Probing Magnetic Ordering in Proposed Altermagnetic RuO₂Thin Films with Exchange Bias and Raman Spectroscopic Measurements

Recent theoretical and experimental work on RuO₂ has sought to describe its magnetic ordering with little concrete agreement. RuO₂ likely does not possess magnetic ordering based on recent neutron diffraction measurements. However, RuO₂ thin films have recently been cited as antiferromagnetic and/or altermagnetic and have also demonstrated superconductivity when grown on TiO₂. Computational results suggest that the resulting magnetic ordering is likely related to Ru vacancies in thin film samples. This study uses Raman spectroscopy and exchange bias measurements to investigate the magnetic ordering in RuO₂ thin films, specifically where RuO₂ has been grown on single crystal MgF₂ substrates with and without a NiFe layer. We observe thickness-dependent unidirectional exchange bias, which emerges below 10 K in 10 nm to 100 nm of RuO₂ films interfaced with about 10 nm of NiFe. Preliminary temperature-dependent magneto-Raman measurements reveal differences in the polarization-dependent phonons and possible low frequency magnon modes emerging below 100 K and at 9 T of applied magnetic field.