In-situ Atomic-Resolution Study of La1-xSrxCoO3 Using Z-contrast Imaging and EELS

The perovskite oxide LaCoO3 has been studied over the last few decades primarily due to its two transitions, namely the transition from a nonmagnetic insulating state to semiconducting one at 80 K and the transition from semiconducting state to metallic phase above 500 K. While the transition at 80 K is believed to be due to a change in the Co 3+ -ion spin state from a low spin (t 6 2g e 0 g S=0) to an intermediate spin (t 5 2g e 1 g S=1), the high-temperature transition stems from a change in the Co 3+ -ion spin-state from the intermediate spin (t 5 2g e 1 g S=1) to the high spin (t 4 2g e 2 g S=2). Previously, it has been shown that epitaxially LaCoO3 films grown on various substrate materials (LaAlO3 or (LaAlO3)(Sr2AlTaO6) substrates) exhibit a ferromagnetic ordering transition at temperatures close to 80 K, which suggests that the biaxial strain induced from the substrate in the LaCoO3 film stabilizes the intermediate Co 3+ -ion spin state at low temperature[1]. On the other hand, tuning the ferromagnetism of LaCoO3 can be achieved at various temperatures by doping bulk sample with smaller atoms, such as Sr. As Sr doping increases the spin polarons which are caused by substituting divalent Sr 2+ ions for trivalent La +3 ions merge and form short-range FM clusters since the neighboring mixed-valance Co ions interact ferromagnatically via double exchange mechanism [2].