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

The MCE

f

i

f

i

H

HHH

ad

H

HH

M

dHT

M

C

TT

dHT

MS

0

0

MCE of Gd, Gschneider, et al. 2005

The MCE is related mostly to the magnetic entropy changes of magnetic materials under the influence of an applied field

Tc where the phenomenon occursS –MCE obtains T of the MCE phenomenon

0, ElLatMagadiabaticTot SSSS

Application of the MCE

B = 0

T=T1

Application of the MCE

B > 0

T>T1

Application of the MCE

B > 0

T=T1

Application of the MCE

B = 0

T<T1

Fe(ab) 0 ~1/4 1/4

Fe(c) 1/3 2/3 0

Si ~3/5 0 1/4

Hexagonal P63/mcm

a = b ~ 6.8 Å

c ~ 4.7 Å

Fe(ab) Fe(c)

Si

Unit cell of Mn5-xFexSi3 at room temperature

Fe(ab) 0 ~1/4 1/4

Fe(c) 1/3 2/3 0

Si ~3/5 0 1/4

Hexagonal P63/mcm

a = b ~ 6.8 Å

c ~ 4.7 Å

Fe(ab) Fe(c)

Si

low cost

non-toxic

Unit cell of Mn5-xFexSi3 at room temperature

Unit cell of Mn5-xFexSi3 at room temperature

Fe(ab) 0 ~1/4 1/4

Fe(c) 1/3 2/3 0

Si ~3/5 0 1/4

Hexagonal P63/mcm

a = b ~ 6.8 Å

c ~ 4.7 Å

Fe(ab) Fe(c)

Si

low cost

non-toxic

structure flexible/ accommo-date various chemical substitutions

understanding how the MCE works

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

MCE in Mn5-xFexSi3

Songlin et al. 2002 Recently at ORNL (X=4)

MCE of Gd5Ge2Si2 (2T)~27 J/kg K

E. Brück et al. 2007

TOF instrument (ideal for atomic occupation / high Q)

high resolution:Δd/d ~0.1%

Neutron the rescue… 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

Schematic view of POWGEN

80000

60000

40000

20000

0 0 0.5 1.0

d [Å]

Inte

nsi

ty

Diamond pattern collected on POWGEN

“unique precision”

X theor % Fe(c) 1 % Fe(ab) x calcul0 0 0 01 0.440 0.038 0.99752 0.754 0.161 1.9933 0.923 0.389 3.01684 0.988 0.673 3.99845 1 1 5

No

rmal

ized

Inte

nsi

ty

Mn5-xFexSi3 (x= 0 to 4)

Fe(ab)

Fe(c)

Si

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

No

rmal

ized

Inte

nsi

ty

0.5 1.0 1.5 2.0 2.5 3.0 3.5

d [Å]

300 K

x=2

Mn5-xFexSi3 (x= 0 to 4)

Candini et al. 2004

Very similar results.

300 K

Our data from SNS

Magnetization Measurements

best MCE candidate

Magnetization Measurements

Moments II c-axis for x=4 in the ferromagnetic phase

LT structure of Mn5-xFexSi3 x=4

Fe(ab)

Fe(c)

Si10K

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

No

rmal

ized

Inte

nsi

ty

0.5 1.0 1.5 2.0 2.5 3.0 3.5

d [Å]

300K

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

No

rmal

ized

Inte

nsi

ty

0.5 1.0 1.5 2.0 2.5 3.0 3.5

d [Å]

P63/mc‘m‘

DFT Calculationsusing program „TB-LMTO-ASA“, R. Tank et al.=> occupations of Mn vs. Fe=> stability of the AF and F phases

Name wyck x y z ___________________M1 2i 0.3333 0.6667 0.0000M2 2i 0.6667 0.3333 0.5000M3 2i 0.7636 0.7636 0.2500M4 2i 0.2364 0.0000 0.2500M5 2i 0.0000 0.2364 0.2500Si1 2i 0.4043 0.4043 0.2500Si2 2i 0.5957 0.0000 0.2500Si3 2i 0.0000 0.5957 0.2500

X=4From group to sub-group

P63/mcm P-1

1Mn for 4Fe

Model 1Mn on Fe(c)

Model 2Mn on Fe(ab)

Fe(ab)

Fe(c)

Si

DFT CalculationsLDA and LSDA calculations have been performed both confirmed that:

Model 1Mn on Fe(c)

Model 2Mn on Fe(ab)is less stable than

by 0.5 eV/unit cell

in agreement with the refined data

magnetic moments of ~1.7 B small magnetization on Si ~0.1B (indirect effect)

Total Density of States

Mn5-xFexSi3 x=0

Negative MCE occurs here

Mn5-xFexSi3 x=0

Mn5-xFexSi3 x=0

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

No

rmal

ized

Inte

nsi

ty

0.5 1.0 1.5 2.0 2.5 3.0 3.5

d [Å]

85 K

AF2 structure

orthorhombic

Mn5-xFexSi3 x=0

Space Group P21/m

a 6.88357(3) Å

b 4.81331(2) Å

c 6.9046(1) Å

β 119.897(2)°

No

rmal

ized

Inte

nsi

ty

d [Å]

60 K

AF1 structure

monoclinic

Acknowledgements

POWGEN at SNS ORNL

Jason Hodges

Ashfia Huq

HB2a at HFIR ORNL

Ovidiu Garlea

Clarina Dela Cruz

JANA Software Vaclav Petricek

Summary and future directionthe combination of experimental data and theoretical tools lead to a better understanding of the MCE property.chemical substitutionssingle crystals for further neutron scattering experiments

Thank you very much for your attention!