M. I. Kobets

Electronic paramagnetic resonance or rare-earth ions Yb3+, Pr3+, Dy3+, and Nd3+ in double molybdates and tungstenates

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.

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.

Antiferromagnetic resonance in Mn2P2S6

The antiferromagnetic resonance (AFMR) spectra along the principal magnetic axes x, y, z of the compound Mn2P2S6 have been investigated in detail in a wide range of frequencies (8–142GHz) and magnetic fields (up to 75kOe) at T=4.2K. It is shown that this compound is a biaxial magnetic substance. At H=0, there are two gaps in the spin-wave spectrum: (101.46±0.1) and (115.52±0.1)GHz. The AFMR data were used to determine the effective magnetic-anisotropy fields: Ha1=0.619kOe and Ha2=0.803kOe. An anomalous interaction of the AFMR branches in Mn2P2S6 has been detected, along with the appearance of coupled spin–spin oscillations of the same symmetry. An additional absorption in the AFMR spectrum is a local mode associated with breakdown of translational order in a low-dimension magnet. It is shown for the first time that the appearance of additional absorption peaks depends on the sample-cooling rate. It is proven that the observed peaks are associated with the internal modes of the domain boundaries.

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.

Sequence of structural phase transitions induced by an external magnetic field in the Jahn–Teller elastic KTm(MoO4)2

Electron paramagnetic resonance and ac magnetic susceptibility studies in the compound KTm(MoO4)2 have revealed a number of features in the microwave absorption and magnetic susceptibility. It is conjectured that there is a sequence of magnetic-field-induced structural phase transitions involving the formation of a superstructure of the crystal lattice in KTm(MoO4)2. Experimental studies are carried out at a temperature of 1.7 K.

Low-temperature magnetic phase transition in aluminum borate TbAl3(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.

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.

EPR in the molecular magnet {Cu6[(MeSiO2)6]2}⋅6DMF

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 (g=2.28,2.28,2.083), split by an axial magnetic field DSz2 (D∕h=9.76GHz).

Magnetoresonance properties of antiferromagnetic TbFe3(BO3)4 at low temperatures

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.

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).