Magnetic properties of a long, thin-walled ferromagnetic nanotube

We consider magnetic properties of a long, thin-walled ferromagnetic nanotube. We assume that the tube consists of isotropic homogeneous magnet whose spins interact via the exchange energy, the dipole-dipole interaction energy, and also interact with an external field via Zeeman energy. Possible stable states are the parallel state with the magnetization along the axis of the tube, and the vortex state with the magnetization along azimuthal direction. For a given material, which of them has lower energy depends on the value \gamma=R^2d/(L \lambda_x^2), where R is the radius of the tube, d is its thickness, L is its length and \lambda_x is an intrinsic scale of length characterizing the ration of exchange and dipolar interaction. At \gamma<1 the parallel state wins, otherwise the vortex state is stable. A domain wall in the middle of the tube is always energy unfavorable, but it can exist as a metastable structure. Near the ends of a tube magnetized parallel to the axis a half-domain structure transforming gradually the parallel magnetization to a vortex just at the edge of the tube is energy favorable. We also consider the equilibrium magnetization textures in an external magnetic field either parallel or perpendicular to the tube. Finally, magnetic fields produced by a nanotube and an array of tubes is analyzed

Anisotropic Transport Properties of Ferromagnetic-Superconducting Bilayers

We study the transport properties of vortex matter in a superconducting thin film separated by a thin insulator layer from a ferromagnetic layer. We assume an alternating stripe structure for both FM and SC layers as found in [7]. We calculate the periodic pinning force in the stripe structure resulting from a highly inhomogeneous distribution of the vortices and antivortices. We show that the transport properties in FM-SC bilayer are highly anisotropic. In the absence of random pinning it displays a finite resistance for the current perpendicular to stripes and is superconducting for the current parallel to stripes. The average vortex velocity, electric field due to the vortex motion, Josephson frequency and higher harmonics of the vortex oscillatory motion are calculated.Comment: 4 pages, 2figures, Submitted to PR

Domain walls in helical magnets

The structure of domain walls determines to a large extent the properties of magnetic materials, in particular their hardness and switching behavior, it represents an essential ingredient of spintronics. Common domain walls are of Bloch and Neel types in which the magnetization rotates around a fixed axis, giving rise to a one-dimensional magnetization profile. Domain walls in helical magnets, most relevant in multiferroics, were never studied systematically. Here we show that domain walls in helical magnets are fundamentally different from Bloch and Neel walls. They are generically characterized by a two-dimensional pattern formed by a regular lattice of vortex singularities. In conical phases vortices carry Berry phase flux giving rise to the anomalous Hall effect. In multiferroics vortices are charged, allowing to manipulate magnetic domain walls by electric fields. Our theory allows the interpretation of magnetic textures observed in helical magnetic structures

Spin Correlations in Quantum Wires

We consider theoretically spin correlations in an 1D quantum wire with Rashba-Dresselhaus spin-orbit interaction (RDI). The correlations of non-interacting electrons display electron-spin resonance at a frequency proportional to the RDI coupling. Interacting electrons on varying the direction of external magnetic field transit from the state of Luttinger liquid (LL) to the spin density wave (SDW) state. We show that the two-time total spin correlations of these states are significantly different. In the LL the projection of total spin to the direction of the RDI induced field is conserved and the corresponding correlator is equal to zero. The correlators of two components perpendicular to the RDI field display a sharp ESR driven by RDI induced intrinsic field. In contrast, in the SDW state the longitudinal projection of spin dominates, whereas the transverse components are suppressed. This prediction indicates a simple way for experimental diagnostic of the SDW in a quantum wire.Comment: 19 pages, 1 figur

Density of states and order parameter in dirty anisotropic superconductors

Journals published by the American Physical Society can be found at http://journals.aps.org/We analyze in detail how the scattering by nonmagnetic impurities affects the shape and amplitude of the order parameter (OF) and the density of states in anisotropic superconductors in the framework of BCS theory. Special attention is paid to the case when the OP is a mixture of d and s waves changing its sign on the Fermi surface. The critical temperature is shown to decay with the increase of the residual resistance according to the power law. At zero temperature impurity scattering gives rise to a peculiar phase transition from a gapless regime to a state with a finite gap in the quasiparticle spectrum

Skyrmion in a real magnetic film

Journals published by the American Physical Society can be found at http://journals.aps.org/Skyrmions are magnetic defects in ultrathin magnetic films, similar to the bubble domains in the thicker films. Even weak uniaxial anisotropy determines their radii unambiguously. We derive equations of slow dynamics for Skyrmions. We show that the discreteness of the lattice in an isotropic two-dimensional magnet leads to a slow rotation of the local magnetization in the Skyrmion and even a small dissipation leads to decay of the Skyrmion. The radius of such a Skyrmion as a function of time is calculated. We prove that uniaxial anisotropy stabilizes the Skyrmion and study the relaxation process. [S0163-1829(98)50438-9]

Spin transitions in time-dependent regular and random magnetic fields

Journals published by the American Physical Society can be found at http://journals.aps.org/We study the transition between Zeeman levels of an arbitrary spin placed into a regular time-dependent magnetic field and a random field with the Gaussian distribution. One component of the regular field changes its sign at some moment of time, whereas another component does not change substantially. The noise is assumed to be fast. In this assumption we find analytically the ensemble average of the spin density matrix and its fluctuations

Stripe Domain-Structures in a Thin Ferromagnetic Film

Journals published by the American Physical Society can be found at http://journals.aps.org/We present a theory of the stripe domain structure in a thin ferromagnetic film with single-ion easy-axis magnetic anisotropy and long-range dipole interactions, for a wide range of temperatures and applied magnetic field. The domains exist at temperatures below the reorientational phase transition from out-of-plane to in-plane magnetization. The system of stripes can be described as a liquid crystal with a preferred domain-wall orientation. The positional order is destroyed by both thermodynamical meandering of domain walls and by the proliferation of dislocations. Spatial anisotropy generated by the fourth-order exchange energy stabilizes the stripe domain structure and pins its orientation. For any temperature below the reorientational phase transition there exists a critical perpendicular-to-plane magnetic field, which separates multidomain and monodomain states of the film. The theory explains recent experimental observations