Magnetic properties of a new one-dimensional vanadium oxide with the hollandite structure

The magnetic properties of the first hollandite-type vanadium oxide containing anions in the 2×2 channels, V_7.22O ₈(OH)₈(Cl)_0.77(H₃O)_2.34, are studied using static (DC) and dynamic (AC) magnetic susceptibilities. From the high-temperature Curie-Weiss behavior the effective magnetic moment is found consistent with the 3+ vanadium oxidation state; the negative Curie-Weiss temperature θ⊖ ≈ -500 K indicates strong antiferromagnetic exchange. The DC magnetic susceptibility shows a rapid increase and the AC susceptibility shows a maximum at about 20 K, indicating magnetic phase transition. The field-cooled and zero-field-cooled susceptibilities diverge below the transition temperature. The real and imaginary components of the AC susceptibility show frequency dependence and shift of maximum toward lower temperatures with decreasing frequency. Analysis of the frequency dependences reveals at least three different relaxation processes existing around and below the transition temperature. The temperature dependences of their relaxation times were obtained using Cole-Cole analysis. We show that the magnetic behavior observed is well explained by the random-field Ising model, with randomness brought on by vacancies in vanadium sites.