Decoupling the spin-state and insulator-metal transitions via heteroepitaxial strain in LaCoO_3-δ

Perovskite LaCoO_3-δ (LCO), because of small energy differences between multiple spin states due to competing crystal-field splitting and Hund’s exchange energies, is highly susceptible to perturbations such as temperature and strain. Bulk LCO, for example, undergoes successive thermally excited spin-state transitions, one at 30-80 K and the other at ~500 K. The latter is accompanied by an insulator-metal transition, although the nature of this transition, and its correlation with spin state, have proven difficult to understand. Here, using heteroepitaxial strain to widely vary the ground spin state, we systematically study the influence on the high-temperature insulator-metal transition in LCO thin films. High-resolution X-ray diffraction and reciprocal space maps confirm fully-strained films (~18 nm thick) on various substrates, with lattice mismatches from -1.4% (compressive) to +2.5% (tensile). As expected, magnetometry shows ferromagnetic ordering (T_C ≈ 80 K) in tensile-strained films, but a bulk-like nonmagnetic ground state under compressive strain. The spin state is thus radically different under tension and compression. Nevertheless, transport measurements to >600 K reveal that the high-temperature insulator-metal transition persists in all films, unambiguously establishing that the LCO insulator-metal transition is essentially decoupled from spin-state physics. The modest trends with heteroepitaxial strain are interpreted in terms of known chemical pressure effects in bulk.