Strain induced ferromagnetism in LaCoO3 and interface coupling in magnetic multilayers

Bulk LaCoO$_{3}$ (LCO) is rhombohedral with the Co$^{3+}$ atoms in a low spin (LS) diamagnetic configuration. Intraatomic exchange splitting is of similar energy to the crystal field of Co$^{3+}$ in an octahedral oxygen environment, and a transition from LS to high-spin (HS) can be induced by epitaxial tensile stress. We have grown ultrathin films of LCO ($\approx $2 nm) on top of SrTiO$_{3}$ (STO, $\approx $1.5 nm) and La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO, $\approx $22 nm). Magnetization and conductive AFM (C-AFM) experiments in the trilayer demonstrate that the ferromagnetic insulating behavior is kept in ultrathin LCO, with a T$_{\mathrm{C}}\approx $90 K, and M$\approx $0.8 $\mu_{\mathrm{B}}$/Co. The magnetization of LCO and LSMO is decoupled by the STO barrier, and so can be independently switched. C-AFM experiments show I-V curves characteristic of tunnel conduction between the ferromagnetic electrodes across the STO barrier. Moreover, the magnetization of both layers can be conveniently coupled/decoupled by changing the order of deposition of the films (either LCO/LSMO or LSMO/LCO). These results show that new approaches for the design of insulating ferromagnets are possible.