E. N. Khatsko

Magnetic properties of the singlet antiferromagnet KTb(WO4)2

X-ray studies of the magnet KTb(WO4)2 have shown that this compound belongs to the monoclinic class C2∕c with a chain structure. The susceptibility and magnetization of KTb(WO4)2 single crystals are investigated experimentally in the temperature region 0.4–100 K and at magnetic fields up to 2 T. The angular dependence of the susceptibility shows that at low temperatures a purely Ising anisotropy is realized in this compound: there exists but one component of the moment, along the b axis. The temperature dependence of the susceptibility has a maximum at T=1.2K, and the M(H) curve obtained at 0.5 K exhibits substantial nonlinearity at low fields, as is characteristic of an ordered state. Analysis of the experimental data in the mean field approximation suggests that this state is a singlet Ising antiferromagnet with TN≈0.7K, a value determined from the susceptibility data. At extremely low temperatures a collinear antiferromagnetic structure with equivalent sublattices is realized in the sample, according t...

Electron paramagnetic resonance study of the singlet magnet KTb(WO4)2

The electron paramagnetic resonance (EPR) spectrum of KTb(WO4)2 is investigated in the frequency range 14–120 GHz and magnetic field range 0–70 kOe at helium temperature. The observed triplet structure of the spectrum is interpreted as a manifestation of resonance in three-site clusters. On this basis a value of the g factor (g≈13.3) and an estimate of the gap (δ≈1 K) are obtained for the quasi-doublet ion Tb3+ in the crystalline field of KTb(WO4)2, and the parameters of the dipole (Id≈1.6 K) and exchange (Iex≈0.9 K) AFM interactions of the nearest neighbors in the chains are determined for the corresponding singlet magnet model. A first-order structural phase transition, induced by an external magnetic field lying in the basal plane of the crystal, is observed.

Phase magnetic segregation and magnetoresistive properties in the manganite nanocompound p-La0.8Mn1.04O3.5

Results of comprehensive, detailed studies of the magnetic and resistive properties of the nanocompound p-La0.8Mn1.04O3.5 at temperatures of 4.2–300 K in magnetic fields up to 9 T are reported. These studies of the temperature, field, and frequency dependences of the magnetization and susceptibility indicate that, below T* ≈ 265 K the nanocomposite is in a magnetic phase-segregated state with ferromagnetic inclusions that form a superparamagnet. A unique interrelation between the magnetic resistance and the magnetization is found. The effect of aging on the magnetic and resistive properties is studied.

The Volleben effect in magnetic superconductors Dy1–xYxRh4B4 (x = 0.2, 0.3, 0.4, and 0.6)

The Volleben effect (“paramagnetic” Meissner effect) was revealed for the first time while measuring the temperature dependence of magnetic moments of Dy1–xYxRh4B4 (x = 0.2, 0.3, 0.4, 0.6) in magnetic fields 1–20 Oe. The effect decreases with increasing magnetic field strength, and at fields below 1 Oe a diamagnetic signal appears. The ferromagnetism of Dy atoms supposedly plays an essential role in the appearance of paramagnetic signal at temperatures below the superconducting transition temperature.

Magnetic resonance studies of the low-dimensional magnet NaFe(WO4)2

Magnetic resonance studies of the low-dimensional monoclinic compound NaFe(WO4)2 are carried out in the frequency range 25–142 GHz and temperature range 1.8–300 K. The EPR data near the phase transition attest to the two-dimensionality of the magnetic structure of NaFe(WO4)2. The frequency-field relation of the AFMR spectrum shows that this compound is a biaxial antiferromagnet. The characteristic parameters of the AFMR energy spectrum are determined: the values of the energy gaps ν1=141 GHz and ν2=168.7 GHz, the anisotropy fields Ha1=10.5 kOe and Ha2=15 kOe, and the exchange field He=121 kOe. The ratio of the intralayer to the interlayer exchange is estimated. Additional absorption is observed due to local modes caused by destruction of the translational order of the magnetic structure.

Neutron scattering study of the layered Ising magnet CsDy(MoO4)2

The quasi-two-dimensional antiferromagnet CsDy(MoO4)2 is studied by neutron diffraction and quasielastic neutron scattering. The crystal structure of two low-temperature phases (below 120 K and below 40 K) is determined. An approximate structure of the magnetically ordered phase (TN=1.36 K) is proposed. In the ordered state the order-parameter critical exponent β=0.17(0.01), the in-plane correlation length exponent ν=0.94(0.07), and the staggered susceptibility critical index γ=1.01(0.04) were determined. Comparing these results to the exact solution for a 2D Ising magnet, we conclude that, although 2D behavior is apparent in CsDy(MoO4)2, there are deviations from the simple 2D Ising model.

Microwave absorption in the frustrated ferrimagnet Cu2OSeO3

The resonance properties of a new Cu2OSeO3 ferrimagnet have been investigated in a wide range of frequencies (17-142 GHz) at liquid helium temperature. The resonance data were used to plot the frequency-field curve of the ferromagnetic spectrum described in the model of an anisotropic two-sublattice ferrimagnet. The effective magnetic anisotropy corresponding to the gap in the spin wave spectrum was estimated (3 GHz). It was found that the spectrum has a multicomponent structure due to the diversity of the types of magnetization precession. As the amplitude of the high-frequency magnetic field increased, additional absorption was observed in an external magnetic field below the main resonance field. The addition absorption detected corresponds to a nonuniform nonlinear parametric resonance due to the nonuniformity of the magnetic structure in the ferrimagnetic crystal Cu2OSeO3.

Low-temperature magnetic properties of the monoclinic magnet RbDy(WO4)2

The temperature dependence of the magnetic susceptibility along the principal magnetic axes of rubidium–dysprosium tungstate RbDy(WO4)2 is measured in the temperature interval 0.5–300 K. Strong anisotropy of the susceptibility is observed at all temperatures in that range. At TN=0.8 K the susceptibility along all three magnetic axes exhibits a sharp anomaly due to the phase transition to a magnetically ordered state. At 0.5 K the measured field dependence of the magnetization along all three axes demonstrates a metamagnetic orientational phase transition in the directions of the magnetic axes y and z. Another type of anomaly of the magnetic susceptibility is observed at temperatures of around 5, 8, and 50 K. Analysis shows that these anomalies correspond to structural phase transitions in this compound.

Kinetics of magnetization processes in a quasi-one-dimensional Ising superantiferromagnet [{CH3}3NH]CoCl3 · 2H2O

The processes of magnetization reversal in a single crystal of CoTAC are investigated in the temperature range 4.2–0.5 K. The relaxation processes as a function of the magnetic field, temperature, and time are studied. It is shown that the results obtained are described within the model of interacting superparamagnetic formations—nanoclusters. The characteristic constants of interaction, the energy of barriers and the size of domains are determined.

Resonance properties of the quasi-one-dimensional Ising magnet [(CH3)3NH]CoCl3⋅2H2O in the paramagnetic and magnetically ordered phases

A study is made of the angular, frequency–field, and temperature dependences of the magnetic resonance of the quasi-one-dimensional Ising magnet [(CH3)3NH]CoCl3⋅2H2O in the paramagnetic phase. The experimental results obtained are explained in a model of spin-cluster resonance in a strongly exchange-coupled spin chain. The frequency–field dependences of the ferromagnetic resonance spectrum measured below the Neel temperature are studied for magnetic-field directions along the crystallographic axes a, b, and c. It is shown that for H→0 the spin-wave spectrum of this quasiferromagnet has two gaps (ν1=70.1 GHz and ν2=52.5 GHz).