Physical properties of the quasi-two-dimensional square lattice antiferromagnet Ba2FeSi2O7

Abstract

We report magnetization (\ensuremath{\chi}, $M$), magnetic specific heat (${C}_{\mathrm{M}}$), and neutron powder diffraction results on a quasi-two-dimensional (2D) $S=2$ square lattice antiferromagnet ${\mathrm{Ba}}_{2}{\mathrm{FeSi}}_{2}{\mathrm{O}}_{7}$ consisting of ${\mathrm{FeO}}_{4}$ tetrahedrons with highly compressive tetragonal distortion (27%). Despite of the quasi-2D lattice structure, both \ensuremath{\chi} and ${C}_{\mathrm{M}}$ present three-dimensional magnetic long-range ordering below the N\'eel temperature ${T}_{\mathrm{N}}=5.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Neutron diffraction data show a collinear ${\mathbit{Q}}_{\mathbit{m}}=$(1,0,1/2) antiferromagnetic (AFM) structure below ${T}_{\mathrm{N}}$ but the ordered moment aligned in the $ab$ plane is suppressed by 26% from the ionic spin $S=2$ value ($4{\ensuremath{\mu}}_{\mathrm{B}}$). 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 ${J}_{\mathrm{intra}}=0$.15 meV in the plane. The characteristic 2D spin fluctuations are recognized in the magnetic entropy release and diffuse scattering above ${T}_{\mathrm{N}}$. This new quasi-2D magnetic system also displays unusual nonmonotonic dependence of ${T}_{\mathrm{N}}$ as a function of magnetic field $H$.