Mario Siewert

Designing shape-memory Heusler alloys from first-principles

The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14%.

Fundamental Aspects of Magnetic Shape Memory Alloys: Insights from Ab Initio and Monte Carlo Studies

Ferromagnetic Heusler alloys like Ni-Mn-Z (Z = Al, Ga, In, Sn, Sb), which undergo a martensitic phase transformation, are on the edge of being used in technological applications involving actuator and magnetocaloric devices. The other class of ferromagnetic full Heusler alloys like Co-Mn-Z (Z = Al, Si, Ga, Ge, Sn) not undergoing a structural phase transition, are half-metals (in contrast to the Ni-based systems) with high spin polarization at the Fermi level and are of potential importance for future spintronics devices. On the basis of recent ab initio calculations, we highlight the main differences between the two classes of Heusler based materials.

Composition-Dependent Basics of Smart Heusler Materials from First- Principles Calculations

The structural and magnetic order are the decisive elements which vastly determine the properties of smart ternary intermetallics such as X2YZ Heusler alloys. Here, X and Y are transition metal elements and Z is an element from the III-V group. In order to give a precise prescription of the possibilities to optimize the magnetic shape memory and magnetocaloric effects of these alloys, we use density functional theory calculations. In particular, we outline how one may find new intermetallics which show higher Curie and martensite transformation temperatures when compared with the prototypical magnetic shape-memory alloy Ni2MnGa. Higher operation temperatures are needed for technological applications at elevated temperatures.

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...

Phase Diagrams of Conventional and Inverse Functional Magnetic Heusler Alloys: New Theoretical and Experimental Investigations

First-principles calculations allow to characterize the electronic and magnetic ground-state properties of the full-Heusler alloys of type X2YZ. Functionality of the materials strongly depends on the type of elements and composition. A half-metallic state with 100% spin polarization at the Fermi level, which is an ideal spintronics material for tunneling devices, is, for instance, achieved for (X = Co, Y = Mn, and Z = Ge and Si). Replacing Co by Ni and Ge or Si by Ga yields the prototypical magnetic shape-memory compound Ni2MnGa, which undergoes a (martensitic) tetragonal distortion at ca. 200 K, where the magnetic shape-memory features can be exploited by an external magnetic field and external stress in the martensitic state. Quite another functionality, the conventional or inverse magnetocaloric effect, is observed in the off-stoichiometric samples of (X = Ni, Y = Mn, and Z = Ga, In, Sn, and Sb), where the efficiency of the magnetocaloric effect depends on the size of the isothermal entropy change across the magnetostructural phase transition in an applied magnetic field. Here, we discuss how some of these material properties can be improved in order to obtain room temperature or higher operation temperatures needed for a technological breakthrough.

Interplay of phase sequence and electronic structure in the modulated martensites of Mn2NiGa from first-principles calculations

We investigate relative stability, structural properties and electronic structure of various modulated martensites of the magnetic shape memory alloy Mn

First-Principles Study of the Influence of (110) Strain on the Ferroelectric Trends of TiO2

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A comparative study of (Fe, Fe3Si)/GaAs and Heusler/MgO for spintronics applications

NiGa by means of density functional theory. We observe that the instability in the high-temperature cubic structure first drives the system to a structure where modulation shuffles with a period of six atomic planes are taken into account. The driving mechanism for this instability is found to be the nesting of the minority band Fermi surface, in a similar way as established for the prototype system Ni

Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles

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