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.
Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS3, a quasi-two dimensional antiferromagnet with spin S = 1 on a honeycomb lattice. The spectra show clear, dispersive magnons with a ∼ 7 meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS3 has an easy axis anisotropy with ∆ = 0.3 meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of J3 = −6.90 meV. The data can be fitted reasonably well with either J1 < 0 or J1 > 0, however the best quantitative agreement with high-resolution data indicate that the nearestneighbour interaction is ferromagnetic with J1 = 1.9 meV and that the second nearest-neighbour exchange is small and antiferromagnetic with J2 = −0.1 meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS3 is close to being unstable.
The electrochemistry of (TPP)Fe"(NO) and (OEP)Fe"(NO) was investigated in nine nonaqueous solvents. In weakly binding solvents, such as dichloromethane or benzonitrile, five diffusion-controlled electron-transfer reactions were observed. Three of these reactions were oxidations. The remaining two reactions involved reversible electroreductions, either at the Fe(I1) center or at the porphyrin ring. In strongly binding solvents, such as Me2S0 or pyridine, similar redox reactions were observed, but a number of chemical reactions were coupled to the electron-transfer steps. Formation of dinitrosyl complexes from either the unnitrosylated or mononitrosyl Fe(I1) and Fe(II1) complexes was characterized, and several stability constants measured. Finally, competitive ligation between different solvents and NO as axial ligand in each solvent system was studied along the series of oxidized, neutral, and reduced complexes.
Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.
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