Real-time imaging of the current-driven vortex-core motion

    Manipulating magnetization by spin currents is attracting scientific interest both due to the intricate physics involved in the interaction between the flowing spins and the localized spins that constitute magnetization and the technological potential to control future nanoscale spintronics devices. Since the efficiency of this spin torque effect is proportional to the spin-polarization of the current flowing in the ferromagnetic material an experimental quantification of the spin-polarization is of paramount importance. So far, the indirect methods used, i.e. transport measurements, such as tunneling spectroscopy, Andreev reflection, and giant magnetoresistance measurements have not shown conclusive results. Recently by using the at beamline 6.1.2 at the Advanced Light Source (ALS) in Berkeley we succeeded in a direct determination of the spin-polarization of the currents from quantitative high resolution X-ray imaging of the current-induced circular motion of a vortex core in a ferromagnetic disk (Figure). We are able to watch the motion of the core position with better than 25 nm spatial resolution with a 70 ps time resolution over a period of several nano-seconds. The spin-polarization of the current is determined to be 0.67 for Permalloy (Fe₁₉Ni₈₁), which is in excellent agreement with an analytical model in the framework of the spin transfer torque.



Figure: Schematic illustration of the experimental setup for imaging the spin dynamics by time- and space-resolved magnetic soft X-ray microscopy of the vortex core motion in a Permalloy disk (diameter: 1.5 μm; thickness: 40 nm).

“Probing the Spin Polarization of Current by Soft X-Ray Imaging of Current-Induced Magnetic Vortex Dynamics ”
by S. Kasai, P. Fischer, M. Im, K. Yamada, Y. Nakatani, K. Kobayashi, H. Kohno, and T. Ono
Phys. Rev. Lett., 101, 237203 (2008).