Experimental Observation of Short-Range Magnetic Correlations in Amorphous Nb$_2$O$_5$ and Ta$_2$O$_5$ Thin Films

TL;DR

This study uses muon spin techniques to reveal that amorphous Nb$_2$O$_5$ and Ta$_2$O$_5$ thin films exhibit distinct magnetic behaviors at low temperatures, with implications for quantum device performance.

Contribution

First detailed investigation of magnetic correlations in amorphous Nb$_2$O$_5$ and Ta$_2$O$_5$ thin films using $bc$SR, highlighting structural dependence of magnetism.

Findings

Amorphous Ta$_2$O$_5$ shows local static magnetism.

Amorphous Nb$_2$O$_5$ exhibits magnetic fluctuations and disorder.

Magnetic differences may impact superconducting qubit performance.

Abstract

We use muon spin rotation/relaxation/resonance ($\mu$SR) to investigate the magnetic properties of niobium pentoxide (Nb$_2$O$_5$) and tantalum pentoxide (Ta$_2$O$_5$) thin films. In both oxides, we observe a magnetic response at the lowest available temperature of 2.8 K. This response appears to be structurally dependent: thermally oxidized Ta$_2$O$_5$ with low crystallinity demonstrates suppressed magnetism, while fully amorphous Ta$_2$O$_5$ demonstrates local static magnetism. In contrast, amorphous Nb$_2$O$_5$ is dominated by magnetic fluctuations and is strongly magnetically disordered compared to Ta$_2$O$_5$. Our results suggest that these fundamental differences in the magnetism of Ta and Nb oxides could explain the performance limitations in superconducting qubits and resonators.