No abstract

Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of FePS 3 , an S = 2 Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of k M = [ 01 1 2 ] and the moments are collinear with the normal to the ab planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were J 1 = 1.46, J 2 = −0.04, and J 3 = −0.96 meV. The single-ion anisotropy is large, = 2.66 meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, J = −0.0073 meV, proving that FePS 3 is a very good approximation to a two-dimensional magnet.

The spin waves in a powdered sample of a quasi-two-dimensional antiferromagnet, FePS(3), have been measured using neutron inelastic scattering. The data could be modelled and the exchange interactions determined using a two-dimensional Heisenberg Hamiltonian with single ion anisotropy. A suitable fit to the data could only be achieved by including magnetic interactions up to the third nearest neighbour, which is consistent with the findings for other members of the MPS(3) family (M=transition metal). The best fit parameters at 6 K were J(1) = 1.49 meV, J(2) = 0.04 meV, J(3) =- 0.6 meV, with an anisotropy of Δ = 3.7 meV. Measurements as a function of temperature give a coarse measure of the behaviour of the anisotropy and the nature of the phase transition.

The long-accepted magnetic structure of FePS 3 has recently been refuted through extensive powder neutron diffraction studies. Single-crystal neutron diffraction, using both quasi-Laue and monochromatic techniques, has now been conducted to reveal more information about the nature of the magnetic ordering in this compound. The magnetic unit cell was found to be twice as large as the crystallographic cell in both the a and b directions and around three times as large along the c direction, giving a propagation vector of ͓ 1 2 1 2 0.34 ͔ .There is incomplete long-range magnetic order along the c * direction due to weak Fe-Fe interactions and large anisotropy in that direction. The variation of the spontaneous moment with temperature closely resembles that for an antiferromagnetic two-dimensional Ising model on a honeycomb lattice.

Neutron powder diffraction studies as a function of temperature on MnPS3 have revealed temperature-dependent features inconsistent with the previously quoted magnetic ordering. The features vanish at approximately 12 K, a temperature well above the accepted Neel temperature of 78 K. The features are consistent with a true Ising-like two-dimensional ordering with the moments being antiferromagnetically coupled and pointing along the (010) direction.