Optical signatures of strain-induced ferromagnetism in a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>LaCoO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math> thin film

Using spectroscopic ellipsometry, we studied the optical conductivity of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>LaCoO</a:mi><a:mn>3</a:mn></a:msub></a:math> with various degrees of strain. The optical response of the compressively strained <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>LaCoO</b:mi><b:mn>3</b:mn></b:msub></b:math> film is qualitatively similar to the one of the unstrained <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>LaCoO</c:mi><c:mn>3</c:mn></c:msub></c:math> polycrystalline sample and exhibits a redistribution of the spectral weight between <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mo>∼</d:mo><d:mn>0.2</d:mn></d:math> and 6 eV, which is most likely related to the thermal excitation of the high-spin (HS) states. The optical response of the ferromagnetic (FM) tensile strained film exhibits clear signatures of the FM state. Below the Curie temperature <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>T</e:mi><e:mi>c</e:mi></e:msub><e:mo>=</e:mo><e:mn>82</e:mn></e:mrow></e:math> K, a spectral weight transfer sets on from high energies (between 3.3 and 5.6 eV) to low energies (between 0.2 and 3.3 eV). The temperature dependence of the low-energy spectral weight can be understood in the framework of the HS biexciton model of Sotnikov [] as corresponding to the increase of the concentration of the HS states that are stabilized below <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>T</f:mi><f:mi>c</f:mi></f:msub></f:math>. The magnitude of the redistribution of the spectral weight due to the formation of the FM state is sizable and corresponds to 0.009 <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mi>e</g:mi></g:math> per Co ion. We discuss it in terms of the effective kinetic energy of Co <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mn>3</h:mn><h:mi>d</h:mi></h:mrow></h:math> bands. Published by the American Physical Society 2024