Talk given by Prof. Heiko Wende Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Germany Abstract: In two examples it will be demonstrated how microscopic, element-specific investigations help to advance the fundamental understanding of 1) magnetic molecular hybrid systems and 2) magnetocaloric systems for solid state refrigeration. The former hybrid systems are developed with the vision to promote the on-going process of electronic device miniaturization. For this purpose the tailoring of the magnetic properties in these nanoscale systems is essential. We make use of hybrid systems that consist of a combination of magnetic molecules, graphene and thin films. By means of X-ray absorption spectroscopy and especially XMCD studies the magnetic coupling of paramagnetic molecules to ferromagnetic surfaces is analyzed (see Fig. 1) [1-3]. These magnetic properties can be tailored by the help of an intermediate layer of atomic oxygen or graphene [4,5]. The fundamental understanding of the relevant interactions in these molecular hybrid systems is possible by combination of experimental and theoretical results utilizing ab initio calculations. Concerning example 2) it is crucial that ferroic materials allow for a significant adiabatic temperature change induced by realistic electrical and magnetic fields, under pressure and external stress. This approves their use in solid state refrigeration concepts, which offer an energy efficient alternative to the classical gas-compressor scheme. By combination of two independent approaches, nuclear resonant inelastic X-ray scattering (NRIXS) and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe13−xSix [6]. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magnetoelastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties. [1] S. Bhandary et al., Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates, Phys. Rev. B 88, 024401 (2013). [2] H. Wende et al., Substrate-induced magnetic ordering and switching of iron porphyrin molecules, Nature Materials 6, 516 (2007). [3] H. Wende, Molecular magnets: How a nightmare turns into a vision, Nature Materials 8, 165 (2009). [4] M. Bernien et al., Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates, Phys. Rev. Lett. 102, 047202 (2009). [5] S. Bhandary et al., Graphene as a Reversible Spin Manipulator of Molecular Magnets, Phys. Rev. Lett. 107, 257202 (2011). [6] M. Gruner et al., Element-resolved thermodynamics of magnetocaloric LaFe 13−xSix, Phys. Rev. Lett., accepted for publication (Jan. 2015). P.S. Dies ist eine freie Informations- und Fortbildungsveranstaltung für externe Gäste und KIT Mitarbeiter.

This event is part of the eventgroup INT Talks

Homepage
https://www.int.kit.edu/events.php

Speaker
Prof. Heiko Wende

Universität Duisburg-Essen
Magnetische Nanostrukturen https://www.uni-due.de/physik/wende/

Organizer
Prof. Dr.-Ing. Horst Hahn
Institute of Nanotechnology (INT)
Karlsruhe Institute of Technology (KIT)
Eggenstein-Leopoldshafen
Mail: horst hahn ∂does-not-exist.kit edu

Targetgroup
Interested / Everyone