Scanning SQUID Susceptometry of a paramagnetic superconductor

TL;DR

This paper develops a theoretical model for scanning SQUID susceptometry to analyze local magnetic properties of superconducting and paramagnetic samples, enabling extraction of physical parameters from experimental data.

Contribution

It provides analytical formulas for the SQUID signal of superconducting slabs with paramagnetic responses, aiding the interpretation of susceptibility measurements.

Findings

Derived analytical expressions for susceptometry signals.

Fitted experimental data for three different samples.

Enabled estimation of paramagnetic spins and superconducting carriers.

Abstract

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine the physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth, on a non-superconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: 1) delta-doped SrTiO3, which has a predominantly diamagnetic response, 2) a thin film of LaNiO3, which has a predominantly paramagnetic response, and 3) a two-dimensional electron layer (2-DEL) at a SrTiO3/AlAlO3 interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements.