Magnetic ordering in the double-layered molecular magnet Cu(tetren)[W(CN)8]: Single-crystal study

The results of the single-crystal studies of the quasi-two-dimensional (2D) copper-octacyanotungstenate coordination polymer {(tetrenH(5))(0.8)Cu-4(II)[W-V(CN)(8)](4)center dot 7.2H(2)O}(n) are presented. It was found that the three-dimensional (3D) magnetic ordering at T-c approximate to 33 K gives rise to antiferromagnetic structure which under relatively small magnetic field changes to ferromagnetic. There is strong easy-plane anisotropy confining the magnetic moments to the ac crystallographic plane. Detailed analysis of the scaling behavior of the dc susceptibility above T-c is performed. For the direction of the external magnetic field parallel to the ac crystallographic plane the ordering process involves one stage only, whereas for the direction parallel to the b crystallographic axis a two-stage process is revealed. The corresponding crossover at about 39 K is from the 2D short-range order state to the 3D long-range one. The well-established short-range order above the transition is consistent with the low value of the entropy amounting to only 15% of the maximal expected value. The scaling behavior of the Berezinski-Kosterlitz-Thouless type has been checked for the temperature dependence of magnetization in the ac plane yielding the value of nu=0.56. The spin-flip field is explained by assuming that below the transition temperature there is a domain structure coupled through the dipole-dipole interactions. The ratio of the interbilayer to the intrabilayer exchange interaction was estimated to amount to 1x10(-4).