Experimental observation of short-range magnetic correlations in amorphous Nb₂O₅and Ta₂O₅ thin films.

Different magnetic behaviors have been observed in niobium and tantalum pentoxides. Using muon spin relaxation, we observed a magnetic response down to 2.8 K. It appeared that amorphous niobium pentoxide exhibits high-frequency magnetic fluctuations with no magnetic order. In contrast, tantalum pentoxide shows local magnetic ordering in the amorphous phase and no magnetic response when the oxide is crystalline. However, the local magnetic field in tantalum pentoxide is significant, which means that both pentoxides are detrimental to the performance of superconducting qubits. Niobium pentoxide is more lossy because it has a broad spectral range that overlaps with the operational range of superconducting qubits, and these fluctuations could likely cause flux noise. This knowledge could help improve the performance of qubits and provide a deeper understanding of sources of loss.