Sign reversal of spin-polarized tunnelling magnetoresistance in 99.95% La0.7Sr0.3MnO3-0.05% Paraffin wax nanocomposite: An effect of spin-flip scattering at intergranular Paraffin wax interface

Spin-dependent electronic transport has been investigated on granular 99.95% La0.7Sr0.3MnO3-0.05% Paraffin wax nanocomposite. Although, the composite exhibits usual signature of spin-polarized tunnelling magnetoresistance (MR) of polycrystalline La0.7Sr0.3MnO3 (LSMO) within the temperature range of 50-300 K; it differs significantly at low temperature region (T < 50 K) where decrease in low field MR (LFMR) with decrease in temperature has been observed. Interestingly, we have even found a change in the sign of LFMR from negative to positive at T = 10 K with the effect gets enhanced with decreasing temperature. Finally, at T = 5 K, MR is found to be positive at the entire applied field regime. Calculated surface or boundary spin susceptibility has been found to follow similar nature of temperature dependence of MR indicating interface magnetization governing MR. Our conjecture is the blocking of superparamagnetic nanoscopic LSMO impurities, possibly present at intergranular Paraffin wax layers, at lower temperature (T < 50 K) resulting in spin-flip scattering of tunnelling electrons. These spin-flipped tunnelling electrons, in turn experience strong spin-dependent scattering at the minority spin band of LSMO. This leads to enhancement of resistance with applied field and hence yields positive MR. Significantly, with the incorporation of cheap, easily available, organic Paraffin wax tunnel barrier, an overall MR response of ∼200% has been achieved at T = 5 K, at 5 T applied magnetic field and with ac excitation of 111 Hz frequency.