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.Work partially supported by the projects FEDER/POCTI no. 155/94, PTDC/CTM/NAN/5414/2014 from Fundacao para a Ciencia e Tecnologia (FCT), Portugal. J.H.B. Silva and M. B. Barbosa thanks FCT for the Grant SFRH/BD/88440/2012 and SFRH/BD/97591/2013, respectively. C.M. acknowledges the support of the Fundación ARAID. This work was supported by the Spanish Ministry of Science (through Project Nos. MAT2014- 51982-C2-R and C1-R, including FEDER funding) and the Aragon Regional government (Project No. E26). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Offce of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Use of the High Magnetic Field Laboratory at Nijmegen was supported by contract no: NMA01-111.Peer reviewe