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

Slow inter-minima relaxation and its consequence for BEC of magnons

Two recent articles of the Munster University experimental team led by S.O. Demokritov displayed several important facts related to the Bose-Einstein condensation of magnons (BECM) under permanent pumping first discovered in 2006. They contradicted existing theories of this phenomenon, which predict the attractive interaction between magnons and strong spontaneous violation of the reflection symmetry. In this article, we show that these theories implicitly assumed all relaxation processes to be fast compared with the lifetime of the magnons, whereas one of them -- relaxation between two minima of energy -- is slow. We classify processes responsible for the inter-minima relaxation and present their analytic theory. We analyze how the slow inter-minima relaxation modifies the anticipated properties of a ferromagnet with the magnon condensate