Double-percolation and magnetoresistance effects of half-metallic ferromagnet-superconductor composites

We have studied the electrical and magneto transport properties of half-metallic ferromagnet-superconductor nanocrystal composites. The materials used in the investigation are CrO2, LSMO and MgB2. The superconducting MgB2 particles were mixed with half-metallic ferromagnetic CrO 2 (or LSMO) particles according to weight ratio and made into pellet samples by cold-pressed method in controlled humidity environment. The samples were characterized by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Four-point resistance measurements were conducted in Quantum Design physical property measurement system (PPMS) at temperature range from 300 to 2 K in applied magnetic field up to 5 T.;When changing the composition ratio of CrO2-MgB2 composites, two electrical percolation thresholds were observed, one is for MgB2 and the other for CrO2. The associated S/I phase transition and M/I phase transition are identified from composition dependent resistance curve, we define this as a new double-percolation effect. The percolation threshold of MgB2 is found to be larger than that of previous numerical calculation, hence particle size and shape are important factors to be taken into account. The magnetoresistance of the composites were observed to have typical powder magnetoreistance (PMR) characteristic. The field dependent resistance curve is butterfly shape as seen in other PMR systems. The largest MR ratio in CrO2-MgB2 composites is 42% at 2K.;In LSMO-MgB2 system, the double-percolation effect was not observed. Only the percolation threshold for MgB2 was measured, and the value is smaller than that in CrO2-MgB2 system.