Strain- and thickness-dependent magnetic properties of epitaxial La_0.67Sr_0.33CoO₃/La_0.67Sr_0.33MnO₃ bilayers

Magnetic properties and interfacial phenomena of epitaxial perovskite oxides depend sensitively on parameters such as film thickness and strain state. In this work, epitaxial La_0.67Sr_0.33CoO₃ (LSCO)/La_0.67Sr_0.33MnO₃ (LSMO) bilayers were grown on NdGaO₃ (NGO) and LaAlO₃ (LAO) substrates with a fixed LSMO thickness of 6 nm, and LSCO thickness (t_LSCO) varying from 2 to 10 nm. Soft x-ray magnetic spectroscopy revealed that magnetically active Co²⁺ ions that strongly coupled to the LSMO layer were observed below a critical t_LSCO for bilayers grown on both substrates. On LAO substrates, this critical thickness was 2 nm, above which the formation of Co²⁺ ions was quickly suppressed leaving only a soft LSCO layer with mixed valence Co³⁺/Co⁴⁺ ions. The magnetic properties of both LSCO and LSMO layers displayed strong t_LSCO dependence. This critical t_LSCO increased to 4 nm on NGO substrates, and the magnetic properties of only the LSCO layer displayed t_LSCO dependence. A non-magnetic layer characterized by Co³⁺ ions and with a thickness below 2 nm exists at the LSCO/substrate interface for both substrates. The results contribute to the understanding of interfacial exchange spring behavior needed for applications in next generation spintronic and magnetic memory devices.