A new approach to describing the magnetic properties of FeRh alloys is proposed. It is based on two assumptions about the properties of 3 d and 4 d electrons in these alloys. The first is the assumption that the 4 d band is submerged under the Fermi level to a depth that ensures its complete filling at a temperature T = 0 K. The second is the assumption that there are two different spatial distributions of 3 d and 4 d electrons that are compatible with one atomic structure. The first assumption makes it possible to explain the absence of uncompensated spins of 4 d electrons in the low-temperature antiferromagnetic (AFM) phase of FeRh. The second assumption is proposed to explain the strong changes in the spin structure of the FeRh alloy upon the AFM–FM transition, while its atomic structure is almost unchanged. Attempts have been made to predict the new properties of the FeRh alloy that follow from these assumptions. Our second assumption proved to be successful and enabled us to predict the existence of local magnetic moments of 3 d electrons in the AFM phase, which are larger than the magnetic moments of Fe ions. Measurements of the magnetic susceptibility of the FeRh alloy in the AFM phase confirmed this prediction.

We propose the mechanism for the optical magnetization reversal of metallic ferrimagnets near the magnetic compensation temperature in a zero magnetic field using a femtosecond linearly polarized optical pumping. The mechanism is based on the spin sublattice model proposed by M. I. Kurkin and N. B. Orlova in 2019. This model assumes the formation of spin sublattices with the obligatory participation of magnetic anisotropy eliminating degeneracy in the spectrum of the exchange interaction operator. The discussed optical magnetization reversal was observed experimentally in the GdFeCo alloy near the magnetic compensation point.

Представлены результаты измерения скорости магнитоупругих волн в гематите, соответствующих их квазифононной ветви. Исследована зависимость данной скорости (V) от магнитного поля (H) при постоянном одноосном давлении P, (VP(H)) и давления P при постоянном H, (VH(P)). В соответствии с существующей теорией магнитоакустических явлений зависимости VP(H) и VH(P) должны иметь минимумы в точках ориентационных фазовых переходов. Такие минимумы обнаружены, но их координаты на плоскости (P, H) оказались несовпадающими. Обсуждаются причины несовпадений и требования к точности измерений, обеспечивающие возможности их использования для определения параметров этих переходов. Ключевые слова: антиферромагнетик, магнитоупругость, ориентационные фазовые переходы, объемная акустическая волна.

The measurement results of the velocity of magnetoelastic waves in hematite corresponding to their quasi-phonon branch have been presented. The dependences of this velocity (V) on the magnetic field (H) at constant uniaxial pressure P, (V-P(H)), and on pressure P at constant H, (V-H(P)), have been studied. In accordance with the existing theory of magnetoacoustic phenomena, dependences V-P(H) and V-H(P) should exhibit minima at the orientation phase transition points. These minima were found, but their coordinates on the plane (P, H) did not coincide. The reasons of non-coincidence and requirements to the measurement accuracy are discussed, which yield possibilities of its use to determine characteristics of these transitions.

The phase composition and magnetic properties of FeNi nanoparticles encapsulated in carbon with a nickel content of about 10% are studied in comparison with FeNi nanoparticles of similar composition without carbon coating. The Fe91Ni9 particles have the body centered cubic structure, whereas the Fe88Ni12@C particles have the face centered cubic (FCC) structure. The Fe88Ni12@C particles are characterized with a rather high magnetization of 60 emu g−1 in the field of 30 kOe at 5 K and 40 emu g−1 at 300 K. The monotonous behavior of magnetization with temperature gives grounds to assume that the Fe88Ni12@C particles retain the FCC structure over the entire temperature range down to 5 K. It is concluded that the stabilization of FCC phase of Fe88Ni12@C and the ferromagnetic state of this FCC phase are related to a large concentration of carbon in the core of the nanoparticles.

In this work, we report the results of comprehensive experimental and theoretical study of magnetic properties of TiO2 nanoparticles (20 nm) doped with Fe at various concentrations ranging from 0.1 to 4.6 at. %. Our electron paramagnetic resonance and magnetic measurements data evidence the mixed magnetic state, where paramagnetic Fe3+ ions coexist with short-range antiferromagnetic correlations caused by negative exchange interaction between neighboring Fe3+ ions. A quantum mechanical model of the Fe-based magnetic cluster represented as a set of dimers with strong similar to(4100-300) K and weak (similar to 1 K) exchange interactions has been proposed. Our model was found to provide a good description of magnetic properties of TiO2:Fe nanopowders. Density-functional theory (DFT) calculations revealed Fe3+ oxidation state of the iron center in the vicinity of an oxygen vacancy in the crystal structure of anatase. DFT calculations confirmed that two types of Fe3+ spin-pairs with weak and strong exchange interactions can be formed in the vicinity of an oxygen vacancy. Accumulation of magnetic moment carriers and formation of magnetic clusters in TiO2 nanoparticles with anatase structure were found to be a general tendency for all studied TiO2:Fe nanopowders.

We present a study of nonlinear NMR and Bose-Einstein condensation (BEC) of nuclear spin waves in antiferromagnetic MnCO3 with coupled electron and nuclear spins. In particular, we show that the observed behavior of NMR signals strongly contradicts the conventional description of paramagnetic ensembles of noninteracting spins based on the phenomenological Bloch equations. We present a theoretical description of the coupled electron-nuclear spin precession, which takes into account an indirect relaxation of nuclear spins via the electron subsystem. We show that the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, which is drastically different from the conventional heating scenario derived from the Bloch equations. This provides strong evidence that the coherent precession of macroscopic nuclear magnetization observed experimentally can be identified with the BEC of nuclear spin waves with k = 0.

The estimation is obtained for the quenching time tau(l) of the non-equilibrium orbital momentum of the electron that can be got after the excitation by the femtosecond optical pump. The quenching process of the l is supposed to be determined by the interaction of l with the crystal field and its deformations (phonons). The time tau(l) is determined not only by the parameters of the crystal field but also by the speed of relaxation of the virtual phonon 1/tau(omega) with the frequency omega approximate to 1/tau(l). Considering the frequency dependence of tau(omega) enlarges the estimated value of tau(l) up to the microsecond scale. This value exceeds greatly the picoseconds times of the ultrafast magnetic dynamics (UMD) processes observed after the action of the femtosecond optical pump. This means that the results obtained in our work are not supposed to be compared with the experimental data on UMD. The presented description of the quenching of l is the solution of one of the auxiliary tasks that can be useful for construction of the UMD theory. (C) 2017 Elsevier B.V. All rights reserved.

A physical mechanism responsible for the relaxation of nuclear spins coupled by the hyperfine interaction to relaxed electron spins in materials with spin ordering is proposed. The rate of such induced nuclear spin relaxation is proportional to the dynamic shift of the nuclear magnetic resonance (NMR) frequency. Therefore, its maximum effect on the NMR signal should be expected in the case of nuclear spin waves existing in the system. Our estimates demonstrate that the induced relaxation can be much more efficient than that occurring due to the Bloch mechanism. Moreover, there is a qualitative difference between the induced and Bloch relaxations. The dynamics of nuclear spin sublattices under conditions of the induced relaxation is reduced to the rotation of m 1 and m 2 vectors without any changes in their lengths (m 1 2 ( t ) = m 2 2 ( t ) = m 0 2 ( t )= const). This means that the excitation of NMR signals by the resonant magnetic field does not change the temperature T n of the nuclear spin system. This is a manifestation of the qualitative difference between the induced and Bloch relaxations. Indeed, for the latter, the increase in T n accompanying the saturation of NMR signals is the dominant effect.

We present a new theoretical description of the coupled electron-nuclear spin systems which takes into account an indirect relaxation of nuclear spins via the electron subsystem. In our theory the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, similar to the Landau-Lifshitz-Gilbert model of relaxation. This is drastically different from the conventional heating scenario based on the phenomenological Bloch equations. The predictions of our theory are compared with the experimental nonlinear NMR signals obtained in a weakly anisotropic antiferromagnetic MnCO$_3$ sample at temperatures below $1,$K and good quantitative agreement is observed. The proposed theory brings together the properties of magnetic nuclear resonance in, on the one hand, the magnetic systems considered here and, on the other hand, the superfluid ${^3}$He where the magnitude of the nuclear magnetization vector is also conserved.

An acoustic signal that is excited without acoustic contact by an excitation electrode in a metal sample has been studied. A constant electric voltage close to the breakdown value was applied between the electrode and the sample. A video pulse of electric field with a duration of 0.4 μs was used to excite sound in the sample. The ratio k of signals that were registered with noncontact and contact excitation proved surprisingly large (k = 0.14) when compared to the value expected from the literature data. A strong dependence of the noncontact acoustic signal on the filling frequency f of the excitation pulse has been observed. For f > 10 MHZ, the signal dropped to the level of noises. An explanation for this frequency dependence is proposed. It is based on the assumption of quasi-atomic thickness of the charged metal layer that is created by the electric field.

The critical analysis of the magnetization reversal processes and magnetooptics of ferromagnets was carried out. The condition of the formation of nonequilibrium orbital momenta lpf after turning off the femtosecond laser pumping was found. The frequency range for processes involving lpf was estimated as 106 Hz to 1015 Hz. Such objects were not known in the magnetism before using the methods of femtosecond optics. The estimated frequency range covers all the processes related to femtosecond magnetism. Our qualitative analysis of these processes allows us to hope that their quantitative description does not require significant changes in the existing theories of magnetism.

A method is proposed that allows one to divide the magnetoresistance (MR) observed in manganites into three mechanisms: dimensional, orientational, and magnetic. The first two mechanisms are associated with the stratification of a substance into ferromagnetic and nonferromagnetic phases, which significantly differ in electric resistivity. The dimensional mechanism of MR is attributed to the effect of a magnetic field on the size of magnetic inclusions. The orientational mechanism of MR is determined by the dependence of electric resistivity on the mutual orientation of the magnetizations of magnetic inclusions. The magnetic mechanism of MR is determined by the properties of the magnetization of a ferromagnet, in particular, by the Curie–Weiss singularity on the temperature dependence of magnetic susceptibility at the Curie point. This mechanism exists in homogeneous substances, although its value may depend on the magnetic properties of inhomogeneities. The method is developed for substances with activation-type conductivity and is applied to the analysis of MR of La0.85Sr0.15MnO3 manganite near the Curie point, where the MR attains its maximum. The dimensional mechanism turns out to be dominant in magnetic fields H greater than the saturation field H s (H > H s ). The orientational, dimensional, and magnetic mechanisms have a comparable effect on the MR for H < H s . The effect of the orientational mechanism on MR is relatively weak (does not exceed the third part of the total MR), although this mechanism determines the giant MR in multilayered metal films. The possibility of application of the method to the analysis of MR near the insulator–metal transition is analyzed.

This paper reports on the results of the experimental investigation of the quasi-phonon (quasi-sound) branch of magnetoelastic waves in hematite (α-Fe 2 O 3 ) in the easy-plane state near the orientation phase transitions in magnetic fields H and under uniaxial pressure P . It has been found that, in an applied external magnetic field H > 1.0 kOe and at a uniaxial pressure P > 40 MPa, the amplitude of the transmitted quasi-sound abruptly increases. The minima of quasi-sound velocities observed at the orientation phase transition points in the magnetic field H are consistent with the predictions of the existing theory of magnetoelastic waves, but their width significantly exceeds the theoretical values. The minima of quasi-sound velocities at a uniaxial pressure P are observed only in magnetic fields H < 3 kOe and, at H > 3 kOe, turn into extended plateaus.

This article is dedicated to Evgenii Akimovich Turov, a well-known scientist in the field of physics of magnetic phenomena and Corresponding Member of the Russian Academy of Sciences. The article includes an analysis of the key problems of the physics of magnetism in the early 21st century, as well as E.A. Turov's and his school's contributions to the science of magnetism. In 2014, we commemorate the 90th anniversary of the birthday of Evgenii Akimovich, and this article is timed to this memorable date. The article also contains a list of the basic works of the scientist.

A comparison has been performed between the Landau-Dzyaloshinskii-Astrov magnetoelectric effects and the electromagnetic effects caused by the electromagnetic Faraday induction and Maxwell displacement currents. The requirement for the spontaneous violation of symmetry relative to space inversion and time reversion is formulated as the condition for the existence of magnetoelectric effects. An analysis is performed of some results obtained by E.A. Turov both personally and in association with colleagues, which made a significant contribution to the development of the science of magnetoelectricity. These results include the development of the scheme of a simplified symmetry analysis for describing collinear spin structures; the use of this scheme for the invariant expansion of thermodynamic potentials for the magnetic materials with different types of magnetic ordering; the formulation of the microscopic model of magnetoelectricity with the use of the relation between spins and electroactive optical phonons; the study of the phenomena of the enhancement of magnetoelectric effects upon the magnetic resonance; the analysis of the opportunities of electrodipole excitation and of the detection of different signals of magnetic resonance; and the study of the manifestations of magnetoelectric effects in magnetoacoustics and optics.

Проводится сравнение магнитоэлектрических эффектов Ландау–Дзялошинского–Астрова с электромагнитными эффектами, обусловленными электромагнитной индукцией Фарадея и токами смещения Максвелла. Формируется требование спонтанного нарушения симметрии относительно пространственной инверсии и обращения времени в качестве условия существования магнитоэлектрических эффектов. Анализируются некоторые результаты, полученные Е.А. Туровым лично и в соавторстве, которые внесли значительный вклад в развитие науки о магнитоэлектричестве. К этим результатам относятся: разработка схемы упрощенного симметрийного анализа для описания коллинеарных спиновых структур; использование этой схемы для инвариантного разложения термодинамических потенциалов для магнетиков с различным типом магнитного упорядочения; формулировка микроскопической модели магнитоэлектричества с использованием связи спинов с электроактивными оптическими фононами; исследование эффектов усиления магнитоэлектрических эффектов при магнитном резонансе; анализ возможностей электродипольного возбуждения и регистрации различных сигналов магнитного резонанса; изучение проявлений магнитоэлектрических эффектов в магнитоакустике и оптике.

Настоящая работа посвящена известному ученому в области физики магнитных явлений, члену-корреспонденту РАН Евгению Акимовичу Турову. Статья включает анализ актуальных вопросов физики магнетизма начала XX1 века, а также вклада Е.А. Турова и созданной им школы в науку о магнетизме. В 2014 г. исполняется 90 лет со дня рождения Евгения Акимовича, и данная статья приурочена к этой памятной дате. В ней приводится список основных трудов ученого.