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Although colossal magnetoresistance (CMR) materials exhibit large changes in electrical resistance (up to 106%), large magnetic fields (several tesla) must be applied. To obtain a sizeable low-field effect (<102% in several millitesla), it is necessary to incorporate structural discontinuities such as grain boundaries, or other types of interfaces. The potential for applications, however, remains limited because structural discontinuities increase electrical resistance by several orders of magnitude and hence create noise. Moreover, it has proven to be difficult to fabricate structural discontinuities reproducibly. We have attempted to investigate discontinuities that are purely magnetic via transport measurements through a precisely controlled number of magnetic domain walls of known area in thin film devices of the ferromagnetic CMR perovskite La0.7Ca0.3MnO3. A sharp low-field switching seen below ∼110 K is ascribed to the formation of a precise number of magnetic domain walls, each with resistance-area product 8×10−14 Ω m2 at 77 K. This is four orders of magnitude larger than expected, suggesting that the domain walls contain an additional structure. Our findings demonstrate that CMR devices are capable of low-noise low-field switching, and suggest the possibility of exploiting a hitherto unexpected intrinsic effect reproducibly and therefore commercially.

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We report investigation of non-linear electronic transport through artificial grain-boundary junctions made on epitaxial films of La 0.7 Ca 0.3 MnO 3 on bicrystal SrTiO 3 substrates. The experiments carried out over the temperature range 4.2 K-300 K in magnetic field up to 3 T allow us to identify some of the conduction mechanisms that may give rise to nonlinear transport in these grain boundary junctions. The nonlinear transport is associated with multistep inelastic processes in the grain-boundary region, which is moderately affected by the applied magnetic field. However the primary effect of the magnetic field is to enhance the zero-bias conductance ͓G 0 ϭ(dI/dV) Vϭ0 ͔. The dominant voltage dependent contribution to the dynamic conductance (GϭdI/dV) comes from a term of the type V 4/3 at lower temperatures. Other voltage dependent contributions to G, which are of higher order in V, appear only for Tу75 K. In addition we found a contribution to G arising from a V 0.5 term, which is likely to arise from the disordered region around the grain boundary ͑GB͒. The magnetoresistance in the GB depends on the bias used and it decreases at higher bias. The bias dependence is found to be reduced as temperature is increased. We discuss the physical origins of the various contributions to the nonlinear conduction.

Articles you may be interested inThree-dimensional strain state and spacer thickness-dependent properties of epitaxial Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 trilayer structure Temperature dependence of magnetoresistance and nonlinear conductance of the bicrystal grain boundary in epitaxial La 0.67 Ba 0.33 MnO 3 thin filmsThe magnetoresistance of grain boundaries in the perovskite manganites is being studied, both in polycrystalline materials, and thin films grown on bicrystal substrates, because of interest in low-field applications. In this article we show that epitaxial films grown on SrTiO 3 bicrystal substrates of 45°misorientation show magnetoresistance behavior which is strongly dependent on the thickness of the film. Thin films, e.g., 40 nm, can show a large low-field magnetoresistance at low temperatures, with very sharp switching between distinct high and low resistance states for fields applied in plane and parallel to the boundary. Thicker films show a more complex behavior of resistance as a function of field, and the dependence on the angle between the applied field and the grain boundary is altered. These changes in magnetoresistance behavior are linked to the variation in morphology of the films. Thin films are coherently strained, due to the mismatch with the substrate, and very smooth. Thicker films relax, with the formation of defects, and hence different micromagnetic behavior.