État de valence de l’ytterbium dans YbMn6Ge6-xSnx et ses dérivés : matériaux magnétocaloriques haute température

Abstract : In the YbMn6Ge6-xSnx system, intermediate valent Yb magnetically orders at an astonishingly high temperature (up to TYb~125 K). This work aims to improve the understanding of the physical properties of these compounds, with an emphasis on the concentration range 4.6~2,8). External pressure experiments showed an analogy between mechanical and chemical pressure effects. Increasing the pressure favors trivalent state and Yb magnetism. In a second part, we investigated the magnetic and magnetocaloric properties of Mn4-xFexGa2Sn and Fe3Sn2. High temperature magnetocaloric applications, such as heat pump or heat conversion, need materials with high Curie temperature. These two compounds have working temperatures above 330 K. The magnetocaloric effect has been evaluated : the magnitude is moderate but close to other compounds with a second order transition near this temperature region.
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Lucas Eichenberger. État de valence de l’ytterbium dans YbMn6Ge6-xSnx et ses dérivés : matériaux magnétocaloriques haute température. Autre. Université de Lorraine, 2015. Français. ⟨NNT : 2015LORR0252⟩. ⟨tel-01754537⟩

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