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

The thickness dependence of electrical transport and magnetization of ultrathin ${\mathrm{La}}_{\mathrm{0.7}}{\mathrm{Sr}}_{\mathrm{0.3}}{\mathrm{MnO}}_{\mathrm{3}}$ films has been investigated using epitaxial ${\mathrm{La}}_{\mathrm{0.7}}{\mathrm{Sr}}_{\mathrm{0.3}}{\mathrm{MnO}}_{\mathrm{3}}{\mathrm{(LSMO)/SrTiO}}_{\mathrm{3}}$ 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 $\text{d=3\hspace{0.05em}}\mathrm{nm},$ accompanied by a strong rise of the coercive field. The in-plane electrical transport changes from metallic $\text{(dρ/dT>0)}$ to insulating at $\text{d∼2.3\hspace{0.05em}}\mathrm{nm}.$ Three different types of magnetoresistance (MR) have been observed for $\text{d<3\hspace{0.05em}}\mathrm{nm}.$ 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.