Recently, Chinese researchers showed the experimental demonstration of the half occupation of the ferromagnetic state in tensile-strained LaCoO₃/SrTiO₃ (LCO) thin films using a home-built magnetic force microscopy (MFM), which successfully explains why the macroscopically measured saturation magnetization is only half of the theoretical value.

The results have been published on Physical Review Materials.

This work was jointly conducted by scientists LU Qingyou, ZHAI Xiaofang and LU Yalin from High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) and University of Science and Technology of China.

Over the past few years, the correlated oxide thin films and their interfaces have attracted much attention for their novel magnetic phenomena which are absent in their bulk samples.

To date, it is commonly accepted that the tensile strain is necessary to induce the ferromagnetism (FM), but the basic principle of the emergence of FM is still under debate.

Furthermore, all experiments in high-quality LCO thin films show a saturation magnetization of only half of the theoretical value. The reasons are still unknown.

To address this issue, the above cooperative group recently grew a high-quality LCO thin film with a uniform structure and oxygen vacancy level below 2% by, say, regulating oxygen pressure and rationally selecting substrate orientation.

The oxygen vacancy level was lower than the lowest measureable value. This sample provides a possibility to study and reveal the origin of FM in LCO.

To this end, the MFM measurements were carried out to image the magnetic phase distribution.

It was found that a submicron-scale phase separation could still be found even in such a uniform high-quality LCO thin film.

In addition, the researchers also found that percolated ferromagnetic regions only occupied about 50% of the entire film even with an applied magnetic field up to 13.4 T at 4.5 K, which can explain long inconsistency between the experiment and theory.

The study thus provides substantial understanding in the intrinsic nature of the emergent ferromagnetism in the LCO thin film.

MFM results obtained by sweeping fields from 0 to 13.4 T at 4. 5 K. (Image by FENG Qiyuan and MENG Dechao)