Tuning Magnetic Properties of α-MnO 2 Nanotubes by K + Doping

α-K x MnO 2 nanotubes with x = 0.10, 0.12, 0.15, and 0.17 have been synthesized through acid and alkali treatments of the precursor α-MnO 2 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 Néel 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 Mn 3+ and Mn 4+ 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 Mn 3+ /Mn 4+ 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 separation of Mn 3+ and Mn 4+ . Our experiments display that the magnetic properties of α-K x MnO 2 can be tuned by varying the K + content.