Unusual Electronic Phase Separation Phenomena in Manganite Ultrathin Films

Ultrathin films often exhibit two-dimensional characteristics that are distinctly different from their bulk counterpart. In this work, we show an unusual behavior in La_0.7Sr_0.3MnO₃ (LSMO) ultrathin films grown on SrTiO₃ (001) (STO) substrate. Combining scanning tunneling microscopy and magneto-optical Kerr effect measurements, we have obtained the evolution of the magnetic state of the LSMO ultrathin films as a function of thickness. Strikingly, the LSMO ultrathin films are electronically phase separated in both lateral and vertical directions in thickness range between 4 and 7 unit cells. Specifically, the films are formed by 3 unit cell thick antiferromagnetic base layers, on top of which the layers consist of ferromagnetic metallic nanodisks in antiferromagnetic matrix. At 8 unit cells, an abrupt shear strain relief occurs by forming twinning patterns along [010] and [100] directions. After the shear strain is relieved, the whole films transform into a uniform ferromagnetic metallic state. Our observation reveals the richness of electronic and magnetic phases for strongly correlated systems even in their two-dimensional limit.