Ferromagnetic metals offer most of the properties desired for a practical spin injecting contact material: a source of electrons rather than holes, high Curie temperatures, low coercive fields, fast switching times. A single-crystal Fe (001) film on GaAs (001) is a promising spin injection heterostructure. 

Very thin Fe (001) films exhibit an unusual uniaxial magnetic anisotropy (UMA) component, which profoundly affects the magnetization reversal process. A fundamental understanding of the evolving magnetic anisotropy remains elusive and is a critical technological issue, since the anisotropy determines the switching characteristics which control the performance of magnetic media, random access memory, and the dynamic response at the nanoscale.  Interface-derived contributions will, in fact, dominate the behavior of nanoscale structures. 

Magnetization-induced Second-Harmonic Generation (MSHG) is a nonlinear optical version of the Magneto-Optical Kerr Effect (MOKE) technique that provides intrinsic interface sensitivity and a very large magneto-optical response. While MOKE averages the magnetic behavior over the entire film thickness, MSHG very selectively detects only the interface contribution due to basic symmetry constraints.

The figure above shows a set of typical MSHG (top panel) and MOKE (bottom panel) magnetization curves for the 10-nm Fe (001) film. In the case of the MSHG curves, one-jump switching is observed for all principal crystallographic axes. In contrast, the MOKE M-H loops exhibit distinct plateaus and two switching fields, revealing a two-jump reversal process. These data show that the magnetization of the interface layer switches in a manner which is distinctly different from that of the bulk, and is not rigidly locked to the bulk by the strong exchange coupling typically associated with ferromagnetic metals. Coherent rotation model predicts only one-jump switching occurs for all crystallographic directions when the uniaxial anisotropy is stronger than the cubic anisotropy, i.e., |r| ³ 1, while two jump switching occurs for  r<1.

For comparison, we utilized the MSHG technique to probe the magnetic anisotropy of the Fe/Cr interface. A 50-nm thick Fe film grown on AlGaAs and capped with a 5-nm Cr layer was used for this experiment. Left figure shows a set of typical longitudinal MSHG (top panel) and MOKE (bottom panel) curves with the magnetic field applied along the hard <110> directions. Both the MSHG and MOKE M-H curves clearly show a two-jump reversal process.

The process of magnetization reversal and dynamics in thin film is of considerable importance in magnetic and magneto-optical recording, and in the context of magneto-electronics. We use Time-Resolved Magneto-Optic Kerr-Effect (TRMOKE) and Time-Resolved Magnetization-induced Second-Harmonic-Generation (TRMSHG) to probe the spin wave excitation in the bulk and interface respectively.

Uniform magnetization precessions are generated by ultrafast optical excitation along the in-plane easy axis [100], as well as along the hard axis [1-10], in epitaxial Fe films grown on AlGaAs (001) over a wide range of applied magnetic fields.

From the temporal evolution of the coherent magnetization precession, we determine the magnetic anisotropy constants and damping parameters which are crucial in designing fast magnetic switching devices and novel spintronics devices.

Interface-selective probing of ultrafast magnetization dynamics is a key issue for the design and realization of spin-electronic junction devices. Here we use the time-resolved magnetization-induced second-harmonic generation technique to initiate and monitor coherent electronic spin precession in the Fe interface layer of a Fe/AlGaAs tunneling junction. The results are directly compared with bulk spin precession as obtained from TR-MOKE measurements.

Our results clearly show:  (a) the coherent precession of the interface magnetization is decoupled from the bulk magnetization precession; (b) higher frequency spin precession occurs at the interface than in the bulk; and (c) the phase of the interface spin precession is opposite to that of the bulk precession at low fields, due to different magnetization switching processes and vanishingly small exchange coupling between the interface magnetization and the bulk Fe.  The higher precessional frequencies observed at the interface for a given field indicate that higher speed performance can be realized in nanoscale magnetic devices where interface properties dominate.

References:

1. H. B. Zhao, D. Talbayev, G. Lüpke, A. T. Hanbicki, C. H. Li, O. M. J. van 't Erve, G. Kioseoglou, B. T. Jonker, Interface Magnetization Reversal and Anisotropy in Fe/AlGaAs (001), Phys. Rev. Lett. 95, 2005, pp. 137202-5.
2. H. B. Zhao, D. Talbayev, Q. G. Yang, G. Lüpke, A. T. Hanbicki, C. H. Li, O. M. J. van 't Erve, G. Kioseoglou, B. T. Jonker, Ultrafast magnetization dynamics of epitaxial Fe films on AlGaAs (001), Appl. Phys. Lett. 86, 2005, pp. 152512-4.
3. H. B. Zhao, Y. H. Ren, B. Sun, G. Lüpke, A. T. Hanbicki, B. T. Jonker, Band Offsets at  CdCr2Se4-(AlGa)As and CdCr2Se4-ZnSe Interfaces, Appl. Phys. Lett. 82, 2003, pp. 1422-4.