Invited Paper MS+GR+MI-ThM5
Methods for Characterizing Variations in Excitation Mode Frequency and Linewidth in Spin Transfer Nanocontact Oscillators

Thursday, November 12, 2009, 9:20 am, Room C3

Resonance probing of magnetoelectronic nanostructures with AC spin torque promises to provide a new means to understand their magnetic behavior, and their interaction with spin-polarized currents. An AC current produces an AC spin polarized current, which in turn produces a time-varying torque. By varying the frequency of the current, the resonance spectrum of the structure can be investigated. By this method, the ferromagnetic resonance mode of metallic and tunnel junction nanopillars has been investigated, and in nanocontact structures, enables probing the ferromagnetic resonance and damping of continuous films at unprecedented length scales.

However, for this tool become the more generally useful, the details of the signals produced by AC spin torque must be better understood. Beyond the ferromagnetic resonance mode, other responses are observed that have not been predicted; in nanocontacts, due to the unbounded geometry, prediction of modes beyond the ferromagnetic resonance in even more difficult. Furthermore, the shape of the ferromagnetic resonance line itself can vary in a complicated way, depending on the sample geometry and materials. As a step towards the goal of developing a robust tool that gives quantitative information about nanocontact spin transfer oscillators, I will present AC spin torque measurements from a variety of field geometries, and of materials with either in- or out-of-plane anisotropy, describing the basic behavior observed in each case. Then, I will compare different methods of ferromagnetic resonance detection (frequency-swept linewidth, field swept linewidth, field or microwave modulation) and discuss the challenges associated with interpreting these results to obtain the damping constant and the zero-field field-swept linewidth.