AVS 51st International Symposium
    Magnetic Interfaces and Nanostructures Tuesday Sessions
       Session MI-TuP

Paper MI-TuP7
Spin Momentum Transfer Induced Dynamics in Magnetic Nanostructures

Tuesday, November 16, 2004, 4:00 pm, Room Exhibit Hall B

Session: Poster Session
Presenter: W.H. Rippard, NIST
Authors: W.H. Rippard, NIST
M.R. Pufall, NIST
S. Kaka, NIST
T.J. Silva, NIST
S.E. Russek, NIST
J.A. Katine, Hitachi Global Storage Technologies
M. Carey, Hitachi Global Storage Technologies
Correspondent: Click to Email
We have directly measured high-frequency precessional dynamics induced by a dc current I injected into patterned nanopillar devices and continuous spin-valve structures through a lithographically defined nanocontact. The induced magnetization dynamics have been studied as a function of current, material, as well as applied field strength H and direction. For in plane applied fields, the excitation frequency is found to linearly decrease with applied current, in qualitative agreement with single domain modeling. The excited frequencies vary between 5 GHz and 40 GHz as a function of applied field and frequencies of excitation can be well-described by the Kittel equation, indicating that the excitations are ones with wavelengths much larger than the contact size. As the angle of the field is varied with respect to the film plane, the dynamics become more complicated. Abrupt shifts in the frequency occur with applied current applied and df/dI can vary strongly with I at a given field. These shifts can be either to an increased or decreased frequency, depending on the field strength and angle. Moreover, the frequency of precession can be multivalued several different, non-harmonically related frequencies being measured at a given field and current with each mode having a linewidth <50 MHz. The power output of the devices is a strong function of the direction of the applied field, increasing by roughly two orders of magnitude as the field is varied from in-plane to out-of-plane. In general, the room-temperature linewidths of the excitations are quite small, on the order of 10 to 50 MHz, but also vary with H and I. For certain field geometries linewidths of < 2MHz are measured, leading to oscillations with quality factors >18,000. Single domain modeling based on an LLG equation modified to contain a spin-torque term captures a number, but not all, of the behaviors we observe.