In-plane anisotropy in the magnetic and transport properties of manganite ultrathin films

To analyze the conducting and magnetic properties near the film and substrate interface in manganites, ultrathin films (thickness <= 100 A) of La0.7Sr0.3MnO3 were epitaxially grown by molecular-beam epitaxy on single-crystal (001) LaAlO3, (110) NdGaO3, and (001) SrTiO3 substrates. Structural, magnetic, and magnetoresistive properties were investigated. All samples exhibit a substrate-independent decrease of the c-lattice parameter for thinnest films. Highly anisotropic behavior in both transport and magnetic properties were measured along the in-plane directions parallel to the substrate crystallographic axes. In particular, for the thinnest films (60 A), the negative magnetoresistance at about 120 K with the average current along one of the crystallographic directions, is larger than the room-temperature colossal value. In the same low-temperature range, with the current along the other in-plane crystallographic direction, the magnetoresistance changes sign (resulting to be magnetic field independent for T similar to 150 K). Such an in-plane anisotropy of transport and magnetic properties is investigated with respect to possible intrinsic and extrinsic physical mechanisms.