Recently, interest in the influence of electrical fields on the resistance of manganites had increased. It has been demonstrated that dc currents induce strong resistance decrease in Pr_0.7Ca_0.3MnO₃ and La_0.82Ca_0.18MnO₃ single crystals. It was observed that pulsed electric fields also change the resistance of thin La_0.67Ca_0.33MnO₃ films. However, the reasons for the origin of this electroresistance (ER) phenomenon are still unclear. In this paper, we present experimental results demonstrating that the main reason for the appearance of this ER in thin manganite films is the existence of ferromagnetic tunnel junctions located at intergranular regions in the polycrystalline films.
Epitaxial, textured and polycrystalline La_0.7Ca_0.3MnO₃ films, having 150 nm thicknesses, were deposited onto (110) NdGaO₃, MgO and lucalox substrates, respectively. The samples were designed in a co-planar shape. Two Ag tapes having width of 1 mm and spaced by a d = 20 μm gap were deposited onto the film. The samples were connected in series to 50 Ω impedance high frequency transmission line. The resistance (R) vs. electric field strength (E) dependence was investigated using 10 ns duration electrical pulses having 0.5 ns rise time and amplitude up to 500 V.
Electroresistance [R(E)-R(0)]/R(0) was investigated using electric field strengths ranging up to 80 kV/cm at temperatures ranging from 300 K to 4.2 K. Strong (up to 93%) negative ER was obtained in polycrystalline La_0.7Ca_0.3MnO₃ films prepared on MgO and lucalox substrates by laser ablation. The epitaxial films grown, using laser ablation technique on NdGaO₃ substrate demonstrated only small resistance change due to Joule heating induced by a current pulse. It was concluded that ER manifests itself in strongly inhomogeneous manganites films exhibiting a large number of ferromagnetic tunnel junctions.

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