High-sensitivity detection of static magnetic field in a SQUID probe microscope

Abstract

We have developed a high-T c SQUID probe microscope. A high permeability probe was used as a flux guide to improve its spatial resolution. The SQUID head with the probe makes it possible to measure samples with high spatial resolution in air at room temperature. To achieve a high spatial resolution, the microscope should have high signal to noise (S/N) ratio. We have investigated a modulation method of the static magnetic field emanating from a sample with the aim of achieving a higher S/N ratio. The sample stage was vibrated perpendicular to the plane of the SQUID with a frequency of 120 Hz by being mounted on a piezo stage attached to a mechanical stage. The oscillation amplitude of the sample was about 44 µm when the piezo stage was vibrated with amplitude 20 µm. The SQUID output was connected to a lock-in amplifier, and the magnetic field signal was obtained using lock-in detection. In order to evaluate the performance of this modulation method, we measured static magnetic field distributions of fine meander lines and fine patterns of a Ni thin film. Observed line scans showed that the improved S/N ratio of the SQUID microscope led to a higher spatial resolution.