Dechao Meng1 2 Yongqi Dong3 4 Qiyuan Feng2 5 Xiang Hu2 Zhangzhang Cui2 6 Hua Zhou4 Hawoong Hong4 Jinghua Guo6 Qingyou Lu5 Xiaofang Zhai2 Yalin Lu2 3 7

1, Microsystem and Terahetz Research Center, CAEP, Chengdu, , China
2, University of Science and Technology of China, Hefei, , China
3, University of Science and Technology of China, Hefei, , China
4, Argonne National Laboratory, Argonne, Illinois, United States
5, Chinese Academy of Sciences, Hefei, , China
6, Lawrence Berkeley National Laboratory, Berkeley, California, United States
7, US Air force Academy, Colorado Springs, Colorado, United States

Great efforts have been taken to reveal the intrinsic origins of emerging ferromagnetism (FM) in strained LaCoO3 (LCO) films, different from LCO bulks. However, even macro magnetic performances of LCO are still not well understood, such as magnetic anisotropy. Understanding magnetic anisotropy might help to find the true causes of FM in turn.
Perpendicular magnetic anisotropy (PMA) is the first time to be directly observed in high quality LCO films with different thickness. The in plane (IP) and out of plane (OOP) remnant magnetic moment ratio of 30 unit cell (u.c.) films is as large as 20. The easy axis lays in the OOP direction with an IP/OOP coercive field ratio of 10. What`s more, the PMA could be simply tuned by changing the thickness. With the thickness increasing, the IP/OOP magnetic moment ratio remarkably decrease with magnetic easy axis changing from OOP to IP. Such a huge and tunable PMA performance exhibit strong potentials in fundamental researches or applications.

What causes PMA is the first concern. More OOP orbitals occupation may be one of the micro reasons of PMA. A cluster-like magnetic domain pattern was found in 30 u.c. with no obvious color contrasts, similar to that of LaAlO3/SrTiO3 films. And the nanosize domains couldn`t be totally switched even at a large OOP magnetic field of 23 T. It indicates strong IP characters or none OOP magnetism of some clusters. The IP magnetic domains might influence the magnetic performance and help to form PMA. Meanwhile some possible nonmagnetic clusters might be the reason why the measured moments of LCO films are smaller than the calculated values 2 μB/Co, one of the biggest confusions in LCO films.

What tunes PMA seems much more interesting. Totally different magnetic domain patterns were found in 180 u.c. films with cluster magnetic domains surrounded by <110> cross-hatch lines. These lines were regarded as structure domain walls (DWs) determined by 3D reciprocal space mapping (RSM). Two groups of in-plane features with fourfold symmetry were observed near the film diffraction peaks in (002) 3D-RSM. One is along <110> directions with a larger intensity, which is well match the lines on the surfaces. The other is much weaker and along <100> directions, which is from the normal lattice titling of films deposited on cubic substrates. The <110> domain features obtained from (103) and (113) 3D-RSMs exhibit similar evolution of the DWs percentages and magnetic behavior. Structure domains and domain walls are believed to tune PMA performances by transform more IP magnetic moments to OOP.

Last but not the least, thick films with lots of structure domains exhibit novel electrical transport behaviors, different from those of thin films. A metal-to-insulator transition (MIT) and an angular dependent negative magnetic resistivity were observed near 150 K, a temperature higher than FM transition temperature (90 K) but similar to that of spin-orbital coupling related 1/4 order diffraction peaks.