Abstract
A series of □-MnO2 samples with different counter ions in the tunnel were synthesized. The X-ray diffraction pattern shows the pure phase of the samples. The content of counter ions were determined by both ICP□AES and EDS. The content depends on the ionic size. The magnetic properties show the superparamagnetic transition for all samples. The blocking temperature depends on the ionic size of counter ions and grain size. The sample with K+ as main cations in the tunnel shows anomaly with an apparent upturn below the transition. In addition, the ZFC susceptibility is larger than the FC susceptibility. We explain the upturn by the suppressing of anisotropy at the bottleneck. The temperature dependence of resistivity of the KxMnO2 bulks with being sintered follows the general variable range hopping formula. The magnetoresistivity of the bulk sintered at 250□C is consistent with the prediction of three-dimensional variable range hopping with Coulomb gap. On the other hand, the exponent in the general variable range hopping formula is nearly independent of magnetic fields up to 7 Tesla.