The fully relativistic disordered local moment (DLM) theory is used to perform calculations of the magnetic torque of tetragonally distorted Fe and fully disordered (A2) Fe 1−x Ga x (0 x 0.2) alloys to describe the temperature dependence of their magnetoelasticity. The finite-temperature magnetoelasticity, in particular the magnetoelastic constant B 1 , is obtained within DLM theory by studying the response of the magnetic torque generated by the magnetocrystalline anisotropy to the application of a tetragonal strain. Calculations of B 1 have been performed on bcc Fe across its ferromagnetic temperature range. Our results show good qualitative agreement with experiment, in particular reproducing the anomalous, nonmonotonic thermal behavior of bcc Fe's magnetostriction, which has been largely unexplained for more than 50 years. The method has also been used to calculate the finite-temperature magnetoelasticity of the A2 phase of Fe 1−x Ga x alloys as a starting point for further investigations into the giant magnetostriction of Galfenol alloys. Our calculations show that homogeneously doping bcc Fe with Ga does not produce an enhancement in magnetostriction and that the nonmonotonic temperature dependence and significant volume dependence are suppressed by increasing Ga content.

The optimal amount of dysprosium in the highly magnetostrictive rare-earth compounds Tb 1−x Dy x Fe 2 for room-temperature applications has long been known to be x = 0.73 (Terfenol-D). Here, we derive this value from first principles by calculating the easy magnetization direction and magnetostriction as a function of composition and temperature. We use crystal-field coefficients obtained within density-functional theory to construct phenomenological anisotropy and magnetoelastic constants. The temperature dependence of these constants is obtained from disordered-local-moment calculations of the rare-earth magnetic order parameter. Our calculations find the critical Dy concentration required to switch the magnetization direction at room temperature to be x c = 0.78, with magnetostrictions λ 111 = 2700 and λ 100 = −430 ppm, close to the Terfenol-D values.

We demonstrate how the many-body potential energy landscape of carbon can be explored with the nested sampling algorithm, allowing for the calculation of its pressure-temperature phase diagram. We compare four interatomic potential models: Tersoff, EDIP, GAP-20 and its recently updated version, GAP-20U. Our evaluation is focused on their macroscopic properties, melting transitions, and identifying thermodynamically stable solid structures up to at least 100 GPa. The phase diagrams of the GAP models show good agreement with experimental results. However, we find that the models’ description of graphite includes thermodynamically stable phases with incorrect layer spacing. By adding a suitable selection of structures to the database and re-training the potential, we have derived an improved model — GAP-20U+gr — that suppresses erroneous local minima in the graphitic energy landscape. At extreme high pressure nested sampling identifies two novel stable structures in the GAP-20 model, however, the stability of these is not confirmed by electronic structure calculations, highlighting routes to further extend the applicability of the GAP models.