Spin Hall voltages from a.c. and d.c. spin currents

ARTICLE Received 6 Nov 2013 | Accepted 31 Mar 2014 | Published 30 Apr 2014 DOI: 10.1038/ncomms4768 OPEN Spin Hall voltages from a.c. and d.c. spin currents Dahai Wei1,*, Martin Obstbaum1,*, Mirko Ribow1,2, Christian H. Back1 & Georg Woltersdorf1,2 In spin electronics, the spin degree of freedom is used to transmit and store information. To this end the ability to create pure spin currents—that is, without net charge transfer—is essential. When the magnetization vector in a ferromagnet–normal metal junction is excited, the spin pumping effect leads to the injection of pure spin currents into the normal metal. The polarization of this spin current is time-dependent and contains a very small d.c. component. Here we show that the large a.c. component of the spin currents can be detected efficiently using the inverse spin Hall effect. The observed a.c.-inverse spin Hall voltages are one order of magnitude larger than the conventional d.c.-inverse spin Hall voltages measured on the same device. Our results demonstrate that ferromagnet–normal metal junctions are efficient sources of pure spin currents in the gigahertz frequency range. 1 Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätsstrae 31, 93053 Regensburg, Germany. 2 Institut für Physik, Martin-Luther-Universität Halle, von-Danckelmann-Platz 3, 06120 Halle, Germany. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to G.W. (email: ). NATURE COMMUNICATIONS | 5:3768 | DOI: 10.1038/ncomms4768 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4768 F or spin electronic technology, the ability to create pure spin currents—that is, without net charge transfer—is essential. Spin pumping is the most popular approach to generate pure spin currents in metals1–5, semiconductors6,7, graphene8 and even organic materials9. When the magnetization vector in a ferromagnet (FM)–normal metal (NM) junction is excited at ferromagnetic resonance (FMR), spin pumping leads to the injection of pure spin currents in the NM. The polarization of this spin current is time-dependent1 and contains a very small d.c. component10, as illustrated in Fig. 1. Spin torque corresponding to the a.c. component has been observed by magneto-optical11 and X-ray methods12, while the spin accumulation because of the d.c. component was observed by light scattering13. Recently, also d.c. voltage signals in ferromagnetic insulator/ferromagnetic conductor bilayers have been interpreted as spin rectification in the ferromagnetic conductor material14. These experiments provide strong evidence for the presence of a large a.c. component of the spin current generated by spin pumping. The d.c. component of the injected spin current has been intensely studied in recent years and given rise to controversial discussions concerning the magnitude of the spin Hall angle, which is a material-dependent measure of the efficiency of spin-to-charge current conversion15,16. However, in contrast to the rather well-understood d.c. component4,5,17 the two orders of magnitude larger a.c. component has escaped experimental detection so far18. The time dependence of the polarization of a spin current injected by spin pumping is related to the dyamics of the magnetization vector m and given by rBm  dm/dt (ref. 1) as illustrated in Fig. 1. The absorption of a spin current in a nonmagnetic metal with a finite spin Hall effect leads to an electric field E and is referred to as the inverse spin Hall effect (ISHE). The voltage UISHE transverse to the spin current JS and spin polarization r is: UISHE  E  JS r: ð1Þ Therefore, the d.c. and a.c.-ISHE voltage components may be measured as shown in Fig. 1. In the following, we demonstrate experimentally the presence of a large a.c. component in the ISHE voltage signal in NM/FM bilayers, where the a.c. spin current is generated by spin pumping at FMR. The magnitude of the a.c.-ISHE signal is measured as a function of frequency, angle and power. In addition, the d.c.- and FM NM m(t) H U(t) σ(t) Udc x z y Figure 1 | Spin pumping and ISHE voltage signal. A spin current is generated by spin pumping at the FM–NM interface (grey arrows). The time-dependent spin polarization of this current (indicated as purple arrow) rotates almost entirely in the y–z plane. The small time-averaged d.c. component (yellow arrow) appears along the x axis. Due to the inverse spin Hall effect both components lead to charge currents in NM and can be converted into a.c. and d.c. voltages by placing probes along the x and y directions, respectively. 2 a.c.-ISHE signals are measured in the same device in order to compare their relative amplitudes. The spectral shape, angular dependence, power scaling behaviour and absolute magnitude of the signals are in line with spin pumping and ISHE effects. Our results demonstrate that FM–NM junctions are very efficient sources of pure spin currents in the GHz frequency range and we believe that our result will stimulate the development of a.c. spintronics18,19. Results Experimental setup. The experimental configuration is shown in Fig. 2a, the NM–FM bilayer stripes are either integrated on top of the signal line or in the gap between the signal and ground lines of a grounded coplanar waveguide (CPW). In these two configurations, the magnetization in the FM is excited by an in-plane and out-of-plane microwave magnetic field hrf, respectively. The difficulty to detect the a.c.-ISHE signal lies in the ability to measure sub-mV GHz signals and isolate them from a large background signal caused by the excitation of FMR at the same frequency. As sketched in Fig. 2a, the microwave signal is transmitted from terminal 1 to terminal 2, where FMR can be measured inductively. In order to measure a.c.-ISHE signals, the NM–FM stripe is connected to a 50-O waveguide (terminal 3). In addition, the sample structure was designed as a transmission line (as microstrip for inplane excitation and as CPW for out-of-plane excitation) such that the a.c.-ISHE voltage signal can propagate along the NM–FM stripe. The microwave signal isolation from terminal 1 to terminal 3 is only about 10 dB and is frequency-dependent (as shown in Supplementary Fig. 1) leading to a large crosstalk a.c. signal amplitude on terminal 3. This signal is 2 orders of magnitude larger than the expected a.c.-ISHE signal. In order to suppress the background signal, an additional reference signal is added in a power combiner where amplitude and phase can be adjusted to almost fully compensate the crosstalk signal. The expected ISHE signal has a magnitude in the mV range allowing for detection by a power meter (detection scheme 1) or by a rectifying diode and a lock-in amplifier (detection scheme 2). For lock-in detection the static magnetic field is modulated with an amplitude of 0.5 mT. (...truncated)