Tuning the caloric response of BaTiO3_3 by tensile epitaxial strain

We investigate the effect of epitaxial strain on the electrocaloric effect (ECE) in BaTiO$_3$ by means of ab initio based molecular dynamics simulations. We show that tensile strain can be used to optimize the operation range for ferroic cooling. Strain in the range of $\leq 1$ % can be used to shift the operation temperature by several hundreds of Kelvin both to higher and lower temperatures, depending on the direction of the external field. In addition, the transformation between multi-domain and mono-domain states, induced by an in-plane electric field, results in an additional peak of the adiabatic temperature change at lower temperatures, and a broad temperature interval where the caloric response scales linearly with the applied field strength, even up to very high fields.Comment: 6 pages, 4 figure

Exploration of all-3d Heusler alloys for permanent magnets: an ab initio based high-throughput study

Heusler alloys have attracted interest in various fields of functional materials since their properties can quite easily be tuned by composition. Here, we have investigated the relatively new class of all-3d Heusler alloys in view of its potential as permanent magnets. To identify suitable candidates, we performed a high-throughput study using an electronic structure database to search for X$_2$YZ-type Heusler systems with tetragonal symmetry and high magnetization. For the alloys which passed our selection filters, we have used a combination of density functional theory calculations and spin dynamics modelling to investigate their magnetic properties including the magnetocrystalline anisotropy energy and exchange interactions. The candidates which fulfilled all the search criteria served as input for the investigation of the temperature dependence of the magnetization and determination of Curie temperature. Based on our results, we suggest that Fe$_2$NiZn, Fe$_2$NiTi and Ni$_2$CoFe are potential candidates for permanent magnets with large out-of-plane magnetic anisotropy (1.23, 0.97 and 0.82 MJ/m$^3$ respectively) and high Curie temperatures lying more than 200 K above the room temperature. We further show that the magnitude and direction of anisotropy is very sensitive to the strain by calculating the values of anisotropy energy for several tetragonal phases. Thus, application of strain can be used to tune the anisotropy in these compounds

Qualitative Analysis of KEA Automatic Indexing Algorithm

While working with documents on the Internet, we commonly rely on search engines to retrieve information for us. Authors of the document often identify keyphrases which would make it easier for people to search conceptually and thematically rather than solely by keywords. In the absence of author identified keyphrases, professional human indexers do this job. With the ever growing number of documents on the Internet and with the rising popularity of digital libraries, it is almost impossible to have professional human indexers assign and manage keyphrases for all of the many documents on the Internet. For this reason, automatic indexers are built. However, accuracy and efficiency are important desirable characteristics of an automatic indexer. This study is a qualitative evaluation of the accuracy of the KEA automatic indexing algorithm. The results of the automatic indexer are compared with the indexing done by professional indexers using a controlled vocabulary

Elastic and Chemical Contributions to the Stability of Magnetic Surface Alloys on Ru(0001)

We have used density functional theory to study the structural stability of surface alloys. Our systems consist of a single pseudomorphic layer of $M_xN_{1-x}$ on the Ru(0001) surface, where $M$ = Fe or Co, and $N$ = Pt, Au, Ag, Cd, or Pb. Several of the combinations studied by us display a preference for atomically mixed configurations over phase-segregated forms. We have also performed further {\it ab initio} calculations to obtain the parameters describing the elastic interactions between atoms in the alloy layer, including the effective atomic sizes at the surface. We find that while elastic interactions favor alloying for all the systems considered by us, in some cases chemical interactions disfavor atomic mixing. We show that a simple criterion (analogous to the Hume-Rothery first law for bulk alloys) need not necessarily work for strain-stabilized surface alloys, because of the presence of additional elastic contributions to the alloy heat of formation, that will tend to oppose phase segregation.Comment: 10 pages, 8 figures Submitted To Phys. Rev.