Vol 164, No 4 (2023)

We construct a nonlinear theory of electric resistance of chiral helimagnets, in which the shape changes and the magnetization spiral starts rotating during the passage of electric current due to the spin transfer torque effect. It is shown that the rotation of the spin spiral under the action of the passing current, the electric resistance of the helimagnet is always lower than the resistance of a helimagnet in which the spin spiral is stationary. It is found that the current–voltage characteristic of the helimagnet in the presence of the spin transfer torque from the conduction electron system to the system of localized electrons can be essentially nonlinear. The possibility of the spin electric bistability effect in helimagnets is predicted for the situation when the spin contribution to electric resistance of a helimagnet can take two different values for the same value of the current passing through it. The possibility of realization of states with a negative differential resistance in helimagnets is demonstrated.

It is shown that the energy of a ferromagnetic film deposited onto a paramagnetic or superconducting substrate acquires a contribution in the form of the Dzyaloshinskii–Moriya interaction. This contribution appears as a result of the magnetostatic interaction of the magnetization of the ferromagnetic film with the magnetization induced by it in a paramagnet or by the supercurrent in the superconductor and leads to the removal of the chiral degeneracy, nonreciprocity of spin waves, and the formation of chiral states such as magnetic skyrmions. Our estimates indicate the possibility of experimental observation of predicted effects.

Dispersion of spin waves in the amorphous ferromagnetic alloy Fe48Ni34P18 can be described within the model of a ferromagnet with random anisotropy: @(q) = Aq2 + gμBH + δω(q), where δω(q) is an additional term linear in |q|. The method of small-angle scattering of polarized neutrons is used to prove the importance of the additional term δω(q) in dispersion. The measurements are carried out for different values of the external magnetic field H and neutron wavelength λ. The scattering map of neutrons represents a circle centered at the point q = 0. The stiffness A of spin waves is derived directly from the λ-dependence of the radius of this circle. The spin-wave stiffness A of the amorphous alloy weakly decreases from 140 to 110 meV Å2 as temperature increases from 50 to 300 K. The field dependence of the radius demonstrates the presence of an additional term δω(q) in the form of an energy gap that is almost independent of field and temperature. The value of the additional term is Δ = 0.015 ± 0.002 meV.

he high-resolution optical absorption spectra of NdFe3(BO3)4 single crystals have been recorded at temperatures from 4 to 40 K in the IR range of f–f transitions in a Nd3+ ion. Light linearly polarized at different angles to the C2 axes in the basal plane has been passed along the trigonal C3 axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (TN ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the C2 axes up to about 17 K, the number of domains with variously directed C2 axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the C2 axis within the ±10° interval.

Cu2FeBO5 ludwigite single crystals have been grown from a solution–melt by spontaneous crystallization. Using the X-ray diffraction method, the crystal structure has been resolved in detail. Cations in sites M2, M3, and M4 have turned out to be structurally disordered. It has been found that oxygen atoms are disordered in one of five nonequivalent sites (O4). As can be seen from Mössbauer spectroscopy data, Fe3+ ions occupy four nonequivalent sites with different distortions of coordination octahedra. In the temperature range 40 K ≤ T ≤ 300 K, the spectra represent a superposition of quadrupole doublets. Static susceptibility measurements have revealed two magnetic features at T1 = 35 K and T2 = 20 K and spin-glass effects. Specific heat measurements in the interval 4–300 K have not discovered magnetic-transition-related anomalies.

A synthetic analog of rare secondary mineral namibite Cu(BiO)2VO4OH has been obtained by the hydrothermal method. The crystal structure of this compound contains isolated uniform chains of vertex-connected copper–oxygen octahedra. Magnetic susceptibility (χ) and magnetization (M) measurements have not indicated the long-range order in the temperature interval 2–300 K. Specific heat (Cp) measurements suggest the formation of a spin-liquid state at low temperatures. X-band electron paramagnetic resonance data recorded at low temperatures have demonstrated only a signal from impurities. First-principles calculations have estimated the exchange interaction in the chains as J = 555 K, whereas exchange interactions between the chains turn out to be one to two orders of magnitude smaller. Thus, namibite represents a rare example of an unordered half-integer spin chain.

We consider the effect of vacancies on the low-energy excitation spectrum of a quantum spin liquid realized in the exactly solvable Yao–Lee model [H. Yao and D.-H. Lee, Phys. Rev. Lett. 107, 087205 (2011)]. Physically, vacancies can appear for different reasons (e.g., because of zero magnetic moments on the lattice, or the presence of nonmagnetic impurities, or a random reduction of local bonds of magnetic moments with the remaining lattice). It is shown numerically that the finite density of random vacancies in this model leads to the accumulation of states near zero energy, which can be detected from the change of the behavior of heat capacity at low temperatures. Moreover, it is shown that the low-energy modes are localized more strongly than remaining eigenmodes. This effect is illustrated using the inverse participation ratio (IPR). In the case of time reversal symmetry breaking (e.g., due to the presence of a magnetic field), a gap is opened in the fermion spectrum of the model, and vacancy-induced localized states appear. The energies of these states depend on the structure of the interactions responsible for the time inversion symmetry breaking.

A double perovskite Sr2CrNbO6 powder compound was studied by using X-ray diffraction, AC and DC magnetization, and ESR measurements. Two transitions in antiferromagnetically ordered regimes were observed through magnetization measurements at T = 5 and 2 K and were confirmed by the linear dependence of the magnetization on the applied magnetic field at these temperatures, approximations of the temperature dependence of the ESR linewidth, and AC magnetization. The zero field cooling curve was approximated by Bonner–Fisher law for quasi one dimensional chain with the exchange integral J/kB = 1 K between chromium spins. An approximation of linear part of magnetic susceptibility temperature dependence was performed using Curie–Weiss law. To describe the obtained effective moment μeff = 3.577μB, the presence Cr3+ and Cr4+ ions is estimated at a respective ratio about 0.8 : 0.2.

Magnetooptical spectroscopy is an effective method for studying the magnetic microstructure of homogeneous and heterogeneous magnets. This review is devoted to analysis of numerous factors affecting the intensity and spectral dependence of a magnetooptical signal of the equatorial Kerr effect in nanocomposites “ferromagnetic metal–dielectric” in the visible and near infrared spectral regions. Examples of the influence of the metal concentration, nanoparticle size and shape, the substrate, the material of the dielectric, the amorphization of grains, the deposition method, and other factors on the magnetooptical spectrum are considered. The differences in the magnetooptical spectra for the superparamagnetic, superferromagnetic, and ferromagnetic states are demonstrated. It is noted that in the presence of fractions with different field dependences of the magnetization in a nanocomposite, the magnetooptical signal is not proportional to the total magnetization. Examples of enhancement and sign inversion of the magnetooptical signal in nanocomposites are considered. The possibility of the description of magnetooptical spectra using the methods of the effective medium (the Bruggeman method and the Maxwell–Garnett symmetrized approximation) is discussed.