We investigate the optomagnetic shift of the ferromagnetic resonance (FMR) frequency in magnets caused by the inverse Cotton-Mouton effect (ICME) under linearly polarized light. Using a Lagrangian description of magnetization dynamics, we derive the equations of motion, and obtain analytical expressions for the resonance frequency in both in-plane and out-of-plane equilibrium configurations. The theory shows that the FMR frequency depends on the polarization angle and propagation direction of light, with ICME producing a frequency shift that can dominate over thermal effects. The analytical results agree well with numerical simulations and with available experimental data for bismuth-substituted yttrium iron garnet, enabling estimation of the ICME contribution. These findings demonstrate that linearly polarized light can be used to control ferromagnetic resonance through magneto-optical effects.

A quantum theory of spin dynamics in the rare-earth orthoferrites excited by terahertz laser pulses is developed. The study demonstrates that dynamic magnetic configurations, triggered by a light pulse, exhibit stability even after the excitation source is ceased. The magnitude of post-excitation oscillations is linked to the ratio between the frequency of rare-earth ion excitations and the frequency of the external source. According to the analysis presented, dynamic response is significantly amplified when the system is exposed to ultrashort terahertz pulses. The physical characteristics of the oscillations emerging after the pulse are determined, and the factors governing their amplitude and phase are identified. The response signal is found to be dependent on the initial part of the pulse, specifically the half-period of the ultrashort light wave, while the subsequent part of the pulse contributes minimally to post-pulse magnetization dynamics. The findings highlight that in DyFeO3, terahertz dynamics primarily result from the influence of the magnetic field of the light, leading to excitations of electrons from the ground state to low-lying electronic levels of Dy3+ions. Additionally, the dynamic magnetoelectric effect excited by the electric field of the pulse is explored, revealing the emergence of odd magnetic modes.

Ultrafast heating of FeRh by a femtosecond laser pulse launches a magneto-structural phase transition from an antiferromagnetic to a ferromagnetic state. Aiming to reveal the ultrafast kinetics of this transition, we studied magnetization dynamics with the help of the magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400 K) and magnetic fields (up to 25 T). Three different types of ultrafast magnetization dynamics were observed and, using a numerically calculated H-T phase diagram, the differences were explained by different initial states of FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that ultrafast heating of FeRh in the canted antiferromagnetic phase launches practically the fastest possible emergence of magnetization in this material. The magnetization emerges on a time scale of 2 ps, which corresponds to the earlier reported time-scale of the structural changes during the phase transition.

Terahertz (THz) electromagnetic pulses offer a promising route for the ultrafast manipulation of magnetization in ferromagnetic materials. While previous studies have demonstrated the excitation of spin dynamics using linearly polarized THz fields, the role of circular polarization and the effects of rapidly oscillating, time-dependent field profiles remained insufficiently understood. We have developed a unified theoretical framework for describing the excitation of spin precession via Zeeman interaction in magnetic materials by high frequency pulses of arbitrary polarization with temporal Gaussian profile. In the regime of long pulses (at least several oscillations are within the pulse duration), a circularly polarized magnetic field acts as an effective rectified magnetic field along the pulse propagation, while linear polarized pulses excite no free precession. In the regime of short pulses (less than one oscillation is within the pulse duration), pulses of any polarization, including linear one can excite free spin precession. There is an optimal pulse duration which maximizes amplitude of the spin precession. It depends on magnetic parameters of the sample and the external magnetic field, as well as on the carrier frequency of the pulse and its amplitude. These findings bridge key gaps in the understanding of THz-induced spin dynamics and provide insights into the design of light-controlled magnetization schemes using tailored electromagnetic pulses.

We demonstrate the peculiarities of the magnetization process in the ferrimagnetic intermetallic compounds of the (R,R')(2)Fe14B-type, their hydrides, as well as complex modified rare earth compounds (R,R')(2)Fe14B (R = Nd, Gd, Dy, Ho). They have been studied theoretically and experimentally in ultrahigh magnetic fields. We observe phase transition induced by external ultrahigh magnetic fields (up to 200 T) and also describe the magnetization process analytically (in terms of critical transition fields) and numerically. In this work, the first and second critical fields of the field-induced magnetic transitions, H-c1 and H-c2, were estimated. Critical field H-c2 predicting the place of transition to the forced-ferromagnetic state was estimated for the first time for various intermetallic compounds.

We report a theoretical study of the phase diagram of a ferrimagnetic iron-garnet with uniaxial anisotropy near a magnetization compensation point in the presence of a two-component magnetic field. The study is performed based on a quasi-antiferromagnetic approximation. The number and stability of the equilibrium states of the Neel vector are analyzed using the effective energy function. It is shown that application of the small out-of-plane magnetic field in addition to the stronger in-plane magnetic field significantly changes the equilibrium states of a ferrimagnet. The possibilities to control the equilibrium Neel vector position and to switch between the monostable and bistable states by tuning the value and ratio of the in-plane and out-of-plane magnetic field components are demonstrated. This opens new possibilities for the utilization of ferrimagnets since the magnetic field could be changed much faster than the temperature.

The development of new computing technologies has given a new stimulus in the study of multiferroics. The use of multiferroics allows the realization of competitive energy efficient scalable logic and storage devices. The low power consumption in Magneto Electric - Spin Orbital logics [1] and Magnetic Random Access Memory components is provided by magnetoelectric switching in multiferroic - based systems using a low-energy electric field. Our work concerns the modelling of the Magneto Electric - Spin Orbital elements with an emphasis on the magnetoelectric component and simulation of magnetization reversal processes in a model system. The use of the proposed approach makes it possible to analyze the influence of dimensional factors (film thicknesses, transverse dimensions, sample shape) affecting the magnetic states of multiferroic nanoelements; taking into account interfacial interactions (magnetic anisotropy and interlayer exchange); energy-efficient external influences that allow switching magnetic states using magnetic and electric fields.

Both temperature (T ) increase and/or application of an external magnetic field (H) are able to induce ferromagnetism in otherwise antiferromagnetic FeRh. We present a theory that allows us to predict H -T phase diagrams for FeRh, with a special emphasis on the canted antiferromagnetic phase. Both cases of in-plane and out-of-plane magnetic anisotropy in FeRh films are studied numerically, and the results of the simulations are compared with the results obtained via magneto-transport studies done in magnetic fields up to 34 T and x-ray magnetic circular dichroism measured at the Rh L2,3 and Fe K absorption edges in magnetic fields up to 17 T. The comparisons suggest examples of the simulated H -T phase diagrams that are most suitable to fit the case of FeRh.

Multiferroic oxides are considered as key elements of energy-consuming devices required for the development of scalable logic and information storage technologies. In this regard, understanding the mechanisms of magnetoelectric switching and finding the optimal way to switch magnetization by an electric field is of crucial importance. In this study, we develop a model for studying magnetic states in a nanoscale exchange-coupled ferromagnetic-multiferroic heterostructure subjected to the action of an electric field. Based on bias effects emerging due to the coupling between a ferromagnetic subsystem and an antiferromagnetically ordered multiferroic material, we explore the magnetic textures and the magnetization reversal processes in a ferromagnet. As the multiferroic material, we consider BiFeO3, where magnetic ordering and ferroelectric ordering are determined by the mutually perpendicular antiferromagnetic (L), weak ferromagnetic (M) and polarization (P) vectors. Application of an electric voltage removes degeneration from eight energetically equivalent positions of P|| 〈111〉, allocates the definite directions of vectors P, M, and L and as a consequence the unidirectional magnetic anisotropy axis in the reference ferromagnetic layer. Our study reveals the features of the magnetic configurations in systems of different geometries, with varying exchange and magnetic anisotropy, necessary to determine the optimal conditions for switching magnetic states in a multiferroic bi-layer by an electric field.

Ultrafast laser-induced heating of ferrimagnetic iron garnet in an external magnetic field triggers magnetization precessional dynamics with a large amplitude. The dynamics is studied as a function of magnetic field, laser fluence, and sample temperature. Exploring the three-dimensional space of these parameters experimentally and computationally, we identify the conditions for which the amplitude of the precession is the largest and even achieves values sufficient for magnetic recording. We found that the range of external magnetic fields and temperatures, which corresponds to the magnetic recording, is rather narrow. Modeling the dynamics, using magnetization as a macroscopic parameter, reveals that this range of parameters is defined by the optimal height of the potential barrier separating two stable states. The barrier needs to be low enough to allow the switching but not so low that the stability of the states is lost.

Electrical model of a magnetic tunnel junction is developed in Verilog-A language, which can use in CAD systems to design an integrated circuit of spintronics devices. In order to check the correct operation of the model verification tests were created and carried out in Cadence ADE. Each test corresponds to the operating mode of the magnetic tunnel junction: switching, generation, rectification. Thus, the developed model can be used to simulate hybrid circuits comprising CMOS elements and magnetic tunnel junctions.

It is shown using the technique of double high-speed photography that an external magnetic field triggers the motion of a GdFeCo domain wall with a velocity up to 1.2 km/s. The domain wall velocity increases and levels off with an increase in the amplitude of the driving magnetic-field pulse. In contrast to the earlier experiments on iron ferrites, no influence of femtosecond laser pulses on the domain wall dynamics has been observed, even when the pump pulse energy is sufficient for magnetization reversal.

Resonant absorption of a photon by bound electrons in a solid can promote an electron to another orbital state or transfer it to a neighboring atomic site. Such a transition in a magnetically ordered material could affect the magnetic order. While this process is an obvious road map for optical control of magnetization, experimental demonstration of such a process remains challenging. Exciting a significant fraction of magnetic ions requires a very intense incoming light beam, as orbital resonances are often weak compared to above‐band‐gap excitations. In the latter case, a sizeable reduction of the magnetization occurs as the absorbed energy increases the spin temperature, masking the non‐thermal optical effects. Here, using ultrafast X‐ray spectroscopy, this work is able to resolve changes in the magnetization state induced by resonant absorption of infrared photons in Co‐doped yttrium iron garnet, with negligible thermal effects. This work finds that the optical excitation of the Co ions affects the two distinct magnetic Fe sublattices differently, resulting in a transient non‐collinear magnetic state. The present results indicate that the all‐optical magnetization switching (AOS) most likely occurs due to the creation of a transient, non‐collinear magnetic state followed by coherent spin rotations of the Fe moments.

Excitation of spin waves of a required frequency and directional spectrum is among the crucial tasks in optomagnonics. Here we investigate the generation of spin waves in an iron garnet thin film by a train of femtosecond laser pulses with ultimately high repetition rate of up to 10 GHz and compare it with the case of 1-GHz repetition rate. The periodic optical excitation with repetition rate close to the frequency of the ferromagnetic resonance amplifies spin waves with particular phase velocity and wavelength, which are tunable across a wide range by small variations of the frequency detuning, and can be adjusted by the magnitude of the applied external magnetic field. For pulses of the same fluence, the 10-GHz pulse rate provides a significant resonant increase of the spin-wave amplitude by 11.5 times with respect to single-pulse excitation while the 1-GHz pulse rate provides only a 1.5 times advancement. Moreover, variation of the detuning frequency provides different regimes of the spin-wave propagation: short-and long-distance propagation along the magnetic field direction and appearance of an "X" line shape in the directionality pattern, making the considered optical approach of spin-wave generation promising for designing magnonic devices.

Multiferroics are materials crucial for energy-efficient scalable electronics. The implementation of an effective combination of ferroic orderings on the nanoscale requires the design of new multiferroic materials. Recently, there have been observations of magnetoelectricity in the antiferromagnetic Ruddlesden-Popper and perovskite oxides with the interfacial Dzyaloshinskii-Moriya interaction. We propose a model for studying magnetic states and magnetoelectric effects in magnetoelectrically coupled antiferromagnetic–ferroelectric bi-layers with the interfacial Dzyaloshinskii–Moriya interaction. The ground magnetic states are calculated for a system on a rectangular lattice, with Heisenberg spins interacting with each other via an antiferromagnetic exchange interaction and a Dzyaloshinskii–Moriya interaction in the absence of an external magnetic field. Our calculations show that the interfacial Dzyaloshinskii-Moriya interaction in the considered system leads to the stabilization of topological skyrmionic states in a zero magnetic field. We explore transformations of magnetic states considering the changes in the in-plane magnetic anisotropy constant and the magnetoelectric coupling parameter. Our findings have shown the possibility of the existence of several magnetic configurations: a skyrmion lattice, a skyrmion state, and a uniform antiferromagnetic ordering realized at a definite ratio of the system parameters. We determine the areas of the phases existence and the conditions required for spin-reorientation phase transitions.

Ferrimagnets possess collinear and noncollinear magnetic phases. Here we identify a crucial influence of the magnetic phase on the excited spin modes. The spin dynamics is launched in a rare-earth iron garnet film by femtosecond laser pulses. The noncollinearity cardinally changes the behavior of spin mode frequencies: the high-frequency one becomes highly sensitive to the external magnetic field while the low-frequency one gets almost field insensitive and even drops with the field growth near the transition to the collinear phase. Near the magnetization compensation point the frequencies of these modes get close to equality.

It is found that single-cycle THz electromagnetic fields efficiently excite a GHz spin resonance mode in ferrimagnetic Tm$_3$Fe$_5$O$_{12}$, despite the near absence of GHz spectral components in the exciting THz pulse. By analyzing how the efficiency of excitation depends on the orientation and strength of the THz electric field, we show that it can be explained in terms of the nonlinear THz inverse Cotton-Mouton effect. Here, the THz electric field gets effectively rectified and acts on the ferrimagnetic spins as a uni-polar effective magnetic field pulse. This interpretation is confirmed by a theoretical model based on the phenomenological analysis of the effective magnetic field, combined with the equations of motion derived from the effective Lagrangian for a ferrimagnet. Moreover, by using the outcome of two-dimensional THz spectroscopy, we conjecture a quantum-mechanical interpretation of the observed effect in terms of stimulated Raman scattering of THz photons by the crystal-field split f-f electronic transitions of Tm$^{3+}$.

The interaction of a single-cycle terahertz electric field with the topological insulator MnBi_{2}Te_{4} triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise noninteracting Raman-active E_{g} and infrared (IR)-active E_{u} phononic modes. Two-dimensional terahertz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multistage process underlying the excitation of the Raman-active E_{g} phonon. In this nonlinear combined photophononic process, the terahertz electric field first prepares a coherent IR-active E_{u} phononic state and subsequently interacts with this state to efficiently excite the E_{g} phonon.

Using the technique of double high-speed photography method, we show that an external magnetic field triggers in GdFeCo domain wall motion with velocities up to 1.2 km / s. The domain wall velocity saturates with an increase of the driving magnetic field. Contrary to earlier experiments on iron garnets, we did not succeed to detect any effect of femtosecond laser pulses on the domain wall velocity, even if the pulses were strong enough to reverse magnetization.