Acoustic excitation of nuclear spin waves in the easy-plane antiferromagnetic material KMnF3

Ultrasound damping at T=4.2 K in single crystal easy-plane antiferromagnetic KMnF₃ is studied experimentally as a function of the magnitude and direction of a constant magnetic field H at frequencies of 640–670 MHz, corresponding to the frequencies of nuclear spin waves. Two experimental situations are examined: in the first, the vector H lies in the easy magnetization plane (001), and in the second, H forms an angle with (001). For longitudinal ultrasound waves propagating along the hard magnetization axis [001], it is found that the damping depends resonantly on the magnitude of the field H. In the first case a single damping maximum is observed, and in the second, two damping peaks that are well resolved with respect to the field. The angular dependence of the resonance damping signals on the direction of the constant magnetic field is found to have a 90° periodicity in all cases. The observed effects are explained by resonant ultrasonic excitation of nuclear spin waves. On the basis of an analysis of the magnetoacoustic interaction energy, it is shown that in the first case, nonzero oscillations of the antiferromagnetism vector L occur only in the basal plane, while in the second, oscillations of L occur both in the basal and a vertical plane, which are associated, respectively, with two branches of the nuclear spin waves. It is also shown that the 90° periodicity in the angular dependence of the damping signals is associated with a fourth order [001] axis. Zh. éksp. Teor. Fiz. 112, 1830–1840 (November 1997)     

Giant magnetoacoustic effect in KMnF3 due to nuclear spin waves

An explanation is proposed for the gigantic magnetoacoustic effect that we observed in KMnF₃ in previous work {Kh. G. Bogdanova, V. A. Golenishchev-Kutuzov, M. I. Kurkin et al., Zh. éksp. Teor. Fiz. 112, 1830 (1997) [JETP 85, 1001 (1997)]}. The effect entails a tenfold amplitude reduction of an acoustic pulse in a magnetic field that varies over the range 0–8 kOe. It is shown that this effect is due to the interference of two nuclear magnetoelastic waves propagating in the sample under magnetoacoustic resonance conditions, if this resonance occurs in the region of strong spatial dispersion of nuclear spin waves. The effect is said to be gigantic because it exceeds in magnitude the magnetoacoustic effects observed previously in magnetically ordered materials even though it is due to nuclear magnetism, which is 10⁵ times weaker than electronic magnetism. We observe a concomitant anomalous dependence of the dispersion of the velocity of sound on the external magnetic field. Zh. éksp. Teor. Fiz. 115, 1727–1739 (May 1999)     

Ferromagnetic resonance and bistability field in a uniaxial magnetic film

Peculiarities of ferromagnetic resonance (FMR) corresponding to bias along the “hard” magnetic axis of a film with 2D uniaxial anisotropy are studied based on numerical solution of magnetic moment dynamics equations. It is shown that an additional resonance peak is formed in the FMR spectrum in the vicinity of “bistability field” H _b . The dependence of this field on the amplitude of the microwave field and damping parameters is analyzed.     

Antiferromagnetic conical refraction of elastic waves in hematite

Conical refraction, which is due to the renormalization of the elastic moduli by the effective magnetoelastic interaction and depends on the static magnetic field, has been experimentally observed in an α-Fe₂O₃ trigonal easy-plane antiferromagnet in addition to the usual internal conical refraction of transverse elastic waves propagating along the trigonal C ₃ axis. It has been shown that the deviation angle θ_η of the energy flux from the C ₃ axis at the internal conical refraction is independent of the magnetic field applied in the basal plane (H ⊥ C ₃) and is a constant determined by the ratio of the elastic moduli C ₁₄ and C ₄₄. The deviation angle of the energy flux at the antiferromagnetic conical refraction increases with the magnetic field and approaches the value θ_η at large H values. The results are well described by the theory of this phenomenon developed by E.A. Turov and confirm its basic conclusions.     

On the nature of the Faraday effect in the rare-earth ortho-aluminate TbAlO3

The magnetic susceptibility χ, the absorption and luminescence spectra, and the Verde constant V along different crystallographic axes of the rhombic crystal TbAlO₃ have been investigated experimentally in the temperature interval 78–300 K. It is found that, in contrast to the magnetic susceptibility χ measured along the [110] axis, the Verde constant V varies in inverse proportion to the temperature in the given range. It is shown that this temperature dependence of the Verde constant measured along the [110] axis is connected with the absence of a contribution to the Faraday rotation of the van Vleck mechanism (in an external field H) of “mixing” of the states of the rare-earth ion Tb³⁺. From a comparison of the optical and magnetic measurements, we have determined the wave functions and magnitudes of the Stark intervals between the lower Stark sublevels for the ⁷ F ₆ ground multiplet of the Tb³⁺ ion in the ortho-aluminate TbAlO₃. Fiz. Tverd. Tela (St. Petersburg) 41, 2047–2052 (November 1999)     

Nuclear magnetic resonance in noncollinear antiferromagnet Mn3Al2Ge3O12

The ⁵⁵Mn nuclear magnetic resonance spectrum of noncollinear 12-sublattice antiferromagnet Mn₃Al₂Ge₃O₁₂ has been studied in the frequency range of 200–640 MHz in the external magnetic field H ‖ [001] at T = 1.2 K. Three absorption lines have been observed in fields less than the field of the reorientation transition H _c at the polarization h ‖ H of the rf field. Two lines have been observed at H > H _c and h ⊥ H. The spectral parameters indicate that the magnetic structure of manganese garnet differs slightly from the exchange triangular 120-degree structure. The anisotropy of the spin reduction and (or) weak antiferromagnetism that are allowed by the crystal symmetry lead to the difference of ≈3% in the magnetization of sublattices in the field H < H _c. When the spin plane rotates from the orientation perpendicular to the C ₃ axis to the orientation perpendicular to the C ₄ axis, all magnetic moments of the electronic subsystem decrease by ≈2% from the average value in the zero field.     

Magnetic tunneling told to ignorants by two ignorants

Magnetic tunneling of a large spin subject to the Hamiltonian is investigated by elementary methods for weak fields H. In zero field (H=0) the tunnel frequency in the ground state is found to be equal to [0pt] multiplied by a quantity whose variation with s is slower than exponential. This result coincides with that of path integral methods [#!schi!#]. For the values of the longitudinal field which allow tunneling, the tunnel frequency is shown to vanish when H _y =0 for certain “diabolic" values of [0pt] , in qualitative agreement with experiments by Wernsdorfer and Sessoli. The particular case H _z =0 was already obtained by Garg by means of path integrals. The diabolic values of are in agreement with numerical results but, as already noticed by Wernsdorfer and Sessoli, they disagree with the experimental ones. This may be an effect of higher order anisotropy terms, which is briefly addressed in the conclusion. Received 17 May 2000     

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.     

Solution of the problem of catastrophic relaxation of homogeneous spin precession in superfluid 3He-B

The quantitative analysis of the “catastrophic relaxation” of the coherent spin precession in ³He-B is presented. This phenomenon has been observed below a temperature of about 0.5 T _c as an abrupt shortening of the induction signal decay. It is explained in terms of the decay instability of the homogeneous transverse NMR mode into spin waves of the longitudinal NMR. Recently, the cross interaction amplitude between the two modes has been calculated by Sourovtsev and Fomin [9] for the so-called Brinkman-Smith configuration, i.e., for the orientation of the orbital momentum of Cooper pairs along the magnetic field, L ‖ H. In their treatment, the interaction is caused by the anisotropy of the speed of the spin waves. We found that, in the more general case of the nonparallel orientation of L corresponding to the typical conditions of the experiment, the spin-orbital interaction provides the additional interaction between the modes. By analyzing the experimental data, we are able to distinguish which contribution is dominating in different regimes. The text was submitted by the authors in English.     

On switching behavior of ferrite substrate with magnetostatic and spin wave exchange term

Due to the generation of magnetostatic and spin waves, switching phenomena of ferrite substrate with a normal magnetic biasing field is presented. Generally below the X-band of microwave range, the Pozar’s quasi TEM waves (extraordinary waves) were studied, but for the study of X-band there should be an inclusion of spin wave exchange term (ωr) in the magnetostatic wave analysis which depends upon the static internal field (Hex). This term is included in analysis because the wavelength of microwave approach is the inter-atomic distance of ferrite material. In this work we synthesize LiTi ferrite through Solid State Reaction Technique (SSRT) and obtained electric and magnetic properties for the analysis. Absorbing and transmission power coefficients have been calculated to obtain the power loss and transmitted power through the substrate, respectively. The absorbing power coefficient verifies the switching behavior of substrate for certain range of applied external magnetic field (H _o) which depends on the resonance line width parameter (ΔH) of ferrite material.     

NMR in 55Mn2+ nuclei in the quasi-one-dimensional antiferromagnetic CsMnBr3

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnetic CsMnBr₃, which has trigonal spin lattice, is investigated in detail. The measurements were performed on a wide-band NMR decimeter microwave-band spectrometer over a wide range of magnetic fields at temperatures 1.3–4.2 K. All three branches of the NMR spectrum previously found by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagnetic resonance spectrum. The experimental results in fields up to 40 kOe are described satisfactorily by an equation obtained by Zaliznyak et al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work that agree very well with experiment at all fields up to the “collapse” field H _c of all sublattices. The unbiased NMR frequency in CsMnBr₃ is determined to be v _n0=416 MHz (T=1.3 K) in zero external magnetic field, and in this way the reduction in the spontaneous moment due to the quasi-one-dimensional nature of the system of Mn²⁺ spins, which according to our data amounts to 28%, is determined more accurately. The field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, confirming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (1988)]. The results on the field dependence of the width and intensities of the NMR lines are discussed, along with three observed anomalies: 1) a strong increase in the NMR frequency for nuclei in sublattices that are perpendicular to the magnetic field; 2) the nonmonotonic temperature dependence of the resonance field for the lower branch of the spectrum; 3) the presence of two branches of the NMR spectrum in large H _c fields, in which the CsMnBr₃ must be a quasi-one-dimensional antiferromagnetic. Zh. éksp. Teor. Fiz. 113, 352–368 (January 1998) Deceased.     

Properties of phase transitions in easy-axis tetragonal antiferromagnets

Properties of phase transitions in a magnetic field H parallel to the easy axis have been investigated, and it has been shown, in particular, that the nature of the transition of the magnetic subsystem from the antiferromagnetic phase to the angular phase depends on the “sign” of the Dzyaloshinskii interaction. The conditions for orientation of the antiferromagnetism vector l in the basis plane in fields larger than the threshold field have been determined. It is shown that the transition from the angular phase to the state where the resulting magnetic moment m is parallel to the easy axis takes place in the field corresponding to a spin-flip transition. From an analysis of the configuration of the magnetic subsystem for arbitrary orientation of the external magnetic field, it follows that the values of the critical angle (ψ _cr ) for which a first-order phase transition takes place satisfy the condition in the case in which the anisotropy constant f in the basis plane is of the order of the first anisotropy constant b. Usually so that the tricritical point in the phase diagram H _y ,H _z satisfies the condition H _y ∼H _z . Fiz. Tverd. Tela (St. Petersburg) 41, 2044–2046 (November 1999)     

Effect of mechanical stresses on the magnetoelastic properties of TmVO4

In this paper the influence of mechanical stress on magnetoelastic properties, i.e., magnetostriction and thermal expansion in the neighborhood of a structural phase transition of the Jahn-Teller crystal TmVO₄ is investigated experimentally and theoretically. It is shown that the magnetoelastic properties of TmVO₄ for a magnetic field H∥[001] do not change the domain structure of the sample, which is rather well described when mechanical stresses in the crystal are taken into account using the parameter . Conversely, for magnetic fields along the direction of spontaneous strain [110] the magnetoelastic properties are primarily caused by reorientation of the Jahn-Teller domains and short-range order effects. It is shown that the “true” magnetostriction of a single-domain crystal for H∥[110] diverges at the phase transition point T _c=2.15 K in the absence of mechanical stresses and is strongly decreased by these stresses. Fiz. Tverd. Tela (St. Petersburg) 40, 701–705 (April 1998)     

Quantum magnets with large single-ion easy-plane anisotropy in magnetic field

We propose a theory describing low-temperature properties of magnets with integer spin and large single-ion easy-plane anisotropy D in magnetic field H directed parallel to the hard axis. Considering the exchange interaction between spins as a perturbation and using the bosonic spin representation proposed in our recent paper [1] we find thermal corrections to the elementary excitation spectrum, magnetization and specific heat in the vicinity of the quantum critical point (QCP) H = H _c1(0) ∼ D in the first nonvanishing orders of the perturbation theory. An expression is found for the boundary of the paramagnetic phase H _c1(T) in the H-T plane. The effective interaction between bosons is derived near the QCP. The proposed theory describes well experimental data obtained in NiCl₂-4SC(NH₂)₂ (DTN).     

Strong coupling in the Kondo problem in the low-temperature region

The magnetic field dependence of the average spin of a localized electron coupled to conduction electrons with an antiferromangetic exchange interaction is found for the ground state. In the magnetic field range μH∼0.5T _c (T _c is the Kondo temperature) there is an inflection point, and in the strong magnetic field range μH≫T _c , the correction to the average spin is proportional to (T _c /μ H)². In zero magnetic field, the interaction with conduction electrons also leads to the splitting of doubly degenerate spin impurity states. Zh. éksp. Teor. Fiz. 115, 1263–1284 (April 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Nonlinear doppler-shifted cyclotron resonance in aluminum

The Doppler-shifted cyclotron resonance in an aluminum plate in the geometry when constant magnetic field H is directed along the [100] crystallographic axis oriented normally to the surface of the plate is studied theoretically. The analysis is performed for a simple model Fermi surface possessing fourth-order symmetry. Capture of holes by the magnetic field of a radio-frequency wave is shown to considerably decrease the effectiveness of cyclotron absorption at large exciting field amplitudes. This suppresses the collisionless damping of dopplerons (propagating modes related to odd cyclotron resonance harmonics). As a result, the sample becomes more transparent to radio-frequency radiation.     

Spin solitons and waves in chiral molecular ferrimagnets

Resonance modes corresponding to a spin-soliton resonance have been found in the electron spin resonance spectra of [Cr(CN)₆][Mn(S)-pnH-(H₂O)]H₂O two-dimensional (2D) chiral single crystals and [Mn{(R/S)-pn}]₂[Mn{(R/S)-pn}₂(H₂O)][Cr(CN)₆]₂ chiral single crystals with a 3D magnetic order. It is also established that the chiral crystals of both types exhibit a spin-wave resonance analogous to the excitation of standing spin waves in thin magnetic films. At the same time, racemic crystals of the first type do not exhibit spin-soliton resonance. The entire body of experimental data indicates that the chirality of crystals influences the spin excitations (standing spin waves and solitons) in these media.     

Magnetization dynamics below of EuO and EuS

Between 4.2 K and the Curie temperatures of the cubic Heisenberg ferromagnets EuS and EuO, their homogeneous dynamic susceptibilities have been investigated by means of a broad-band reflectometer operating from 0.1 GHz to 40 GHz. For internal magnetic fields larger than the anisotropy fields H A ( T ) of both materials, their static susceptibilities exhibit a -divergence, which reveals quantitatively the dominance of dipolar-anisotropic spin-wave fluctuations. displays a Lorentzian shape the damping frequency of which obeys scaling in terms of .The scaling function agrees quantitatively with work by Frey and Schwabl [#!FS88!#] for dipolar Heisenberg ferromagnets at temperatures above T_c. Building upon their approach, the resonance frequency of the Lorentzian can be related to a memory effect in the damping determined by the large value of the relaxation rate of the longitudinal magnetization fluctuations . For EuS, this relation is substantiated directly by inelastic neutron scattering. All these features reveal the hitherto uncovered importance of the dipolar anisotropic fluctuations below T_c of ferromagnets. Received: 4 March 1998 / Accepted: 12 May 1998     

Splitting of NMR signals in parallel fields in the easy-plane antiferromagnet FeBO3

Experimental observations are reported of the splitting of NMR lines of ⁵⁷Fe into two absorption peaks in a static magnetic field H ₀ parallel to a variable field H ₁ in the basis plane. The field dependence of the intensity and the variation in the resonance frequencies of the absorption peaks with H ₀ are studied. These results can be used to explain some features of the layered domain structure of iron borate. Fiz. Tverd. Tela (St. Petersburg) 41, 290–292 (February 1999)