Ferromagnetic insulating substrate for magnetic proximity studies: LaCoO₃thin film

Ferromagnetic insulators (FMIs) are intriguing not only due to their rare nature, but also due to their potential applications in spintronics and various electronic devices. One of its key promising applications is based on an FMI-induced magnetic proximity effect, which can impose an effective time-reversal symmetry breaking on the target ultrathin layer to realize novel emergent phenomena. Here, we conduct systematic studies on thin film LaCoO₃, an insulator known to be ferromagnet under tensile strain, with varying thicknesses, to establish it as an FMI platform to be integrated in heterostructures. The optimal thickness of the LaCoO₃layer, providing a smooth surface and robust ferromagnetism with large remanence, is determined. A heterostructure consisting of an ultrathin target layer (2 uc SrRuO₃), the LaCoO₃FMI layer, and the La_0.5Sr_0.5CoO₃conducting layer has been fabricated and the angle-resolved photoemission spectroscopy measurement on the multi-layer system demonstrates a sharp Fermi edge and a well-defined Fermi surface without the charging effect. This demonstrates the feasibility of the proposed heterostructure using LaCoO₃thin film as the FMI layer, and further lays a groundwork to investigate the magnetic proximity induced phases in quantum materials.