The Magnetic and Transport Properties of MxMnO2.yH2O with the alpha-MnO2-type structure

The $\alpha $-MnO$_{2}$, also called Hollandite, is an interesting material because it has 0.46 nm pore in its crystal structure, forming a one-dimensional tunnel. A series of $\alpha $-MnO$_{2}$ samples with different counter ions in the tunnel were synthesized. 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 powdery sample with K$^{+}$ as main cations in the tunnel shows anomaly with an apparent upturn below the blocking temperature. 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 K$_{x}$MnO$_{2}$ bulks with being sintered follows the general variable range hopping formula. The magnetoresistivity of the bulk sintered at 250\r{ }C is consistent with the prediction of three-dimensional variable range hopping with Coulomb gap. On the other hand, for the bulk sintered at 350\r{ }C, the exponent in the general variable range hopping formula is nearly independent of magnetic fields up to 7 Tesla.