Changing Exchange Bias in Spin Valves with an Electric Current Z. Wei, 1 A. Sharma, 2 A. S. Nunez, 1 P. M. Haney, 1 R. A. Duine, 1,3 J. Bass, 2 A. H. MacDonald, 1 and M. Tsoi 1 1 Physics Department, University of Texas at Austin, Austin, Texas 78712, USA 2 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA 3 Institute for Theoretical Physics, Utrecht University, Leuvenlaan 4, 3584 CE Utrecht, The Netherlands (Received 22 September 2006; published 16 March 2007) We show that a high-density electric current, injected from a point contact into an exchange-biased spin valve, systematically changes the exchange bias. The bias can either increase or decrease depending upon the current direction. This observation is not readily explained by the well-known spin-transfer torque effect in ferromagnetic metal circuits, but could be evidence for the recently predicted current-induced torques in antiferromagnetic metals. DOI: 10.1103/PhysRevLett.98.116603 PACS numbers: 72.25.Ba, 73.63.Rt, 75.47.m, 85.75.d An electrical current can transfer spin angular momen- tum to a ferromagnet [1– 6]. This novel physical phenome- non, called spin transfer or spin torque, offers unprece- dented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. It was recently predicted [7] that current- induced torques are a general property of magnetic metals not limited to ferromagnets (FM) and, in particular, that spin torques act on the order parameter of antiferromag- netic (AFM) circuit elements. Unlike spin torques in a FM metal, which follow from conservation of total spin and act only near interfaces, current-induced torques in AFM met- als are not related to total spin conservation and have a bulk contribution [7]. In this Letter we show that a high-density dc current injected from a point contact into an exchange-biased spin valve (EBSV) [8] can systematically change the exchange bias [9 –11], increasing or decreasing it depending upon the current direction. From observations of qualitatively simi- lar behaviors in spin valves (SV) with equal (symmetric SV) and unequal (asymmetric SV) FM-layer thicknesses, we infer that the source of these changes is almost certainly not the usual spin torque exerted by the current on the FM layers. Our data can instead be explained qualitatively in terms of a current-induced torque acting on magnetic mo- ments in the AFM component of SV structure. This new effect could be used to control the magnetic state of spin- valve devices, e.g., in magnetic memory applications. Because of their exceptional responsiveness to magnetic fields, EBSVs are chosen for devices such as magnetic field sensors, read heads in hard drives, and galvanic isolators. Our EBSVs consist of two Co(9%Fe) FM layers separated by a nonmagnetic (N) Cu-spacer thick enough that ex- change coupling between the FM layers should be small. The magnetization direction of one FM layer is ‘‘pinned’’ in a fixed direction by the presence of an adjacent FeMn AFM layer [9 –11], while the magnetization direction of the other FM layer is free to switch from parallel (P) to antiparallel (AP) to that of the first layer. Such spin valves exhibit giant magnetoresistance [12]; i.e., the SV resist- ance is smallest for P alignment of the two magnetizations and largest for AP alignment. Switching from P to AP is achieved by applying an external magnetic field in the plane of the layers. To generate a high-density electrical current, we use point contacts. Point contacts were instrumental both for the original observation of spin transfer in ferromag- netic materials [3] and in probing high-frequency mani- festations of this phenomenon [13 –15]. The extremely small size, less than a trillionth of a square cm, quali- fies point contacts as the smallest probes of spin trans- fer phenomena, enabling current densities up to 10 13 A=m 2 . Our point contacts were made with a standard system [3,16], using a sharpened Cu wire and a differential screw mechanism to move the Cu tip toward a FeMn=CoFe=Cu=CoFe EBSV. The spin-valve structures were sputtered onto Si substrates using techniques de- scribed previously [17], and had a 5 nm thick Au protective cap and a thick (50 or 100 nm) Cu underlayer. The latter was used to secure a closely perpendicular-to-plane flow of the current (CPP) from the point contact, across the spin valve, and into the Cu buffer. Three standard SVs: (I) FeMn8 nm=CoFe3 nm=Cu10 nm=CoFe10 nm, (II) FeMn3 nm=CoFe3 nm=Cu10 nm=CoFe10 nm, (III) FeMn8 nm=CoFe3 nm=Cu10 nm=CoFe3 nm and two inverted SV structures: (IV) CoFe10 nm= Cu10 nm=CoFe3 nm=FeMn8 nm, (V) CoFe3 nm= Cu10 nm=CoFe3 nm=FeMn8 nm were studied. SVs (I), (II), and (IV) are asymmetric, and SVs (III) and (V) are symmetric. The samples were cooled through the Ne ´el temperature of FeMn (T N  400 K) in the presence of a static magnetic field (  18 mT) and zero applied current, to pin the magnetization direction of the neighboring CoFe. A total of 29 point contacts with resistances from 0:7–5 showed the characteristic behaviors that we describe with the help of representative data from a 0:92  contact to sample (I), a 2:72  contact to sample (III), and a 1:59  contact to sample (IV). At room temperature and in magnetic fields B up to 0.1 T applied along the exchange-bias direction, we have mea- sured the magnetoresistance (MR) of point contacts at PRL 98, 116603 (2007) PHYSICAL REVIEW LETTERS week ending 16 MARCH 2007 0031-9007= 07=98(11)=116603(4) 116603-1  2007 The American Physical Society