Peter Entel

Modelling the phase diagram of magnetic shape memory Heusler alloys

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

Surface energies of stoichiometric FePt and CoPt alloys and their implications for nanoparticle morphologies

We have calculated surface energies and surface magnetic order of various low-indexed surfaces of monoatomic Fe, Co, and Pt, and binary, ordered FePt, CoPt, and MnPt using density-functional theory. Our results for the binary systems indicate that elemental, Pt-covered surfaces are preferred over Fe and Co covered and mixed surfaces of the same orientation. The lowest energy orientation for mixed surfaces is the highly coordinated (111) surface. We find Pt-covered (111) surfaces, which can be realized in the

Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys

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First-principles investigations of homogeneous lattice-distortive strain and shuffles in Ni2MnGa

structure only, to be lower in energy by about 400 meV/atom compared to the mixed

Designing shape-memory Heusler alloys from first-principles

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Monte Carlo simulations of the magnetocaloric effect in magnetic Ni–Mn–X (X = Ga, In) Heusler alloys

(111) surface. We conclude that in small nanoparticles this low surface energy can stabilize the

e∕a dependence of the lattice instability of cubic Heusler alloys from first principles

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Vibrational properties of nanoscale materials: From nanoparticles to nanocrystalline materials

structure, which is suppressed in bulk alloys. From the interplay of surface and bulk energies, equilibrium shapes of single-crystalline ordered nanoparticles and crossover sizes between the different orderings can be estimated.

Mastering hysteresis in magnetocaloric materials.

First-principles calculations are used in order to investigate phonon anomalies in non-magnetic and magnetic Heusler alloys. Phonon dispersions for several systems in their cubic L2

A guideline for atomistic design and understanding of ultrahard nanomagnets.

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