Injection of a Nonequilibrium Spin into a Helicoidal Ferromagnet
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Injection of a Nonequilibrium Spin into a Helicoidal Ferromagnet E. A. Karashtina, b, * a
Institute for Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod, 603087 Russia b Lobachesky University, Nizhny Novgorod, 603105 Russia *e-mail: [email protected] Received March 26, 2020; revised March 26, 2020; accepted April 2, 2020
Abstract—The features of spin injection into a helicoidal ferromagnet are investigated. Two methods of spin injection are considered: injection of a spin-polarized current and spin pumping. For the case when the axis of the helicoid is perpendicular to the interface through which the spin injection occurs, the conditions are determined under which electrons fill the specified spin subband. For the case when the axis of the helicoid is parallel to the interface, the occurrence of an effect similar to the topological Hall effect is demonstrated. In the second geometry of spin pumping, we discovered the phenomenon of conversion of the injected spin current into an electric current, flowing along the axis of the helicoid, due to the exchange interaction in a ferromagnet. Keywords: noncollinear ferromagnet, spin pumping, topological Hall effect, spin injection, exchange interaction DOI: 10.1134/S1063783420090127
1. INTRODUCTION A nonequilibrium spin can be injected into a conducting medium in several ways. One of them is passing an electric current through a system consisting of two ferromagnets (a conductor into which it is necessary to inject a spin, and a uniformly magnetized medium used as a polarizer of free charge carriers along the spin), separated by a conducting or tunnel layer [1–5]. Such a method is used, for example, in tunneling magnetic contacts to read information or remagnetize them by the current [6–9]. In this case, the nonequilibrium spin is injected from one uniformly magnetized ferromagnet to another. Such a spin-dependent injection of charge carriers makes the resistance to depend on the mutual orientation of the magnetizations (ferromagnetic or antiferromagnetic state). At a high electric current density, the injected spin entrains the magnetization of the ferromagnet, which leads to the magnetization reversal of the ferromagnetic layer. Another possible way is the spin pump phenomenon, which consists of the occurrence of a spin flux from a ferromagnet to a metal adjacent to it upon excitation of ferromagnetic resonance (FMR) in the magnetic material. This effect was first proposed in 2002 [10, 11] and since then, it has been studied quite a lot both theoretically and experimentally [12]. The first works showed that this effect broadens the line of absorption of microwave radiation by a ferromagnet at ferromagnetic resonance (FMR). In the medium into which the spin is injected, a potential difference arises
in the direction perpendicular to the equilibrium magnetization vector of the spin source and the normal to the medium interface. The mechanisms of this phenomenon can be the inverse spin Hall effect [13] or the Rashba–Edelstein effect [14]. The
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