Direct measurement of the low-temperature spin-state transition in epitaxially strained LaCoO$_{3}$ thin films

The perovskite oxide LaCoO$_{3}$ exhibits an anomaly in its magnetic susceptibility at 80 K associated with a thermally excited transition of the Co$^{3+}$-ion spin. We will show that atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy in combination with ab-initio first-principles DFT calculations can be utilized to measure the spin-state transition in LaCoO$_{3}$. In particular, we utilize in-situ cooling experiments in a transmission electron microscope to demonstrate that the O K-edge pre-peak is sensitive to the Co$^{3+}$-ion spin-state. Our experimental results will be compared to first-principles calculations, and we will conclude that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state. Next, we will examine the effects of bi-axial strain and point defects in LaCoO$_{3}$ thin-films on the Co$^{3+}$-ion spin-state. We will show that a single-crystal pseudo-cubic LaCoO$_{3}$ (001) film can be successfully grown on LaAlO$_{3}$ (001). Moreover, we will show that the epitaxially strained LaCoO$_{3}$ film exhibits a ferro-magnetic transition at low temperature that was not observed in bulk LaCoO$_{3}$. We will discuss the origin of this transition and the possibility of stabilizing different Co$^{3+}$-ion spin-states in LaCoO$_{3}$ using interfacial strain.