Maximization of ferromagnetism in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>LaCo</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> films by competing symmetry

Spin state transition in perovskite cobaltite is delicately controlled by the competition between exchange interaction energy and crystal-field energy. The latter is mainly governed by the change of bond length and bonding angle. Previous work has revealed that the electronic configuration associated with spin state transition in $\mathrm{LaCo}{\mathrm{O}}_{3}$ (LCO) thin films can be effectively modulated by epitaxial strain. However, a systematic study on the spin state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions in LCO films with different crystallographic symmetry is still missing. Here, keeping the in-plane strain unchanged, we report that the magnetization of LCO films can be manipulated with crystallographic symmetry. The ultrathin LCO layers, constrained by the cubic substrate, have pseudotetragonal structure and small magnetization. Upon increasing the layer thickness, the monoclinic structure dominates the LCO film and maximizes its ferromagnetism. For the LCO films with a thickness beyond 35 unit cells, the symmetry relaxes gradually towards its rhombohedral bulk form, and meanwhile the magnetization reduces. These results highlight the importance of spin-lattice entanglement in a ferroelastic material and provide a concise way to maximize its functionality using symmetry engineering.