Nonlinear spin wave instability processes in manganese substituted zinc y-type hexagonal ferrites
Cox, Richard Garner
MetadataShow full item record
The large magnetocrystalline anisotropy observed in hexagonal ferrites makes these materials ideally suited for high frequency millimeter-wave applications. However, the large microwave losses observed at low-power levels and the high-power handling capabilities of hexagonal ferrites need to be addressed prior to their wide acceptance in real devices. In order to address the above issues, measurements and analyses of the microwave field amplitude (hcrit) required to parametrically excite nonlinear spin wave amplitude growth were performed on single crystal easy plane disks of Mn substituted Zn Y-type hexagonal ferrites at 9 GHz and room temperatures. Plots of the hcrit dependence on the static magnetic field, termed "butterfly curves," were obtained and analyzed for the resonance saturation (RA), subsidiary absorption (SA), and parallel pumping (PP) configurations. In order to obtain the butterfly curve data and perform the analyses: (1) a state-of-the-art computer-controlled high-power microwave spectrometer was constructed, (2) the classical spin wave instability theory, originally developed by Suhl and Schloemann, was extended, and (3) instability measurements were performed on multiple Zn Y-type hexagonal ferrites samples for several pumping configurations and static field settings. The measurements and analyses performed here constitute the first time RS, SA, and PP spin wave instability butterfly curve analysis have all been performed in planar hexagonal ferrite samples. This work also corresponds to the first time that resonance saturation measurements and analyses were performed for static magnetic fields both at and in the vicinity of the ferromagnetic resonant field in a hexagonal ferrite. The data obtained as part of this work show that the microwave field amplitude required to parametrically excite nonlinear spin wave amplitude growth in hexagonal ferrites is similar to polycrystalline cubic ferrites, which are currently in use in microwave devices. Follow-up measurements, motivated by this work, revealed that hcrit can be varied by manipulating the sample dimensions. The analyses performed here indicate that two-magnon scattering is likely not the dominant source of the large low-power microwave losses observed in these hexagonal ferrites; rather that these losses may be an intrinsic property of the material. The theoretical work performed here identified a sign problem with the anti-Larmor uniform mode complex damping terms in several past publications and provides an improved methodology of treating the uniform mode anti-Larmor complex frequency damping.