Spintronics is a new technological discipline which refers to studying the role played by an electron spin in solid state physics and aims at developing a revolutionary new class of electronic devices based on the spin degree of freedom of the electron in addition to, or in place of, the charge. Spintronics is built on a complementary set of phenomena in which the magnetic configuration of a system influences its transport properties and vice versa. In ferromagnetic (F) systems these interconnections are exemplified by Giant Magnetoresistance (GMR) – where the system’s resistance depends on the relative orientation of magnetic moments in constituent F-parts [1], and Spin Transfer Torque (STT) – in which an electrical current can perturb the system’s magnetic state [2]. Recently, corresponding effects were proposed [3] to occur in systems where F-components are replaced with antiferromagnets (AFM), thus leading to antiferromagnetic GMR and STT effects.
In this talk I will focus on STT which may be the method of choice to control and manipulate magnetic moments in future spintronic devices. I will describe several experiments where transport currents alter the magnetic state in various solid state systems:
(i) In ferromagnetic/nonmagnetic (F/N) multilayers a dc electrical current can switch and/or drive its constituent F parts into high-frequency precession [2] which is of interest for microwave and magnetic recording technologies. Interestingly, application of high-frequency currents can also drive the multilayer, e.g. into STT-driven ferromagnetic [4] and parametric [5] resonances.

(ii) In antiferromagnetic (AFM) systems I will focus on our experiments with exchange-biased spin valves [6] where extreme current densities were found to affect the exchange bias at F/AFM interfaces. As exchange bias is known to be associated with interfacial AFM magnetic moments, this observation can be taken as the first evidence of STT in AFM materials and first step towards all-AFM spintronics.

(iii) Finally I will discuss STT associated with an electric current traversing a magnetic domain wall that can drive it into motion [7]. The inverse effect, in which an emf is induced by a moving domain wall, has also been predicted [8]. We recently detected a small voltage generated during the field-driven motion of a single domain wall in a Permalloy nanowire [9]. Our observations confirm the theoretical predictions and can be used to extract information about the wall motion.

In collaboration with (i) T. Staudacher, C. Wang, H. Seinige, (ii) Z. Wei, A. Sharma, J. Basset, A. S. Nunez, P. M. Haney, R. A. Duine, J. Bass, A. H. MacDonald, (iii) S. A. Yang, G. S. D. Beach, C. Knutson, D. Xiao, Q. Niu, J. L. Erskine. Supported in part by NSF Grant No. DMR-06-45377, DoE, and the Welch Foundation.

[1] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988); G. Binasch et al., Phys. Rev. B 39, 4828 (1989); [2] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996); M. Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998); [3] A. S. Núñez et al., Phys. Rev. B 73, 214426 (2006); [4] A. A. Tulapurkar et al., Nature 438, 339 (2005); J. C. Sankey et al., Phys. Rev. Lett. 96, 227601 (2006); [5] C. Wang et al., to be published; [6] Z. Wei et al., Phys. Rev. Lett. 98, 116603 (2007); [7] G. S. D. Beach et al., Phys. Rev. Lett. 97, 057203 (2006); [8] L. Berger, Phys. Rev. B 33, 1572 (1986); [9] S. Yang et al., Phys. Rev. Lett. 102, 067201 (2009).

For more information, please contact: Dr. Anzhong Wang x 2276