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Theory of Current-Driven DomainWall Motion: Spin Transfer versus Momentum Transfer
Tatara, G. & Kohno, H., 2004. Theory of Current-Driven DomainWall Motion: Spin Transfer versus Momentum Transfer. Phys. Rev. Lett. 92, 086601
Essay about this article
As early as the 1980’s Luc Berger of Carnegie Mellon University focused his attention on the effect of electric currents on domain walls, the interfacial region between ferromagnetic domains in which the magnetization rotates away from one domain and towards another. At first he focused on the charge current transferring its momentum to the domain wall and thereby pushing the wall [Newton’s third law of action and reaction], but in the 1990’s he also considered how the effect of the spin component of the current might cause the wall to move. This was clarified over the years by Shufeng Zhang, now at the University of Arizona, and Gen Tatara of Metroplitan University of Tokyo; their articles are cited above. The central idea of how the spin current moves a wall is that the plane of polarization of the current should track the rotating background magnetization in a domain wall, as best it can. When the spin current’s polarization rotates it creates a back reaction back on the wall much like a cork screw, i.e., as the spin current moves forward while its polarization rotates, it produces a backward thrust on the domain wall.
These concepts would lead one to expect that the motion of the wall is proportional to the strengths of the charge and spin currents; however experiments produced a different result. The first in a new series of experiments on the role of spin currents on the motion of domain walls were done by Fert’s group; these were rapidly followed by other studies by Parkin’s group at IBM and Ono’s group in Kyoto; the latter highlighted that a threshold had to be overcome before walls moved. The threshold current for motion is due to the fact that domain walls are usually pinned in the solid by defects and anisotropy forces. Inasmuch as their presence impedes the motion of the wall across a ferromagnet, because it requires one overcoming energy barriers; this in turn means there will be a threshold current before a domain wall budges.
Recently attention has been focused by Peter Weinberger on the role of anisotropic magnetoresistance (AMR) in domain walls. The idea is that the residual orbital component of the angular momentum alters the scattering cross section for electrons in the rotating wall [1]. To study the AMR effect in domain walls Dr. Weinberger used a relativistic ab-initio code; of note, he found that, compared to thehomogeneous infinite systems, the AMR is reduced in the presence of a domain wall. His work was inspired by research on a new form of memory, known as race track memories, which was pioneered by Stuart Parkin of IBM [2]. The idea is to do away with all mechanically moving parts, as in hard disk drives, and move domains past sensors, read heads, by moving the domain walls. Recent experiments by Cowburn’s group, now at Imperial College in London, have probed some aspects of the feasibility of Parkin’s idea [see Cowburn’s article cited above].
References
[1] P. Weinberger, Phys. Rev. Lett.98, 027205 (2007); ibid 100, 017201 (2008).
[2] S. S. P. Parkin, U.S. Patent No. US 683 400 5 2004; also see S. A. Wolf, D. Treger, and A. Chtchelkanova, MRS Bull. 31, 400 (2006).
See Also:
L. Berger, J. Appl. Phys. 71, 2721 (1992); Phys. Rev. B 54, 9353 (1996)
J. Grollier, P. Boulenc, V. Cros, A. Hamzic, A. Vaurès, A. Fert, and G. Faini, Appl. Phys. Lett. 83, 509 (2003)
A. Yamaguchi, T. Ono, S. Nasu, K. Miyake, K. Mibu, and T. Shinjo, Phys. Rev. Lett. 92, 077205 (2004)
S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004)
D. A. Allwood, G. Xiong, C.C. Faulkner, D. Atkinson, D. Petit, R.P. Cowburn, Science 309, 1688 (2005).
Discussion Questions
a. What differences are there in the mechanisms by which spin currents, as compared to charge currents, induce domain wall motion?
b. Compare the domain wall motion induced by currents to those generated by an applied magnetic field.
The above article is reprinted with permission from the author(s) of Tatara, G. & Kohno, H., 2004. Theory of Current-Driven DomainWall Motion: Spin Transfer versus Momentum Transfer. Phys. Rev. Lett. 92, 086601. Copyright (2004) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.
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