Seminar organized by Graduate Program in Spintronics“Magnetic domain walls inthin-film nanostructures:
statics, dynamics, andemerging applications”
Speaker: Professor Geoffrey Beach(Massachusetts Institute of Technology)

HERE

February 4 (Thu.) 2016
16:00-17:30
Venue: Room A401
Laboratory for
Nanoelectronics andSpintronics, RIEC
(Katahira campus)

Contact to Shunsuke Fukami
E-mail: s-fukami@csis.tohoku.ac.jp, Phone: 022-217-5555

Magnetic domain walls in thin-film nanostructures:
Statics, dynamics, and emerging applications
Geoffrey BeachMIT Department of Materials Science and Engineering, Cambridge, MA, USA

Domains of opposing magnetization direction in a ferromagnet are separated by narrowtransitions called domain walls. Ferromagnetic nanowires provide a well-defined conduit for
domain walls, which may be manipulated by magnetic fields or electric currents and used in avariety of information storage and processing schemes [1]. This seminar will describe thestatic and dynamic properties of domain walls, their interactions with spin currents, and how
these properties can be engineered in nanoscale materials and device geometries.
In thicker magnetic films the magnetostatic energy dominates: domains lie in the plane andthe structure and dynamics [2] of nanowire-guided domain walls are dictated principally by thegeometry. In this case the domain walls are typically quite wide, and while they can be driven
by current through bulk spin-transfer torques [3,4], the high critical currents and low velocities
present a challenge to eventual applications. In ultrathin films, broken symmetry and spinorbit coupling (SOC) at interfaces can give rise to an interfacial contribution to the magnetic
anisotropy that can dominate if the film is sufficiently thin, pulling the magnetization out of theplane and leading to domain walls with a width of just a few nm. While the influence ofinterfacial SOC on magnetic anisotropy has been recognized for decades, the impacts on spintransport and spin textures in ultrathin-film heterostructures have only recently to come to light.
In nonmagnetic heavy metals such as Pt and Ta with strong SOC, conduction elecrons exhibit
a left-right scattering asymmetry such that spin “up” and spin “down” electrons pile up oneither side of a material when a charge current flows through it. This spin Hall effect can beused to create pure spin currents at the interface that can efficiently drive magnetization
switching and domain wall motion in an adjacent ferromagnetic film [5,6]. At the same time,broken inversion symmetry and spin-orbit coupling can lead to an antisymmetric exchangeinteraction known as the Dzyaloshinskii-Moriya (DM) interaction, which competes with theusual Heisenberg exchange to generate nonuniform spin textures with a preferred chirality [6].
This seminar will describe the origin and consequences of this interaction on domain walls andtheir dynamics [6,7], as well as other topologically nontrivial textures such as magnetic
skyrmions that have very recently been observed at room temperature [8].
1. S.S.P. Parkin, M. Hayashi, and L. Thomas, Science 320, 190 (2008).
2. G. S. D. Beach, C. Nistor, C. Knutson, M. Tsoi, and J. L. Erskine, Nature Mater. 4, 741 (2005).
3. G.S.D. Beach, C. Knutson, C. Nistor, M. Tsoi, and J. L. Erskine, Phys. Rev. Lett. 97, 057203 (2006).
4. G. S. D. Beach, M. Tsoi, and J. Erskine, J. Magn. Magn. Mater. 320, 1272 (2008).
5. L. Liu, C.-F. Pai, Y. Li, H.W. Tseng, D.C. Ralph, and R.A. Buhrman, Science 336, 555 (2012).
6. S. Emori, U. Bauer, S.-M. Ahn, E. Martinez, and G. S. D. Beach, Nature Materials 12, 611 (2013).
7. E. Martinez, S. Emori, N. Perez, L. Torres and G. S. D. Beach, J. Appl. Phys. 115, 213909 (2014).
8. S. Woo, et al., arXiv:1502.07376 (2015); to be published in Nature Materials.