Surface phases of transition metal oxides

A signature of transition metal oxides is the ability to accommodate stoichiometry variations by forming thermodynamically stable structures. This means that the overall composition is sensitive to thermochemical history and composition gradients near surfaces and interfaces can exist at equilibrium. Since the phase diagram of most transition metal oxides is critically dependent upon the chemical composition, the surface should be a ‘different material’. Thus a thorough knowledge of the surface composition and structure is of primary importance for understanding the electronic and magnetic properties of transition metal oxides surfaces and interfaces. Here, differences in the chemical composition, lattice structure, and electronic properties of the surface, as compared to those of the bulk, have been investigated in crystalline films of La _0.65Sr_0.35MnO₃. The surface chemical composition has been investigated by angle-resolved X-ray photoelectron spectroscopy and has been found to be significantly different from that of the bulk because of a copious Sr segregation. The angular dependence of the core level relative intensities has been analyzed using a semiemperical method. The analysis shows that a major restructuring of the surface region occurs. The new surface structure is reminiscent of a Ruddlesden-Popper phase (La,Sr)_n+1Mn_n O_3n+1 with n = 1. These major changes in the surface composition and structure pave the way for a dramatic modification of the surface electronic properties. We have observed a novel surface phase transition for this important class of colossal magnetoresistance materials. The surface phase transition occurs at 240 K compared to 370 K for the bulk and is fundamentally different. In the bulk a ferromagnetic metal to paramagnetic bad-metal transition occurs, while the lower temperature surface transition is from an insulator to a semimetal. The surface of this manganese perovskite is electronically and compositionally quite different from the bulk with important implications for the behavior of artificially grown layered transition metal oxides and for the use of surface sensitive techniques to probe the bulk.