Experimental observation of short-range magnetic correlations in amorphous Nb₂O₅ and Ta₂O₅ thin films

We use muon spin rotation/relaxation/resonance (μSR) to investigate the magnetic properties of niobium pentoxide (Nb₂O₅) and tantalum pentoxide (Ta₂O₅) 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 Ta₂O₅ with low crystallinity demonstrates suppressed magnetism, while fully amorphous Ta₂O₅ demonstrates local static magnetism. In contrast, amorphous Nb₂O₅ is dominated by magnetic fluctuations and is strongly magnetically disordered compared to Ta₂O₅. 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.