Transcript Magnetocaloric Effect Materials - uni

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
STot ,adiabatic  SMag  SLat  SEl  0
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
 S –MCE obtains
 T of the MCE phenomenon
S M  0 
Hf
Hi
Tad   0 
Hf
Hi
 M 

 dH
 T  H
T  M 

 dH
C H  T  H
MCE of Gd, Gschneider, et al. 2005
Application of the MCE
T=T1
B=0
Application of the MCE
T>T1
B>0
Application of the MCE
T=T1
B>0
Application of the MCE
T<T1
B=0
Unit cell of Mn5-xFexSi3 at room
temperature
Hexagonal P63/mcm
Si
Fe(ab)
Fe(c)
a = b ~ 6.8 Å
c ~ 4.7 Å
Fe(ab)
0
~1/4
1/4
Fe(c)
1/3
2/3
0
Si
~3/5
0
1/4
Unit cell of Mn5-xFexSi3 at room
temperature
Hexagonal P63/mcm
 low cost
 non-toxic
Si
Fe(ab)
Fe(c)
a = b ~ 6.8 Å
c ~ 4.7 Å
Fe(ab)
0
~1/4
1/4
Fe(c)
1/3
2/3
0
Si
~3/5
0
1/4
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(c)
a = b ~ 6.8 Å
c ~ 4.7 Å
Fe(ab)
0
~1/4
1/4
Fe(c)
1/3
2/3
0
Si
~3/5
0
1/4
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
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
 TOF instrument (ideal for
atomic occupation / high Q)
 high resolution:Δd/d ~0.1%
80000
Intensity
Diamond pattern collected on POWGEN
60000
40000
20000
Schematic view of POWGEN
0
0
0.5
1.0
d [Å]
Mn5-xFexSi3 (x= 0 to 4)
300 K
1.0
0.9
0.8
x=2
Normalized IntensityNormalized Intensity
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
d [Å]
X theor
0
1
2
3
4
5
“unique precision”
% Fe(c) 1
0
0.440
0.754
0.923
0.988
1
% Fe(ab)
0
0.038
0.161
0.389
0.673
1
x calcul
0
0.9975
1.993
3.0168
3.9984
5
Si
Fe(ab
)
Fe(c
)
Mn5-xFexSi3 (x= 0 to 4)
300 K
Candini et al. 2004
Our data from SNS
Very similar results.
Magnetization Measurements
best MCE candidate
Magnetization Measurements
LT structure of Mn5-xFexSi3 x=4
1.0
0.9
0.8
Normalized Intensity
0.7
300K
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.5
1.0
1.5
1.0
2.0
d [Å]
2.5
3.0
3.5
P63/mc‘m‘
0.9
0.8
Si
Normalized Intensity
0.7
10K
0.6
0.5
0.4
0.3
Fe(ab
)
0.2
Fe(c
)
0.1
0.0
0.5
1.0
1.5
2.0
d [Å]
2.5
3.0
3.5
Moments II c-axis for x=4 in
the ferromagnetic phase
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
1Mn for 4Fe
P63/mcm
Si
Name wyck
x
y
___________________
M1 2i 0.3333 0.6667
M2 2i 0.6667 0.3333
M3 2i 0.7636 0.7636
M4 2i 0.2364 0.0000
M5 2i 0.0000 0.2364
Si1 2i 0.4043 0.4043
Si2 2i 0.5957 0.0000
Si3 2i 0.0000 0.5957
z
0.0000
0.5000
0.2500
0.2500
0.2500
0.2500
0.2500
0.2500
Fe(ab
)
Fe(c
)
P-1
Model 1
Model 2
Mn on Fe(c) Mn on Fe(ab)
DFT Calculations
LDA and LSDA calculations have been performed both confirmed that:
Model 1
Mn on Fe(c)
is less stable than
by 0.5 eV/unit cell
Model 2
Mn on Fe(ab)
in agreement with the refined data
 magnetic moments of ~1.7 B
 small magnetization on Si ~0.1 B (indirect effect)
Total Density of States
Mn5-xFexSi3 x=0
Negative MCE occurs here
Mn5-xFexSi3 x=0
Mn5-xFexSi3 x=0
AF2 structure
orthorhombic
1.0
0.9
0.8
85 K
0.7
Normalized Intensity
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.5
1.0
1.5
2.0
2.5
d [Å]
3.0
3.5
Mn5-xFexSi3 x=0
AF1 structure
monoclinic
Normalized Intensity
60 K
d [Å]
Space Group
P21/m
a
6.88357(3) Å
b
4.81331(2) Å
c
6.9046(1) Å
β
119.897(2)°
Acknowledgements
POWGEN at SNS ORNL
HB2a at HFIR ORNL
Jason Hodges
 Ovidiu Garlea
Ashfia Huq
 Clarina Dela Cruz
JANA Software
 Vaclav Petricek
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!