  The enigma of the emergent ferromagnetic state in tensile-strained LaCoO3
thin films remains to be explored because of the lack of a well agreed
explanation. The direct magnetic imaging technique using a low-temperature
magnetic force microscope (MFM) is critical to reveal new aspects of the
ferromagnetism by investigating the lateral magnetic phase distribution. Here
we show the experimental demonstration of the rare halved occupation of the
ferromagnetic state in tensile-strained LaCoO3 thin films on SrTiO3 substrates
using the MFM. The films have uniformly strained lattice structure and minimal
oxygen vacancies (less than 2%) beyond the measurement limit. It is found that
percolated ferromagnetic regions with typical sizes between 100 nm and 200 nm
occupy about 50% of the entire film, even down to the lowest achievable
temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Preformed
ferromagnetic droplets were still observed when the temperature is 20 K above
the Curie temperature indicating the existence of possible Griffiths phase. Our
study demonstrated a sub-micron level phase separation in high quality LaCoO3
thin films, which has substantial implications in revealing the intrinsic
nature of the emergent ferromagnetism.
