Near-Field Nonlinear Microwave Microscope for Fundamental Superconducting Studies

This article presents the implementation of a near-field magnetic microwave microscope integrated into a cryostat with a base temperature of 3.6 K. The probe of the microwave microscope is a magnetic writer capable of generating a strong microwave magnetic field within a sub-micron region. A microwave signal in the GHz range is directed to the magnetic writer. The magnetic writer then generates a local RF field to stimulate the superconducting sample. The sample creates screening currents and the local response is collected by the same magnetic writer. Nonlinear components of the sample response, particularly the third harmonic response, are extracted using a spectrum analyzer. We employ this microwave microscope to investigate a Nb film, a type-II superconductor with a transition temperature around 9.3 K. By analyzing the third harmonic response of the Nb film, we explore the sample's surface defects, especially those that nucleate RF magnetic vortices.