Nanoparticle geometrical effects on percolation, packing density, and magnetoresistive properties in ferromagnet-superconductor-insulator nanocomposites

The influence of nanoparticle geometrical and percolation effects on the electrical transport and magnetotransport properties of the system are investigated in a number of binary networks composed of superconducting (${\mathrm{MgB}}_{2}$), ferromagnetic (${\mathrm{CrO}}_{2}$ or ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{MnO}}_{3}$), and insulating (${\mathrm{LiCoO}}_{2}$ or ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$) nanoparticles. For all-metal ${\mathrm{CrO}}_{2}/{\mathrm{MgB}}_{2}$ binary composites an anomalously high resistance state with two distinct percolation thresholds is found within a narrow range of the constituent composition, as a result of the strong suppression of heterointerfacial conductance between superconducting and half-metallic (${\mathrm{CrO}}_{2}$) nanoparticles and an unexpectedly large value of the percolation threshold of ${\mathrm{MgB}}_{2}$. The latter can be attributed to the geometric mismatch between the constituent nanoparticles and the related particle packing and excluded volume effects. The filling factor in this binary composite attains a maximum near the same percolation threshold and exhibits a power-law scaling behavior, which would indicate another geometric phase transition. The geometric effect of constituent particles is also verified through the large changes of percolation threshold in different types of combinations of binary components. Interestingly, the percolation transitions in these binary nanocomposites, either in the case of single or double percolation, are found to be governed by an unconventional scaling behavior of resistance just below the percolation threshold, due to mechanisms determined by heterogeneous interfaces of particles that are absent in conventional single-component percolation systems. In addition, magnetotransport is investigated in ${\mathrm{CrO}}_{2}/{\mathrm{MgB}}_{2}$ junctions, demonstrating nonhysteretic low-temperature magnetoresistance at the half-metal/Type II superconductor interface with the highest values around $42%$ at liquid helium temperatures obtained at volume fraction between the two percolation thresholds. The results show large variations of magnetoresistance that are highly sensitive to the constituent composition and the sample temperature near the percolation thresholds, demonstrating the interrelation between the structural or geometrical property of the system and materials functionality.