From molecular spin hybrids to magnetocaloric systems: fundamental understanding by element specific investigations
Prof. Dr. Heiko Wende (Universität Duisburg-Essen, Fakultät für Physik)

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 or single molecular magnets to ferromagnetic surfaces is analyzed (Fig. 1) [1-4]. These magnetic properties can be tailored by the help of an intermediate layer of atomic oxygen or graphene [5-7]. 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 the magnetocaloric systems 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 [8]. 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 [9,10]. 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] A. Candini et al., Spin-communication channels between Ln(III) bis-phthalocyanines molecular nanomagnets and a magnetic substrate, Sci. Rep. 6, 21740 (2016).
[5] M. Bernien et al., Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates, Phys. Rev. Lett. 102, 047202 (2009).
[6] S. Bhandary et al., Graphene as a Reversible Spin Manipulator of Molecular Magnets, Phys. Rev. Lett. 107, 257202 (2011).
[7] S. Marocchi et al., Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates, ACS Nano 10, 9353-9360 (2016).
[8] O. Gutfleisch et al., Mastering hysteresis in magnetocaloric materials, Phil. Trans. R. Soc. A 347, 20150308 (2016).
[9] M. Gruner et al., Element-resolved thermodynamics of magnetocaloric LaFe13−xSix, Phys. Rev. Lett., 114, 057202 (2015).
[10] M. Gruner et al., Moment-Volume Coupling in La(Fe1−xSix)13, Phys. Status Solidi B, 1700465. doi:10.1002/pssb.201700465 (2017).