Effects of pressure and isotopic substitution on the Raman spectrum of α-Fe2O3: Identification of two-magnon scattering

We report measurements of the effects of high pressure and O-isotope substitution on the Raman spectrum of \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$. Spectra for the pressure effects were obtained from single crystals in a diamond-anvil cell; those for isotopic substitution from polycrystalline films grown in a controlled atmosphere. The Gr\"uneisen parameters and isotope shifts of the first-order phonons are found to be comparable to the same quantities for the strong 1320-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ line, consistent with its assignment by McCarty [Solid State Commun. 68, 799 (1988)] as a phonon overtone. We identify the two-magnon scattering as a weak and broad feature at \ensuremath{\sim}1525 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, showing a temperature dependence consistent with a magnetic origin. As expected, the magnon peak does not shift with isotopic substitution, and its frequency is in good agreement with that predicted by an Ising-model calculation (1540 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$).