Ultrafast magnetization reversal

The speed of magnetization reversal is a key feature in magnetic data storage. The data rate in longitudinal recording is currently 100 MB/s and continues to double every two years. This means that magnetization reversal takes place in times approaching one nanosecond. In recent experiments we have shown that magnetization reversal can be initiated by magnetic field pulses of ps duration in thin in-plane magnetized Co films. The field pulses are generated by the electron beam at the Stanford Linear Accelerator, and are most effective for magnetization reversal if the field is applied perpendicular to the initial magnetization.

Ultrafast magnetic field pulses as short as 2 picoseconds are able to reverse the magnetization in thin, in-plane, magnetized cobalt films [1,2], as observed by spin-SEM. The field pulses are applied in the plane of the film, and their direction encompasses all angles with the magnetization. At a right angle to the magnetization, maximum torque is exerted on the spins, leading to the double-ring structure of reversed magnetization, Figure 1. In this geometry, a precessional magnetization reversal can be triggered by fields as small as 184 kA/m [2]. At the direction where field and original magnetization are parallel or antiparallel no reversal can be induced. The topography image shows the impact "crater" produced by the high-intensity, high-energy electron beam.

If the pulse length is reduced even further to below 100 fs, the pattern changes in a pronounced way, Figure 2. It exhibits a marked up-down asymmetry, strongly deviating from the more circular patterns observed for ps pulses [3]. We attribute this change to an electric field-induced magnetic anisotropy in the thin-film metallic ferromagnet. The strong (~10⁹ V/m) and short (70 fs) electric field pulse is large enough to distort the valence charge distribution in the metal, yet its duration is too brief to change the atomic positions. This purely electronic structure alteration of the sample generates a new type of transient anisotropy axis and strongly influences the magnetization dynamics. The successful creation of such an anisotropy opens the possibility for all-electric-field-induced magnetization reversal in thin metallic films — a greatly desired yet unachieved process.

[1] H.C. Siegmann, E.L. Garwin, C.Y. Prescott, J. Heidmann, D. Mauri, D. Weller, R. Allenspach, W. Weber, J. Magn. Magn. Mater. 151, L8 (1995).

[2] C.H. Back, R. Allenspach, W. Weber, S.S.P. Parkin, D. Weller, E.L. Garwin, H.C. Siegmann, Science 285, 864 (1999).

[3] S.J. Gamble, M.H. Burkhardt, A. Kashuba, R. Allenspach, S.S.P. Parkin, H.C. Siegmann, and J. Stöhr, Phys. Rev. Lett. 102, 217201 (2009).