Abstract
We report magnetization (χ, M), magnetic specific heat (CM), and neutron powder diffraction results on a quasi-two-dimensional (2D) S=2 square lattice antiferromagnet Ba2FeSi2O7 consisting of FeO4 tetrahedrons with highly compressive tetragonal distortion (27%). Despite of the quasi-2D lattice structure, both χ and CM present three-dimensional magnetic long-range ordering below the Néel temperature TN=5.2K. Neutron diffraction data show a collinear Qm=(1,0,1/2) antiferromagnetic (AFM) structure below TN but the ordered moment aligned in the ab plane is suppressed by 26% from the ionic spin S=2 value (4μB). Both the AFM structure and the suppressed moments are well explained by using Monte Carlo simulations with a large single-ion in-plane anisotropy D=1.4 meV and a rather small Heisenberg exchange Jintra=0.15 meV in the plane. The characteristic 2D spin fluctuations are recognized in the magnetic entropy release and diffuse scattering above TN. This new quasi-2D magnetic system also displays unusual nonmonotonic dependence of TN as a function of magnetic field H.
