Josephson spin-valve realization in the magnetic nodal-line topological semimetal Fe$_3$GeTe$_2$

TL;DR

This study demonstrates a Josephson spin-valve effect in Fe$_3$GeTe$_2$, a magnetic topological semimetal, revealing unusual magnetic field-dependent supercurrent behavior linked to spin polarization and magnetic domain effects.

Contribution

First observation of Josephson spin-valve phenomena in a magnetic topological nodal-line semimetal Fe$_3$GeTe$_2$, highlighting spin polarization effects on supercurrent in such materials.

Findings

Asymmetric $I_c(B)$ pattern depending on magnetic field sweep direction.

Reversal of $I_c(B)$ pattern when magnetic field direction is inverted.

Presence of aperiodic $I_c(B)$ fluctuations in in-plane magnetic fields.

Abstract

Three-dimensional van der Waals ferromagnet Fe$_3$GeTe$_2$ (FGT) is regarded as a candidate for the magnetic topological nodal line semimetal. We investigate lateral electron transport between two 3~$\mu$m spaced superconducting In leads beneath a thick three-dimensional FGT exfoliated flake. At low 30~mK temperature, we observe Josephson supercurrent that exhibits unusual critical current $I_c$ suppression by the magnetic field $B$. The overall $I_c(B)$ pattern is asymmetric in respect to the B sign. We demonstrate, that the asymmetry is defined by the magnetic field sweep direction, so the $I_c(B)$ pattern is strictly reversed (as $B$ to $-B$ inversion) for the opposite sweeps. We also observe an interplay between maximum and minimum in $I_c(B)$ in normal magnetic fields, while there are fast aperiodic $I_c(B)$ fluctuations for the in-plane ones. These effects can not be expected for homogeneous superconductor-ferromagnet-superconductor junctions, while they are known for Josephson spin valves. The mostly possible scenario for Josephson spin valve realization in FGT is the misalignment of spin polarizations of the Fermi arc surface states and ferromagnetic FGT bulk, but we also discuss possible influence of spin-dependent transport between magnetic domains.