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**Experimental and theoretical Studies of the Magnetocaloric Effect (MCE) in the Mn5-xFexSi3 Series**

Michael Gottschlich1 Olivier Gourdon1, Michael Ohl1, Jörg Perßon2 1 Jülich Centre for Neutron Science outstation at the SNS at ORNL 2 Peter Grünberg Institut Forschungszentrum Jülich

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The MCE The MCE is related mostly to the magnetic entropy changes of magnetic materials under the influence of an applied field Tc where the phenomenon occurs DS –MCE obtains DT of the MCE phenomenon MCE of Gd, Gschneider, et al. 2005

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Application of the MCE T=T1 B = 0

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Application of the MCE T>T1 B > 0

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Application of the MCE T=T1 B > 0

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Application of the MCE T<T1 B = 0

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**Unit cell of Mn5-xFexSi3 at room temperature**

Hexagonal P63/mcm Si Fe(ab) Fe(ab) ~1/4 1/4 Fe(c) 1/3 2/3 Si ~3/5 Fe(c) a = b ~ 6.8 Å c ~ 4.7 Å

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**Unit cell of Mn5-xFexSi3 at room temperature**

Hexagonal P63/mcm low cost non-toxic Si Fe(ab) Fe(ab) ~1/4 1/4 Fe(c) 1/3 2/3 Si ~3/5 Fe(c) a = b ~ 6.8 Å c ~ 4.7 Å

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**Unit cell of Mn5-xFexSi3 at room temperature**

Hexagonal P63/mcm low cost non-toxic structure flexible/ accommo-date various chemical substitutions understanding how the MCE works Si Fe(ab) Fe(ab) ~1/4 1/4 Fe(c) 1/3 2/3 Si ~3/5 Fe(c) a = b ~ 6.8 Å c ~ 4.7 Å

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**Synthesis: Mn5-xFexSi3 (x=0,1,2,3,4)**

polycrystalline samples prepared by inductive crucible free melting structures preliminary confirmed by x-ray diffraction magnetization measurements chemical Analysis

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**MCE in Mn5-xFexSi3 MCE of Gd5Ge2Si2 (2T)~27 J/kg K Songlin et al. 2002**

Recently at ORNL (X=4) MCE of Gd5Ge2Si2 (2T)~27 J/kg K E. Brück et al. 2007

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**Neutron the rescue… …POWGEN at the SNS**

to elucidate the coloring problem: Fe vs. Mn on both sites (no discernable by X-ray) to investigate the magnetic ordering …POWGEN at the SNS TOF instrument (ideal for atomic occupation / high Q) high resolution:Δd/d ~0.1% 80000 60000 40000 20000 d [Å] Intensity Diamond pattern collected on POWGEN Schematic view of POWGEN

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**Mn5-xFexSi3 (x= 0 to 4) 300 K x=2 “unique precision” Si Fe(ab) Fe(c)**

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 x=2 Normalized Intensity Normalized Intensity Fe(ab) Fe(c) Si d [Å] “unique precision” X theor % Fe(c) 1 % Fe(ab) x calcul 1 0.440 0.038 0.9975 2 0.754 0.161 1.993 3 0.923 0.389 3.0168 4 0.988 0.673 3.9984 5

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**Mn5-xFexSi3 (x= 0 to 4) 300 K Very similar results.**

Candini et al. 2004 Our data from SNS Very similar results.

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**Magnetization Measurements**

best MCE candidate

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**Magnetization Measurements**

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**LT structure of Mn5-xFexSi3 x=4**

300K 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Normalized Intensity d [Å] P63/mc‘m‘ 10K 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Normalized Intensity d [Å] Si Fe(ab) Fe(c) Moments II c-axis for x=4 in the ferromagnetic phase

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**DFT Calculations using program „TB-LMTO-ASA“, R. Tank et al.**

=> occupations of Mn vs. Fe => stability of the AF and F phases From group to sub-group X=4 Si 1Mn for 4Fe Name wyck x y z ___________________ M i M i M i M i M i Si i Si i Si i Fe(ab) Fe(c) P63/mcm P-1 Model 1 Mn on Fe(c) Model 2 Mn on Fe(ab)

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**DFT Calculations Model 1 Mn on Fe(c) Model 2 Mn on Fe(ab)**

LDA and LSDA calculations have been performed both confirmed that: Model 1 Mn on Fe(c) Model 2 Mn on Fe(ab) is less stable than by 0.5 eV/unit cell in agreement with the refined data magnetic moments of ~1.7 mB small magnetization on Si ~0.1 mB (indirect effect) Total Density of States

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Mn5-xFexSi3 x=0 Negative MCE occurs here

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Mn5-xFexSi3 x=0

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**Mn5-xFexSi3 x=0 AF2 structure orthorhombic 85 K 1.0 0.9 0.8 0.7 0.6**

0.5 0.4 0.3 0.2 0.1 0.0 85 K Normalized Intensity d [Å]

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**Mn5-xFexSi3 x=0 AF1 structure monoclinic 60 K Space Group P21/m a**

(3) Å b (2) Å c 6.9046(1) Å β (2)° Normalized Intensity d [Å]

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**Acknowledgements POWGEN at SNS ORNL Jason Hodges Ashfia Huq**

HB2a at HFIR ORNL Ovidiu Garlea Clarina Dela Cruz JANA Software Vaclav Petricek

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**Summary and future direction**

the combination of experimental data and theoretical tools lead to a better understanding of the MCE property. chemical substitutions single crystals for further neutron scattering experiments Thank you very much for your attention!

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