The theoretical study considers chiral spin textures induced in a 2D electron gas (2DEG) by magnetic skyrmions. We calculate the electron gas spin density as a linear response to the exchange interaction between the 2DEG and the magnetization field of a magnetic skyrmion. Two physically distinct regimes occur. When the size of the skyrmion is larger than the inverse Fermi wavevector kF-1, the spin density response follows the magnetization profile of the skyrmion. In the opposite case of a small skyrmion the emerging spin structure of 2DEG has a characteristic size of kF-1 and the response becomes non-local, it can be viewed as chiral Friedel oscillations. At that, the emerging spin structure of the oscillations appears to be more complex than that of the skyrmion itself.

We provide a theoretical description of frustrated multiferroic MnI2 with a spiral magnetic ordering in magnetic field **h**. We demonstrate that subtle interplay of exchange coupling, dipolar forces, hexagonal anisotropy, and the Zeeman energy account for the main experimental findings observed recently in this material (Kurumaji et al., Phys. Rev. Lett. **106** (2011) 167206). We describe qualitatively the non-trivial evolution of electric polarization **P** upon **h** rotation, changing **P **direction upon *h* increasing, and disappearance of ferroelectricity at *h > hc*, where *hc* is smaller than the saturation field.

A quadruple quantum-dot (QQD) cell is proposed as a spin filter. The transport properties of the QQD cell were studied in linear response regime on the basis of the equations of motion for retarded Green's functions. The developed approach allowed us to take into account the influence of both intra- and interdot Coulomb interactions on charge carriers' spin polarization. It was shown that the presence of the insulating bands in the conductance due to the Coulomb correlations results in the emergence of spin-polarized windows (SPWs) in magnetic field leading to the high spin polarization. We demonstrated that the SPWs can be effectively manipulated by gate fields and considering the hopping between central dots in both isotropic and anisotropic regimes.

Ferromagnetic materials with exchange fields E_ex smaller or of the order of the superconducting gap Delta are important for applications of corresponding (s-wave) superconductor/ ferromagnet/ superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small E_ex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal - ferromagnet - insulator - superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=E_ex. Thus measuring the subgap conductance one can reliably determine small E_ex < Delta. In the opposite case E_ex > Delta one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=E_ex. This peak can be only visible for small enough exchange fields of the order of few Delta.

We investigate electron cooling based on a clean normal-metal/spin-filter/superconductor junction. Due to the suppression of the Andreev reflection by the spin-filter effect, the cooling power of the system is found to be extremely higher than that for conventional normal-metal/nonmagnetic-insulator/superconductor coolers. Therefore we can extract large amount of heat from normal metals. Our results strongly indicate the practical usefulness of the spin-filter effect for cooling detectors, sensors, and quantum bits.

Structural, magnetic and magnetotransport properties of (Bi1-xEux)2Se3 thin films have been studied experimentally as a function of Eu content. The films were synthesized by MBE. It is demonstrated that Eu distribution is not uniform, it enter quint-layers forming inside them plain (pancake-like) areas containing Eu atoms, which sizes and concentration increase with the growth of Eu content. Positive magnetoresistance related to the weak antilocalization was observed up to 15 K. The antilocalization was not followed by weak localization as theory predicts for nontrivial topological states. Surprisingly, the features of antilocalization were seen even at Eu content x = 0.21. With the increase of Eu content the transition to ferromagnetic state occurs at x about 0.1 and with the Curie temperature 8 K, that rises up to 64 K for x = 0.21. At temperatures above 1–2 K, the dephasing length is proportional to T^-1/2 indicating the dominant contribution of inelastic e-e scattering into electron phase breaking. However, at low temperatures, the dephasing length saturates, that could be due to the scattering on magnetic ions.

A set of thin film Mn_{xSi1-x alloy samples with different manganese concentration x≈0.44−0.63 grown by the pulsed laser deposition (PLD) method onto the Al2O3(0001) substrate was investigated in the temperature range 4–300 K using ferromagnetic resonance (FMR) measurements in the wide range of frequencies () and magnetic fields (). For samples with x≈0.52−0.55, FMR data show clear evidence of ferromagnetism with high Curie temperatures . These samples demonstrate a complex magnetic anisotropy described phenomenologically as a combination of the essential second order easy plane anisotropy contribution and the additional fourth order uniaxial anisotropy contribution with easy direction normal to the film plane. The observed anisotropy is attributed to a polycrystalline (mosaic) structure of the films caused by the film-substrate lattice mismatch. The existence of extra strains at the crystallite boundaries initiates a random distribution of local in-plane anisotropy axes in the samples. As a result, the symmetry of the net magnetic anisotropy is axial with the symmetry axis normal to the film plane. The principal features of the observed anisotropy are explained qualitatively within the proposed microscopic model.}

We present the magnetic properties of (Y1-xNdx) 2BaNiO5 (x=1, 0.15) investigated by means of specific heat, magnetic susceptibility, and spectroscopic measurements. Magnetic ordering occurs at 47 K and 13 K in the compounds with x=1 and x=0.15, respectively. We estimate the magnetic contribution of the neodymium subsystem to magnetization and specific heat using temperature dependences of the splitting of ground Kramers doublet of Nd3+ ion obtained from spectroscopic experiment. We show that both Nd-Ni and Nd-Nd interactions should be taken into account. An origin of the observed spin-glass state below ∼5 K and a contribution of the Ni chain breaks to the magnetization and specific heat are discussed. © 2012 Elsevier B.V. All rights reserved.

We report on the high-resolution spectroscopic study of multiferroic ErFe3(BO3)4. The energies of all eight Kramers doublets of the ground 4I15/2 multiplet of the Er3+ ion were determined by the high-resolution 4I13/2 → 4I15/2 infrared luminescence spectra. The spectroscopically determined temperature dependence of the splitting of the ground Kramers doublet was used to calculate the contribution of the erbium subsystem into the specific heat and the magnetic susceptibility of erbium iron borate. The analysis of the thermodynamic properties based on these calculations allowed us to suggest the domain structure in the easy-plane antiferromagnetically ordered iron subsystem, with two magnetically nonequivalent erbium positions in each domain.

We report on the high-resolution spectroscopic study of multiferroic ErFe3(BO3)4. The energies of all eight Kramers doublets of the ground 4I15/2 multiplet of the Er3+ ion were determined by the high-resolution 4I13/2 → 4I15/2 infrared luminescence spectra. The spectroscopically determined temperature dependence of the splitting of the ground Kramers doublet was used to calculate the contribution of the erbium subsystem into the specific heat and the magnetic susceptibility of erbium iron borate. The analysis of the thermodynamic properties based on these calculations allowed us to suggest the domain structure in the easy-plane antiferromagnetically ordered iron subsystem, with two magnetically nonequivalent erbium positions in each domain.

Motivated by recent experiments on quantum magnet NiCl2-4SC(NH2)2 (DTN) and its Br-doped counterpart DTNX we propose a theoretical description of optical magnon branch in the antiferromagnet with large single-ion anisotropy in the magnetically ordered phase. In the framework of the 1/S expansion we derive analytical expressions for optical magnon with \bf{k} = 0 energy magnetic field dependence \Delta(h). It is shown that in the linear spin wave approximation \Delta(h) is monotonic without extrema whereas first order in 1/S corrections makes it drastically different function with a minimum near the center of magnetically ordered phase. The latter behaviour was observed in ESR experiments. Moreover, we show that \Delta(h) has nontrivial dependence on the system parameters. It solves the discrepancy between inelastic neutron scattering data where the growth of interaction constants in DTNX with small Br concentration was observed and ESR experiments showing almost unchanged \Delta(h) in comparison with pure DTN.

We discuss theoretically a frustrated Heisenberg antiferromagnet in magnetic field close to the saturation one. It is demonstrated that a small biaxial anisotropy and/or the magnetic dipolar interaction produce a delicate balance between phases with a commensurate canted, incommensurate helical (conical), and fan spin orderings. As a result, different sequences of phase transitions are realized depending on values of these small anisotropic interactions. We derive analytical expressions for critical fields and ground-state energies of the phases which are in a quantitative agreement with our and previous Monte-Carlo simulations.

We investigate the critical temperature *T**c* of F2/F1/S trilayers (Fi is a ferromagnetic metal and S is a singlet superconductor), where the long-range triplet superconducting component is generated at noncollinear magnetizations of the F layers. In this paper we demonstrate a possibility of the spin-valve effect mode selection (standard switching effect, the triplet spin-valve effect or reentrant Tc(α)Tc(α) dependence) by the variation of the F2/F1 interface transparency.

The analytical theory of density of states (DOS) in three-dimensional quantum magnets with the bond disorder is proposed based on the self-consistent T-matrix approximation (SCTMA). It successfully describes the DOS both for resonant and non-resonant scattering, whose emergence is governed by the ratio of scattering length and the average distance between impurities, which concentration is denoted as c. Corrections to the quasiparticle band gap in these cases are shown to scale as c^(2/3) and c, respectively. Moreover, the theory yields a semi-circle form of the DOS for the bound states inside the gap, which results in highly nontrivial DOS in the intermediate parameter region between the two limiting cases when the band DOS and the semi-circle overlap. Long-wavelength excitations are discussed. In the resonant regime their damping scales as c^(2/3), which, according to Ioffe-Regel criterion, corresponds to their localization. Applicability of the theory is illustrated by its quantitative agreement with the recent experimental data on spin-dimer system Ba_(3-x)Sr_xCr_2O_8.

Quasi-one-dimensional superconducting channels host sound-like plasma modes propagating along the which are associated with fluctuations of the phase of the superconducting order parameter (Mooiji and 1985) [5]. Interaction between these electromagnetic excitations and charge carriers affects the electron density of states (DOS). I-V characteristics of tunnel S1-I-S2 junctions, where superconducting S2 electrode nanowire in the regime of quantum fluctuations have been studied. The observed broadening of the I-V dependencies at the gap edge is interpreted as the renormalization of DOS. The results are in reasonable agreement with the model, taking into consideration plasma modes in quasi-one-dimensional superconductors.

Ferromagnetic-insulator (FI) based Josephson junctions are promising candidates for a coherent superconducting quantum bit as well as a classical superconducting logic circuit. Recently the appearance of an intriguing atomic-scale 0-pi transition has been theoretically predicted. In order to uncover the mechanism of this phenomena, we numerically calculate the spectrum of Andreev bound states in a FI barrier by diagonalizing the Bogoliubov-de Gennes equation. We show that Andreev spectrum drastically depends on the parity of the FI-layer number L and accordingly the pi (0) state is always more stable than the 0 (pi) state if L is odd (even).

Collective plasmon excitations in a helical electron liquid on the surface of strong three-dimensional topological insulator are considered. The properties and internal structure of these excitations are studied. Due to spin-momentum locking in helical liquid on a surface of topological insulator, the collective excitations should manifest themselves as coupled charge- and spin-density waves.

It is known that Dirac nodes can be present at high-symmetry points of Brillouin zone only for certain space groups. For these cases, the eect of strain is treated by symmetry considerations. The dependence of strain-induced potentials on the strain tensor is found. In all but two cases, the pseudomagnetic eld potential is present. It can be used to control valley currents.

The non-stationary response of local magnetic moment to abrupt switching ‘‘on” and ‘‘off” of external magnetic ﬁeld was studied for a single-level quantum dot (QD) coupled to a reservoir. We found that transient processes look different for the shallow and deep localized energy level. It was demonstrated that for deep energy level the relaxation rates of the local magnetic moment strongly differ in the case of magnetic ﬁeld switching ‘‘on” or ‘‘off”. Obtained results can be applied in the area of dynamic memory devices stabilization in the presence of magnetic ﬁeld

A review of our recent results on the spin valve effect is presented. We have used a theoretically proposed spin switch design F1/F2/S comprising a ferromagnetic bilayer (F1/F2) as a ferromagnetic component, and an ordinary superconductor (S) as the second interface component. Based on it we have prepared and studied in detail a set of multilayers CoO*x*/Fe1/Cu/Fe2/S (S=In or Pb). In these heterostructures we have realized for the first time a full spin switch effect for the superconducting current, have observed its sign-changing oscillating behavior as a function of the Fe2-layer thickness and finally have obtained direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the magnetizations of the Fe1 and Fe2 layers.

Static and dynamic magnetic properties of a ferrimagnetic [Fe(35Å)/Gd(50Å)]12 superlattice were investigated in a wide 4–300 K temperature range using magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR) techniques. The multilayer structure was sputtered on a transparent glass substrate which made it pos- sible to perform MOKE measurements on both Fe and Gd terminated sides of the superlattice. These experiments allowed us to detect a transition between field-aligned and canted magnetic states on both sides of the film and to distinguish between the bulk and surface twisted phases of the superlattice. As a result, the experimental H − T magnetic phase diagram of the system was obtained. FMR studies at frequencies 7–36 GHz demonstrated a complex evolution of absorption spectra as temperature decreased from room down to 4 K. Two spectral branches were detected in the sample. Theoretical simulations show that the observed spectral branches correspond to different types of inhomogeneous resonance modes in the multilayer with non-uniform magnetization precession inside Gd layers.