High-sensitivity in-plane vector magnetometry using the alternating gradient force method

Abstract

The alternating gradient force magnetometer is a highly sensitive tool particularly suited for thin films magnetometry. The measurement technique is based upon the alternating force generated on a magnetized sample by a set of field-gradient coils. The so-induced sample oscillation is directly proportional to the sample’s magnetization. High sensitivity measurements are achieved by mounting the sample at the end of a cantilever attached to a piezoelectric bimorph element, and by tuning the excitation frequency close to the mechanical resonance of the sample-cantilever assembly. Here we describe a new design that allows to measure both in-plane components of the magnetization of a thin film sample, for any direction of the external magnetic field within the sample’s plane. By rotating the sample-probe assembly, we find the output signal to be proportional to the projection of the alternating force along the sense axis of the piezoelectric bimorph. Besides, the resonance frequency of the system remains unchanged. Thus, hysteresis loops can be measured accurately for various angles between the applied field and an in-plane anisotropy axis. The signal only vanishes when the alternating force is orthogonal to the bimorph axis. Moreover, we have designed a set of two pairs of gradient coils, whose axis are orthogonal to one another. By varying the excitation current within these two pairs of coils, it is possible to rotate the alternating gradient direction, to detect magnetization components along or perpendicular to the external field.