Thickness-Driven First-Order Phase Transitions In Manganite Ultrathin Films

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.