<title>Field dependence of FMR in ferrites and implications for microwave absorber design</title>

Abstract

There are numerous potential uses for a microwave absorbent material whose absorption can be modified by an applied stimulus. This paper presents work to exploit the dependence of the microwave properties of ferrites on applied magnetic fields, and to develop such an absorbent material.Ferromagnetic resonance (FMR) is known to result in absorption of incident microwave energy at a characteristic frequency for any given ferrite. The FMR frequency is dependent on the anisotropy field H, which in turn can be modified by chemical doping. Therefore, ferrite RAMs can be produced to operate at required frequencies through judicious choice of the ferrite. Furthermore, if H is supplemented by an applied magnetic field Ha1,,, the FMR frequency will be increased by an amount dependent on the strength of Ha,,, fld on the sense of anisotropy.The microwave permeability and permittivity spectra of a range of ferrites were measured as functions of Han,, using a vector network analyser. These were used as input to a computer model for calculating absorber performance, and the effects of bias fields on a variety of absorber designs were predicted. A novel system was developed to apply large fields in test panels, for validating performance predictions.

Approaching soft X-ray wavelengths in nanomagnet-based microwave technology

“…To decrease to the few 10-nm regime, the nanomagnets must provide an even larger resonance frequency than CoFeB. It is worth noting that a hexaferrite can exhibit an FMR eigenfrequency f 0 of 53 GHz in zero magnetic field 25 . Exploiting such a material as a transducer, a spin-wave wavelength of ∼60 nm would be realized in YIG even without applying a magnetic field.…”

Magnetic interfaces as sources of coherent spin waves

“…Hence, a question arises as to how the resonance frequency of one of the two media could be increased. Possible strategies include the use of confined exchange spin waves [31,32], hexaferrites [8,33], or even antiferromagnets [34] and weak ferromagnets [35]. In particular, the use of antiferromagnets is capable of pushing the excitation frequencies to the THz domain but will require one to reconsider the choice of stimuli employed to excite the sample.…”