Superlong-Range Magnetic Coupling and Ferromagnetic Spin Freezing in Mechanoluminescent Semiconductor Eu:SrAl₂O₄

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_xSr_{1- x}Al₂O₄ (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 Eu²⁺. Muon spin spectroscopy demonstrates intrinsic spin freezing with a spontaneous internal field developed below T_C of ≈3 K. The neighboring magnetic Eu²⁺ 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.