(LaCo O3)n/(SrCo O2.5)n superlattices: Tunable ferromagnetic insulator

Ferromagnetic insulators have great potential for spintronic applications. For such applications, it is essential to find materials with a robust and controllable ferromagnetic insulating phase. However, because ferromagnetism in functional transition-metal oxides is usually coupled to metallicity, ferromagnetic insulators are very rare and independent control of their magnetic and electrical properties is difficult. In this study, the electrical, magnetic, and optical properties of (LaCoO3)n/(SrCoO2.5)n superlattice films are investigated for the manipulation of the ferromagnetic insulating phase. While the superlattices remain insulating irrespective of the periodicity n, the electronic structure and magnetic state vary drastically. Superlattices with large periodicities n of 10 and 20 show a ferromagnetic transition at a critical temperature TC of ∼80K. With decreasing periodicity and increasing interface density of the superlattices, system with n=4 becomes almost nonmagnetic, while in systems with n=2 and 1, a reentrant ferromagnetic phase is observed at TC of ∼180 and ∼225K, respectively. Optical spectroscopy reveals that the fine control of the magnetic ground state is achieved by the modified electronic structure associated with the spin-state transition. Our results suggest an important design principle to create and manipulate the ferromagnetic insulating properties of Co-based oxide thin films.