Altermagnetism for spintronics and superconductivity: anomalous Hall effect, transport spin polarization, and Josephson junctions

Altermagnets are an emerging class of antiferromagnets predicted to exhibit many key phenomena of interest for spintronics and superconductivity, including anomalous Hall effect (AHE) and finite transport spin polarization despite zero net magnetization, as well as unconventional Josephson coupling when used as a barrier in junctions. We report experimental progress on each of these fronts using two candidate altermagnetic materials, RuO₂ and Mn₅Si₃. In high-temperature sputtered epitaxial RuO₂ films, we observe orientation-dependent AHE: ultrathin 1.8 nm [110]-oriented films show a clear Hall response, while [001]-oriented films do not. To quantitatively probe transport spin polarization, we employ point-contact Andreev reflection (PCAR) with superconducting Nb tips, revealing a robust ~55% transport spin polarization in thicker 60 nm RuO₂ films at 4.2 K. Complementary studies on polycrystalline Mn₅Si₃ films, sputtered near room temperature, likewise show both AHE and finite transport spin polarization. Finally, we demonstrate Nb—Pt/Mn₅Si₃/Pt—Nb Josephson junctions, providing a first platform to explore supercurrent transport through an altermagnet. Collectively, the results define our superconductivity-based metrology-to-device roadmap for altermagnetism research: use superconducting spectroscopy to quantify transport spin polarization beyond AHE, then exploit Josephson junctions to realize and test predicted π-states and supercurrent diode behavior.