Microstrip resonator for GHz measurements of microscale magnetic objects (abstract)

Abstract

The integration of thin ferromagnetic films for applications above 1 GHz is expected to require micron scale, thin film objects (∼100 μm×100 μm×0.1 μm) with volumes less than ∼10−15 m3. With maximum relative susceptibilities of ∼100, the measurement of these micro-objects by small perturbative techniques is experimentally challenging. Here, we describe the design, calibration, and performance of a microstrip resonator with a cavity volume of <10−6 m3 and a cross-sectional area <1 mm2. We have designed and built a gap-coupled, transmission-type, linear microstrip resonator. At even (odd) harmonics of the fundamental resonance, maxima of the electric (magnetic) field are established at the midpoint of the linear resonator. This resonator was designed to measure the magnetic responses of ensembles of micron scale ferromagnetic objects in the 0.4–10 GHz frequency range. The lowest resonant frequency (400 MHz) intentionally overlapped the upper operating frequency range of our lower frequency measurement systems, to permit comparison and calibration. The center conductor of the microstrip was photolithographically patterned in a 6 μm thick gold film electroplated on a high quality microwave alumina substrate. A quasi transverse electric mode (TEM) was launched on the microstrip from a well-matched transition to a 3.5 mm coaxial connector. The microstrip resonator was excited by coupling to the microstrip transmission line through two 50 μm capacitive gaps (∼0.07 pF) in the center conductor. The measurements were performed with HP 8510 network analyzer. The equivalent circuit of the microstrip transmission resonator and coupling capacitors were used to compute the 22 transmission resonances expected in the 0.4–10 GHz range. The results compare well with the measured resonances indicating the applicability of the quasi static approximation to 6 GHz. A perturbation theory for calculating the permeability from the measured cavity parameters was formulated for the microstrip resonator. Initial measurements on microscale magnetic objects are in good agreement with those obtained from other measurements.