Magnetotransport and Magnetic Properties of La_(0.7)MnO_(3-delta) and Pr_(0.65)Ba_(0.05)Ca_(0.3)MnO_(3-delta) Superlattices

摘要

In an effort to achieve high magnetoresistance ratios at high temperature and low fields, we have fabricated superlattice structures consisting of La_(0.7)MnO_(3-delta) (LMO) and Pr_(0.65)Ba_(0.05)Ca_(0.3)Mno_(3-delta)(PBCMO) systems where La_(0.7)MnO_(3-delta) is believed to act as a ferromagnetic biasing source to Pr_(0.65)Ba_(0.05)Ca_(0.3)MnO_(3-delta. 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_(0.7)MnO_(3-delta) is fixed and that of Pr_(0.65)Ba_(0.05)Ca_(0.3)MnO_(3-delta) varied from 1 to 8 unit cells, have been grown in situ on (100) LaAlO_3 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_(0.65)Ba_(0.05)Ca_(0.3)MnO_(3-delta) show a transition temperature of 240 K, 230 K, 150 K and 160 K and MR ratio of 540 percent, 592 percent, 3150 percent and 2875 percent 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.