We use muon spin rotation/relaxation/resonance $\left(\mu \mathrm{SR}\right)$ to investigate the magnetic properties of niobium pentoxide $\left({\mathrm{Nb}}_2{\mathrm{O}}_5\right)$ and tantalum pentoxide $\left({\mathrm{Ta}}_2{\mathrm{O}}_5\right)$ 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 ${\mathrm{Ta}}_2{\mathrm{O}}_5$ with low crystallinity demonstrates suppressed magnetism, while fully amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ demonstrates local static magnetism. In contrast, amorphous ${\mathrm{Nb}}_2{\mathrm{O}}_5$ is dominated by magnetic fluctuations and is strongly magnetically disordered compared to ${\mathrm{Ta}}_2{\mathrm{O}}_5$. 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.

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• Defects
• Magnetic coupling
• Magnetism
• Amorphous materials
• Thin films
• Transition metal oxides
• Muon spin relaxation & rotation