RESUMEN
The magnetic state of a ferromagnet can affect the electrical transport properties of the material; for example, the relative orientation of the magnetic moments in magnetic multilayers underlies the phenomenon of giant magnetoresistance. The inverse effect--in which a large electrical current density can perturb the magnetic state of a multilayer--has been predicted and observed experimentally with point contacts and lithographically patterned samples. Some of these observations were taken as indirect evidence for current-induced excitation of spin waves, or 'magnons'. Here we probe directly the high-frequency behaviour and partial phase coherence of such current-induced excitations, by externally irradiating a point contact with microwaves. We determine the magnon spectrum and investigate how the magnon frequency and amplitude vary with the exciting current. Our observations support the feasibility of a spin-wave maser' or 'SWASER' (spin-wave amplification by stimulated emission of radiation).
RESUMEN
The measured voltage derivative of the nonlinear resistance of tiny point contacts can be separated into a phonon-emission effect (alpha(2)F) and an analytic functional form (background effect). The alpha(2)F's show structure coincident with bulk phonon densities of states. Values of the integral of 2 alpha(2)F/omega are closely related to literature values. The background effect is related to the impurity concentration of the materials.
RESUMEN
We have built an electron spin echo spectrometer operating at 604 GHz, extending the frequency limit of existing spectrometers by more than a factor of 4. In order to handle this high frequency we have used optical techniques, i.e., molecular gas lasers for the excitation pulses and far infrared techniques for the heterodyne detection system. The different components of the spectrometer are described in detail and first experimental results are given.