Abstract
A first-principles-based computational tool for simulating phonons of magnetic random solid solutions including thermal magnetic fluctuations is developed. The method takes fluctuations of force constants due to magnetic excitations as well as due to chemical disorder into account. The developed approach correctly predicts the experimentally observed unusual phonon hardening of a transverse acoustic mode in Fe–Pd an Fe–Pt Invar alloys with increasing temperature. This peculiar behavior, which cannot be explained within a conventional harmonic picture, turns out to be a consequence of thermal magnetic fluctuations. The proposed methodology can be straightforwardly applied to a wide range of materials to reveal new insights into physical behaviors and to design materials through computation, which were not accessible so far.
Highlights
Magnetic random solid solutions represent a large and important class of crystalline materials ranging from structural materials such as steels,[1,2,3,4] including Invar alloys,[5,6,7,8,9] up to multicomponent magnetic high-entropy alloys.[10,11,12] The simultaneous presence of chemical disorder and thermal magnetic fluctuations as well as their couplings to lattice vibrations play pivotal roles in many of these alloys
In the following we focus on (i) the impact of mass and force constants (FCs) variations among atomic sites as well as the performance of the itinerant coherent-potential approximation (ICPA) and the band unfolding to incorporate it and, in particular, (ii) the impact of thermal magnetic fluctuations on the phonon spectra
We propose a first-principles-based method to incorporate both thermal magnetic fluctuations and chemical disorder into a unified computational framework of phonon calculations for magnetic random solid solutions
Summary
Magnetic random solid solutions represent a large and important class of crystalline materials ranging from structural materials such as steels,[1,2,3,4] including Invar alloys,[5,6,7,8,9] up to multicomponent magnetic high-entropy alloys.[10,11,12] The simultaneous presence of chemical disorder and thermal magnetic fluctuations as well as their couplings to lattice vibrations play pivotal roles in many of these alloys. A computational scheme that simulates the lattice vibrations in magnetic random solid solutions by properly taking into account both, magnetic fluctuations as well as chemical disorder (as sketched in Fig. 1), is of genuine importance. A prominent example is the hardening of a transverse acoustic phonon mode and elastic constants with increasing temperature in Invar alloys.[15,16] Since thermal expansion—usually dominating the temperature dependence of phonon modes—is in such alloys negligible,[5,6,7,8,9] the inclusion of explicit temperature-dependent excitations, such as magnetic fluctuations, is critical to resolve such peculiarities
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