PHYSICAL REVIEW B 88, 214405 (2013) Pure spin current-induced domain wall motion probed by localized spin signal detection Nils Motzko, 1 Bj¨ orn Burkhardt, 1 Nils Richter, 1 Robert Reeve, 1 Piotr Laczkowski, 2 Williams Savero Torres, 2 Laurent Vila, 2 Jean-Philippe Attan´ e, 2,3 and Mathias Kl¨ aui 1,* 1 Institut f ¨ ur Physik, Johannes Gutenberg-Universit¨ at Mainz, 55099 Mainz, Germany 2 INAC, CEA Grenoble, 17 avenue des Martyrs, 38054 Grenoble, France 3 Universite Joseph Fourier, BP 53, 38041 Grenoble, France (Received 28 September 2013; published 10 December 2013) We demonstrate the displacement of domain walls via pure diffusive spin currents in a nonlocal spin valve geometry, without any externally applied fields. We implement a localized detection of the domain wall position by simultaneous nonlocal spin signal measurements using contacts on both sides of the spin current conduit, which allows us to determine the domain wall position even underneath the spin current conduit. Using this detection method, we probe the domain wall position as it moves across the spin current conduit when sweeping a field or on current application. Injecting pure spin currents in the nonlocal architecture, we find that in our optimized geometry we can displace a transverse head-to-head or tail-to-tail domain wall without any externally applied fields at effective spin currents <10 10 A/m 2 , showing that this method can be a viable avenue to low-power domain wall manipulation. DOI: 10.1103/PhysRevB.88.214405 PACS number(s): 72.25.−b, 75.60.Ch The manipulation of magnetization using spin currents currently receives significant interest due to the fundamental interaction of spin currents with magnetization that leads to spin transfer torques and, on the application side, due to the favorable scaling of this approach for devices in terms of energy requirements. 1 The now well-established spin transfer torque using spin-polarized charge currents was predicted some time ago, 2,3 and was more recently experimentally shown to be able to displace spin structures such as domain walls 4–6 using charge currents that become spin polarized in the magnetic material. For the spin torque effect, however, one is only interested in the spin current with the charge current only leading to unwanted ohmic losses and resulting Joule heating, which can disturb the spin structure due to heating of the material close to its Curie temperature. 7 So approaches that reduce the Joule heating have been sought and, in recent years, the focus has shifted to pure diffusive spin currents without net charge currents flowing. While this still entails a certain amount of Joule heating during the spin current generation, no net charge current flows at the position of the domain wall and thus the heating at the domain wall position is reduced. In general, to use such diffusive spin currents, they need to be first generated in a spin current source, transported, for instance, in a nonmagnetic spin current conduit and eventually used to manipulate magnetization. The magnetization acts as a spin current sink, absorbing the spin current, which reciprocally leads to the exertion of a torque on the magnetization. There are various ways to generate such spin currents. Sources include the spin Hall effect, where a spin accumulation is generated at the surfaces of materials with large spin-orbit coupling, and this can then be injected into a spin current conduit that carries the spin current. 8 Dynamic spin pumping can inject pure spin currents from ferromagnets into spin conduits 9 and femtosecond laser excitations can generate superdiffusive spin currents, 10 which were shown to manipulate, for instance, domain wall profiles on ultrafast time scales. 11 The most widely used approach to date is nonlocal spin injection in a lateral spin valve [see Fig. 1(a)]. In this configuration, two ferromagnetic elements are connected by a nonmagnetic spin current conduit. By injecting a combined spin and charge current from one ferromagnet (in our case, FM1 at the bottom) into the paramagnetic spin current conduit (SCC), a spin accumulation is generated at the interface between the ferromagnet and the SCC, and this accumulation diffuses as a pure diffusive spin current in all directions, including along the SCC towards the top ferromagnet FM2. 12–14 At the interface between the SCC and FM2, the spin current is absorbed into the ferromagnet due to the low resistance of the ohmic contact between the SCC and FM2 leading to a low spin resistance of the ferromagnet. 15 This absorbed spin current then exerts a torque on the magnetization in the absorbing ferromagnet. It has been shown that the absorbed spin current can reverse the magnetization of a small disk. 16 We demonstrated a high efficiency for domain wall motion, 17 as the spins in the spin current exhibit a large angle with respect to the magnetization direction in the small domain wall volume where they are absorbed, thereby exerting a large torque. This can lead to an efficiency that can be orders of magnitude larger than for combined spin and charge currents flowing in the ferromagnet. 17 Of course the domain wall motion using pure diffusive spin currents can only be induced while the domain wall is (at least partially) underneath the spin conduit because the spin-diffusion length in permalloy (Py) is of the order of a few nm. 18 Previously, we detected pure spin current-assisted depinning of a domain wall from a position underneath the SCC using a local-detection scheme based on the anisotropic magnetoresistance (AMR) in the ferromagnet. 17 While a very high spin transfer efficiency was found, this measurement was only able to detect a displacement of the domain wall from the area below the SCC to the outside area between the SCC and one of the adjacent contacts. This is due to the measurement scheme, where we probed the resistance of the FM area between the SCC and an adjacent contact and this resistance only changes if the wall moves into this area and is no longer underneath the SCC. Thus, such a measurement would not be able to detect a motion of the domain wall underneath the spin conduit as needed to demonstrate pure spin current-induced motion. Such pure 214405-1 1098-0121/2013/88(21)/214405(7) ©2013 American Physical Society