Antiferromagnetic resonance in ferroborate NdFe3_3(BO3_3)$_4

The AFMR spectra of the NdFe$_3$(BO$_3$)$_4$ crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of 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 $H_{a1}$=1.14 kOe and $H_{a2}$=60 kOe and magnetic excitation gaps $\Delta\nu_1$=101.9 GHz and $\Delta \nu_2$=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

Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry

The magnetic and magnetoresonance properties of a new single-crystal compound Ni₅(TeO₃)₄Cl₂ are studied. The measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5–300 K) made it possible to conclude that Ni₅(TeO₃)₄Cl₂ is a quasi-two-dimensional antiferromagnet with the easy magnetization axis a* directed perpendicular crystallographic plane bc and a magnetic ordering temperature TN≈21 K. The resonance measurements at 4.2 K in wide range of frequencies (25–105 GHz) and magnetic field (up to 200 kOe) permitted us to obtain the frequency–field dependence of AFMR spectrum for a field applied along the easy magnetization axis a*. It is shown that the magnetic field directed along the antiferromagnetism axis (H||a*) induces the magnetic phase transition of a spin-flop type which is found to be Hsf≈120 kOe. The magnetic resonance experimental data are described qualitatively in model of the biaxial antiferromagnet

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 frequencyfield dependence of the ferrimagnetic spectrum described within the model of an anisotropic two-sublattice ferrimagnet. The effective magnetic anisotropy corresponding to the gap in the spin wave spectrum has been estimated (3 GHz). It is found that the spectrum has a multicomponent structure which is due to the diversity of the types of magnetization precession. As the amplitude of the high-frequency magnetic field increased, an additional absorption was observed in the external magnetic field lower than the field of the main resonance. The detected additional absorption corresponds to the nonuniform nonlinear parametric resonance, connected with nonuniformity of magnetic structure in the ferrimagnetic crystal Cu₂OSeO₃