Thickness-dependent ferromagnetic metal to paramagnetic insulator transition in La$_{0.6}$Sr$_{0.4}$MnO$_3$ thin films studied by x-ray magnetic circular dichroism

摘要

Metallic transition-metal oxides undergo a metal-to-insulator transition(MIT) as the film thickness decreases across a ritical thickness of severalmonolayers (MLs), but its driving mechanism remains controversial. We havestudied the thickness-dependent MIT of the ferromagnetic metalLa$_{0.6}$Sr$_{0.4}$MnO$_3$ by x-ray absorption spectroscopy and x-ray magneticcircular dichroism. As the film thickness was decreased across the criticalthickness of the MIT (6-8 ML), a gradual decrease of the ferromagnetic signalsand a concomitant increase of paramagnetic signals were observed, while the Mnvalence abruptly decreased towards Mn$^{3+}$. These observations suggest thatthe ferromagnetic phase gradually and most likely inhomogeneously turns intothe paramagnetic phase and both phases abruptly become insulating at thecritical thickness.