Abstract
We report thickness dependent inhomogeneous antiferromagnetic (AFM) to ferromagnetic (FM) phase transitions in ultrathin La0.7Sr0.3MnO3 (LSMO) films grown on the SrTiO3 substrate. When the films are 4-7 unit cells (UCs) thick, FM domains appear as isolated nanodisks in the AFM matrix that together floats on top of the three AFM base UCs, leading a superparamagnetic blocking behavior. Our first principles calculations unravel the rather counterintuitive physical origin of this mixed phase state; the formation of FM/AFM domain boundaries is energetically favorable. At 8 UCs, an abrupt shear strain relief occurs in the LSMO thin film and twinning patterns with two unit cell periodicity form along the [010] and [100] directions. Our studies reveal the complexity of the magnetic phase transition at the nanometer scale and open a door for the development of quantum devices and statistical theories.