Ferromagnetic order in epitaxially strained LaCoO₃ thin films

LaCoO₃ films grown epitaxially on ⟨001⟩ oriented (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 substrates by pulsed laser deposition exhibit ferromagnetic ordering below a critical temperature, T_c, of 85 K. Polycrystalline films of LaCoO₃ prepared in the same way did not show ferromagnetic order down to T≈5 K, and their temperature dependent susceptibility was identical to that of bulk LaCoO₃. The ferromagnetism in epitaxial films is not simply a property of the surface region, rather it extends over the complete film thickness, as shown by the linear increase of the saturated magnetic moment with increasing film thickness. We discuss this surprising result in terms of epitaxial tensile strain via the properly chosen substrate inducing ferromagnetic order.