First-principles simulation of magnetocaloric and electrocaloric effects in nanostructured films

• Applicant: Professorin Dr. Anna Grünebohm
• Subject Area: Synthesis and Properties of Functional Materials ; Theoretical Condensed Matter Physics
• Term: from 2012 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 227381001

In this project we will continue our hitherto successful investigation and optimization of the adiabatic temperature changes in ferroelectric and ferromagnetic materials for applied external magnetic and/or electric fields. Besides the individual caloric effects, we focus on the coupling between magnetic, ferroelectric, and lattice degrees of freedom in the quest for optimizing caloric effects and exploring new ways to utilize multiferroic cross-couplings.We aim at a better understanding of the physical processes giving rise to giant temperature changes, or hampering them, and at the optimization of the caloric response at room temperature.Our main focus are thin films and heterostructures based on the ferroelectric materials (Ba,Sr)TiO3, metamagnetic Heusler alloys (Ni-Mn-Sn,Ga,In), and the multiferroic system (Sr,Ba)MnO3.We use density functional theory in order to investigate the electronic and magnetic structure of these materials on an "ab initio" level. Furthermore, we model materials at finite temperature and/or external fields by means of Monte-Carlo and molecular dynamics simulations. For both the magnetic and ferroelectric materials we employ models (spin-lattice model, effective Hamiltonian based on the ferroelectric modes) which are parametrized by ab initio simulations. Both methods describe the ferroic phase diagrams in good agreement with experiment.These methods allow us to investigate and optimize the caloric response depending on epitaxial strain and changes of the chemical composition. For example Co or Cr doping can be used to shift the optimal adiabatic response of NiMn(Sn,Ga) to lower temperatures and at the same time increase its magnitude. We will continue this successful approach of materials design for both classes of ferroics. In particular, the use of multiferroic materials has a huge potential for enhanced changes of temperature and entropy, if the magnetic and ferroelectric degrees of freedom order simultaneously. In addition, one can for instance use multiferroic coupling to trigger the giant caloric response of metamagnetic materials by an electric field.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1599: Caloric Effects in Ferroic Materials: New Concepts for Cooling
• International Connection: Switzerland
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Cooperation Partner: Professor Dr. Claude Ederer