Direct Imaging Revealing Halved Ferromagnetism in Tensile-Strained LaCoO₃ Thin Films

The enigma of the emergent ferromagnetic state in tensile-strained LaCoO₃ thin films remains to be explored since there is a longstanding controversy between the theory and experiments. It is that all experiments can only find at most half of the magnetic moment predicted by the theory. Here we show the experimental evidence of the rare halved occupation of the ferromagnetic state in tensile-strained LaCoO₃ thin films by the direct magnetic imaging technique using a low-temperature magnetic force microscope. The film was grown by the atomic layer-by-layer laser molecular beam epitaxy and exhibits the coherently strained lattice structure and the stoichiometric composition. The direct magnetic imaging revealed that percolated ferromagnetic regions with typical sizes between 100 nm and 200 nm occupy about 50% of the entire film, down to the lowest achievable temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Our study demonstrated the halved occupation of the ferromagnetic phase in the LaCoO₃ ultrathin films as the real microscopic state of the emergent ferromagnetism.