Experimental Observation of Short-Range Magnetic Correlations in Amorphous Nb₂O₅ and Ta₂O₅ Thin Films

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.