Ferromagnetic order can be induced in LaCoO_3 (LCO) thin films by epitaxial strain. Here, we show that the magnetic properties can be "tuned" by epitaxial strain imposed on LCO thin films by the epitaxial growth on various substrate materials, i.e., (001) oriented SrLaAlO_4, LaAlO_3, SrLaGaO_4, (LaAlO_3)_(0.3)(Sr_2AlTa0_6)_(0.7), and SrTiO_3. The lattice mismatch at room temperature of the in-plane lattice parameters between the substrate, a_s, and bulk LCO, a_b, ranges from -1.31% to +2.63%. Single-phase, <001> oriented LCO thin films were grown by pulsed laser deposition on all these substrates. Due to the difference of the thermal-expansion coefficients between LCO and the substrates, the films experience an additional tensile strain of about +0.3% during the cooling process after the deposition at T_s=650 °C. The film lattice parameters display an elastic behavior, i.e., an increase of the in-plane film lattice parameter with increasing a_s. From the ratio between the out-of-plane and in-plane strain, we obtain a Poisson ratio of v≈ 1/3. All films show a ferromagnetic transition as determined from magnetization measurements. The magnetization increases strongly with increasing tensile strain, whereas the transition temperature T_C after a rapid initial rise appears to saturate at T_C≈85 K above a = 3.86 A. The effective magnetic moment μ_(eff) in the paramagnetic state increases almost linearly as a function of the mean lattice parameter , indicating an enhanced population of higher spin states, i.e., intermediate- or high-spin states. The experimental results are discussed in terms of a decrease of the octahedral-site rotation with increasing tensile strain.
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