Self-compensation in manganese-doped ferromagnetic semiconductors

We present a theory of interstitial Mn in Mn-doped ferromagnetic semiconductors. Using density-functional theory, we show that under the non-equilibrium conditions of growth, interstitial Mn is easily formed near the surface by a simple low-energy adsorption pathway. In GaAs, isolated interstitial Mn is an electron donor, each compensating two substitutional Mn acceptors. Within an impurity-band model, partial compensation promotes ferromagnetic order below the metal-insulator transition, with the highest Curie temperature occurring for 0.5 holes per substitutional Mn.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let

Ferromagnet/superconductor layered superlattices

New 0π and ππ states with an antiparallel orientation of magnetization in the adjacent FM layers are predicted for ferromagnetic metal/superconductor (FM/S) superlattices. If the thickness ds.of the S layers is less than the threshold value dπs, these states have a higher critical temperature Tc than the earlier known 00 and π0 LOFF states. A new type of logic device combining the advantages of superconducting and magnetic recording channels in one sample is proposed on the basis of FM/S superlattices. Good quantitative agreement with known experimental phase diagrams is obtained

FM/S/FM/S system as the simplest superlattice logical device with two separating recording channels

We theoretically study the coexistence and mutual influence of magnetism and superconductivity in the ferromagnetic metal/superconductor (FM/S) superlattices (SLs). We found a physically interesting region of the theory parameters, making possible to manage the FM/S SLs superconducting and magnetic properties. We propose the principal scheme of the simplest FM/S/FM/S device which allows one to record information in two separated channels (superconducting and magnetic ones) in a common sample. It is shown that the SLs have more logically different variants of data record than the known FM/S/FM trilayer spin switch. © 2002 Elsevier Science B.V. All rights reserved

Multicritical behavior of the phase diagrams of ferromagnet/superconductor layered structures

For ferromagnet/superconductor (F/S) layered structures, new 3D Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states are predicted. In most cases, these states are characterized by a higher critical temperature Tc than the known 1D LOFF states. It is shown that the nonmonotonic behavior of Tc is determined by the oscillations of the Cooper pair flux through the F/S boundary, which occur as a result of the 3D-1D-3D phase transitions at the Lifshits triple points. The appearance of the new 3D LOFF states and the presence of nonmagnetic impurities leads to a strong damping of the 1D oscillations of the LOFF pair amplitude and to a considerable smoothing of the dependence of Tc on the F layer thickness df. An interpretation of the behavior of the experimental dependences Tc(df) obtained for F/S structures is proposed. © 2000 MAIK "Nauka/Interperiodica"

Adjustment of superconductivity and ferromagnetism in the few-layered ferromagnet-superconductor nanostructures

The phase diagrams of the few-layered nanosystems consisting of dirty superconducting (S) and ferromagnetic (F) metals are investigated within the framework of the modern theory of the proximity effect taking into account the boundary conditions. The F/S tetralayer and pentalayer are shown to have considerably richer physics than the F/S bi- and trilayer (due to the interplay between the 0 and π phase superconductivity and the 0 and π phase magnetism and nonequivalence of layers) and even the F/S superlattices. It is proven that these systems can have different critical temperatures and fields for different S layers. This predicted decoupled superconductivity is found to manifest itself in its most striking way for F/S tetralayer. It is shown that F/S/F'/S' tetralayer is the most perspective candidate for use in superconducting spin nanoelectronics

π-phase magnetism in ferromagnet-superconductor superlattices

New 0π and ππ Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states with antiferromagnetic orientation of magnetizations in the neighboring layers of a ferromagnetic metal (FM) are predicted for FM/superconductor (FM/S) superlattices. Under certain conditions, the critical temperature Tc of these states is higher than for the known 00 and π0 LOFF states with ferromagnetic ordering of the FM layers. It is shown that the nonmonotonic behavior of Tc in the FM/S superlattices with S-layer thickness ds less than the threshold value dπ s is due to the phase transition cascade 0π-ππ-0π. At ds > dπ s, the Tc oscillations are caused by the 00-π0-00 transitions. New logic elements based on the FM/S structures and combining the advantages of the superconducting and magnetic data-record channels in a single sample are proposed. © 2001 MAIK "Nauka/Interperiodica"

Competition between superconductivity and magnetism in ferromagnet/superconductor heterostructures

The mutual influence of superconductivity and magnetism in FS systems, i.e. systems of alternating ferromagnetic (F) and super-conducting (S) layers, is comprehensively reviewed. For systems with ferromagnetic metal (FM) layers, a theory of the proximity effect in the dirty limit is constructed based on the Usadel equations. For a FM/S bilayer and a FM/S superlattice, a boundary-value problem involving finite FM/S boundary transparency and the diffusion and wave modes of quasi-particle motion is formulated; and the critical temperature Tc is calculated as a function of FM- and S-layer thicknesses. A detailed analysis of a large amount of experimental data amply confirms the proposed theory. It is shown that the superconducting state of an FM/S system is a superposition of two pairing mechanisms, Bardin - Cooper - Schrieffer's in S layers and Larkin - Ovchinnikov - Fulde - Ferrell's in FM ones. The competition between ferromagnetic and antiferromagnetic spontaneous moment orientations in FM layers is explored for the 0- and π-phase superconductivity in FM/S systems. For FI/S structures, where FI is a ferromagnetic insulator, a model for exchange interactions is proposed, which, along with direct exchange inside FI layers, includes indirect Ruderman - Kittel - Kasuya - Yosida exchange between localized spins via S-layer conduction electrons. Within this framework, possible mutual accommodation scenarios for superconducting and magnetic order parameters are found, the corresponding phase diagrams are plotted, and experimental results explained. The results of the theory of the Josephson effect for S/F/S-contacts are presented and the application of the theory of spin-depending transport to F/S/F contacts is discussed. Application aspects of the subject are examined

Ferromagnet/superconductor superlattices as logical devices with two recording channels

For the ferromagnetic metal/superconductor (FM/S) superlattices, the new π magnetic states with antiferromagnetic ordering of the FM layers magnetizations are predicted. If the S layers thickness ds is less than the threshold value ds π, these new states have a higher critical temperature Tc than the earlier known ferromagnetic states (the 0 magnetic states). Therefore, the Tc oscillations origin at ds < ds π is due to the transitions cascade between the 0-π-0 types of superconductivity at π magnetism conditions. A new type of logical device combining the advantages of the superconducting and magnetic recording channels in one sample is offered on the FM/S superlattices base

Transverse instabilities of multiple vortex chains in superconductor-ferromagnet bilayers

Using scanning tunneling microscopy and Ginzburg-Landau simulations we explore vortex configurations in magnetically coupled NbSe$_2$-Permalloy superconductor-ferromagnet bilayer. The Permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes non-linear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices `levitating' in the anti-pinning channels.Comment: accepted in PRB-Rapid

Adjustment of superconductivity and ferromagnetism in few-layered ferromagnet-superconductor nanostructures

The phase diagrams of few-layered nanosystems consisting of dirty superconducting (S) and ferromagnetic (F) metals are investigated within the framework of the modern theory of the proximity effect taking into account the boundary conditions. The F/S tetralayer and pentalayer are shown to have considerably richer physics than the F/S bi- and trilayer (due to the interplay between the 0 and π phase superconductivity and the 0 and π phase magnetism and nonequivalence of layers) and even the F/S superlattices. It is proven that these systems can have different critical temperatures and fields for different S layers. This predicted decoupled superconductivity is found to manifest itself in its most striking way for F/S tetralayer. It is shown that F/S/F′/S′ tetralayer is the most promising candidate for use in superconducting spin nanoelectronics. © 2006 American Institute of Physics