Abstract
We report a comprehensive study of the electrical and magneto-transport 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 0.3–5 K 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 magneto-resistance (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 inter-grain 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 gap-like 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.
Highlights
Electrical conduction in nanostructured materials consisting of granular metals dispersed in insulating oxide matrix has been investigated since the 1970s1
The measurements done to low temperatures allow us to obtain the value of the activation energy ∆ unambiguously. ∆ for transport in a granular medium at low temperatures is related to the Coulomb charging energy EC of a metallic grain, they are distinctly different quantities[1]
The studies, done in the temperature range down to 0.3 K and magnetic fields upto T, allow us to investigate the effect of spin polarization in a regime where the inter-grain transport is dominated by tunnelling in Coulomb blockade regime in such nanocrystals with sizes down to nm
Summary
Electrical conduction in nanostructured materials consisting of granular metals dispersed in insulating oxide matrix has been investigated since the 1970s1. In arrays of magnetic nanocrystals separated by an insulating tunnelling barrier, one would expect the phenomenon of SPT to be strongly modulated by the contribution from Coulomb charging at temperatures T< EC /kB The interplay of these two phenomena leads to interesting physics where the transport in the Coulomb blockade region can be effectively controlled by an applied magnetic field[6]. In arrays of CoFe magnetic nanoparticles where the nanoparticles have been separated by a thin organic insulating layer have been studied to understand the transport in the CB regime[7] In this context, systems made up of ferromagnetic metallic oxide nanoparticles, those of perovskite manganites and cobaltates, present an interesting class of systems. If the ferromagnetic nanocrystals have sufficient spin polarization, one would expect contribution from SPT as the electron tunnels between neighbouring ferromagnetic nanocrystals
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