Tuning Magnetic Properties of α-MnO2 Nanotubes by K+ Doping

α-KxMnO2 nanotubes with x = 0.10, 0.12, 0.15, and 0.17 have been synthesized through acid and alkali treatments of the precursor α-MnO2 nanotubes. For the samples with x = 0.10 and 0.12, a large divergence of the ZFC and FC susceptibilities and pronounced hysteresis at low temperature are observed. However, the samples with x = 0.15 and 0.17 are antiferromagnetic with Neel temperatures of 25.4 and 25.3 K, respectively. The reciprocal susceptibility and electrical conductivity indicate that the samples with x = 0.15 and 0.17 undergo a charge separation transition from mixed/averaged valence state manganese ions to Mn3+ and Mn4+ between 250 and 300 K. Below half occupancy of the K+ site (x ≤ 0.125), the strong geometrical frustration due to the triangular lattice configuration and the mixture of Mn3+/Mn4+ result in the spin-glass behavior of the samples with x = 0.10 and 0.12. Above half occupancy of the K+ site, the samples with x = 0.15 and 0.17 exhibit an antiferromagnetic feature due to the charge separ...