Opposite Sublattices Research Articles

Scattering of Light by One- and Two-Magnon Excitations

We present details of the theory of light scattering by one- and two-magnon excitations, and compare predictions of the theory with our experimental results in the tetragonal antiferromagnets Mn${\mathrm{F}}_{2}$ and Fe${\mathrm{F}}_{2}$. Two mechanisms are considered for first-order (one-magnon) light scattering: one involving a direct magnetic-dipole coupling and the other involving an indirect electric-dipole coupling which proceeds through a spin-orbit interaction. Experimental results on the intensity and polarization selection rules of the first-order scattering show that the spin-orbit mechanism is the important one. On the other hand, second-order (two-magnon) scattering is observed to be even stronger than first-order scattering in these antiferromagnets, implying that the process is not due to the spin-orbit mechanism taken to a higher order in perturbation theory. A theory of second-order scattering based on an excited-state exchange interaction between opposite sublattices is given. When coupled with group-theoretical requirements for the ${{D}_{2h}}^{12}$ crystals, the mechanism predicts the intensity, the polarization selection rules, and the magnetic field dependence of the second-order spectrum. Features of the second-order spectra are related quantitatively to magnons at specific points in the Brillouin zone. Analysis of both first- and second-order magnon scattering has thus enabled determination of the complete magnon dispersion relation for Fe${\mathrm{F}}_{2}$.

Magnetic Properties of Antiferromagnetic GdAlO3

Specific-heat and magnetic-susceptibility measurements on GdAlO3 show that it undergoes a transition to antiferromagnetism at 3.69°K. Magnetization measurements on a single crystal at temperatures down to 0.6°K in fields up to 70 kOe show that the ordered state is a simple two-sublattice structure with the orthorhombic b axis as the axis of alignment. The spin flop, exchange, and anisotropy fields have been found to be 11.2 kOe, 20 kOe to 25 kOe, and 2.3 kOe to 3.0 kOe, respectively. No parasitic ferromagnetism is observed. Optical absorption studies of Er3+ and Dy3+ in GdAlO3 observe the spin flop and confirm that in the ordered structure the distorted simple cubic lattice of Gd3+ ions divides into two sublattices with each ion surrounded by six nearest neighbors belonging to the opposite sublattice.