Imaging magnetic phase transitions with nanoscale superconducting quantum interference devices
Magnetic microscopy allows direct visualization of spins in solid state systems. However, common techniques including magnetic force microscopy, scanning SQUID and Hall probes, or NV center microscopy typically suffer from low spatial resolution, low spin sensitivity, a restricted operating range in temperature and magnetic fields, or are highly invasive. I will describe the design and qualification of a magnetic microscope based on a nanoscale SQUID fabricated at the tip of a quartz pipette. The resulting microscope offers 50nm spatial resolution and single spin sensitivity, in combination with topographic and thermal imaging capabilities. As an example of its utility, I will describe measurements of the magnetization dynamics in quantum anomalous Hall insulator Cr-BiSbTe thin films. Originally believed to be ferromagnetic, microscopic measurements reveal that magnetization reversal occurs not by domain wall propagation but instead via reversal of uncorrelated microscopic dipoles—the films are in fact superparamagnets. Using in situ imaging and transport measurements of relaxation dynamics induced by electrostatic gate voltages, we further demonstrate that magnetic interactions are mediated by conduction electrons, again contra conventional wisdom of this material and with implications for the use of these materials as stable, zero-magnetic field resistance standards.