S=1/2 Ising behavior in the two-dimensional molecular magnet Fe(NCS)2(pyrazine)2 -: art. no. 224405

The magnetic ordering and critical behavior of antiferromagnetic Fe(NCS)(2)(pyrazine)(2) has been studied by neutron powder diffraction (NPD), inelastic neutron scattering (INS), Mossbauer spectroscopy, and magnetic measurements. The system can be regarded as a two-dimensional (2D) antiferromagnet even in the ordered phase, given that long-range magnetic ordering between the layers simply follows a necessary consequence of the establishment of long-range ordering within the planes. The INS data, which were taken on a cold neutron time-of-flight spectrometer, reveals that when the temperature is lowered towards T-N, the correlation length within the 2D layers increases and ultimately crosses over from two- to three-dimensional (3D) behavior. Indeed, 3D long-range antiferromagnetic order, associated with a propagation vector [1,0,1/4+epsilon], is observed in the NPD data below 6.8 K. Furthermore, in agreement with the behavior of both chi(T) and C-m(T) data, the order parameter follows the exact Osanger solution for a 2D S=1/2, Ising system.