Tanja Graf

Half-Heusler compounds: novel materials for energy and spintronic applications

Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as future energy applications and for spintronics. The semiconducting Heusler compounds can be identified by the number of valence electrons. The band gap can be tuned between 0 and 4 eV by the electronegativity difference of the constituents. Magnetism can be introduced in these compounds by using rare-earth elements, manganese or ‘electron’ doping. Thus, there is a great interest in the fields of thermoelectrics, solar cells and diluted magnetic semiconductors. The combination of different properties such as superconductivity and topological edge states leads to new multifunctional materials, which have the potential to revolutionize technological applications. Here, we review the structure, the origin of the band gap and the functionalities of semiconducting half-Heusler compounds.

Electronic structure and transport properties of the Heusler compound Co2TiAl

The properties of the Heusler compound Co2TiAl were investigated in detail by experimental techniques and theoretical methods. X-ray diffraction measurements indicate that as-cast samples of the compound exhibit the L21 structure with a small amount of B2-type disorder. This leads to a reduced saturation magnetization per formula unit of 0.747 μB. The Curie temperature is approximately 120 K. The transport properties are influenced by the change in the electronic structure at the Curie temperature, as revealed experimentally by conductivity, thermal transport and specific heat measurements. Different theoretical models based on ab initio calculations of the electronic structure are used to explain the experimental observations.

Heusler Compounds—A Material Class With Exceptional Properties

The class of Heusler compounds, including the XYZ and the X2YZ compounds, has not only an endless number of members, but also a vast variety of properties can be found in this class of materials, ranging from semiconductors, half-metallic ferromagnets, superconductors, and topological insulators to shape memory alloys. With this review article, we would like to provide an overview of Heusler compounds, focusing on their structure, properties, and potential applications.

Efficient spin injector scheme based on Heusler materials.

We present a rational design scheme intended to provide stable high spin polarization at the interfaces of the magnetoresistive junctions by fulfilling the criteria of structural and chemical compatibilities at the interface. This can be realized by joining the semiconducting and half-metallic Heusler materials with similar structures. The present first-principles calculations verify that the interface remains half-metallic if the nearest interface layers effectively form a stable Heusler material with the properties intermediately between the surrounding bulk parts. This leads to a simple rule for selecting the proper combinations.

Spin configurations in Co2FeAl0.4Si0.6 Heusler alloy thin film elements

We determine experimentally the spin structure of half-metallic Co2FeAl0.4Si0.6 Heusler alloy elements using magnetic microscopy. Following magnetic saturation, the dominant magnetic states consist of quasi-uniform configurations, where a strong influence from the magnetocrystalline anisotropy is visible. Heating experiments show the stability of the spin configuration of domain walls in confined geometries up to 800 K. The switching temperature for the transition from transverse to vortex walls in ring elements is found to increase with ring width, an effect attributed to structural changes and consequent changes in magnetic anisotropy, which start to occur in the narrower elements at lower temperatures.

Crystal-Facet-Dependent Metallization in Electrolyte-Gated Rutile TiO2 Single Crystals

The electric-field-induced metallization of insulating oxides is a powerful means of exploring and creating exotic electronic states. Here we show by the use of ionic liquid gating that two distinct facets of rutile TiO2, namely, (101) and (001), show clear evidence of metallization, with a disorder-induced metal-insulator transition at low temperatures, whereas two other facets, (110) and (100), show no substantial effects. This facet-dependent metallization can be correlated with the surface energy of the respective crystal facet and, thus, is consistent with oxygen vacancy formation and diffusion that results from the electric fields generated within the electric double layers at the ionic liquid/TiO2 interface. These effects take place at even relatively modest gate voltages.

Electronic structure of fully epitaxial Co2TiSn thin films

Electronic structure of fully epitaxial Co 2 TiSn thin films Markus Meinert, ∗ Jan Schmalhorst, Hendrik Wulfmeier, and G¨ nter Reiss u Thin Films and Physics of Nanostructures, Department of Physics, Bielefeld University, 33501 Bielefeld, Germany Elke Arenholz Advanced Light Source, Lawrence Berkeley National Laboratory, CA 94720, USA Tanja Graf and Claudia Felser arXiv:1010.5754v1 [cond-mat.mtrl-sci] 27 Oct 2010 Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University, 55128 Mainz, Germany (Dated: October 28, 2010) In this article we report on the properties of thin films of the full Heusler compound Co 2 TiSn prepared by DC magnetron co-sputtering. Fully epitaxial, stoichiometric films were obtained by deposition on MgO (001) substrates at substrate temperatures above 600 ◦ C. The films are well ordered in the L2 1 structure, and the Curie temperature exceeds slightly the bulk value. They show a significant, isotropic magnetoresistance and the resistivity becomes strongly anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with nearly 1 µ B on the Co atoms, and a small antiparallel Ti moment, in agreement with theoretical expectations. From comparison of x-ray absorption spectra on the Co L 3,2 edges, including circular and linear magnetic dichroism, with ab initio calculations of the x-ray absorption and circular dichroism spectra we infer that the electronic structure of Co 2 TiSn has essentially non-localized character. Spectral features that have not been explained in detail before, are explained here in terms of the final state band structure. PACS numbers: 75.70.-i, 78.70.Dm, 73.61.At, 81.15.Cd I. INTRODUCTION The materials class of Co 2 YZ Heusler compounds (with Y a transition metal and Z an sp element) has been the subject of extensive studies in the context of spintronics during the last decade. They are of interest because many of them are predicted as half-metallic fer- romagnets with full spin polarization at the Fermi edge. The Heusler compound Co 2 TiSn (CTS) is of partic- ular interest for applications. It is predicted to be a half-metallic ferromagnet with a magnetic moment of 2 µ B /f.u. and it has a high formation energy of the Co- Ti site-swap defect. 1,2 Making use of Heusler compounds which exhibit low disorder or high tolerance of the ground state properties against disorder is highly desired. Co 2 TiSn has been the subject of many experimental and theoretical studies. The ground state properties ob- tained by density functional theory (DFT) depend sensi- tively on the choice of the DFT method. 1–8 The poten- tial has strong non-spherical components and thus only a full-potential treatment in connection with the general- ized gradient approximation (GGA) to the density yields a half-metallic ground state. 1,4 Experiments conducted on bulk CTS find a lat- tice parameter of 6.07 ˚ , a magnetic moment of about A 1.95 µ B /f.u. and a Curie temperature (T C ) around 355 K. 1,9,10 Further, it is found to have a strongly anoma- lous temperature dependence of resistivity, the temper- ature coefficient becomes negative above the Curie tem- perature. A large negative magnetoresistance reveals the importance of spin fluctuations in the compound. 11 A rather new development aims at the magnetocaloric properties of Co 2 TiSn, which has a large and constant Seebeck coefficient of −50 µV/K above T C in the bulk. 10 There have been some efforts to understand the unusual transport properties of CTS by ab initio band structure and semi-classical transport theory. 10,12 These properties make CTS interesting for a possible application in spin caloritronics, which attempt to make use of the interac- tions between heat and spin. An implementation into thin films is of particular importance for such applica- tions. Only two studies on thin films of CTS are available as far as we know. Gupta et al. applied pulsed laser ablation to grow CTS on Si (001) substrates from a stoichiometric target at growth temperatures up to 200 ◦ C. 13 The au- thors found off-stoichiometric, polycrystalline films with (022) texture. Suharyadi et al. utilized an atomically controlled alternate deposition technique based on elec- tron beam evaporation. 14 They have grown (001) ori- ented, L2 1 ordered films on Cr buffered MgO (001) sub- strates at growth temperatures up to 600 ◦ C and investi- gated them by nuclear resonant scattering. In this paper we present a successful preparation tech- nique based on DC magnetron co-sputtering. We present data on the structural and magnetic properties of our films. Further, we characterize the electronic transport properties which make CTS a particularly interesting compound. Finally we discuss the electronic structure of our CTS films based on soft x-ray absorption spec- troscopy and ab initio electronic structure calculations.

Anomalous transport properties of the half-metallic ferromagnets Co2TiSi, Co2TiGe and Co2TiSn

In this work, the theoretical and experimental investigations of Co2TiZ (Z=Si, Ge or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfil the Slater–Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic-like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting-like behaviour. A large negative magnetoresistance (MR) of 55 per cent is observed for Co2TiSn at room temperature in an applied magnetic field of μ0H=4T, which is comparable to the large negative MRs of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is −52 μVK−1 for Co2TiSn, which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations.

High-throughput screening of inorganic compounds for the discovery of novel dielectric and optical materials

Dielectrics are an important class of materials that are ubiquitous in modern electronic applications. Even though their properties are important for the performance of devices, the number of compounds with known dielectric constant is on the order of a few hundred. Here, we use Density Functional Perturbation Theory as a way to screen for the dielectric constant and refractive index of materials in a fast and computationally efficient way. Our results constitute the largest dielectric tensors database to date, containing 1,056 compounds. Details regarding the computational methodology and technical validation are presented along with the format of our publicly available data. In addition, we integrate our dataset with the Materials Project allowing users easy access to material properties. Finally, we explain how our dataset and calculation methodology can be used in the search for novel dielectric compounds.

Heusler Compounds at a Glance

The class of Heusler compounds, including the XYZ and the X 2 YZ compounds, does not only have an endless number of members, but also a vast variety of properties can be found in this class of materials, ranging from semi-conductors, half-metallic ferromagnets, superconductors, and topological insulators to shape memory alloys. With this chapter, we would like to provide an overview of Heusler compounds, focusing on basis design principles, their properties and potential applications.