Determining the vibrational entropy change in the giant magnetocaloric material ${\mathrm{LaFe}}_{11.6}{\mathrm{Si}}_{1.4}$ by nuclear resonant inelastic x-ray scattering

Journal Article · 18 July 2018 · Physical Review B

DOI: https://doi.org/10.1103/PhysRevB.98.024417 · OSTI ID:1480690

Landers, J. ^[1]; Salamon, S. ^[1]; Keune, W. ^[1]; Gruner, M. E. ^[1]; Krautz, M. ^[2]; Zhao, J. ^[3]; Hu, M. Y. ^[3]; Toellner, T. S. ^[3]; Alp, E. E. ^[3]; Gutfleisch, O. ^[4]; Wende, H. ^[1]

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1. Univ. of Duisburg-Essen (Germany). Faculty of Physics. Center for Nanointegration Duisburg-Essen (CENIDE)
2. Leibniz Inst. for Solid State and Materials Research (IFW Dresden), Dresden (Germany)
3. Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source
4. Technical Univ. of Darmstadt (Germany). Materials Science

Magnetocaloric $${\mathrm{LaFe}}_{13{-}x}{\mathrm{Si}}_{x}$$-based compounds belong to the outstanding materials with potential for efficient solid-state refrigeration. We have performed temperature-dependent $$^{57}\mathrm{Fe}$$ nuclear resonant inelastic x-ray scattering measurements (in a field $${{\mu}}_{0}H$$ of $${\sim}0.7$$ T) of the vibrational (phonon) density of states, VDOS, in $${\mathrm{LaFe}}_{11.6}{\mathrm{Si}}_{1.4}$$ across the metamagnetic isostructural first-order phase transition at $${T}_{C}{\sim}192$$ K from the low-temperature ferromagnetic (FM) to the high-temperature paramagnetic (PM) phase, in order to determine the change in thermodynamic properties of the Fe lattice at $${T}_{C}$$. The experimental results are compared with density-functional-theory-based first-principles calculations using the fixed-spin moment approach. Our combined experimental and theoretical results reveal distinct and abrupt changes in the VDOS of the Fe sublattice across $${T}_{C}$$, occurring within a small temperature interval of $$\mathrm{{\Delta}}T{\le}12$$ K around $${T}_{C}$$. This indicates that strong magnetoelastic coupling (at the atomic scale) is present up to $${T}_{C}$$, leading to a pronounced lattice softening (phonon redshift) in the PM phase. These changes originate from the itinerant electron magnetism associated with Fe and are correlated with distinct modifications in the Fe-partial electronic density of states $$D({E}_{F}$$) at the Fermi energy $${E}_{F}$$. From the experimental VDOS we can infer an abrupt increase (jump) in the Fe-partial vibrational entropy $$\mathrm{{\Delta}}{S}_{\mathrm{vib}}$$ of $$+6.9\pm{}2.6$$ J/(kg K) and in the vibrational specific heat $$\mathrm{{\Delta}}{C}_{\mathrm{vib}}$$ of $$+2.7\pm{}1.6$$ J/(kg K) upon heating. The increase in magnitude of the vibrational entropy $$|\mathrm{{\Delta}}{S}_{\mathrm{vib}}|=6.9$$ J/(kg K) of the Fe sublattice at $${T}_{C}$$ upon heating is substantial, if compared with the magnitude of the isothermal entropy change $$|\mathrm{{\Delta}}{S}_{\mathrm{iso}}|$$ of 14.2 J/(kg K) in a field change $$\mathrm{{\Delta}}B$$ from 0 to 1 T, as obtained from isothermal magnetization measurements on our sample and using the Maxwell relation. Finally, we demonstrate that $$\mathrm{{\Delta}}{S}_{\mathrm{vib}}$$ obtained by nuclear resonant inelastic x-ray scattering is a sizable quantity and contributes directly and cooperatively to the total entropy change $$\mathrm{{\Delta}}{S}_{\mathrm{iso}}$$ at the phase transition of $${\mathrm{LaFe}}_{13{-}x}{\mathrm{Si}}_{x}$$.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States); University of Duisburg-Essen (Germany); Leibniz Inst. for Solid State and Materials Research (IFW Dresden), Dresden (Germany); Technical Univ. of Darmstadt (Germany)
• Sponsoring Organization: USDOE Office of Science (SC); German Research Foundation (DFG)
• Grant/Contract Number: AC02-06CH11357; GR3498/3-2; GU514/6-2; WE2623/12-2; WE2623/7-1; WE2623/13-2
• OSTI ID: 1480690
• Alternate ID(s): OSTI ID: 1460757
• Journal Information: Physical Review B, Vol. 98, Issue 2; ISSN 2469-9950
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English