Optical absorption edge in α–Fe2O3: The exciton–magnon structure

Abstract

Transmission spectra of synthetic and natural hematite (α-Fe2O3) crystals are measured at temperatures 10, 25, and 300 K in the wavelength range 500–1100 nm, and the absorption spectra are computed. Pure exciton and exciton–magnon d–d transition bands are revealed, the corresponding wavelengths at 10 K being λ0=1020 nm and λ1=965 nm respectively. The half-widths and oscillator forces are g0=84 cm−1, f0=4×10−9, g1=60 cm−1, f1=1.4×10−7 for 10 K, g0=85 cm−1, f0=5×10−9, g1=110 cm−1, f1=2.1×10−7 for 25 K. The mechanisms of band formation for weakly allowed d–d transitions in hematite are analyzed.

Magnetic-field-induced color change inα-Fe2O3single crystals

“…Both temperature (T M ) and magnetic field (B C = 6.8 T, B C⊥ = 16.0 T) 12,13,17 drive a spin flop to the basal plane 18 , and in the high temperature/field phase, α-Fe 2 O 3 is weakly ferromagnetic due to a slight (∼10 −4 degree) spin canting 19,20 . Previous spectroscopic work revealed the electronic structure and identified the exciton and associated magnon sideband on the leading edge of the 6 A 1g → 4 T 1g on-site excitation 21,22 . No exciton fine structure was resolved, which prevented an analysis of magnetic symmetry, and magnetic field effects in this iconic material have not been explored from an optical properties point of view.…”

“…Spectra were collected using a Bruker Equinox 55 Fourier transform infrared spectrometer equipped with a microscope attachment (600-17000 cm −1 ). 21,22 . These d-d excitations are formally forbidden although they appear in many oxides due to spin-orbit coupling, exchange interaction, and odd parity phonons that hybridize states and break inver- , respectively.…”

“…This is a consequence of the larger inter-sublattice coupling energy in α-Fe 2 O 3 . The feature labeled MS1 is assigned as the magnon sideband 22 . Based on the position, shape, and polarization behavior, we associate it with the M1σ exciton 38 .…”

Magnetochromic sensing and size-dependent collective excitations in iron oxide nanoparticles

“…Our suite of hematite nanoparticles provides a superb platform for such tests. [20,47,57] Figure 3 summarizes how the collective excitations in nano-hematite change with size. An obvious linear correlation exists between particle size and magnon sideband position in the absence of magnetic field [ Figure 3 (a, d)].…”

“…The latter arises from the dipoleallowed combination of an exciton and a magnon (ω MS = ω E + ω M ) and is commonly observed on the leading edge of a d-to-d band. [20,47,49,54,55] Exciton splitting and magnon sideband trends are incisive probes of symmetry, and their behavior can even be used to develop temperature-magnetic field phase diagrams. [20,55,56] While much is known about excitons, magnons, and magnon sidebands in antiferromagnets under external stimuli, there have been few opportunities to reveal finite length scale effects on the behavior of collective excitations.…”