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Title: Determining the vibrational entropy change in the giant magnetocaloric material LaFe 11.6 Si 1.4 by nuclear resonant inelastic x-ray scattering

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 $${\mathrm{LaFe}}_{13{-}x}{\mathrm{Si}}_{x}$$.
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [3] ;  [3] ;  [3] ;  [3] ;  [4] ;  [1]
  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
Publication Date:
Grant/Contract Number:
AC02-06CH11357; GR3498/3-2; GU514/6-2; WE2623/12-2; WE2623/7-1; WE2623/13-2
Type:
Accepted Manuscript
Journal Name:
Physical Review B
Additional Journal Information:
Journal Volume: 98; Journal Issue: 2; Journal ID: ISSN 2469-9950
Publisher:
American Physical Society (APS)
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States); Univ. of Duisburg-Essen (Germany); Leibniz Inst. for Solid State and Materials Research (IFW Dresden), Dresden (Germany); Technical Univ. of Darmstadt (Germany)
Sponsoring Org:
USDOE Office of Science (SC); German Research Foundation (DFG)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; entropy; magnetic phase transitions; magnetism; phonons; specific heat; first-principles calculations; resonant inelastic x-ray scattering
OSTI Identifier:
1480690
Alternate Identifier(s):
OSTI ID: 1460757

Landers, J., Salamon, S., Keune, W., Gruner, M. E., Krautz, M., Zhao, J., Hu, M. Y., Toellner, T. S., Alp, E. E., Gutfleisch, O., and Wende, H.. Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6Si1.4 by nuclear resonant inelastic x-ray scattering. United States: N. p., Web. doi:10.1103/PhysRevB.98.024417.
Landers, J., Salamon, S., Keune, W., Gruner, M. E., Krautz, M., Zhao, J., Hu, M. Y., Toellner, T. S., Alp, E. E., Gutfleisch, O., & Wende, H.. Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6Si1.4 by nuclear resonant inelastic x-ray scattering. United States. doi:10.1103/PhysRevB.98.024417.
Landers, J., Salamon, S., Keune, W., Gruner, M. E., Krautz, M., Zhao, J., Hu, M. Y., Toellner, T. S., Alp, E. E., Gutfleisch, O., and Wende, H.. 2018. "Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6Si1.4 by nuclear resonant inelastic x-ray scattering". United States. doi:10.1103/PhysRevB.98.024417.
@article{osti_1480690,
title = {Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6Si1.4 by nuclear resonant inelastic x-ray scattering},
author = {Landers, J. and Salamon, S. and Keune, W. and Gruner, M. E. and Krautz, M. and Zhao, J. and Hu, M. Y. and Toellner, T. S. and Alp, E. E. and Gutfleisch, O. and Wende, H.},
abstractNote = {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 ${\mathrm{LaFe}}_{13{-}x}{\mathrm{Si}}_{x}$.},
doi = {10.1103/PhysRevB.98.024417},
journal = {Physical Review B},
number = 2,
volume = 98,
place = {United States},
year = {2018},
month = {7}
}

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