A presentation of experimental study results obtained by antiferromagnetic resonance (AFMR) of the high-frequency properties of the multiferroic Nd0.75Ho0.25Fe3(BO3)4 in a broad range of temperatures and frequencies. We studied the effect of substituting the Nd3+ ions with Ho3+, on the resonance properties of the Nd0.75Ho0.25Fe3(BO3)4 solid solution. In addition, we investigated the particularities of magnetic-field induced spin-reorientation phase transitions for H||c and H||a directions, in which the anisotropy of the magnetic system is measured from “easy-axis” anisotropy to “easy-plane.” The AFMR modes of a Fe3+ subsystem are revealed. New information about the most important characteristics of AFM is obtained, including: frequency-field dependence of the AFMR spectrum, gaps in the spin-wave spectrum, effective magnetic anisotropy values, and fields of spin-reorientation transitions. It is shown that “easy-plane” anisotropy prevails in this magnet, with a weak anisotropy in the basal plane. For the first time there are observed features in the AFMR spectra that could be tied to the presence of a spatially modulated spin structure (incommensurate phase) in AFM Nd0.75Ho0.25Fe3(BO3)4.

Magnetic ordering temperature, initial splitting and effective g-factor of the ground quasi-doublet of a Tb3+ ion were determined by investigating the heat capacity and ESR in a TbAl3(BO3)4 single crystal. The parameters of the magnetic interaction were calculated.

The effect of replacing Nd3+ ions with Dy3+ ions on the resonance properties of Nd0.75Dy0.25Fe3(BO3)4 is studied. The antiferromagnetic resonance (AFMR) method is used to detect an easy axis-easy plane magnetic spin-reorientation phase transition owing to competing exchange interactions of Nd–Fe and Dy–Fe, and the frequency-field dependence of the AFMR spectrum before and after the phase transition is studied. At 4.2 K, the detected resonance spectra in the H||c direction are AFMR modes of iron, and their frequency-field dependences correspond to easy-axis (H < 15 kOe) and easy-plane (H > 16 kOe) magnetic structures with magnon excitation energy gaps of 77.2 and 100.3 GHz, respectively. The effective magnetic anisotropies responsible for these gaps are 0.7 and 1.2 kOe, respectively. Substitution by Dy3+ ions in crystalline Nd0.75Dy0.25Fe3(BO3)4 in the paramagnetic region produces an additional static internal field owing to polarization, which leads to a shift in the resonance field of the EPR absorption line and a change in the g-factor of the Fe3+ ions.

The magnetic resonance and field dependence of magnetization were studied in a single crystal of TbFe3(BO3)4 at temperatures from 2 to 13 K and frequencies from 18 to 142 GHz. Two pairs of lines with different intensities were found in the EPR spectrum. The found lines can be assigned to two types of centers: the Tb3+ ions which neighbor with the Bi and Mo growth impurities. The initial splitting of the lowest quasi-doublet of such Tb3+ ions by the crystal field and exchange field acting on the rare-earth ions from the iron sublattices were determined. The amount of these centers was estimated.

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.

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.

The results of detailed investigations of magnetic properties of Pr0.4Bi0.3Ca0.3MnO3 manganites at low temperatures are presented. The performed investigations of temperature, field and frequency dependences of magnetization and susceptibility as well as the magnetization relaxation processes indicate that below TC = 44 K the manganite Pr0.4Bi0.3Ca0.3MnO3 is in the magnetic phase-segregated state with ferromagnetic inclusions forming a cluster glass.

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 results of measurements of thermal properties (specific heat) of potassium holmium double tungstate KHo(WO4)(2) as a function of temperature (from 0.5 to 300 K) and magnetic field (up to 2T) are presented. The total specific heat without the phonon and Schottky contributions is found to have the anomaly with maximum at T-SPT similar to 5K. This anomaly is likely related with the structural phase transition (SPT) caused by the cooperative Jahn-Teller effect. The increase of specific heat at very low temperatures and its shift towards high temperatures with increasing magnetic field are observed. The origin of this behaviour can be connected with possible magnetic phase transition induced by magnetic field.

The potassium holmium double tungstate was prepared by using top seeded solution growth technique. Structural investigations have been performed at room temperature. The KHo(WO(4))(2) single crystal belongs to the monoclinic space group C2/c with the unit-cell parameters: a = 10.624(2) angstrom, b = 10.352(2) angstrom, c = 7.5434(15) angstrom, beta = 130.78(3)degrees, and Z = 4. The atomic coordinates, isotropic and anisotropic displacement parameters and interionic distances for the studied structure were determined.

The AFMR spectra of the NdFe3(BO3)4 crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of its magnetic anisotropy the compound is an “easy-plane” antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields Ha1=1.14 kOe and Ha2=60 kOe and magnetic excitation gaps Δν1=101.9 GHz and Δν2=23.8 GHz. It is shown that commensurate–incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width. At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.

The low-frequency ferromagnetic resonance (FMR) spectra of nanocomposite manganites p-La0.78Mn0.99O3.5 and p-La0.80Mn1.04O3.5 with the cubic perovskite structure and hole conductivity were studied. The samples were prepared under pressure of 7.5 GPa in oxygen atmosphere. It was found that the temperature dependence of FMR parameters in the vicinity of the Curie point is similar to that of magnetization. The characteristic time of decay of metastable magnetic phase above T* (metal-semiconductor transition) and the time of its formation below T* were determined.

Resonance studies of single crystals of the molecular magnet {Cu6[(MeSiO2)6]2}⋅6DMF have been performed in a wide range of frequencies 18–142GHz and magnetic fields 0–7.5T at liquid-helium temperature. Two nonequivalent magnetic centers of copper nanoclusters with turn angle of the local axes (50±2)° have been found. The ground state of each magnetic center (magnetic molecules, containing a ring of six ferromagnetic interacting ions Cu2+ (S=1∕2)) can be represented as a system of energy levels with effective particle spin S=3 (ĝ=2.28,2.28,2.083), split by an axial magnetic field DSz2 (D∕h=9.76GHz).

The particulars of the EPR spectrum of magnetically concentrated crystals of double molybdates and tungstenates with a crystal structure of low symmetry were studied at liquid-helium temperatures in the frequency range 15–120GHz. The directions of the principal magnetic axes were determined and the principal values of the g tensors were obtained for all experimental crystals KYb(MoO4)2, CsPr(MoO4)2, RbDy(MoO4)2, and RbNd(WO4)2. Two non-equivalent geometric centers were found in KYb(MoO4)2 and RbDy(MoO4)2 crystals; the local axes of the centers were turned in different directions relative to the crystallographic axes a and c by 34° and 25°. The dipole-dipole interaction energy was estimated as Edd∼0.1cm−1 for KYb(MoO4)2. It was shown that the lowest states of CsPr(MoO4)2 are close-lying singlets (quasidoublet) with splitting Δ∼0.2cm−1.

The magnetic and thermal properties of the nanocomposite compound GdNiO3 was investigated in temperature interval from room temperature to 0.5 K and magnetic fields up to 14 T. Anomalies of static, dynamic magnetization and specific heat were detected at 0.7, 13, and 1.6 K, respectively. Detected anomalies are associated with magnetic phase transitions.

Resonance studies of single crystals of the molecular magnet {Cu-6[(MeSiO2)(6)](2)}center dot 6DMF have been performed in a wide range of frequencies 18-142 GHz and magnetic fields 0-7.5 T at liquid-helium temperature. Two nonequivalent magnetic centers of copper nanoclusters with turn angle of the local axes (50 +/- 2)degrees have been found. The ground state of each magnetic center (magnetic molecules, containing a ring of six ferromagnetic interacting ions Cu2+ (S=1/2)) can be represented as a system of energy levels with effective particle spin S=3 (g=2.28,2.28,2.083), split by an axial magnetic field DSz2 (D/h=9.76 GHz). (C) 2010 American Institute of Physics. [doi: 10.1063/1.3426295]

A system of ultrasmall nanocrystals of zinc-substituted magnetite has been synthesized by chemical condensation. Indications of a transition of the particles into a superparamagnetic state have been found by two methods—1) measurements of the temperature dependence of the magnetization in the temperature interval 4.2–250K for fixed values of the external magnetic field from 10Oeto2kOe and 2) the major hysteresis loop at 300K. The dependence of the blocking temperature on the magnetic field strength is obtained.

The resonance properties of a new Cu2OSeO3 ferrimagnet have been investigated in a wide range of frequencies (17–142GHz) 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 (3GHz). 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.