GHz ferromagnetic resonances in laminated microscale objects (abstract)

Abstract

Recent research1 has indicated that ferromagnetic laminates are potentially useful in the design of integrated monolithic microwave integrated circuit (MMIC) devices operating at GHz frequencies. Laminates with n=10–50 insulated, uniaxial magnetic films can have large permeabilities and device fill factors, and are process compatible with monolithic integration. The effect of lamination disappears, however, for frequencies greater than the frequency of a dimensional resonance (DR) associated with the easy axis dimension of the laminated object.23 Thus, when the ferromagnetic resonance (FMR) occurs at a frequency greater than the DR, the FMR spectra of the magnetic films may be distorted or, possibly, not observed. The bandwidths of both the DR and the FMR will be significantly broadened, if either lies above the eddy current relaxation (ECR) frequency of the individual magnetic films. Prior models23 of the permeability of laminated objects were developed for magnetic recording head laminates operating at frequencies below 10 MHz. At these frequencies, the effects of FMR are negligible, and easy axis dimensions e>1 cm are required to observe DR. In this work we modified our equivalent circuit model (ECM)3 to include the effects of DR, FMR, and the ECR in each layer. FMR spectra in the GHz frequency range, were computed for laminates with different dimensions, a variable number of different magnetic and insulating films, and a range of film thicknesses. A single domain, coherent rotational model was used to determine the layer permeability. The effect of the dc and rf shape demagnetizing fields of the laminated object on the FMR of the individual layers was included. The results show that the FMR spectra are shifted and distorted when the DR is near the FMR, and significantly broadened for insulator resistivities <1000 Ω cm and, as expected, when the FMR>ECR. To eliminate DR in the GHz range, the laminate must have e<100 μm.