p-Orbital Ferromagnetism Arising from Unconventional O⁻ Ionic State in a New Semiconductor Sr₂AlO₄ with Insufficiently Bonded Oxygen

Oxygen in solids usually exists in an O²⁻ ionic state. As a result, it loses its magnetic nature of a single atom, wherein two unpaired electrons exist in its outer 2p orbitals. Here, it is shown that an unconventional stable ionic state of O⁻ is realized in a new semiconductor material Sr₂AlO₄, 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 Sr₂AlO₄ 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.