Element resolved thermodynamics of magnetocaloric LaFe13−xSix

Heiko Wende, CENIDE and University of Duisburg-Essen, Duisburg, Germany

Ferroic materials allow for a significant adiabatic temperature change induced by realistic electrical and magnetic fields, under pressure and external stress [1]. This approves their use in solid state refrigeration concepts, which offer an energy efficient alternative to the classical gas-compressor scheme. A good cooling material is characterized by a large isothermal entropy change, which determines the latent heat to be taken up during a first-order transformation in conjunction with a high adiabatic temperature change, which controls the operating range. Apart from the prototype Gd-based systems, a large number of suitable materials have evolved, which undergo a magnetic first-order transition and perform well with respect to both quantities, and LaFe13−xSix-based compounds are found among the best [2, 3]. They consist of largely abundant components and are significantly less expensive than Gd- or Rh-based alloys. 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 [4, 5, 6]. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic 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.

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