Spin-glass behavior in KxRu4-yNiyO8 hollandite materials

We report the synthesis and comprehensive ac and dc susceptibility measurements of KxRu4-yNiyO8 hollandite. The value of the relative frequency shift, delta T-f , has been determined as 0.025 which is within the range expected for spin-glass systems (0.005-0.06). Additionally, the characteristic flipping time of a single spin flip, tau(0), and the dynamical critical exponent, -zv, were determined to have values of 5.82x 10(-8)s and 6.1(3), respectively from the power law. While the value of tau(0) is comparatively very large, -zv is consistent with what is expected for spin-glass systems. Field-cooled hysteresis behavior demonstrates a small increase in the remnant magnetization (at 2 K) on increasing the strength of the cooling field, suggesting that the degree of short-range correlations increases consistent with the formation of larger spin clusters. Thermoremnant magnetization data indicate an exponential-like decay of the magnetization as a function of time with the remnant magnetization remaining nonzero. However, it is clear from these data that multiple components contribute to the decay behavior. Collectively, these data confirm spin-glass character for K0.73(3)Ni1.9(5)RU2.1(5)O8 and clearly demonstrate that the magnetic behavior of this material is far from simplistic.