p ‐Orbital Ferromagnetism Arising from Unconventional O ⁻ Ionic State in a New Semiconductor Sr ₂ AlO ₄ with Insufficiently Bonded Oxygen

Abstract Oxygen in solids usually exists in an O ²⁻ ionic state. As a result, it loses its magnetic nature of a single atom, wherein two unpaired electrons exist in its outer 2 p orbitals. Here, it is shown that an unconventional stable ionic state of O ⁻ is realized in a new semiconductor material Sr ₂ AlO ₄ , leading to an intrinsic p ‐orbital ferromagnetism stable until ≈900 K. Experimental and theoretical investigations have clarified that one‐fourth of the oxygen atoms in Sr ₂ AlO ₄ are insufficiently bonded in the crystal structure, resulting in a unique O ⁻ ‐state and p ‐orbital ferromagnetism. To date, the O ⁻ state is reported to exist only in non‐equilibrium conditions, and p ‐orbital magnetism is only suggested in impurity bands with small ferromagnetic moments. The present work provides a new route for creating ferromagnetism in semiconductors and exploring new p ‐orbital physics and chemistry. In addition, the material shows elastic‐mechanoluminescence that may enable unprecedented mechano‐photonic‐spintronics.