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alpha-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 alpha-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 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 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 alpha-K(x)MnO(2) can be tuned by varying the K(+) content.