Magnetotransport of La0.7Sr0.3MnO3/SrTiO3 multilayers with ultrathin manganite layers

The thickness dependence of electrical transport and magnetization of ultrathin La0.7Sr0.3MnO3 films has been investigated using epitaxial La0.7Sr0.3MnO3(LSMO)/SrTiO3 multilayers prepared by pulsed laser deposition. Layer thicknesses range from 1.9 to 10 nm for both constituents. Microstructural characterization by x-ray diffraction and transmission electron microscopy has confirmed a well-defined superlattice structure, coherent growth, and fairly smooth interfaces of the samples. Magnetization measurements show a pronounced drop of the ferromagnetic Curie temperature below a LSMO thickness of d = 3 nm, accompanied by a strong rise of the coercive field. The in-plane electrical transport changes from metallic (d rho /dT >0) to insulating at d similar to2.3 nm. Three different types of magnetoresistance (MR) have been observed for d <3 nm. In the sequence of decreasing d, the first resembles the grain boundary MR of polycrystalline LSMO, the second is positive, and the third is associated with a magnetic field-induced insulator-to-metal transition. Magnetic and transport results are discussed in the framework of a mixed-phase state of ferromagnetic metallic and less magnetically ordered insulating clusters in the ultrathin LSMO films. (C) 2001 American Institute of Physics.