Strontium-doped Lanthanum Cobalt Oxides Studied by XPS

Abstract

La1−xSrxCoO3 is a perovskite-type magnetic material. LaCoO3 is a nonmagnetic semiconductor at low temperatures but it undergoes a gradual transition from the nonmagnetic ground state to a paramagnetic state above ∼90 K and then to a metal above ∼500 K. It has remained controversial whether the 90 K transition is a low spin-to-high spin transition or not. One aim of this research was to elucidate the origin of the magnetic transition and our conclusion was that it is not a low spin-to-high spin transition, but low spin-to-intermediate spin transition. By replacing La with Sr, the system changes from the nonmagnetic semiconductor to a ferromagnetic metal with increasing Sr concentration. This is similar to the colossal magnetoresistance (CMR) system La1−xSrxMnO3, and the system shows fairly large MR, in fact. However, in contrast to the manganese system, the present system shows only about a half of the full moment of the high spin Co ion. The other aim of this research was to understand the magnetic state of this system using photoemission spectroscopy. The samples were polycrystalline and were prepared by solid-state reaction. XPS measurements were carried out using Mg Kα source at liquid nitrogen temperature (∼80 K). In order to obtain a fresh clean surface, the samples were scraped with a diamond file in situ, and surface aging was monitored by observing the intensity of a tail at the higher binding-energy side of the O 1s peak. The surface typically lasted for 1–2 h.