Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

Magnon Raman scattering in CoBr2

The one-magnon Raman spectrum of CoBr₂ has been investigated as a function of temperature, and peak frequency, integrated intensity and width parameters obtained. The results obtained for the band energy at low temperature (22.2 ± 0.2 cm^-1 at 5.7.K) are in good agreement with AFMR and neutron scattering results. The one-magnon energy renormalises relatively slowly with increasing temperature and is about 15 cm^-1 at T_N = 19 K, whereas the integrated intensity approaches zero like the magnetization at T_N and the width diverges. A low intensity band at 26.8 ± 1 cm^-1 (7.6K) may be due to two-magnon scattering from spin waves along the c-axis.     

Polarisation dependent Raman study of single-crystal nickel oxide

The magnetic domain structure and Raman scattering have been studied in NiO single-crystals with three different (100), (110) and (111) orientations. Twin-domain structure was observed in NiO(100) and NiO(110) single-crystals using cross-polarized optical microscopy. We found that the ratio of the two-magnon (at 1500 cm⁻¹) to the two-phonon (2LO, at 1100 cm⁻¹) Raman bands intensity is sensitive in a particular way to the type of the twin-domain pattern.     

Antiferromagnetic fluctuations in water-intercalated YBa2Cu3O6.8

Raman scattering by phononic and electronic excitations, as well as the magnetic susceptibility of water-intercalated YBa₂Cu₃O_6.8, has been studied. Direct evidence of the incorporation of OH^?-groups into the structure of the crystal has been obtained. The observed changes in the phonon spectrum and the appearance of the spectrum of antiferromagnetic fluctuations with an increased exchange coupling constant imply the localization of carriers and suppression of superconductivity. Change in the resonance behavior of the two-magnon scattering of light indicates the transformation of the electronic structure in crystallites absorbing water.     

Two-magnon relaxation reversal in ferrite spheres

The reversal of two-magnon relaxation associated with linear scattering of oscillations of uniform magnetization precession from sample nonuniformities is studied theoretically and experimentally in ferrite spheres of yttrium iron garnet (YIG). Relaxation reversal is performed by parametric phase conjugation of dipole-exchange spin waves formed as a result of scattering of uniform precession from inhomogeneities. As a result of two-magnon backward scattering of dipole-exchange spin waves with a certain time delay, magnetization oscillations are renewed with an amplitude that could exceed the initial amplitude of uniform precession. The relaxation reversal is due to crystallographic anisotropy of the sample and is manifested most strongly when a YIG sphere is magnetized along the intermediate axis [110]. Experiments were carried out on YIG spheres of diameter 0.65–1.05 mm for a parallel pumping frequency ω _p /2π ≈ 9.4 GHz, which is about twice the uniform precession frequency. The maximal delay time for the restored signal of uniform precession was about 2 μs, while the maximal amplitude exceeded the initial uniform precession amplitude by a factor of about 5. The “latent” relaxation parameters of ferrites, e.g., the natural ferromagnetic resonance linewidth associated with many-particle processes and the linewidth associated with two-magnon scattering at bulk nonuniformities, are determined experimentally.     

Low frequency excitations in multiferroic MnWO(4)

Dynamic anomalies have been found in the magnetically ordered phases of multiferroic MnWO(4) using polarized Raman scattering. Strong phonon damping is observed for several B(g) modes within the ferroelectric phase and has been attributed to spin-phonon interactions. Moreover, a new low frequency excitation was detected near 33?cm(-1) that grows in intensity on cooling into the antiferromagnetic phases. It is argued that this signal is most probably due to a two-magnon process.     

Neutron inelastic scattering study of transverse spin fluctuations in CsNiF3: A soliton-only central peak

We have observed a quasi-elastic contribution to the spectrum of the transverse spin fluctuations S^perp;(Q, ω), perpendicular to an applied magnetic field in the easy plane of the one-dimensional ferromagnet CsNiF₃. According to the present theoretical understanding this contribution is due solely to soliton quasi-particles and it should not contain two-magnon scattering. The observed dependence on momentum transfer is as expected for soliton scattering with zero intensity at q_c = 0 rising through a maximum with increasing q_c.     

Two-magnon Raman scattering in a spin density wave antiferromagnet

We present the results for a model calculation of resonant two-magnon Raman scattering in a spin density wave (SDW) antiferromagnet. The resonant enhancement of the two-magnon intensity is obtained from a microscopic analysis of the photon-magnon coupling vertex. By combining magnon-magnon interactions with ‘triple resonance’ phenomena in the vertex function the resulting intensity line shape is found to closely resemble the measured two-magnon Raman signal in antiferromagnetic cuprates. Both, resonant and non-resonant Raman scattering are discussed for the SDW antiferromagnet and a comparison is made to the conventional Loudon-Fleury theory of two-magnon light scattering.     

Electrical transport in La1−x Ca x MnO3 thin films at low temperatures

We report here the low-temperature resistivity of the chemical solution deposited La_1−x Ca _x MnO₃ (x=0.2, 0.3 and 0.33) thin films on LaAlO₃ substrates. The films were post-annealed in atmosphere at 850°C. The low temperature resistivity data has been studied in order to understand the nature of low-temperature conduction processes. The data showed T ² dependence from 60 K to 120 K consistent with the single magnon scattering process. The deviation from this quadratic temperature dependence at low temperatures is attributed to the collapse of the minority spin band. The two-magnon and electron-phonon processes contribute to scattering of carriers in the temperature range above 120 K.     

Relation between the superconducting gap energy and the two-magnon raman peak energy in Bi(2)Sr(2)Ca(1-x)Y(x)Cu(2)O(8+delta)

The relation between the electronic excitation and the magnetic excitation for the superconductivity in Bi(2)Sr(2)Ca(1-x)Y(x)Cu(2)O(8+delta) was investigated by wide-energy Raman spectroscopy. In the underdoping region the B(1g) scattering intensity is depleted below the two-magnon peak energy due to the charge-spin interactions. The depleted region decreases according to the decrease of the two-magnon peak energy, as the carrier concentration increases. This two-magnon peak energy also determines the B(1g) superconducting gap energy as 2Delta approximately alphaPlanck's over 2piomega(two-magnon) approximately J(eff) (alpha = 0.34-0.41) from under to overdoping hole concentration.     

YIG和BCVIG中铁磁共振线宽各向异性的可能起因

由于磁弹耦合,应力等效为各向异性场并对磁化强度分布发生影响。借助这一机制阐明了机械抛光的YIG和BCVIG单晶球铁磁共振线宽的各向异性行为,指明机械抛光造成的表面损伤层引起的应力通过磁致伸缩的各向异性使弛豫时间各方向不同,即产生了线宽的各向异性。若损伤层主要由位错组成,按微磁学理论和双磁振子散射模型可得到位错密度、磁致伸缩、磁化强度等与线宽的关系。从测量到的线宽各向异性估算了3μm抛光的YIG的表面层位错密度为5.4×10¹⁰/cm²。该弛豫机制表明,进一步降低多晶小线宽材料的线宽,减小磁致伸缩系数可能是有效途径。 关键词：     

Raman scattering from phonons and magnons in antiferromagnetic Fe3BO6

Raman scattering by phonons and by magnon pairs has been observed in Fe₃BO₆. Of the predicted 60 Raman-active modes, 39 have been identified and classified according to their symmetries. The two-magnon band shows a strong decrease in intensity with increasing temperature, and almost vanishes close to T_N = 508 K. The origin of this effect is attributed to the existence of a nearly dispersionless magnon branch.     

Theory of two-magnon dissipation in disordered ferromagnets I. Relation to sample statistics

The theory of two-magnon dissipation in disordered ferromagnetic materials is developed. Fluctuations of one-ion anisotropy energy due to the disorder are assumed to be the source of the linewidth broadening (relaxation of magnons by two-magnon scattering). Ordinary two-magnon theory is reconsidered on the basis of the sample statistics. The main idea of it is to describe the macroscopic properties of materials by means of quantities, whose relative fluctuations over the ensemble of configurations are negligibly small for sufficiently large sample. The energies of magnons — modes with well defined wave vectork — and their lifetimes are shown to be such variables. Further, possible models for a two-component ferromagnet are considered in the approximation of high external field. The energies of magnons in mixed crystal are obtained as linear combinations of those in parent crystals. Finally, the dependence of magnon lifetimes on the ordering is separated from the dependence on the magnitude of anisotropy variations.     

Temperature-dependent light scattering from magnetic excitons in TlCoF3

Raman spectra of antiferromagnetic thallium cobaltous fluoride have been obtained with 4579A argon ion laser excitation at temperatures from 4°K to T_N = 94 ± 2°K. The features observed consist of six Co²⁺ excitons ranging in energy from 325 to 1070 cm^-1, at two-magnon peak with low-temperature energy of 315 cm^-1, and a one-magnon feature whose 4°K energy is 37 cm^-1. The energy and linewidth of the one-magnon scattering has been measured from 4°K to about 0.8 T_N; it is found that the magnon becomes critically damped at about 0.8 T_N, in good agreement with our previous observations on RbCoF₃. The Co²⁺ excitons observed at 325, 380, 410, 730 (weak), 960, and 1070 cm^-1 agree in energy quite well with the KCoF₃ levels calculated by Buyers, Holden et al. as 340, 400, 467, 767, 967 and 1050 cm^-1.     

On the two-magnon scattering of light and neutrons

The effect of magnon-magnon interaction on the two-magnon scattering of light and neutrons has been calculated at finite temperatures. Neglecting life-time effects the temperature dependence of the magnetization and that of the spin wave energy as well as the Bose functions for the magnon occupation number have been taken into account. In the low temperature region, where this spin wave approximation is valid, we have gotten a shift of the two-magnon peak toward smaller energies with increasing temperature, which scales with the temperature dependence of the magnon energy and a broadening of the peak as well. These features are in agreement with the result of optical experiments.     

One-dimensional orbital excitations in vanadium oxides

The d electron orbital is a hidden but important degree of freedom controlling novel properties of transition-metal oxides. A one-dimensional orbital system is especially intriguing due to its enhanced quantum fluctuation. We present a combined experimental and theoretical study on the Raman scattering spectra in perovskite oxides NdVO(3) and LaVO(3) to prove that the quasi-one-dimensional orbital chain described by fermionic pseudospinons bears orbital excitations exchanging occupied orbital states on the neighboring sites, termed a two-orbiton in analogy with two-magnon.     

Determination of zone-boundary magnon energy and damping in RbMnF3 by means of light scattering experiments

New experimental data of two-magnon Raman scattering on the cubic antiferromagnet RbMnF₃ are used to obtain information on the single spin dynamics in all the ordered phase. The comparisons with the existing second order theory and with the neutron scattering data shows that the light scattering gives correct values for energy and lifetime of the zone boundary magnetic excitations.     

Raman Spectroscopic Analysis of Perovskite-Structured Alkali Metal Fluorides Containing Nickel and Copper Ions

ABSTRACT

The mixed metal fluorides containing alkali metals have a range of important applications in optical and electronic devices. Raman spectrums of two such fluorides were examined. Raman spectrum of KCuF₃ at 300 K exhibited bands at 261, 295, 363, 468, 519, and 549 cm^?1, indicating site symmetry (orthorhombic) lower than the tetragonal symmetry as observed from the powder X-ray diffraction pattern. Cubic KNiF₃ showed bands at 410, 468, and 657 cm^?1. The first two bands were attributed to the second-order phonon scattering, and the band at 657 cm^?1 was assigned to two-magnon peak.     

Magnetic Raman optical activity in FeF2

Raman optical activity (ROA) of magnons and phonons in antiferromagnetic FeF₂ (T_N=78 K) has been studied as a function of temperature and in applied magnetic field. For exciting light incident along the rutile-structure c-axis, ROA is observed for magnons but not phonons. In zero field, the small anisotropy-induced splitting (0.09 cm⁻¹) of the two acoustic-magnon branches is observed by light scattering for the first time. The splitting in applied magnetic field is found to reduce with increasing temperature in accordance with theory. No ROA was detected for two-magnon excitations.     

Effect of ordering of mobile oxygen on spectra of two-magnon and electronic Raman scattering of light in YBa2Cu3O6+x crystals with different doping levels

The effect of ordering of mobile chain-site oxygen in YBa₂Cu₃O_6+x crystals at different doping levels x on the kinetics of the intensity change of the two-magnon line and the extended structureless electronic continuum in optical Raman spectra and on the superconducting transition temperature T _c , has been studied in detail. An increase in the chain-site oxygen content x leads to a higher contribution of free carriers to the electronic continuum in Raman spectra. The kinetics of the electronic continuum becomes slower with x, whereas the relaxation rate of the two-magnon scattering is a nonmonotonic function of the stoichiometric index. Computer simulations of the relaxation of nonequilibrium states using the Monte Carlo technique qualitatively describe the kinetics observed in experiments. Our results lead us to a conclusion about local inhomogeneities in the electronic and spin systems in CuO₂ planes with scales of several lattice constants. Zh. éksp. Teor. Fiz. 116, 684–703 (August 1999)