Rock salt-type HoO epitaxial thin film as a heavy rare-earth monoxide ferromagnetic semiconductor with a Curie temperature above 130 K

Ho monopnictides (HoPn: Pn = N, P, As, Sb, and Bi) and monochalcogenides (HoCh: Ch = S, Se, and Te) are known to possess a nontrivial magnetic structure, while solid-phase HoO has not been synthesized yet. In this study, a rock salt-type HoO epitaxial thin film was grown by using the pulsed laser deposition method, which had a bandgap of 0.11 eV and tunable electrical conduction via its oxygen nonstoichiometry. Its Curie temperature, above 130 K, was much higher than those of HoPn and HoCh, most likely due to an enhanced direct exchange interaction. Similar to HoPn and HoCh, a HoO thin film showed a metamagnetic behavior below the Curie temperature in spite of its dominant ferromagentic ordering.