FINITE-SIZE SPIN-WAVE THEORY OF A COLLINEAR ANTIFERROMAGNET

The ground-state and low-energy properties of the two-dimensional J1-J2 Heisenberg model in the collinear phase are investigated using finite-size spin-wave theory [Q. F. Zhong and S. Sorella, Europhys. Lett.21, 629 (1993)], and Lanczos exact diagonalizations. For spin one-half — where the effects of quantization are the strongest — the spin-wave expansion turns out to be quantitatively accurate for J2/J1≳0.8. In this regime, both the magnetic structure factor and the spin susceptibility are very close to the spin-wave predictions. The spin-wave estimate of the order parameter in the collinear phase, m†≃0.3, is in remarkable agreement with recent neutron scattering measurements on Li 2 VOSiO 4.

The magnetization of [Fe(OD 2 ) 6 ] 2+ in ammonium ferrous Tutton salt studied by polarized neutron diffraction - a canted paramagnet

A polarized neutron diffraction experiment has been done on deuterated ammonium ferrous Tutton salt at 1.5 K, 4.6 T with orientations of the magnetic field along the crystal b and c * axes. The flipping ratios of 303 and 280 reflections respectively were used, after correction for extinction, to give 121 and 118 unique values of the magnetic structure factor F M, Z (hkl) (eff). Those values were used in refinements of models for a description of the magnetization density in the crystal. All models resulted in substantial (37° and 45°) canting of the magnetization direction in the paramagnet away from the magnetic field, to an almost constant direction with respect to the O 6 ligand framework, indicating large magnetic anisotropy at the iron atom sites. There is delocalization of magnetization density away from the iron atom into the Fe-O overlap region ( — 4.5%) and onto the OD 2 ligands (6.5%), values comparable with the delocalization of spin from the metal atom in other Tutton salts studied. An earlier ligand field model for the electronic structure of the ion based upon spectroscopic and magnetic data is shown to be inadequate, because it is incompatible with the observed anisotropy in the magnetization around the iron atoms.

MAGNETIC ORDERING IN A TWO-DIMENSIONAL HUBBARD MODEL

We have used the fourth order perturbation expansion to construct the magnetic phase diagram of the two-dimensional Hubbard model. To the contrary of the three-dimensional case, a stable antiferromagnetic phase is found at the limit of (t/U)≃0 if the electron density n lies in the region 0.95<n<1.05. Our results for n=0.9 are compared to the recent indecisive Monte-Carlo investigation on the magnetic structure factor regarding the antiferromagnetic ordering.