Infrared spectra and lattice vibrations of the spin-chain compound LiCuVO 4

The infrared spectra of the one-dimensional antiferromagnet LiCuVO₄ are measured in the frequency range from 10 cm^-1 to 10 000 cm^-1 and at temperatures from 2 K to 300 K, for the electric field vector E of the radiation polarized either along the a- or along the b-crystallographic directions. For each polarization six infrared active phonon modes are observed in accordance with factor group analysis of the crystal structure of LiCuVO₄. The theoretical group analysis of the possible spinel low-symmetry phases is performed within the framework of Landau's theory of phase transitions. The parameters of several phonon lines show noticeable anomalies around 150 K where the magnetic correlations appear in the copper chains, which may indicate a finite interaction between the phonon and the magnon subsystems in LiCuVO₄. Received 19 February 2001 and Received in final form 26 June 2001     

Raman scattering from magnetic excitations in Cd1−xMnxTe semiconductor alloys

We have observed antisymmetric Raman scattering from low frequency magnetic excitations in Cd_1?xMn_xTe alloys with 0.1 ≤ × ≤ 0.7. Crystals with x = 0.65 and 0.7 exhibit a magnon line in the antiferromagnetic phase, centered at 10.5 and 12.0 cm^-1 respectively, at 5 K. The frequency and the intensity of these lines decrease markedly with increasing temperature, finally disappearing in the paramagnetic phase. Crystals with 0.4 ≤ × ≤ 0.6 exhibit a similar feature at 5 K over the frequency range 4.5 – 5.5 cm^-1. These are attributed to spin waves originating from antiferromagnetically ordered clusters within their spin-glass phase. All the alloys show a distinct inelastic wing in the paramagnetic phase, which may be related to the magnetic excitations within residual clusters of coupled Mn⁺⁺ spins.     

One-magnon light scattering and spin-reorientation transition in epitaxial BiFeO3 thin films

Raman scattering from one-magnon excitation has been observed for the first time in epitaxial BiFeO₃ thin films grown on (1 1 1) SrTiO₃ substrates. The intensities and the frequency of the magnon mode at 18.9 cm⁻¹ (M₁) showed a discrepancy at the characteristic temperatures of ∼140 and 200 K and the magnon mode at 27.9 cm⁻¹ (M₂) disappeared at ∼200 K suggesting spin-reorientation (SR) transition in the epitaxial BFO film. The dc susceptibility measurement showed a large discrepancy near these two temperatures evidently elucidating the spin-reorientation transition mechanism. The partial spectral weight of the magnon modes is believed to be transferred to the lowest phonon mode appearing at 72.8 cm⁻¹ and higher magnon mode M₂ disappearing near 200 K reveal magnon-phonon coupling near to SR transition.     

Temperature dependent splitting of the 59Co hyperfine field in GdCo2

We have studied the temperature dependence of the ⁵⁹Co NMR in the magnetically ordered state in GdCo₂ between 1.3 K and 335 K. A clearly resolved splitting in the ⁵⁹Co spectrum is observed to occur at 200 K. This splitting is interpreted as due to a change in easy axis from [100] to a detection which is not a major symmetry axis. Furthermore, a sharp change in the intensity ratio of the two clearly resolved lines in the spectrum occurs at about 300 K.     

Magnetic circular dichroism and Faraday rotation of YCrO3 in the R-lines region

The rich structure in magnetooptical spectra of ⁴A₂ → ²E transition in YCrO₃ was observed at liquid helium temperature. The magnetooptical lines are correlated with adsorption structure and attributed to magnon and phonon-assisted exciton transitions.     

Experimental determination of the spin-wave spectrum of the two-dimensional ferromagnet K2CuF4

K₂CuF₄ is a quasi-bidimensional ferromagnet with an easy magnetization plane. The Curie temperature is 6.25 K. We have measured the spin-wave spectrum from 2 to 14 K, and found an exchange integral of 11.4 K between the first neighbours in the basal plane. No measurable dispersion was found along the axis perpendicular to that plane. We have observed a shift in the energy of the magnons, and a broadening of the magnon peak both of which behave differently below and above the Curie temperature.     

Magnetic phase transitions in the 1D ising antiferromagnet CsCoCl3

We report on Raman scattering from the magnons in CsCoCl₃ with particular emphasis on two magnetic phase transitions occurring at T_N = 20.82 K and at 8 K. The T → 0 magnon spectrum is fitted by a $\text{S =}\text{1}\text{2}$ anisotropic Heisenberg model. With a simple cluster model we can explain the temperature behaviour of the magnon frequencies and intensities. The physics of the 8 K phase transition is also discussed.     

Temperature dependence of the fine structure of the C and E absorption bands in RbMnF3 below the Néel temperature

The variation in the parameters (width, position, intensity) of the fine structure lines in the C[⁶ A _1g → ⁴ A _1g , ⁴ E _g(⁴ G)] and E[⁶ A _1g → ⁴ E _g(⁴ D)] bands in RbMnF₃ with temperature is studied in the temperature range 10–70 K. In the C band, two narrow (<6 cm^?1) lines are are distinguished at distances of 77 and 80 cm^?1 from the exciton line at T = 10 K. The other lines in the C band and all lines in the E band are more than 20 cm^?1 wide. It is demonstrated that the narrow lines become allowed because of the spin-exchange interaction within a long-range magnetic order model and originate from the excitation of exciton-magnon bound states and that the other lines are made allowed by the exchange-vibronic mechanism within a short-range magnetic order model and originate from the excitation of bound states composed of an exciton, magnon, and oddparity phonon. The vibrational replicas of the main exciton-magnon-phonon lines are due to the quadratic vibronic interaction with odd-parity vibrations. Variations of the intensities and widths of the absorption lines with temperature indicate that these parameters are affected by relaxation and delocalization of the bound states.     

Magnon Raman scattering in the quasi 1-D antiferromagnet CsCoBr3

Raman scattering from magnons has been observed in the three magnetic phases of CsCoBr₃. In the 1-D Ising phase T > 28K a broad band at 96 cm^?1 is observed. This band grows in intensity but shows little renormalisation (100.5 cm^?1 at 14K) in the partially disordered antiferromagnetic phase 14K < T < 28K. For T < 14K additional structure at 111.5, 123.5, 133, and 141 cm^?1 is attributed to magnon-magnon combination bands. Two extra magnon branches are expected for this ferrimagnetic phase. One of these has an energy of ≈ 11 cm^?1.     

Spectre d'absorption et effet Zeeman de l'ion Mn2+ dans un sulfate acide de manganese et de potassium MnK4H2(SO4)4, 2H2O a basse temperature

We present aborption spectra of an oriented single crystal of MnK₄H₂(SO₄)₄, 2H₂O at temperatures between 20 and 1.6 K. This salt has orthorhombic symmetry and gives, both in the visible and ultraviolet regions, an absorption spectrum which changes considerably when the temperature decreases from 20 to 4.2 K. The spectrum has sharp lines at 1.6 K in the two absorption regions corresponding to teh excited levels ⁴A₁⁴E(⁴G) and ⁴T₂(⁴D). A study has been made of the Zeeman splitting of these sharp lines; a discontinuity has been observed in the Zeeman splitting as a function of magnetic field at 1.6 K. This discontinuity is a consequence of a magnetically-ordered phase.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Temperature dependence of the localized magnon mode in MnF2 : Co

The localized magnon mode in MnF₂ : Co was studied by far infrared resonance techniques. The halfwidth at 8 K is 1 cm^-1, about three times smaller than that obtained by other experimental methods. Therefore, accurate measurements of the temperature dependence are possible between 8 and 40 K. The temperature dependence of the effective g-factor is attributed to spin wave renormalization. The half width is magnetic field dependent at higher temperatures and can be approximated by a T² law.     

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 satellite bands in the optical spectrum of antiferromagnetic Rb<Subscript>2</Subscript>MnCl<Subscript>4</Subscript>

The evolution of optical absorption in a two-dimensional antiferromagnet is investigated in the range of the transition ⁶A_1g → ⁴A_1g, ⁴E _g (⁴G) observed in manganese ions in an external magnetic field inducing noncollinearity of the magnetic structure. It is revealed that hot and cold satellites of the exciton-magnon bands appear in the optical absorption spectrum and then increase in intensity. The shapes of the magnon satellite bands corresponding to a two-dimensional magnetic structure are calculated. It is demonstrated that magnons at the inner points of the Brillouin zone appreciably contribute to the absorption. The zero-point magnetic oscillations play a decisive role in the absorption associated with the magnon decay at low temperatures.     

Raman spectrum of strontium titanate

The Raman spectrum of strontium titanate has been recorded using λ 4358 of mercury as exciter. The observed spectrum consists of 7 Raman lines, one of which is of low frequency, as expected from the recent theory of Cochran. 6 of these Raman lines have been interpreted as the first order spectrum arising from a small deviation of the cubic strontium titanate from its idealized symmetry. It has been shown that one normal mode of SrTiO₃ neglected by J.T.Last, will be really active in infrared absorption in the region of 440 cm^?1 and that it has to be taken into account in the interpretation of the infrared spectra of titanates. The four vibrational modes of the unit cell of SrTiO₃ correspond to frequencies of 90, 335, 441 and 620 cm^?1 observed in Raman effect. The large width of the Raman lines and the additional lines at 256 cm^?1 and 726 cm^?1 have been attributed to a splitting of the longitudinal and transverse optical modes. With the observed frequencies it has been found possible to account for in a satisfactory manner the specific heat of SrTiO₃ in the range 54·84° K to 1800° K.     

Fine Structure of the Optical Spectrum and Multiparticle Excitations in Rb2MnCl4

Results of experimental investigations into the optical absorption spectrum of Rb₂Mn_xCd_1–x Cl₄ solid solutions corresponding to ⁶ A _1g ⁴ A _1g ⁴ E _g transitions in manganese ions are presented. The spectra were measured at variable temperatures, magnetic fields, and concentrations x. The magnetic phase transitions accompanying variations of these parameters cause considerable changes in the spectrum. The exciton and exciton-magnon bands, their cold and hot magnon analogs, and the phonon satellite bands have been detected. The specific features of the optical absorption spectrum caused by the low-dimensional magnetic order are elucidated.     

Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

Optical investigation of ErFeO3

The absorption spectrum of single crystals of ErFeO₃ has been investigated in the red and near infrared spectral region in the temperature range between 1.2 °K and 4.2 °K and at 20 °K and 77 °K. Between 77 °K and 4.2 °K a constant splitting of the absorption lines is observed. Below the Néel-temperature of the erbium sublattice at 4.5 °K the splitting of the absorption lines increases; the saturation value extrapolated to 0 °K of the splitting of the lowest crystal field level of the⁴ I _15/2 groundterm is (6.08±0.30) cm^?1. By measuring the Zeeman effect the groundstate magnetic moment is determined asμ=(6.6±0.2)μ _B. The measured temperature dependence of the splitting of the lowest crystalfield level of the⁴I_15/2 groundterm is compared with that calculated by a Monte Carlo method.     

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.     

Optic-mode coupling in ferroelectric gadolinium molybdate

Separate measurements of the A₁(TO) and A₁(LO) Raman spectra of ferroelectric gadolinium molybdate at 80°K and above have elucidated the origin of the anomalous temperature dependence of the two lowest frequency lines in the A₁(TO) spectrum. The observed behavior is postulated to be the result of coupling among modes at 44.5, 51.5, and 83 cm^?1 (at 80°K). The 44.5 and 83 cm^?1 modes become the degenerate, soft zone-boundary modes of the paraelectric phase while the 51.5 cm^?1 mode changes to B₂ symmetry. The two lowest frequency lines are the same as those observed previously in i.r. absorption.