Spin-Superfluidity and Spin-Current Mediated Nonlocal Transport

Abstract

Some strategies for reducing energy consumption in information processing devices involve the use of spin rather than charge to carry information. This idea is especially attractive when the spin current is a collective one carried by the condensate of a magnetically ordered state rather than a quasiparticle current carried by electrons or magnons. In this chapter, we explain how easy-plane magnets can be viewed as Bose-Einstein condensates (BECs) of magnons, defined in terms of quanta of the spin-component perpendicular to the easy plane, and how they can carry dissipationless spin-currents that induce nonlocal interactions between electrically isolated conducting channels. We comment specifically on important differences between superconductivity in normal/superconducting/normal circuits and spin-superfluidity in normal/magnetic/normal circuits. Introduction Spintronics, the study of the interplay between the electrical transport and magnetic properties of magnetically ordered solids, has made steady progress over the past few decades. Spintronics involves both phenomena such as giant magnetoresistance, in which transport properties are influenced by magnetic order configurations, and phenomena such as spin-transfer torques in which transport currents can be used to modify magnetic configurations. Pure spin currents, which do not involve charge flow, are routinely detected via the spin-transfer torques they exert on magnetic condensates and the electrical signals they give rise to when spins accumulate near sample boundaries or at electrodes. There are hopes that spin currents have advantages over charge currents that can be exploited to enable faster or lower-power electronic devices. In this chapter, we discuss the notion of spinsuperfluidity in thin film magnetic systems, either ferromagnetic or antiferromagnetic and either metallic or insulating, that have approximate easy-plane magnetic order [1, 2, 3, 4, 5, 6, 7]. In spintronics, spin-superfluidity refers to the capacity for spin currents to be carried without dissipation by a metastable configuration of a magnetic condensate rather than by an electron or magnon quasiparticle current. Our chapter is organized as follows. In Section 27.2, we introduce the concept of spin superfluidity using the common language of magnetism researchers by applying Landau-Lifshitz equations to easy-plane magnets.