Electrical transport properties of nanostructured ferromagnetic perovskite oxides La0.67Ca0.33MnO3 and La0.5Sr0.5CoO3 at low temperatures (5 K ⩾ T ⩾ 0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magnetotransport properties of nanocrystals of La0.67Ca0.33MnO3 (LCMO) (with size down to 15 nm) and La0.5Sr0.5CoO3 (LSCO) (with size down to 35 nm) in the temperature range 0.3-5K and magnetic fields up to 14 T. The transport, magneto-transport and nonlinear conduction (I -V curves) were analysed using the concept of spin-polarized tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (ΦB). The magneto-transport data were used to find the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the magnetoresistance (MR) contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field, leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to barrier height persists. The idea of intergrain tunnelling has been validated by direct measurements of the nonlinear I -V data in this temperature range, and the I -V data were found to be strongly dependent on magnetic field. We made the important observation that a gaplike feature (with magnitude ∼EC, the Coulomb charging energy) shows up in the conductance g(V) at low bias for the systems with the smallest nanocrystal size at the lowest temperatures (T ⩽ 0.7 K). The gap closes when the magnetic field and temperature are increased. [ABSTRACT FROM AUTHOR]