An ab initio based framework for quantitatively assessing the phonon contribution due to magnonphonon interactions and lattice expansion is developed. The theoretical results for bcc Fe are in very good agreement with high-quality phonon frequency measurements. For some phonon branches, the magnonphonon interaction is an order of magnitude larger than the phonon shift due to lattice expansion, demonstrating the strong impact of magnetic short-range order even significantly above the Curie temperature. The framework closes the previous simulation gap between the ferro-and paramagnetic limits. DOI: 10.1103/PhysRevLett.113.165503 PACS numbers: 63.20.K-, 63.20.dd, 63.20.dk, 75.50.Bb An understanding of the mutual interaction between different temperature-induced excitations in solids is a pivotal challenge for the simulation of thermodynamic properties of many materials. Experimental studies of phonons at elevated temperatures can help elucidate magnon-phonon coupling. Neutron scattering experiments of phonon dispersions have provided important data at selected temperatures [11]. Nuclear resonant inelastic x-ray scattering measurements are more amenable for showing thermal trends with measurements at many temperatures, and we have recently performed nuclear resonant inelastic x-ray scattering measurements of the phonon density of states of bcc Fe at 38 temperatures through the Curie transition [12]. Nonharmonic changes in the phonon DOS and vibrational entropy were found to track the change in magnetization with temperature. Since experimental analysis of phonon DOS broadening suggests explicit anharmonic contributions from phonon-phonon interactions to be an order of magnitude smaller, these new results are suggestive of large magnon-phonon interactions in bcc Fe.Parameter-free electronic structure calculations like density functional theory (DFT) in principle provide access to interatomic forces, spin-polarized energetics, and their interactions. Force-constant calculations and spin simulations have indeed been performed for decades [10,13]. However, most studies have been restricted to separate investigations of the two effects, whereas their mutual coupling could only be addressed in recent years [1,[14][15][16]. The T ¼ 0 K limit of a ferromagnetic system like Fe is the most straightforward case [17] since calculating force constants for a single magnetic configuration with all spins pointing in the same direction is sufficient ("FM limit" in Fig. 1). The infinite-temperature limit of a paramagnetic system with fully disordered spins ("PM limit") is significantly more challenging due to the large magnetic phase space that needs to be sampled for an accurate prediction of the coupling. Significant progress has been made only very recently with techniques based on DLM and spin molecular dynamics [1,18], a spin-spiral approach [15], dynamical mean field theory [14], and a spin-space averaging procedure [16].Given the complexity of the problem, present day methods are currently applied only at very low temperat...

The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the 57 Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys.

Equimolar FeCoCrNi alloys have been the topic of recent research as "high-entropy alloys," where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi 3 was prepared in the same manner to act as a control. High-entropy alloys (HEAs) are multicomponent alloys where the atomic fraction of each of the elements is nearly equal.~ These materials have been studied for their high hardness, resistance to wear, and corrosion resistance. Wang et al. showed that ordered NiAl forms in the alloy FeCoCrNiA1Cu,2 whichwas cited as the main cause for strengthening in the alloy. Indeed, superlattice reflections indicative of chemical ordering have been observed in the x-ray diffraction (XRD) patterns presented in several studies.

Synchrotron x-ray diffraction (XRD) measurements, nuclear forward scattering (NFS) measurements, and density functional theory (DFT) calculations were performed on L1_{2}-ordered Pd3Fe. Measurements were performed at 300 K at pressures up to 33 GPa, and at 7 GPa at temperatures up to 650 K. The NFS revealed a collapse of the 57Fe magnetic moment between 8.9 and 12.3 GPa at 300 K, coinciding with a transition in bulk modulus found by XRD. Heating the sample under a pressure of 7 GPa showed negligible thermal expansion from 300 to 523 K, demonstrating Invar behavior. Zero-temperature DFT calculations identified a ferromagnetic ground state and showed several antiferromagnetic states had comparable energies at pressures above 20 GPa.

Phonon density of states (DOS) curves were measured on alloys of face-centered-cubic (fcc) Au-Fe using nuclear resonant inelastic x-ray scattering (NRIXS) and inelastic neutron scattering (INS). The NRIXS and INS results were combined to obtain the total phonon DOS and the partial phonon DOS curves of Au and Fe atoms, from which vibrational entropies were calculated. The main effect on the vibrational entropy of alloying comes from a stiffening of the Au partial phonon DOS with Fe concentration. Force constants were calculated from first principles for several compositions and show a local stiffening of Au-Au bonds close to Fe atoms. The calculated phonon DOS curves reproduce the experimental trend. The stiffening is attributed to two main effects comparable in magnitude: 1) an increase in electron density in the free-electron-like states, and 2) stronger sd-hybridization.