Dependence Of The High-Field Grain-Boundary Magnetoresistance Of Ferromagnetic Manganites On Curie Temperature

We analyze the high-field magnetoresistance of polycrystalline ferromagnetic manganites of varied composition in magnetic fields up to mu H-0=47 T. Small to medium deviations from a linear field dependence of the conductance {as discovered for a La0.7Sr0.3MnO3 film recently, [N. Kozlova , J. Magn. Magn. Mater. 261, 48 (2003)]} are typical. The conductance is always well described by a quadratic polynomial G(H)=a+bH+cH(2) with temperature-dependent coefficients a, b, and c. Both b and c increase with decreasing Curie temperature (T-C) of the samples. The conductance slope 1/G(0) dG/dH is related to an average susceptibility of the magnetically disturbed layer at the grain boundaries. The nonsaturated magnetic order of grain-boundary (GB) spins at 5 K and 47 T means the presence of strong antiferromagnetic interactions. The observation of a systematic correlation between GB magnetoresistance and bulk T-C indicates that magnetic order at GBs is governed by parameters such as doping and average ionic size (electronic band width). Furthermore, the temperature dependence of linear and quadratic magnetoconductance contributions is investigated; here, the weak temperature dependence for compounds with high T-C is notable.