Insights on the origin of the Tb5Ge4 magnetocaloric effect

In this report the magnetic, atomic structures and spin-lattice coupling have been thoroughly studied through high magnetic field magnetometry, Synchrotron X-ray diffraction under applied magnetic field and magnetostriction measurements in the Tb5Ge4 compounds. A field induced phase transition from an antiferromagnetic towards a ferromagnetic ordering was confirmed but with absence of structural transformation. This absence has been confirmed experimentally through synchrotron x-ray diffraction under applied field (up to 30 T). Moreover, this absence was explained via a thermodynamic free energy model: first principles calculations determined a large energy gap (ΔE=0.65 eV) between the two possible structures, O(I) and O(II). From magnetic and structural properties, a H-T phase diagram has been proposed for Tb5Ge4. Finally it was observed a large magnetostriction (up to 600 ppm) induced by ∆H=7 T.