Magnetotransport and magnetic properties of La_0.7MnO_3-δ and Pr_0.65Ba_0.05Ca_0.3MnO_3-δ superlattices

In an effort to achieve high magnetoresistance ratios at high temperature and low field, we have fabricated superlattice structures consisting of La_0.7MnO_3-δ (LMO) and Pr_0.65Ba_0.05Ca_0.3MnO_3-δ (PBCMO) systems where La_0.7MnO_3-δ is believed to act as a ferromagnetic biasing source to Pr_0.65Ba_0.05Ca_0.3MnO_3-δ. LMO and PBCMO individually transform to ferromagnetic states at 240 K and 60 K respectively. A series of samples, in which the thickness of La_0.7MnO_3-δ is fixed and that of Pr_0.65Ba_0.05Ca_0.3MnO_3-δ varied from 1 to 8 unit cells, have been grown in situ on (100) LaAlO₃ substrates using a pulsed laser deposition technique. Microstructural characterization carried out on these films show the presence of characteristic intense satellite peaks indicating the chemical modulation of the superlattice structure. The insulator-to-metal transition and the MR ratio, defined as [R(0)-R(H)/R(H)], where R(0) and R(H) are resistances in zero and applied fields, is found to vary with the number of unit cells. The samples with 1, 2, 5 and 8 unit cells of Pr_0.65Ba_0.05Ca_0.3MnO_3-δ show a transition temperature of 240 K, 230 K, 150 K and 160 K and MR ratio of 540%, 592%, 3150% and 2875 % respectively. We have observed an enhancement of magnetoresistance ratio in case of superlattices with thickness of PBCMO greater than 5 unit cells. We attribute this enhancement to a ferromagnetic biasing provided by the LMO layers acting as a ferromagnetic film below its transition temperature.