Phonons in Nanocrystalline57Fe

Abstract

We measured the phonon density of states (DOS) of nanocrystalline Fe by resonant inelastic nuclear g-ray scattering. The nanophase material shows large distortions in its phonon DOS. We attribute the high energy distortion to lifetime broadening. A damped harmonic oscillator model for the phonons provides a low quality factor, Q u , averaging about 5, but the longitudinal modes may have been broadened most. The nanocrystalline Fe also shows an enhancement in its phonon DOS at energies below 15 meV. The difference in vibrational entropy of the bulk and nanocrystalline Fe was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies.[ S0031-9007(97) PACS numbers: 76.80. + y, 61.72. -y, 63.20. -e Over the past decade there has been much interest in nanocrystalline materials, generally defined as materials composed of crystallites smaller than 100 nm. Unusual mechanical properties and soft magnetic properties were topics of numerous investigations on metallic nanocrystals [1]. Very recently, neutron inelastic scattering measurements have shown some differences in the phonon density of states (DOS) of nanocrystalline and bulk materials [2-6]. One such effect was an enhancement of the phonon DOS at low energies [3][4][5][6][7]. A broadening of the peak from the longitudinal modes in the phonon DOS was also observed and attributed to the lifetime broadening of phonons in small crystals [5,6]. Unfortunately, it was not possible to measure accurately the shape of the longitudinal peak, owing to statistical and background limitations of the neutron inelastic scattering technique.In this Letter we show how a recently developed experimental technique, resonant inelastic nuclear g-ray scattering [8,9], provides new information on the shape of the phonon DOS of nanocrystalline Fe. In particular, the excellent signal-to-noise ratio of the data makes it possible to examine quantitatively the high energy tail of the phonon DOS in small samples. In our resonant inelastic nuclear g-ray scattering measurements, 14.41 keV g rays were directed onto a foil specimen, and 6.4 keV conversion x-ray radiations from the specimen were detected. This scattering is incoherent, so the data provide information on the velocity-velocity correlation function of individual 57 Fe nuclei. The experiments were performed at the undulator beamline 3-ID at the Advanced Photon Source. A high-heat-load monochromator, which consists of two symmetric silicon (1 1 1) reflections in a nondispersive setting, and a high-resolution, nested monochromator, as described previously [10], were used to provide the 14.413 keV radiation onto the specimen. The highresolution monochromator operates with asymmetric silicon (4 2 2) and symmetric silicon (10 6 4) reflections and produces a constant energy bandwidth of 5.5 meV over the tuning range. The energy of the incident radiation was tuned by rotating the (10 6 4) channel-cut crystal in steps of 2 meV. The photons were incident on the sample at 5 3 10 9

Enhancement of the electronic contribution to the low-temperature specific heat of an Fe/Cr magnetic multilayer

“…Measurements on sputtered Fe films with different thickness [24] and nanocrystalline films [25] showed that an enhancement of the low energy phonon DOS correlates with increasing disorder. This enhancement of the low energy DOS would result in a lower Θ D as observed in our measurements.…”

High-pressure studies with x-rays using diamond anvil cells

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Phonon DOS has been determined for a number of iron compounds and alloys: FeO [519,520], FeS [521], Fe-Ni [522], Fe-Si alloys [287,522], FeH [523], Fe3S [524], Fe3C [525][526][527], Fe2O3 [528], (Mg,Fe)O [497], and (Mg,Fe)SiO3 enstatite [529]. For nanocrystalline Fe, large distortions were found in its phonon DOS [530], showing a lifetime broadening at high energies and an enhancement in its phonon DOS at energies below 15 meV.…”

Vibrations of Micro-eV Energies in Nanocrystalline Microstructures

“…Enhanced intensity above the high-energy cutoff of the bulk material has been attributed to phonon lifetime broadening caused by phonon interactions with grain boundaries [9][10][11] and recent measurements on iron have isolated a contribution from surface oxides [12]. The increased number of phonon modes at low energies [5][6][7][8][9][10][11] is less well understood. The phonon density of states (DOS) at low energies is up to a factor of 5 times larger than the DOS of control samples of material composed of larger crystals [10].…”

“…More recently, neutron and x-ray inelastic scattering measurements revealed that nanocrystalline materials have an increased number of vibrational modes at the highest and lowest energies of their measured spectra, compared to materials with crystals of conventional sizes [5][6][7][8][9][10][11][12]. Enhanced intensity above the high-energy cutoff of the bulk material has been attributed to phonon lifetime broadening caused by phonon interactions with grain boundaries [9][10][11] and recent measurements on iron have isolated a contribution from surface oxides [12]. The increased number of phonon modes at low energies [5][6][7][8][9][10][11] is less well understood.…”