M. Y. Hu

Inelastic nuclear resonant scattering with sub-meV energy resolution

With an undulator-based synchrotron source and a tunable x-ray monochromator, we produce an x-ray beam with a flux of 4×108u2009photons/s in an energy bandwidth of 920±110u2009μeV (ΔE/E≈6.2×10−8) at a photon energy of 14.4 keV. The tunability of the monochromator allows for a measurement of lattice excitations in α-Fe using the technique of inelastic nuclear resonant scattering from 57Fe. The phonon density-of-states are extracted from the measurement and compared to the calculated phonon density-of-states derived from neutron-scattering measurements.

Element-Resolved Thermodynamics of Magnetocaloric LaFe 13−x Si x

By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe(13-x)Si(x). These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.

Phonon damping in thin films of Fe

The phonon density of states (DOS) in thin films of polycrystalline α-Fe was measured by inelastic nuclear resonant scattering of synchrotron radiation. The thin-film DOS exhibits significant deviations from the DOS of bulk Fe, which we attribute to phonon lifetime broadening in the confined geometry. The measured DOS can be described with a damped harmonic oscillator model for the phonons with different quality factors Q=25(2) and Q=13(1) for layer thicknesses of 28 and 13 nm, respectively.

Introduction to nuclear resonant scattering with synchrotron radiation

The concepts leading to the application of synchrotron radiation to elastic and inelastic nuclear resonant scattering are discussed. The resulting new experimental techniques are compared to conventional Mössbauer spectroscopy. A survey of situations that favor experiments with synchrotron radiation is offered.

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic x-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phases, which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the

Source and optics considerations for new generation high-resolution inelastic X-ray spectrometers

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Phonon density of states in epitaxial Fe/Cr(001) superlattices

point depend critically on the Fe magnetic moment and atomic volume. Analyzing these nonvibrational modes leads to the discovery of a stable monoclinic ground-state structure, which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy, and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling.

Ultrahigh-resolution x-ray monochromators for elastic and inelastic x-ray scattering studies (abstract) (invited)

The high-resolution inelastic X-ray scattering technique has evolved rapidly at the third-generation synchrotron sources. It is now possible to measure collective excitations with 2 meV resolution. The next level of experiments may require even more stringent conditions in terms of energy resolution, sample size and environment. For example, microscopic single crystals, thin films or multilayers, confined liquids, and samples under high pressure are outside the domain of momentum resolved inelastic X-ray scattering. Inelastic nuclear resonant scattering can measure partial phonon density of states from such samples, provided that the samples contain suitable M . ossbauer isotopes. Based on the experience obtained at the high-resolution X-ray scattering beamline (SRI-CAT 3-ID) of the APS, we present some new perspectives for X-ray sources, monochromators and analyzers to improve the performance of the spectrometer by an order of magnitude. # 2001 Elsevier Science B.V. All rights reserved.

Ultra‐stable sub‐meV monochromator for hard X‐rays

Incoherent nuclear resonant absorption of synchrotron radiation at the 14.413 keV nuclear resonance of 57Fe was employed to measure directly the Fe-projected (partial) phonon density of states (DOS) in epitaxial FeCr(0 0 1) superlattices and in an 57Fe0.03Cr0.97(0 0 1) alloy film MBE-grown on MgO(0 0 1). The measurements were performed at 300 K with 2.3 meV energy resolution around 14.413 keV. At the interfaces, longitudinal vibrations of Fe atoms are suppressed, and a strong resonance phonon mode appears near 23 meV.

Microfocusing options for the inelastic X-ray scattering beamline at sector 3 of the Advanced Photon Source.

High-resolution spectroscopies that use hard x rays like nuclear resonant scattering and inelastic x-ray scattering require monochromatization to the meV level. Currently, tunable x-ray monochromators using silicon allow one to achieve sub-meV energy bandwidths in the 10–30 keV energy range. Attempts to improve the energy resolution beyond 108 encounter issues such as stability, efficiency, and crystal-quality-limited resolution. Cryogenically cooling the silicon can potentially mitigate the problems related to thermal stability, as well as improve the efficiency. Cooling high-resolution monochromators, while maintaining the requisite 10 nrad angular control over the crystals, poses a challenging technical problem. Recent attempts to improve the limits of energy resolution and to improve efficiency and stability of high-resolution monochromators with cryogenic cooling will be discussed.