| AVS 54th International Symposium | |
| Magnetic Interfaces and Nanostructures | Wednesday Sessions |
| Session MI-WeA |
| Session: | Nanomagnetic Imaging and Spectroscopy |
| Presenter: | J.P. Strachan, Stanford Univ. |
| Authors: | J.P. Strachan, Stanford Univ. Y. Acremann, Stanford Synchrotron Radiation Lab. V. Chembrolu, Stanford Univ. X.W. Yu, Stanford Univ. A. Tulapurkar, Stanford Synchrotron Radiation Lab., Stanford Univ. T. Tyliszczak, Lawrence Berkeley National Lab. J. Katine, Hitachi Global Storage Tech. M. Carey, Hitachi Global Storage Tech. H.C. Siegmann, Stanford Synchrotron Radiation Lab., Stanford Univ. J. Stöhr, Stanford Synchrotron Radiation Lab., Stanford Univ. |
| Correspondent: | Click to Email |
Spin-torque (or spin-transfer torque) is a novel phenomenon involving the transfer of angular momentum from a spin-polarized current to a ferromagnet. There is much excitement in the use of this effect for developing non-volatile, high density magnetic RAM, as well as for DC current-driven microwave oscillators. Indeed, steady-state precessional modes as well as full magnetization reversal of nanoscale magnetic elements driven by spin-torque have been observed. These observations have been via giant magneto-resistance measurements, using a reference "fixed" magnetic layer, which also serves as the spin-polarizer. Given the experimental challenges in probing thin, buried nanomagnets, the detailed magnetic configuration of the element has remained unknown. I describe a high resolution, time-resolved x-ray microscopy technique which provided the first direct images of the nanostructure during the switching process. Motion pictures with 200 ps time resolution and 35 nm spatial resolution reveal that the process is based on the transient formation of a vortex configuration. The vortex moves across the magnetic element, leaving behind a switched magnetization in its wake. A physical understanding of this unexpected mechanism is discussed, as well as the dependence on sample size and shape. It is seen that the sample dimensions are well within the single-domain regime. The highly non-uniform magnetic configuration which is transiently taken is initiated by the presence of the Oersted field, but primarily formed by the spin-torque. It is seen that other non-uniform switching mechanisms may dominate for smaller length scales.