Uniform and localized magnetic modes in the random mixture with competing spin anisotropies,Fe(1−x)CoxBr2

Electron spin resonance experiments at millimeter and submillimeter wavelengths have been performed on the randomly mixed antiferromagnet with competing spin anisotropies, Fe_(1?x)Co_xBr₂. Two antiferromagnetic resonance (AFMR) modes and extra resonances have been observed in the Co-rich concentration region. From the analysis of the AFMR, the anisotropy in the mixtures is shown to become small with concentration. This is a clear evidence for the competition of the anisotropies in these alloys. The extra modes are qualitatively explained as arising from a localized excitation of an Fe²⁺ spin.     

Metamagnetic transitions of Lu2Cu2O5 studied by submillimeter-wave ESR measurement

Lu₂Cu₂O₅ is a related substance of Y₂Cu₂O₅ which is an unusual copper oxide with two successive metamagnetic transitions. We have performed the antiferromagnetic resonance (AFMR) measurements of Lu₂Cu₂O₅ in the millimeter and submillimeter wave region with the pulsed magnetic field up to 16 T at 1.8 K. Our measurements this time are very detailed frequency dependence measurements compared to the previous measurement and we found new AFMR modes which were not observed previously. From the analysis of the observed AFMR modes, we found that the magnetic phase aboveH _cl = 3.1 T in Lu₂Cu₂O₅ is different from that in Y₂Cu₂O₅.     

Development and applications of high-frequency ESR up to 55 T

A multiextreme (high-field, low-temperature, high-pressure and nanoscale) electron spin resonance (ESR) measurement system is under development in Kobe. In this connection, our recent development is introduced and two applications of our high-frequency high-field ESR are described. High-frequency high-field ESR measurements of dioptase (Cu₆Si₆O₁₈·6H₂O), which has an interesting antiferromagnetic Cu²⁺ network, have been performed using a pulsed magnetic field of up to 55 T. Antiferromagnetic resonances (AFMR) are clearly observed at 4.2 K with the light sources of up to 1017 GHz. However, a deviation from the conventional two-sublattice AFMR theory is observed in the high field. Temperature dependence of the X-band and high-frequency ESR has been also observed in the triangular lattice antiferromagnet EtMe₃P[Pd(dmit)₂]₂ which shows the spin-Peierls-like transition below T _c = 25 K. The preliminary field dependence of the spin gap estimated from the analyses of our ESR results has been shown in connection with the previous magnetic susceptibility results.     

Magnetostatic modes in antiferromagnetic resonance

Measurements of the antiferromagnetic resonance (AFMR) spectra of MnF₂ spheres are reported, and the principal magnetostatic modes (MSMs) in these spectra are identified. To interpret the observed spectra it is necessary to take into account, in addition to the theoretical MSM solution, the effects of impurities, finite sample size (propagation effects), and radiation damping. Expressions for the predicted relative intensities of the modes are developed for particular choices of symmetry of the exciting rf field and compared to observations. The observed shifts and intensities are larger than predicted. This discrepancy is explained by analysis of the role of the magnetic susceptibility of a sample near resonance. New observations are reported which improve the understanding of radiation damping in AFMR.     

Investigation of submillimeter-wave spin-precession modes in Fe3BO6

AFMR in Fe₃ BO₆ is investigated by quasioptical method in submillimeter-wave band. Depending on conditions of excitation two resonance modes were observed. One mode, which is identified as a quasiferromagnetic, was investigated both in phases ₁ and ₁. Also the quasiantiferromagnetic AFMR mode was observed and investigated. The carried researches evidence in favour of two-sublattice Fe₃ BO₆ model.     

Antiferromagnetic resonance and dielectric properties of rare-earth ferroborates in the submillimeter frequency range

The magnetoresonance and dielectric properties of a number of crystals of a new family of multiferroics, namely, rare-earth ferroborates RFe₃(BO₃)₄ (R = Y, Eu, Pr, Tb, Tb_0.25Er_0.75), are studied in the sub-millimeter frequency range (ν = 3–20 cm⁻¹). Ferroborates with R = Y, Tb, and Eu exhibit permittivity jumps at temperatures of 375, 198, and 58 K, respectively, which are caused by the R32 → P3₁21 phase transition. Antiferromagnetic resonance (AFMR) modes in the subsystem of Fe³⁺ ions are detected in the range of anti-ferromagnetic ordering (T < T _N = 30–40 K) in all ferroborates that have either an easy-plane (Y, Eu) or easy-axis (Pr, Tb, Tb_0.25Er_0.75) magnetic structure. The AFMR frequencies are found to depend strongly on the magnetic anisotropy of a rare-earth ion and its exchange interaction with the Fe subsystem, which determine the type of magnetic structure and the sign and magnitude of an effective anisotropy constant. The basic parameters of the magnetic interactions in these ferroborates are found, and the magnetoelectric contribution to AFMR is analyzed.     

Antiferromagnetic resonances and magnetization of a canted antiferromagnet

A two-sublattice model has been applied to a canted antiferromagnet including both Dzyaloshinsky-Moria interaction and double-exchange. Static magnetization and frequencies of the antiferromagnetic resonances (AFMR) were calculated for static magnetic fields along the principal crystallographic axes of an orthorhombic crystal. The experimental data for La(0.95)Sr(0.05)MnO3, which include magnetization, positions and excitation conditions of the AFMR modes can be well accounted for using the model calculations.     

Antiferromagnetic resonance in Bi2CuO4

Magnetic resonance of the low-frequency spin-wave branch in the Bi₂CuO₄ antiferromagnet with an easy-plane anisotropy has been studied. Angular, frequency, and temperature dependences of the position and width of the antiferromagnetic resonance (AFMR) line have been measured. Our measurements combined with earlier data [H. Ochta, K. Yoshida, T. Matsuya, T. Nanba, M. Motokawa, K. Yamada, Y. Endon, and S. Hosoya, J. Phys. Soc. Jpn. 61, 2921 (1992); E. W. Ong, G. H. Kwei, R. A. Robinson, B. L. Ramakrishna, and R. B. von Dreele, Phys. Rev. B 42, 4255 (1990) [ have allowed us to determine anisotropy constants of this material and to account for the unusual character of its static susceptibility anisotropy. The AFMR line shifts to the high-field side and broadens in a temperature range of 10–15 K, and the cause of this has remained unclear. In the low-temperature range the line shows a hysteresis corresponding to a static field magnitude several times as large as the spin-flop field. The position and width of the AFMR line depend sensitively on the sample preparation technique. Zh. éksp. Teor. Fiz. 113, 2244–2255 (June 1998)     

Effect of dipolar interaction on the antiferromagnetic resonance spectra of NiO

We have investigated the antiferromagnetic (AF) resonance modes (AFMR) of NiO, theoretically using a model that includes the effects of exchange, dipolar coupling, and a small cubic anisotropy, and experimentally using Brillouin scattering. Using only superexchange between next nearest Ni atoms the model accounts for the observed AF structure with a [112] spin orientation. The model predicts that there are four, weakly coupled, AF lattices that should therefore exhibit eight AFMR modes. Because of degeneracies, only five distinct frequencies are predicted by the model. Three of these frequencies are consistent with the doublet observed by Raman scattering and the central peak reported in Brillouin experiments. Using Brillouin scattering we report the observation of the two missing modes.     

Investigation of iron borate on the submillimeter waves

Antiferromagnetic resonance (AFMR) in iron borate at the frequency of 300 GHz at the room temperature in weak (down to 0.1T) external magnetic fieldH has been studied. The loss in AFMR line increased or decreased depending on the direction and magnitude ofH. Such properties ofFeBO ₃ monocrystals allows to design undirectional and modulation devices.     

The Investigation of Plasmon-Longitudinal Optical Phonon Coupling and Surface Polariton Modes in Donor Doped GaAs-Al0.33Ga0.67As Multi Quantum Wells

Infrared Fourier transform spectroscopy has been used to investigate phonon, plasmon, surface polariton and plasma-longitudinal optical phonon coupling in highly donor doped multi quantum wells (GaAs/Al_0.33Ga_0.67As) and direct band gap n- type Al_XGa_1-XAs thin layer on GaAs substrate. Using different samples with different concentration of free carriers. The dispersion equation of coupling modes have been calculated by using the condition which the dielectric functions of samples are zero for longitudinal coupled modes and experimental papameters which have been obtained from the best fit p-polarized oblique incidence far infrared reflection spectra. In MQW samples, the free carriers confined to the well and carriers are quasi two dimensional. So, plasmon- LO phonon coupling occur in the well (GaAs). In n- type Al_XGa_1-XAs thin layer, the coupled modes consist of three branches of the high, intermediate and low frequency modes. Their frequencies depend on both concentration and alloy composition. To analyses the surface polariton modes we carry out attenuated total reflection (ATR) measurements. In order to support our assignment the magnetic field profiles and surface polariton dispersion curves have been calculated.     

Collective rearrangement of spin excitation spectrum of antiferromagnet with magnetic impurities in external magnetic field

Antiferromagnetic resonance (AFMR) in Co_1?cMn_cF₂ (c ～ 10^-4) in the external magnetic field is investigated. It was established that coherent and incoherent rearrangements (CR and ICR) of spectrum in the region of AFMR and impurity lines crossing are possible in the same sample depending on field orientation. Topology of the phase diagram of spin excitation (SE) spectrum in variables c-H is obtained.     

Magnetic ordering in Co<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>Mg<Subscript>1−<Emphasis Type="Italic">c</Emphasis></Subscript>O solid solutions

The influence of dilution by diamagnetic ions on the magnetic ordering in single-crystal Co _c Mg_1−c O solid solutions was studied by Raman spectroscopy and magneto-optical microscopy in a wide range of temperatures (6 < T < 200 K). Far infrared absorption measurements of antiferromagnetic resonance (AFMR) were also performed for pure CoO. It was found that the domain structure and the contribution from Brillouin zone center magnons to Raman scattering and AFMR disappear well below the Néel temperature, whose value was determined from neutron diffraction and magnetic susceptibility measurements. The text was submitted by the authors in English.     

Electron paramagnetic resonance experiments at high frequencies and low temperatures in one dimensional TMMC

We report the existence of collective excitations of spins in TMMC at high magnetic fields and low temperatures, but higher than T_N at zero magnetic field. The main features of the resonances can be attributed to those of AFMR. Three kinds of crystallographic domains obtained from twining are established. Two magnon excitations may have also been observed.     

Polarized Raman Study On LiNbO3

Polarized Raman was used to study the lattice vibrations in LiNbO₃ single crystals which temperature range from 291K to 11K. The 13 vibrational modes expected from group-theoretical analysis were obtained. The nine internal modes and four external modes of NbO₆ octahedra have been assigned for the first time.     

The infrared and laser Raman spectra of K2Zn(SO4)2 · 6H2O

The Raman spectra of the single crystal of K₂Zn(SO₄)₂·6H₂O belonging toC _2h ⁵ space group in the 40–1200 cm⁻¹ region in different scattering geometries and their spectra ofthe microcrystalline salt in the 1500-50 cm⁻¹ region have been reported. The dynamics of the crystal has been described in terms of 186 phonon modes under the unit cell approximation. The weak bands in the region 400–900 cm⁻¹ have been assigned to the libratory modes of H₂O molecules in contradiction to the assignments reported by Ananthanarayanan. The ambiguities existing in the literature about the assignments ofν ₂ ^c andν ₅ ^c modes of [Zn(H₂O)₆]²⁺ have also been removed. The translatory and libratory modes of different units of the crystal have been identified and assignments are made using farir and Raman data on various isomorphous tutton salts. It has been inferred that both SO ₄ ²⁻ tetrahedron and [Zn(H₂O)₆]²⁺ octahedron undergo linear as well as angular distortions from their free state symmetries in the crystal.     

Precession modes and resonance absorption linewidth in orthorhombic antiferromagnet Fe3BO6

Resonance absorption in Fe₃BO₆ has been investigated for the low-frequency AFMR branch as a function of magnetic RF-field component direction in the bc crystal plane. Anisotropic character of absorption is shown to be due to complex character of weak ferromagnetic moment precession in orthorhombic crystals. Relation between the true resonance line width, and that observed in the quasi-optical experiment is discussed.     

Infrared spectra of BeSO4.4H2O and its deuterated analogue in 4000–1200 cm−1 region

IR spectra of BeSO₄.4H₂O and its deuterated analogue at ∼300 K and ∼110 K are reported in the region 4000–1200 cm⁻¹ using thin film and nujol mull techniques. The observed bands have been assigned as the internal modes of the water and the overtones and combinations of various modes using the recently revised assignments of SO₄ ²⁻ and Be(aq)₄ fundamentals in the region 1200–250 cm⁻¹ (Srivastavaet al 1976). The splitting of the internal modes of water has been discussed in the light of the effects of deuteration and cooling and it is shown that all the water molecules in a unit cell are asymmetric but crystallographically equivalent.     

Inelastic neutron scattering and lattice dynamics of NaNbO<Subscript>3</Subscript> and Sr<Subscript>0.70</Subscript>Ca<Subscript>0.30</Subscript>TiO<Subscript>3</Subscript>

NaNbO₃ and (Sr,Ca)TiO₃ exhibit an unusual complex sequence of temperature- and pressure-driven structural phase transitions. We have carried out lattice dynamical studies to understand the phonon modes responsible for these phase transitions. Inelastic neutron scattering measurements using powder samples were carried out at the Dhruva reactor, which provide the phonon density of states. Lattice dynamical models have been developed for SrTiO₃ and CaTiO₃ which have been fruitfully employed to study the phonon spectra and vibrational properties of the solid solution (Sr,Ca)TiO₃.