Measurement of Spin-Wave Dispersion in NiO by Inelastic Neutron Scattering and Its Relation to Magnetic Properties

TLDR

Inelastic neutron scattering has been used to measure spin‑wave dispersion in the fcc antiferromagnet NiO at 78 K. The authors determined NiO exchange parameters by fitting spin‑wave energies to resolution‑corrected data and then used these parameters to compute magnon density of states, transition temperature, and thermomagnetic properties via molecular‑field and random‑phase‑approximation Green’s‑function methods. The measured dispersion shows a steep initial slope (~250 meV Å) and a high maximum (~117 meV), with a low zone‑boundary energy in certain directions; the dominant antiferromagnetic next‑nearest‑neighbor exchange J₂=221 K (19.01 meV) and a much smaller ferromagnetic nearest‑neighbor exchange J₁=–15.9 K (–1.37 meV) explain the limited domain resolution, and the exchange values confirm NiO as a weakly covalent insulator.

Abstract

Inelastic neutron scattering techniques have been used to measure the spin-wave dispersion relations at 78\ifmmode^\circ\else\textdegree\fi{}K in the fcc antiferromagnet NiO. The energy dispersion has a steep initial slope (\ensuremath{\sim}250 meV \AA{}) and a high maximum energy (\ensuremath{\sim}117 meV) and is further characterized by a relatively low zone boundary energy in certain directions. The exchange parameters defined by ${\mathcal{H}}^{1,2}={J}_{j}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}^{(1)}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}^{(2)}$ were determined by fitting the theoretical expression for the spin-wave energies to the experimental data corrected for instrumental resolution effects. The predominant interaction is a large antiferromagnetic exchange ${J}_{2}=221\ifmmode^\circ\else\textdegree\fi{}$K (19.01 meV) between next-nearest neighbors, which are linked by a 180\ifmmode^\circ\else\textdegree\fi{} superexchange path. The interaction between nearest neighbors, linked by a 90\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{Ni}}^{2+}$---${\mathrm{O}}^{2\ensuremath{-}}$---${\mathrm{Ni}}^{2+}$ configuration, is much smaller and ferromagnetic in sign, ${J}_{1}=\ensuremath{-}15.9\ifmmode^\circ\else\textdegree\fi{}$K (-1.37 meV). A consequence of the relatively small value of ${J}_{1}$ is that the spin waves from the four domains present in the sample can only be resolved in a limited region of reciprocal space. These values of exchange interactions are in accord with simple ideas of covalency and overlap, and the results emphasize the behavior of NiO as a weakly covalent insulator. The density of magnon states, estimates of the transition temperature, and several thermomagnetic properties of NiO have been calculated from the measured exchange parameters using molecular field and random-phase-approximation Green's-function formulas.