Robust spin glass state with exceptional thermal stability in a chemically complex alloy

Spin glasses (SGs) arise from the frustration of competing magnetic interactions without long-range order; hence they tend to be destabilized by thermal fluctuation and exhibit a rather low glass transition temperature, presenting a major challenge for SG research and applications. Here, we report an unusual SG state in quaternary Fe-Co-Ni-Mn chemically complex alloys (CCAs). The SG exhibits an ultrahigh freezing temperature above room temperature, well exceeding that of conventional bulk SGs, as well as a unique and fast relaxation dynamics. The thermally stable SG state can be attributed to the strong frustration of exchange interactions owing to the high concentration of magnetic atoms and their chemical randomness in the solid-solution lattice. In addition, owing to the high phase stability of CCAs, the SG is robust over a wide compositional range, enabling a variety of magnetic phase transitions and largely tunable glass properties. These properties make CCAs important for understanding the nature of the SG state and intriguing for practical applications of SGs in spintronics.