The effects of high pressure on the vibrational and magnetic properties of iron-based materials.

摘要

High pressure experimental methods are demonstrated for studying pressure-dependent material properties and solid phases unattainable at ambient pressure with synchrotron nuclear resonance techniques.The phonon density of states (DOS) of nanocrystalline 57Fe was measured under pressures up to 28 gigapascals (2.8 x 105 atm) using the nuclear resonant inelastic x-ray scattering (NRIXS) technique. The nanocrystalline material exhibited an enhancement in its DOS at low energies by a factor of 2.2. This enhancement persisted throughout the entire pressure range, even across the pressure-induced bcc-to-hcp phase transformation at 13 GPa. At higher energies, the van Hove singularities in both samples were coincident in energy at all pressures, indicating that interatomic forces in nanocrystalline materials are similar to those in bulk crystals. Subsequent neutron inelastic scattering measurements at ultra-low energies (2 to 18 mueV) also observed enhancement in the vibrational sepctrum of the nanocrystalline material. This enhancement is partly attributed to novel microstructural modes, characterized by cooperative dynamics of individual crystallites.Recent density functional theory (DFT) investigations have identified a static antiferromagnetic structure with negligible hyperfine fields for the high-pressure hcp (epsilon) phase of iron. This structure exhibits a perfect cancellation of core electron polarization at the nucleus by an equally large and oppositely oriented conduction electron polarization. To test this hypothesis, an alloy of composition Fe92Ni8 was subjected to synchrotron Mossbauer spectrometry (SMS) measurements at 20 GPa and 11 K. The addition of nickel was expected to disrupt the precise balance of core and conduction electron polarization in the alloy, and to result in a measurable hyperfine field in the presence of significant magnetic moments. Full-potential DFT calculations in the generalized gradient approximation (GGA) verified this effect for a Fe7Ni1 hcp supercell, which exhibited calculated hyperfine fields of nearly 70 kG. However, SMS measurements were unable to detect a hyperfine field. This disparity may be a result of quantum spin fluctuations on the geometrically frustrated hcp lattice with a period much shorter than the lifetime of the nuclear excited state. Alternately, the result is evidence of a significant flaw in the handling of exchange coupling by the GGA exchange-correlation functional.