Denis Comtesse

First-principles calculation of the instability leading to giant inverse magnetocaloric effects

The structural and magnetic properties of functional Ni-Mn-Z (Z=Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

Basic Properties of Magnetic Shape-Memory Materials from First-Principles Calculations

The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the low-temperature martensite phase with body-centered-tetragonal (bct) or rather face-centered-tetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds.

First-principles investigations of caloric effects in ferroic materials

We study the magnetic interactions in Ni-Mn-based Heusler alloys which are suitable candidates for refrigeration based on magnetocaloric, barocaloric, and elastocaloric effects, where the adiabatic temperature change of the Heusler material is induced by applying a magnetic field, hydrostatic pressure, or compressive strain. The predominantly ferromagnetic interactions of the Heusler alloys with austenite cubic structure at high temperatures are modified by the appearance of antiferromagnetic interactions in the alloys with Mn-excess because of the much shorter distances between the Mn-excess atoms and those on the original Mn-sublattice. This leads to a larger entropy change across the magnetostructural transformation in Ni50Mn25+x(Ga,In,Sn,Sb)25−x alloys and is also responsible for the appearance of the inverse magnetocaloric effect in the martensitic phase. In Ni-excess Ni-Mn-Ga alloys the influence of antiferromagnetic correlations is weaker and the large entropy change across the magnetostructural tr...

Optimizing the Magnetocaloric Effect in Ni–Mn–Sn by Substitution: A First-Principles Study

We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization.

Simulation of Cluster Sintering, Dipolar Chain Formation, and Ferroelectric Nanoparticulate Systems

Magnetic and ferroelectric nanoparticles are subjects of increasing basic research for future technologies. In this work Fe and Ni clusters and near-ferroelectric TiO\(_2\) clusters have been chosen as representatives in order to discuss fundamental issues such as sintering of magnetic and near-ferroelectric clusters as well as configurations resulting from cluster agglomeration due to magnetic dipolar interactions.