Reflection and refraction of acoustic waves from the insulator-magnetoacoustic material interface

The reflection and refraction of longitudinal and transverse acoustic waves by the plane interface between an insulator and an easy-plane antiferromagnet undergoing a magnetic-field-induced orientational phase transition are analyzed. The angles of refraction, as well as all four coefficients of wave conversion, can be effectively controlled by varying the field. Conditions for the appearance of the critical angles of internal reflection and the effect of the magnetic field on these angles are considered. A glancing wave radiated into the material is shown to be a possibility near the phase transition.     

Reflection and refraction of an acoustic beam at the boundary between a fluid and a magnetoacoustic material near the orientational phase transition

Reflection and refraction of an acoustic beam at the boundary between a liquid and a magnetoacoustic material is considered in the region of the orientational phase transition of the material in magnetic field. For the boundary between water and hematite, the possibility of controlling the angles of refraction and the wave transformation amplitudes is numerically demonstrated. The longitudinal displacement and the splitting of an acoustic beam at the boundary between the aforementioned media are investigated as functions of the external magnetic field.     

Reflection of elastic waves in a crystal of Heusler alloy Ni<Subscript>2</Subscript>MnGa in the phase transition range

The reflection of longitudinal and transverse acoustic waves from the free surface of the ferromagnetic shape memory alloy Ni₂MnGa that is located in the ranges of the premartensite and martensite phase transformations is considered. The propagation directions and amplitudes of the waves reflected in the (001) plane of the crystal are determined. They acquire the character of substantially quasi-longitudinal and quasi-transverse vibrations rather than being pure modes. The angles of wave reflection and conversion are shown to be effectively controlled by temperature and a magnetic field due to the colossal acoustic anisotropy of the crystal over the wide range of its phase transitions. Beginning from a certain critical angle of incidence of a quasi-transverse wave, the quasi-longitudinal wave having appeared upon reflection becomes an accompanying surface vibration, and it can be emitted into the bulk of the crystal when the phase transition point is approached. Two angles of full conversion of an incident quasi-longitudinal wave into a quasi-transverse wave are established, and their temperature dependences are found. Trivisonno’s experimental data for the ultrasound velocity and absorption in an Ni₂MnGa crystal are used to numerically estimate these acoustic effects.     

Phase transitions and reflection positivity for a class of quantum lattice systems

We prove a form of reflection positivity in planes containing sites for a class of quantum lattice systems. As an application, a proof is given of a phase transition for the Fisher-stabilized Ising antiferromagnet in an external magnetic field with parallel and transverse components, both by the method of infrared bounds and by a suitable version of the Peierls argument. We also discuss the spherical model in an appendix.     

Monte Carlo simulation of a random-field Ising antiferromagnet

Phase transitions in the three-dimensional diluted Ising antiferromagnet in an applied magnetic field are analyzed numerically. It is found that random magnetic field in a system with spin concentration below a certain threshold induces a crossover from second-order phase transition to first-order transition to a new phase characterized by a spin-glass ground state and metastable energy states at finite temperatures.     

Phase diagram of the helical structure of a two-subsystem frustrated antiferromagnet

The expansion of the thermodynamic potential for the two-subsystem antiferromagnet with frustrated intersubsystem isotropic exchange is obtained. It is demonstrated that this expansion contains the first derivatives with respect to the antiferromagnetic vectors of the subsystems, i.e., the Lifshitz invariant. The equation for the temperature-field boundary of the helical phase for the two-subsystem frustrated antiferromagnet is derived by linearizing the variational equations for the minimum free energy within the mean-field approximation. Relationships are obtained for the critical field at T = 0, the angle of canting of moments of the antiferromagnetic sublattices, and the temperature of spontaneous appearance of helical ordering in the absence of an external field. It is revealed that there is a second higher temperature of formation of the helical magnetic structure induced by the magnetic field with the wave vector of the helix nonmonotonically depending on the external field. The phase boundary of the helical phase and the temperature dependence of the orientation of moments of the magnetic subsystem with weak exchange interaction are determined using numerical minimization of the free energy. It is shown that the transition to the commensurate phase is a first-order transition with a small magnetization jump. A comparative analysis of models with different spatial displacements of ions in the subsystems along the direction of the vector of the helical structure is performed. A criterion is proposed for the choice of the direction of the vector of the incommensurate magnetic structure.     

Bulk spin-wave electrodynamics of an antiferromagnetic plate in a constant external electric field

Using the example of a plate of an easy-axis centrosymmetric antiferromagnet, the conditions are determined under which an external constant electric field qualitatively changes not only the character of refraction of a bulk TM wave incident from vacuum onto the antiferromagnet surface but also the structure of the spectrum of propagating bulk magnons. The relation of the discovered effects to the shape of the refraction surface of TM polaritons is studied.     

Local spin resonance and spin-peierls-like phase transition in a geometrically frustrated antiferromagnet

Inelastic magnetic neutron scattering reveals a localized spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated cubic antiferromagnet ZnCr2O4. The resonance develops abruptly from quantum critical fluctuations upon cooling through a first order transition to a co-planar antiferromagnet at T(c) = 12. 5(5) K. We argue that this transition is a three dimensional analog of the spin-Peierls transition.     

Nonequilibrium Phase Transition in Spin-S Ising Ferromagnet Driven by Propagating and Standing Magnetic Field Wave

The dynamical response of spin-S(S=1, 3/2, 2, 3) Ising ferromagnet to the plane propagating wave, standing magnetic field wave and uniformly oscillating field with constant frequency are studied separately in two dimensions by extensive Monte Carlo simulation. Depending upon the strength of the magnetic field and the value of the spin state of the Ising spin lattice two different dynamical phases are observed. For a fixed value of S and the amplitude of the propagating magnetic field wave the system undergoes a dynamical phase transition from propagating phase to pinned phase as the temperature of the system is cooled down. Similarly in case with standing magnetic wave the system undergoes dynamical phase transition from high temperature phase where spins oscillate coherently in alternate bands of half wavelength of the standing magnetic wave to the low temperature pinned or spin frozen phase. For a fixed value of the amplitude of magnetic field oscillation the transition temperature is observed to decrease to a limiting value as the value of spin S is increased. The time averaged magnetisation over a full cycle of the magnetic field oscillation plays the role of the dynamic order parameter. A comprehensive phase boundary is drawn in the plane of magnetic field amplitude and dynamic transition temperature. It is found that the phase boundary shrinks inwards for high value of spin state S.Also in the low temperature(and high field) region the phase boundaries are closely spaced.     

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     

Direct transition from a disordered to a multiferroic phase on a triangular lattice

We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)(2). Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.     

Multipath reflection of light from plane-parallel crystalline plates in the presence of additional exciton-polariton waves

The reflectance and transmittance of light by plane-parallel crystal plates were found in the exciton-transition spectral region upon oblique incidence of light with polarization in which two transverse and one longitudinal normal exciton-polariton wave are excited. The reflection spectra calculated for a model crystal with the parameters of the CdSe crystal are analyzed for different thicknesses of the plates, angles of incidence, and damping constants. It is shown that accounting for additional exciton-polariton waves and multiple reflections of light inside the plate results in the possibility of an abrupt change in the phase (by more than 2π) in the vicinity of the exciton transition.     

The theory of acoustic self-induced transparency in magneto-ordered mediums

Possible acoustic self-induced transparency of magneto-ordered medium is under investigation in the paper. Heisenberg model of a ferromagnet and antiferromagnet in magnetic field is considered as an example. The allowance made for exchange interaction between electron spins is shown to result in qualitative change of self-induced transparency picture; in particular, the dispersion low for the carrier sound wave varies along with the character of phase modulation and the shape of the stationary pulse. The possibility of experimental observation of the above-mentioned phenomenon is discussed.     

Phase diagram of a geometrically frustrated triangular-lattice antiferromagnet in a magnetic field

The magnetic phase diagram of a geometrically frustrated triangular-lattice antiferromagnet is evaluated as a function of magnetic field and anisotropy using a trial spin state built from harmonics of a fundamental ordering wave vector. A noncollinear incommensurate state, observed to be chiral and ferroelectric in CuFeO2, appears above a collinear state with 4 sublattices (SLs). The apparent absence of multiferroic behavior for predicted chiral, noncollinear 5-SL states poses a challenge to theories of the ferroelectric coupling in CuFeO2.     

Two-dimensional quantum antiferromagnet in a strong magnetic field

The behaviour of the two-dimensional quantum antiferromagnet of arbitrary spin in a strong transversal magnetic field on a square lattice is studied in terms of the equivalent Bose gas problem. The existence of phase transition from the state characterized by “quasi-long-range” magnetic order to the disordered ferromagnetic state is demonstrated. The expressions for correlation functions, thermodynamical and magnetic characteristics are derived.     

The phenomenon of conical refraction in a thin-layer periodic semiconductor-dielectric structure in a magnetic field

The propagation of electromagnetic waves along the optical axes of a thin-layer periodic semiconductor-dielectric structure in an external magnetic field (i.e., under conditions of external and internal conical refraction) has been investigated. It is shown that the conditions for conical refraction can be implemented in certain regions by changing the external magnetic field, wave frequency, and thickness of the layers forming the structure. By varying the above-mentioned characteristics, one can efficiently control the conical refraction parameters; in particular, the opening angles of the cone of internal and external conical refraction and the inclination of the optical axes with respect to the periodicity axis can be varied in a wide range. The results of this study may be useful for designing millimeter-wave, submillimeter-wave, and IR devices.     

Ground state of an antiferromagnet with uniform antisymmetric exchange in a magnetic field

A system of two nonlinear differential equations for sublattice angles is proposed to describe the spin orientation distribution in a planar antiferromagnet with uniform antisymmetric exchange in a magnetic field. This system involves the initial symmetry of the problem and is reduced to a single delay differential equation. The solutions of this system are parameterized by the initial condition imposed on the angle of one sublattice at the hyperbolic singular point of the phase space. The numerical analysis of the stability boundary of soliton solutions demonstrates that the transition to the commensurate phase takes place outside the region where the stochastic solutions appear and is accompanied by the magnetization jump Δm ∼ 10⁻¹ m.     

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提出碰撞吸收型等离子体的等效折射指数,重新计算了折射角,并与利用常规方法得到的折射角进行了比较。分析了等离子体密度和碰撞频率对折射角的影响。考察了斜入射电磁波在吸收型等离子体中的折射特性,并以一维非均匀碰撞吸收型等离子体为例,对斜入射电磁波的反射特性进行了考察。结果表明,等效折射指数的概念涵盖了入射角度的影响,在考察斜入射时更准确、便捷。     

A multisublattice magnetic phase induced by external field in a singlet magnet

The theory of magnetization in a longitudinal magnetic field is developed for an easy-plane multisublattice antiferromagnet with a singlet ground state and a strong single-ion anisotropy exceeding the magnitude of exchange interaction. The magnetic-field-induced phase transition from the singlet (magnetically dis-ordered) state to a multisublattice antiferromagnetic state represents a displacive magnetic phase transition. At T=0, this transition proceeds continuously and belongs to second-order phase transitions, while at T ≠0, the behavior changes to jumplike and the process becomes the first-order phase transition.     

Induced moment antiferromagnet in an external magnetic field—I: Molecular field theory

The behavior of a two-level induced moment antiferromagnet in an external magnetic field is investigated in the molecular field approximation. A significant reduction in the critical field and in the sublattice magnetizations is shown. However, the total magnetization rises more rapidly with field and can remain at large value in an external field even at T = 0. The magnetic susceptibility also remains finite at T = 0 in contrast to the case of a permanent moment Ising antiferromagnet. The effects of a ferromagnetic next-nearest neighbor interaction are then examined. It is shown that, in contrast to the usual Ising antiferromagnets, the ferromagnetic coupling has to exceed a certain value depending on the crystal field strength and the antiferromagnetic interaction, to allow for a first order phase transition in a field to occur even at zero temperature.