THRESHOLD MICROWAVE-FIELD AMPLITUDE FOR UNSTABLE GROWTH OF SPIN WAVES UNDER OBLIQUE PUMPING

The threshold microwave-field amplitude hcrit required for the unstable growth of spin waves has been investigated for oblique pumping with a linearly polarized microwave field applied at an arbitrary angle with respect to the static magnetic field. The experiment was performed on YIG (1% Dy) spheres at 9.2 GHz, using a rectangular TE102 cavity and a pulsed magnetron source. Data were obtained at static external fields of 400, 600, 1100, and 1500 Oe. In general, hcrit increases with the pump angle. Theoretical expressions for the oblique pumping hcrit have been derived for ferromagnetic insulators of ellipsoidal shape and with axial symmetry about the direction of the static magnetic field sufficient to saturate the sample. The threshold field is determined by maximizing 1/h=cosψ/h ∥±sinψ/h⊥ with respect to the angle between the internal field and the spin-wave propagation direction. Here, h ∥ and h⊥ are the parallel and perpendicular pump threshold fields of Schlömann and Suhl. For external fields sufficient to magnetize the spherical samples to saturation, the theoretical predictions are in good agreement with the experimental data. For small fields, the agreement is poor. However, fair agreement is obtained by assuming a transverse demagnetizing factor Nt greater than the value of 1/3, which is appropriate for saturated spherical samples. The value of Nt for the best theoretical fit depends on the way in which the sample is demagnetized. This result indicates that for samples not magnetized to saturation, the magnetic structure consists of elongated domains, and that the oblique-pumping data can be explained from the theory for saturated systems, assuming demagnetizing factors appropriate to the domain structure. © 1969 The American Institute of Physics.