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.     

Antiferromagnetic-resonance spectrum in charge-ordered R<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganites (R=La,Pr, Tb): The effect of orbital and charge structures

A theoretical study is made into the effect of the crystal, orbital, and charge structures on the magnetic structure and spin-wave spectra and on the antiferromagnetic resonance (AFMR) for R_0.5Ca_0.5MnO₃ crystals of monoclinic structure. The model assumes fixed crystal, charge, and orbital structures and enables one to determine the orbitally dependent exchange interaction and single-ion anisotropy for R = La, Pr, Tb. A 16-sublattice weakly noncollinear magnetic CE-structure without a ferromagnetic component is obtained. The behavior of magnetic structure in an external magnetic field is simulated, and the values of fields of spin-flop-transition for different Rs are obtained. The law of spin-wave dispersion and the field dependence of the antiferromagnetic-resonance spectrum are calculated.     

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)     

Reappraisal of the published neutron diffraction data on amorphous ferromagnetic alloys

A reanalysis of the previously reported neutron diffraction data on glassy ferromagnetic alloys suggest that the spin-wave energies of some glasses renormalize in accordance with the predictions of the itinerant-electron model. This observation is shown to have a direct bearing on the discrepancy between the spin-wave stiffness coefficient values D_s (neutron diffraction) and D_m (magnetization) found in certain amorphous ferromagnets.     

The spin-wave resonance spectrum and the structure of cermet Fe - SiO films

The results of electron-microscopic investigation of the structure and spin-wave resonance of thin ferromagnetic Fe_100-x(SiO)_x films are presented in the paper. The correlation radii of fluctuations of the exchange interaction parameter are determined from the analysis of the experimental results of spin-wave resonance. These radii are compared with characteristic sizes of the structural clusters in the films.     

Magnetic excitations in ferromagnetic semiconductors,and their study by light scattering

The magnetic excitation spectrum of degenerate ferromagnetic semiconductors, described by the s-f interaction model, is investigated by calculating various Green functions which provide the dynamic response of the system. In addition to the usual acoustic spin-wave mode, there are also an optical spin-wave branch and the Stoner-like continuum of excitations. Some estimates are made for the first order light scattering from ferrmagnetic semiconductors at low temperatures T?T_c.     

Spin-wave theory and the field-dependent critical behaviour of the antiferromagnetic perpendicular susceptibility in (C2H5NH3)2 CuCl4 and CoBr2·6H2O

The spin-wave theory for the field-dependent bahaviour of the perpendicular susceptibility of intiferromagnets is tested with the aid of two widely different antiferromagnetic systems, (C₂H₅NH₃)₂ CuCl₄ and CoBr₂·6H₂O. The first consists of ferromagnetic layers that are very weakly coupled antiferromagnet. magnetically, the second is an example of the 2-dimensional XY antiferromagnet. Consequently, these compounds serve as limiting cases, both of which are found to be excellently described by spin-wave theory.Interestingly, the experiment on CoBr₂·6H₂O yields a verification of the existence of zero-point spin deviations in antiferromagnets with planar anisotropy, and a prediction for the reduction of the perpendicular susceptibility at T = 0 caused by these deviations.     

Collinear light scattering on dipole-exchange spin waves in inhomogeneous ferromagnetic films

A theory is developed for the collinear TE-TM scattering of optical waveguide modes on dipoleexchange spin waves in perpendicularly magnetized ferromagnetic films that are inhomogeneous across their thickness. It is found in homogeneous ferromagnetic films and in films with small deviations from homogeneity that the TE-TM scattering on higher spinwave modes is strongest when the synchronism conditions for the transverse phases and for the longitudinal and transverse wave vectors are satisfied. When the thickness of the planar optical waveguide does not match the thickness of the ferromagnetic film, the phase synchronism condition is violated with the resultant appearance of an oscillating type of dependence of the TE-TM scattering on the spin-wave mode number. The scattering of light on spin-wave modes in films with a magnetization gradient is investigated in the presence of turning points for the magnetostatic potential. It is found that the existence of a turning point in the region of the antinode for the optical modes leads to an increase in the scattering amplitude. The formation of inhomogeneous magnetooptical structures and superlattices based on (Lu,Y,Bi)₃(Fe,Ga)₅O₁₂ is discussed. Zh. Tekh. Fiz. 68, 78–84 (June 1998)     

Spin-pair correlations of the classical Heisenberg ferromagnet above and below the critical point: Results of self-consistent Monte Carlo calculations

The spin-pair correlation functions of the classical Heisenberg ferromagnet with simple cubic lattice have been calculated in the paramagnetic and ferromagnetic temperature range using the self-consistent Monte Carlo method. The results agree with high-temperature series expansions aboveT _c , for low temperatures with spin-wave theory. By two different approaches the divergence of the ferromagnetic homogeneous susceptibility in zero field throughout the ferromagnetic temperature range could be verified. The functional dependence of the static susceptibilityχ _T (k) upon the inverse correlation lengthκ ₁ is discussed above and belowT _c and a Fourier transform for the explicit dependence of the spin correlations upon correlation length belowT _c is given. According to these results the scaling assumptionv=v′ for the exponents of the correlation length in the critical region is consistent with a divergent ferromagnetic susceptibility.     

Spin-wave resonance in [Co x Ni1 − x ]N and [Co x P1 − x ]N gradient films

Gradient films of ferromagnetic 3d metals with prescribed magnetic potential profile along the film thickness are obtained. It is found that the spin-wave resonance spectrum in these films is characterized by anomalous dependences of resonance fields of spin-wave modes H _r on the mode number: H _r(n) ～ n, H _r(n) ～ n ^2/3.     

Magnetic-breakdown oscillations of the thermoelectric field in layered conductors

We consider the quasi-two-dimensional pseudo-spin-1/2 Kitaev–Heisenberg model proposed for A₂IrO₃ (A = Li, Na) compounds. The spin-wave excitation spectrum, the sublattice magnetization, and the transition temperatures are calculated in the random phase approximation for four different ordered phases observed in the parameter space of the model: antiferromagnetic, stripe, ferromagnetic, and zigzag phases. The Néel temperature and temperature dependence of the sublattice magnetization are compared with the experimental data on Na₂IrO₃.     

Spin-wave dispersion softening in the ferromagnetic Kondo lattice model for manganites

Spin dynamics is calculated in the ferromagnetic (FM) state of the generalized Kondo lattice model taking into account strong on-site correlations between e_g electrons and antiferromagnetic (AFM) exchange among t_2g spins. Our study suggests that competing FM double-exchange and AFM super-exchange interaction lead to a rather nontrivial spin-wave spectrum. While spin excitations have a conventional Dq² spectrum in the long-wavelength limit, there is a strong deviation from the spin-wave spectrum of the isotropic Heisenberg model close to the zone boundary. The relevance of our results to the experimental data are discussed.     

Intrinsic Localized Modes in Quantum Ferromagnetic XXZ Chains in an Oblique Magnetic Field

A semiclassical study of intrinsic localized spin-wave modes in a one-dimensional quantum ferromagnetic XXZ chain in an oblique magnetic field is presented in this paper. We quantize the model Hamiltonian by introducing the Dyson-Maleev transformation, and adopt the coherent state representation as the basic representation of the system. By means of the method of multiple scales combined with a quasidiscreteness approximation, the equation of motion for the coherent-state amplitude can be reduced to the standard nonlinear Schrödinger equation. It is found that, at the center of the Brillouin zone, when θ < θ _c a bright intrinsic localized spin-wave mode appears below the bottom of the magnon frequency band and when θ > θ _c a dark intrinsic localized spin-wave resonance mode can occur above the bottom of the magnon frequency band. In other words, the switch between the bright and dark intrinsic localized spin-wave modes can be controlled via varying the angle of the magnetic field. This result has potential applications in quantum information storage. In addition, we find that, at the boundary of the Brillouin zone, the system can only produce a dark intrinsic localized spin-wave mode, whose eigenfrequency is above the upper of the magnon frequency band.     

Spin-wave resonance spectrum of a ferromagnetic boundary layer with unsymmetrical boundary conditions

A study is presented on the effects of unsymmetrical (d₁ =–d₂) attachment of surface spins and regards the excitation spectrum for a conducting ferromagnetic layer with spin-system damping. The conductivity causes nonuniformity in the high-frequency field in the film, which alters the shape of the spin-wave spectrum.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 119–124, July, 1974.     

Ferromagnetic resonance linewidth in exchange-biased ferromagnet/antiferromagnet bilayers with ac field-cooling procedure

Out-of-plane ferromagnetic resonance spectra of Co₂₀Ni₈₀/FeMn bilayers with a ground state consisting of stripe ferromagnetic domains as accommodated by ac field-cooling process were measured and analyzed by the Landau–Lifshitz–Gilbert equation. One weak resonance peak that might be the surface spin-wave was observed beside the main resonance peak when the applied field was perpendicular to the film plane. In order to explain the linewidth, the extrinsic magnetic relaxation must be considered, in addition to the intrinsic Gilbert damping effect and the broadening induced by magnetic inhomogeneity. The characteristic of the extrinsic magnetic relaxation is consistent with the two-magnon scattering model. PACS 75.70.-i; 75.70.Cn; 76.50.+g; 75.30.Gw     

Schottky-like anomaly in the low-temperature specific heat of single-crystal NdMnO3

The specific heat of single-crystal NdMnO₃ was investigated from 2 to 20 K under different magnetic fields up to 8 T. All the specific heat data show a Schottky-like anomaly, which becomes more indistinctive as increasing magnetic field. The experiment data were successfully fitted by taking into account factors such as crystal-field splitting, the two-level Schottky anomaly, the lattice vibration, and type-A antiferromagnetic (A-AF) spin waves. It was found that the splitting of the ground state doublet of Nd³⁺ ion increases linearly with magnetic field. The above phenomena can be interpreted in terms of the model of unchanged effective molecular field at Nd³⁺ site caused by the ferromagnetic component of A-AF structure of Mn spins. This ferromagnetic component is likely caused by the GdFeO₃-type octahedron rotation. In addition, it was also found that the magnetic field increases the spin-wave stiffness coefficient, but reduces the Debye temperature.     

Photoacoustic detection of ferro and paramagnetic resonance

A commercial ESR spectrometer was modified to detect the photoacoustic signal of modulated magnetic resonance absorption, operating at room temperature and at atmospheric pressure. Both magnetically ordered materials (YIG and FeBO₃) and paramagnetic substances were investigated. The measurements include various phenomena of ferromagnetic resonance: uniform precession, magnetostatic modes and spin-wave instabilities. Paramagnetic resonance was observed in the concentrated system MnF₂ and in the diluted system ZnS : Mn (1%), with well resolved hyperfine splitting.     

Ferromagnetic phases in spin-Fermion systems

Spin-Fermion systems which obtain their magnetic properties from a system of localized magnetic moments being coupled to conducting electrons are considered. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 Fermi operators of itinerant electrons. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferromagnetic phases in the system: low temperature phase 0 < T < T*, where all electrons contribute the ordered ferromagnetic moment, and high temperature phase T* < T < T _C , where only localized spins form magnetic moment. The magnetization as a function of temperature is calculated. The theoretical predictions are utilize to interpret the experimentally measured magnetization-temperature curves of UGe₂.     

The magnetization of GdAl2

The magnetization of a single crystal of GdAl₂ has been measured parallel to the easy direction as a function of field (maximum field 1.7 T) within the temperature range 4.2–300 K. The main emphasis was placed on the results obtained for the ferromagnetic phase. From an analysis based on molecular field theory it is deduced that the magnetic moment at 0 K is 7.2 μ_B per Gd ion and that the molecular field cannot be represented by a simpler polynomial than λ₁M + λ₂M³ + λ₃M⁵. The same data is analysed using spin-wave theory from which it is deduced that the spin-wave stiffness is 18 meV Ų and that the conduction band susceptibility is approximately 2.6 x 10^-6 emu g^-1. The conduction electron polarization, parallel to the Gd ion moment, amounting to 0.2 μ_B per Gd ion implies the presence of an internal field acting on the conduction electrons of approximately 200 T at 0 K.