Spin Pumping and Spin-Transfer Torques in Antiferromagnets

Abstract

Spin pumping and spin-transfer torques are two reciprocal phenomena widely studied in ferromagnetic materials. However, pumping from antiferromagnets and its relation to current-induced torques have not been explored. By calculating how electrons scatter off a normal metal-antiferromagnetic interface, we derive pumped spin and staggered spin currents in terms of the staggered field, the magnetization, and their rates of change. For both compensated and uncompensated interfaces, spin pumping is of a similar magnitude as in ferromagnets with a direction controlled by the polarization of the driving microwave. The pumped currents are connected to current-induced torques via Onsager reciprocity relations. PACS numbers: 76.50.+g, 72.25.Mk, 75.50.Ee A major task of spintronics is understanding the mutual control of spin transport and magnetic properties. This inspires intense studies in fundamental physics which opens new avenues in, e.g., magnetic recording technologies. A new direction in this field aims at harnessing spin dynamics in materials with a vanishing magnetization, such as antiferromagnets (AFs) with compensated magnetic moments on an atomic scale. As compared to ferromagnets (Fs), AFs operate at a much higher frequency in the Tera Hertz (THz) ranges [1-3] which makes it possible to perform ultra fast information processing and communication. At the same time, since there are no stray fields in AFs, they are more robust against magnetic perturbations, an attractive feature of AFs for use in next-generation data storage material. However, to build a viable magnetic device using AF, it is vital to find observable effects induced by the rotation of the order parameter. The recent discovery of tunneling anisotropic magnetoresistance in AF may potentially fulfill this demand [4,5]. Nevertheless, in such experiments, the AF is dragged passively by an adjacent F, which is rotated by a magnetic field. Will an AF interact directly with (spin) currents without the inclusion of a F or a magnetic field?Partial answers are available from recent investigations. While the observation of a current-induced change of the exchange bias on a F|AF interface indicates spintransfer torques (STTs) in AFs [6,7], theoretical models of STT have been developed in a variety of contexts [8][9][10][11][12][13][14][15]. To achieve a general understanding of spintronics based on AFs, we recall a crucial insight from well-established ferromagnetic spintronics: STT and spin pumping are two reciprocal processes intrinsically connected [16][17][18]; they are derivable from each other [19]. To the best our knowledge, all existing studies on AF have focused on STT, whereas spin pumping has received no attention because it seems to be naively believed that the vanishing magnetization spoils any spin pumping in an AF.Spin pumping is the generation of spin currents by the precessing magnetization [18,19]. When the magnetization m of a F varies in time, a spin current proportional to m ×ṁ is pumped into an adjacent normal (N) metal. In contrast, m...

Spin pumping and inverse spin Hall voltages from dynamical antiferromagnets

“…Unlike the treatment of Ref. [20], in NiO, there are two significant anisotropies to consider. As a starting point in the exploration of high-frequency spintronics, it is also important to tune the resonance frequencies to a lower gigahertz range for easier detection by conventional electronics.…”

“…Our main findings are that, when applying an external magnetic field along the easy axis close to the spin-flop transition, we can decrease the resonance frequency while simultaneously significantly increasing the inverse spin Hall signal. The increase in the signal can even overcome the previously anticipated limiting factor in antiferromagnet spin pumping: the ratio of the anisotropic energy to the exchange energy [20].We consider a small antiferromagnet in the macrospin limit whereby all spin excitations are homogeneous. The antiferromagnet has two sublattices, with temporal magnetizations M 1 and M 2 .…”

“…* Corresponding author: oyvinjoh@ntnu.no Furthermore, there is a large variety of antiferromagnets and external field configurations that require knowledge beyond the first predictions of the magnitude of the pumped spin current of Ref. [20]. Recently, researchers explored spin transport through, e.g., the insulating antiferromagnets NiO and MnF 2 .…”

“…Furthermore, this result is valid even when the magnetic system is insulating. The firm electron-magnon coupling at the interface opens the door for electrical probing of the ultrafast spin dynamics in antiferromagnets [20,21].…”

Magnon-photon coupling in antiferromagnets

“…18 It was recently reported that spin pumping exists also at the interface of AFM/normal metal. 25 In fact, AFM layer may even enhance the spin pumping. Thus, electrical detection of the coupling signal in AFM is also possible in the hybridized structures.…”