Superlong‐Range Magnetic Coupling and Ferromagnetic Spin Freezing in Mechanoluminescent Semiconductor Eu:SrAl ₂ O ₄

Abstract Magnetic transition in nonmetals requires the presence of a considerable proportion of magnetic spins. A new type of ferromagnet named dilute ferromagnetism that contradicts this well‐established concept is proposed for semiconductors of ZnO etc. but has remained experimentally unproven. In this study, an unconventional superlong‐range magnetic coupling and ferromagnetic spin freezing are reported, which can be viewed as an experimental realization of an intrinsic dilute ferromagnetism, in mechanoluminescent material of Eu _x Sr _{1‐ x} Al ₂ O ₄ ( x = 0.2−2%), wherein Eu is sparsely incorporated into the lattice to substitute Sr. Ferromagnetic coupling appears below ≈80 K and fully saturated ferromagnetic magnetization appears below ≈3 K, with an unusually large magnetic moment of ≈14 µ _B per Eu ²⁺ . Muon spin spectroscopy demonstrates intrinsic spin freezing with a spontaneous internal field developed below T _C of ≈3 K. The neighboring magnetic Eu ²⁺ ions in the lattice have an exceptionally large separation more than one order of magnitude larger than those in conventional magnets, marking it as a unconventional magnetic order over a superlong distance. Bound magnetic polarons arising from electrons trapped at oxygen vacancies may account for this unconventional ferromagnetism. Magnetization under light radiation supports this scenario.