V. A. Pashchenko

Magnetic properties of the antiferromagnetic spin-1/2 chain system beta-TeVO4

The magnetic susceptibility and magnetization of high-quality single-crystal beta-TeVO4 are reported. We show that this compound, made of weakly coupled infinite chains of VO5 pyramids sharing corners, behaves as an S = 1/2 one-dimensional Heisenberg antiferromagnet. From magnetic experiments, we deduce the intrachain antiferromagnetic coupling constant, J/k(B) = 21.4 +/- 0.2 K. Below 5 K, a series of three phase transitions at 2.26, 3.28, and 4.65 K is observed.

Rotational magnetocaloric effect in TbAl3(BO3)4

A rotational magnetocaloric effect is predicted in the TbAl3(BO3)4 crystal based on the results of measuring field dependences of magnetization and the temperature dependence of heat capacity. The effect is modeled using the quasi-doublet approximation. Changes to the crystal entropy at constant temperature are determined, as well as at varying temperature under adiabatic conditions. The refrigerant capacity of the crystal is estimated. It is shown that terbium aluminum borate is a promising material for magnetic cooling.

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).

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).

Magnetic resonance properties of the low-dimensional antiferromagnet Mn[C10H6(OH)(COO−)]2∙2H2O

Magnetic resonance studies of the metalorganic antiferromagnet Mn[C10H6(OH)(COO−)]2∙2H2O are carried out over a wide range of frequencies (25–12GHz) and temperatures (1.8–300K). The data on the temperature dependence of the magnetic susceptibility attest to a low-dimensional magnetic structure of this crystal. The results of resonance studies at a temperature of 1.8K are used to construct the frequency–field diagram of the antiferromagnetic resonance spectrum of this compound; it is described by the model of a biaxial rhombic antiferromagnet with the following parameters: ν01=14.1±0.1GHz, ν02=44.2±0.1GHz, Ha1=0.25kOe, Ha2=2.53kOe, Hsf=5.1±0.1kOe, 2He=98±1kOe. Two-magnon and nonresonance three-magnon absorption is observed, and the weakest interchain exchange interaction is estimated as J′=6.2×10−3K.

Magnetic properties of a GdCr3(BO3)4 single crystal

The temperature dependence of the magnetic susceptibility χ (T) of the GdCr3(BO3)4 single crystal was studied in the temperature range of 2–300 K for two orientations of the external magnetic field: H ǁ c and H ± c. It was established that gadolinium chromium borate is ordered antiferromagnetically at TN = 7 K. It was found that, at T > 10 K, the magnetic properties of the crystal are isotropic, which is characteristic of the pure spin states of Cr3+ and Gd3+ ions. It was demonstrated that the magnetic susceptibility χ(T) of the GdCr3(BO3)4 compound at T > TN is well described by a one-dimensional spin model for a system of ferromagnetic chains of Cr3+ ions with ferromagnetic exchange 2J/k = −13.6 K and antiferromagnetic interchain interaction of 2J′/k = + 0.72 K. The gadolinium subsystem is antiferromagnetically biased due to the exchange f-d-interaction, which gives θ Gd = − 2.8 K. It was assumed that the GdCr3(BO3)4 compound is an antiferromagnet with “easy-plane” type anisotropy in the magnetically or...

Pressure effect on magnetic susceptibility of LaCoO3

The effect of pressure on magnetic properties of LaCoO3 is studied experimentally and theoretically. The pressure dependence of magnetic susceptibility χ of LaCoO3 is obtained by precise measurements of χ as a function of the hydrostatic pressure P up to 2 kbar in the temperature range from 78 K to 300 K. A pronounced magnitude of the pressure effect is found to be negative in sign and strongly temperature dependent. The obtained experimental data are analysed by using a two-level model and DFT+U calculations of the electronic structure of LaCoO3. In particular, the fixed spin moment method was employed to obtain a volume dependence of the total energy difference Δ between the low spin and the intermediate spin states of LaCoO3. Analysis of the obtained experimental χ(P) dependence within the two-level model, as well as our DFT+U calculations, have revealed the anomalous large decrease in the energy difference Δ with increasing of the unit cell volume. This effect, taking into account a thermal expansion, can be responsible for the temperatures dependence of Δ, predicting its vanishing near room temperature.The effect of pressure on magnetic properties of LaCoO3 is studied experimentally and theoretically. The pressure dependence of magnetic susceptibility χ of LaCoO3 is obtained by precise measurements of χ as a function of the hydrostatic pressure P up to 2 kbar in the temperature range from 78 K to 300 K. A pronounced magnitude of the pressure effect is found to be negative in sign and strongly temperature dependent. The obtained experimental data are analysed by using a two-level model and DFT+U calculations of the electronic structure of LaCoO3. In particular, the fixed spin moment method was employed to obtain a volume dependence of the total energy difference Δ between the low spin and the intermediate spin states of LaCoO3. Analysis of the obtained experimental χ(P) dependence within the two-level model, as well as our DFT+U calculations, have revealed the anomalous large decrease in the energy difference Δ with increasing of the unit cell volume. This effect, taking into account a thermal expansion,...

Antiferromagnetic resonance in crystalline PrFe3(BO3)4

Experimental AFMR studies of crystalline PrFe3(BO3)4 over a wide frequency range of 10–143 GHz at a temperature of 4.2 K have been carried out. The high-frequency properties of praseodymium ferroborate are well described in terms of a model of a two-sublattice antiferromagnet with an “easy axis” anisotropy. An energy gap of 134.3 ± 0.5 GHz is determined and the magnitude of the effective magnetic anisotropy field is estimated to be 1.9 ± 0.1 kOe. An analysis indicates that the spin-orientational phase transition in this compound is a first order transition.

Antiferromagnetic resonance in a GdCr3(BO3)4 crystal

An experimental study of AFMR in a GdCr3(BO3)4 single crystal within the frequency range of 17–38 GHz, at the temperature 4.2 K, was performed. The determined value of the energy gap in spin-wave spectrum equals (25.5 ± 0.5). It was shown that the high-frequency properties of gadolinium chromium borate are well defined within the framework of a two-sublattice antiferromagnet model with “easy-plane” anisotropy.

Heat capacity properties of quasi-one-dimensional magnet β-TeVO4

We studied the temperature dependence of heat capacity CP(T) of a quasi-one-dimensional magnetic system β-TeVO4, in a zero magnetic field and a temperature region of 0.1 K ≤ T ≤ 300 K. The CP(T) dependence shows phase transitions at temperatures of 4.65, 3.28, and 2.32 K. Is it established that at a temperature of TN = 4.65 K there is a phase transition from a paramagnetic to an antiferromagnetic state. We provide a theoretical description of the heat capacitys temperature dependence.