Construction of a $^3$He magnetic force microscope with a vector magnet

TL;DR

This paper reports the development of a $^3$He magnetic force microscope capable of operating at 300 mK under a vector magnetic field, utilizing fiber optic interferometry for high-sensitivity magnetic imaging.

Contribution

The paper introduces a novel $^3$He magnetic force microscope with vector magnetic field capability and fiber optic detection, enabling advanced imaging of magnetic phenomena at sub-Kelvin temperatures.

Findings

Imaging coexistence of magnetism and superconductivity in ErNi$_2$B$_2$C.

Probing the dipole shape of a single Abrikosov vortex.

Achieving near-thermodynamic noise levels in detection.

Abstract

We constructed a $^3$He magnetic force microscope operating at the base temperature of 300 mK under a vector magnetic field of 2-2-9 T in the $x-y-z$ direction. Fiber optic interferometry as a detection scheme is employed in which two home-built fiber walkers are used for the alignment between the cantilever and the optical fiber. The noise level of the laser interferometer is close to its thermodynamic limit. The capabilities of the sub-Kelvin and vector field are demonstrated by imaging the coexistence of magnetism and superconductivity in a ferromagnetic superconductor (ErNi$_2$B$_2$C) at $T$=500 mK and by probing a dipole shape of a single Abrikosov vortex with an in-plane tip magnetization.