnetizations in each layer is affected by quantum fluctuations, thermal fluctuations and frustration of spins.     

Magnetic-field dependence of the specific heat in planar spin chains

The specific heat of theXY model develops a characteristic double peak as a function of an applied magnetic field at relatively low temperatures. We argue that such a behavior is generic for planar spin chains, including chains with single-site anisotropy, and should be amenable to experimental observation. We also show that anharmonic corrections in the standard spin-wave theory exhibit certain infrared anomalies which limit the validity of perturbative calculations.     

The spin-wave contribution to the heat capacity of ferromagnetic thin films with variable interlayer couplings

A Heisenberg model is solved within the spin-wave theory for thin films in which ferromagnetic monolayers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have included also a magnetic anisotropy in each manolayer. The temperature dependence of the spin-wave contribution to the heat capacity C _m in such composite systems is derived for different interlayer couplings and for different film thickness.     

阻挫对准一维非对称子格反铁磁链自旋波激发的影响

子格对称性破缺使得s=1/2准一维海森伯自旋链具有三支自旋波激发谱(其中一支属于声学模，两支属于光学模). 计算表明：阻挫导致两支光学模能隙简并解除；阻挫引起声学模自旋波激发谱软化，但并不导致光学模激发谱软化；阻挫导致两支光学模激发谱中的一支明显下移，这意味着阻挫使光学模激发变得重要且容易实现；阻挫引起的自旋偏离对于属于不同子格的自旋是不同的；阻挫对于系统基态磁性长程序的削弱随着阻挫增强变得越来越明显. 通过与严格对角化方法和DMRG方法的数值结果比较，还分析了自旋波近似的合理性和不足之处. 关键词： 准一维反铁磁自旋系统 自旋波激发 光学模 能隙简并     

Angular dependences of spin-wave resonance spectra of inhomogeneous films with orthorhombic anisotropy

Zero spin-wave mode in inhomogeneous magnetic films with orthorhombic anisotropy has been found to exhibit a change of its localization region in two of three typical angular dependences of resonance fields of high-intensity modes. It has been shown that the anisotropy fields on both sides of the film can be determined from the resonance fields of the zero and uppermost high-intensity spin-wave modes of spin-wave resonance spectra.     

Brillouin light scattering of CoFe/IrMn-based top and bottom spin-valves

The spin-wave Brillouin light scattering (BLS) was observed from CoFe/IrMn-based bottom and top spin-valve structures. The magnitude of perpendicular magnetic anisotropy of these structures was investigated by means of BLS. The experimental spin-wave frequency was simulated using a simple model which includes crystallographic, exchange, in-plane uniaxial, and in- and out-of-plane anisotropies and takes the exchange interaction between the two ferromagnetic layers into account. This enabled us to determine, in addition to the other magnetic parameters, both in- and out-of-plane anisotropy constants near the surface. It was found that the top spin-valve is in a strong out-of-plane anisotropy whereas in-plane anisotropy is dominant for the bottom spin-valve.     

The q-Deformed SU(2) Quantum Antiferromagnetic Model

In this paper we set up a q-analogous Holstein- Primakoff transformation (HPT) and obtain the.q-deformed spin-wave solutions. We apply a mean-field treatment that is arranged to preserve some nonlinearity of the Hamiltonian, and the results turn out to be interesting,e.g., a gapful spectrum of the elementary excitation spectrum is found, which is different from the spin-wave results for the conventionid model, and the specific heat is therefore modified greatly. The properties of the Néel state and the ground state are investigated.     

DC current effect on the magnetic phase and transport properties of polycrystalline La0.7Ca0.3MnO3

The influence of DC current on the resistivity and phase transition of polycrystalline La_0.7Ca_0.3MnO₃ has been investigated. The specific heat measurement found that charge carriers and ferromagnetic spin-wave contributions were changed after applied DC current. Applying high electric fields leads to the formation of ferromagnetic regions. The resistivity drops abruptly once the percolating current path is established. As current through the sample disappears, the larger ferromagnetic (FM) clusters, however, remain and are frozen in giving a measurable contribution to the specific heat of the system. The larger clusters should give rise to the value of spin-wave stiffness constant (D), as it is expected to increase the strength of the ferromagnetic coupling. The metallic ferromagnetic regions would make the charge carrier delocalization and attribute to specific heat linear term γ.     

Density functional theory simulation of liquid helium-4 in aerogel

Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient D in the spin-wave spectrum E _k ～ Dk ² vanishes. Within the Dyson-Maleev formalism, it is found that, at low temperatures, thermal fluctuations stabilize ferromagnetism by increasing the value of D. For not overly strong frustration, this leads to an unusual “concave” shape of the temperature dependence of magnetization, which is in agreement with experimental data on europium chalcogenides. The phase diagram is constructed by means of Monte Carlo simulation, and suppression of the magnetization and Curie temperature is found in comparison with the results of the spin-wave theory. This effect is explained by the presence of nonanalytical corrections to the spin-wave spectrum which are represented in the lowest order by the term ～(T/S)² k ²lnk.     

Energy gaps in the spin-wave dispersion of the linear antiferromagnet TMMC

By means of neutron scattering, we have measured magnetic excitations in the linear-chain antiferromagnet TMMC at T =1.8 K. An energy gap is observed at q=0 associated with zz-spin correlations, thus giving direct evidence for the importance of the dipolar xy anisotropy for the spin dynamics at low temperatures. A finite magnetic field applied perpendicular to the chains creates a second q=0 gap. There are substantial discrepancies between field dependence of the spin-wave dispersion and the predictions of linear spin-wave theory.     

Internal energy and specific heat in a ferromagnetic- antiferromagnetic double layers

The internal energy and specific heat of a Heisenberg ferro- antiferromagnetic double-layer system are studied by using spin-wave theory and the retarded Green function method at low temperatures. Numerical results show that the antiferromagnetic intralayer coupling $J_2 $ has an important influence on internal energy and specific heat for a four-sublattice system with antiferromagnetic (or ferrimagnetic) interlayer couplings.     

Surface modes with biquadratic coupling and uniaxial anisotropy in semi-infinite Heisenberg antiferromagnet

We address the role and the influence of both biquadratic exchange and uniaxial anisotropy in the study of bulk and surface magnetic excitations for a two-sublattices semi-infinite Heisenberg antiferromagnet. The calculations apply to combined effect and are based on a matching technique within the framework of both non-interacting spin-wave theory and random phase approximation. Emphasis has been laid upon the advantage of handling bulk and surface spin precessional fields, characterized by their symmetry in particular. Analytic expressions for the bulk and surface equations of motion are also derived which enables us to analyze qualitatively the nature of the modes by observing the response of the energy branches to the variation of both biquadratic exchange and single-ion anisotropy field as well as exchange parameters occurring on the surface. Two numerical applications of the theoretical formalism developed in this work are described. First concerns the situation when the free surface model is assumed. The second application considers the existence of surface perturbations and treats the effects of the presence of biquadratic exchange term on the bulk and surface spin-wave spectra showing two kinds of surface propagating modes which depend on the presence or not of the uniaxial anisotropy contribution and also on the variations of the bulk-surface exchange parameters. A significant conclusion which this work reached relates to the checking of the results obtained in the literature by using our own theoretical formalism. We show that this formalism is very well adapted to calculations of localized bulk and surface spin-wave modes associated with the simultaneous presence of biquadratic exchange and uniaxial anisotropy. Also, the results obtained for the bulk spin fluctuations field as well as for the magnetic surface waves, are found to be in qualitative agreement with those obtained in the literature. The analytic formulation presented in this work provides an accurate and practical scheme for calculating the surface spin dynamics under the influence of surface exchange parameters, biquadratic coupling and uniaxial anisotropy terms.     

Unidirectional anisotropy and the roughness of the ferromagnet-antiferromagnet interface

Magnetization curves of a ferromagnetic film on an antiferromagnetic substrate are investigated with allowance for the frustration caused by roughness of the interface. The conditions for unidirectional anisotropy are determined, and its dependence on the degree of roughness of the film-substrate interface is found.     

On the spin-wave resonance in bulk ferromagnetic conductors

The shape of the absorption curve of ferromagnetic experimental permeability is found and relations are derived for the resonance line width, the magnitude of the resonance field and the correction of the g-factor for the spin-wave resonance of ferromagnetic conductors for an arbitrary value of the surface anisotropy. The relations are used to discuss the results of the measurements and to show the dominant role of the skin-effect in the ferromagnetic resonance of bulk monocrystalline conductors with a small number of defects.     

Reflection of electromagnetic waves from the surface of a cubic ferrite

The reflectivity of electromagnetic waves incident on the surface of a semi-infinite nonconducting cubic ferrite was found both analytically and numerically with inclusion of spin-wave damping. The frequency and field dependences of the reflectivity were found for various values of the damping parameter and the anisotropy and magnetostriction constants in a region far from the point of the orientational phase transition and at this point. It is shown that the reflectivity has peaks near the frequencies of ferromagnetic, magnetoacoustic, and magnetostatic resonances. The peak amplitude decreases with increasing spin-wave damping. At frequencies below the magnetoelastic gap, the reflectivity can take on anomalously small (down to zero) and anomalously large (up to unity) values. These frequencies can lie in the microwave range.     

The Complex Magnetism in the Breathing Pyrochlore LiIn(Cr_(1-x)Rh_x)_4O_8

We perform a detailed investigation of the new 'breathing' pyrochlore compound LiInCr_4O_8 through Rh substitution with measurements of magnetic susceptibility,specific heat,and x-ray powder diffraction.The antiferromagnetic phase of LiInCr_4O_8 is found to be slowly suppressed with increasing Rh,up to the critical concentration of x=0.1 where the antiferromagnetic phase is still observed with the peak in specific heat T_p=12.5K,slightly lower than T_p=14.3 K for the x=0 compound.From the measurements of magnetization we also uncover evidence that substitution increases the amount of frustration.Comparisons are made with the LiGa_yIn_(1-y)Cr_4O_8system as well as other frustrated pyrochlore-related materials and comparable amounts of frustration are found.The results of this work show that the engineered breathing pyrochlores present an important method to further understand the complex magnetism in frustrated systems.     

Spin-wave contribution to the nuclear spin-lattice relaxation in triplet superconductors

We discuss collective spin-wave excitations in triplet superconductors with an easy axis anisotropy for the order parameter. Using a microscopic model for interacting electrons, we estimate the frequency of such excitations in Bechgaard salts and ruthenate superconductors to be 1 and 20 GHz, respectively. We introduce an effective bosonic model to describe spin-wave excitations and calculate their contribution to the nuclear spin-lattice relaxation rate. We find that, in the experimentally relevant regime of temperatures, this mechanism leads to the power law scaling of 1/T1 with temperature. For two- and three-dimensional systems, the scaling exponents are 3 and 5, respectively. We discuss experimental manifestations of the spin-wave mechanism of the nuclear spin-lattice relaxation.     

Parametric excitation of spin waves in strongly anisotropic uniaxial ferrites

The thresholds of parametric spin-wave excitation are measured in a Ba₂Zn₂Fe₁₂O₂₂ easy-plane ferrite at a pumping frequency of 36 GHz in the temperature range from 5 to 350 K at different angles between the external dc magnetic field and the anisotropy axis, where the pumping is, in a general case, oblique. The dependences of the spin-wave damping parameter on wave number, temperature, and the angle between the spin-wave propagation direction and the anisotropy axis have been found. The results of measuring the ferromagnetic resonance parameters in this ferrite and an easy-axis Ba(Fe_0.95Sc_0.05)₁₂O₁₉ ferrite are presented. Conclusions on the relaxation processes in strongly anisotropic hexagonal ferrites are made. Fiz. Tverd. Tela (St. Petersburg) 41, 1652–1659 (September 1999)