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Crystallographic studies
on correlated electron
systems
Tbilisi, July 8th 2014
Karen Friese
1
Structure - Property Relationships
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Principle of Neumann
The symmetry elements of any physical
property of a crystal must include the
symmetry elements of the point group of the
crystal
Examples
Superconductors: HoNi2B2C
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Magnetocalorics: Mn5-xFexSi3
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Example 1
HoNi2B2C
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Superconductivity and Magnetism in HoNi2B2C
Structure
300 K
Properties
I4/mmm
~8K
paramagnetic
superconducting
~ 6.25 K
2nd incommens. phase
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~5K
reenters normal state
orthorhombic?
~ 2-4 K
co-existence of
commens. AFM Phase
+ incommen. spiral state
incommens. phase
suppressed;
commens. AFM phase
persists
superconducting
based on Schneider et. al, Phys. Rev. B74, 104426(2006), Lynn et.al., Phys. Rev. B55, 6585 (1997)
Open Questions?
Symmetry of the nuclear structure in the commensurate antiferromagnetic
phase?
Direction of the magnetic moments?
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Do both Ho and Ni contribute to the magnetism?
based on Schneider et. al, Phys. Rev. B74, 104426(2006), Lynn et.al., Phys. Rev. B55, 6585 (1997)
HEIDI
Single crystal diffractometer on hot source
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Heinz Maier-Leibnitz Zentrum, Garching
Local Contacts: Martin Meven, Andrew Sazonov
λ=0.79Å, 1036 reflections, room temperature and 2K
Structure determination combining
neutron and x-ray data
C
a=3.5177(1) Å
c=10.5278(3) Å
Ho
Ni
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B
I4/mmm
I4mm
Structure determination combining
neutron and x-ray data
a=3.5177(1) Å
c=10.5278(3) Å
C
Ho
Ni
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B
I4/mmm
Structure is polar!
I4mm
Low temperature structure of HoNi2B2C
Extinction rules:
Ambient temperature
2K
I-centered: h+k+l=2n
No extinctions: Primitive lattice
Two possible interpretations:
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1. Contribution to new reflections is exclusively magnetic
2. New reflections have also a contribution from the nuclear structure →
structural phase transition
Refinement of the magnetic structure
Nuclear structure stays I-centered tetragonal
Magnetic
Symmetry
all
P[I]4nc
C[B]mc21
P[I]mn21
16.11
8.28
8.81
nuclear
magnetic
6.85
6.67
7.36
39.64
12.64
12.42
reflections NP
717/718
1200/1208
1148/1152
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14
16
Direction of
magnetic moments
Mz
Mx=My
My
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Primitive nuclear structure → structural phase transition
Cm´m2´
Pm´m2´
P2´
all
6.71
7.28
6.62
h+k+l=2n
6.83
6.90
6.04
h+k+l=2n+1
6.45
8.13
8.13
1200/1208
1148/1152
1846/1865
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26
33
Mx=My
My
Mx,My
Magnetic models for HoNi2B2C
Z=1.0
Z=0.5
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6.71%
7.28%
Z=0.0
Cmm´2´
Pmm´2´
Spherical Neutron Polarimetry
POLI
Heinz Maier-Leibnitz Zentrum,Garching
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Local Contact: Vladimir Hutanu
• Direction of magnetic moments
• Volume fractions of different domains
Simulation of polarization matrices
Calculated polarization matrix for the magnetic reflection (00l)
Models Cmm2 :
Models Pmm2:
Single domain with magnetic moment along [110]:
1
0
0
0
0
1
0
1
0
Single domain with magnetic moment along [-110]:
-1
0
0
0
0
-1
0
-1
0
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distribution of domains 50%/50%
-1
0
0
0
0
0
0
0
0
Terms depending on the domain population are off- diagonal
Reflection 0 0 3
Single domain with magnetic moment along [100]:
-1
0
-1,00(3)
0
1
-0,01(1)
0
0
0
0.01(1)
0.03(1)
0
-0,01(1)
0.13(1)
-1
Single
domain with magnetic
moment along
[010]:
0,04(1)
0,11(1)
0,03(1)
-1
0
0
-1
domain
0
0
0
0
population:
1
0.45:0.55
distribution of domains 50%/50%
-1
0
0
0
0structure
0
0
0
0
refinement:
0.40:0.60
Terms depending on the domain population are on the diagonal
Magnetic structure of HoNi2B2C at 2.2 K
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Cmm´2´
a=b=5.497Å
c=10.522 Å
•
•
•
•
refined magnetic moments on Ho: 7.98(10) μB
no significant contribution from Ni
Symmetry of the nuclear structure is broken: I4mm (Z=1)→ P4mm (Z=1)
Polar character of the nuclear structure increases
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Example 2
Mn5-xFexSi3
Magnetocaloric Effect
and Magnetocaloric Cooling
Magnetic field changes lead to
→ changes of the isothermal
magnetic Entropy
→ changes of the adiabatic
temperature
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→ 20-30% higher efficiency
potential compared to vapor
cycle refrigeration
[modified from: O. Tegus et. al., Let. Nat. (2002)]
System Mn5-xFexSi3
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x
For x=4
After Songlin et al., J. Alloys Compds 334 (2002)
249-252]
• Modestly large MCE ≈ 2.9J/Kg K at ΔB=0-2T
• Tc=299.6(1.0) K
• No rare earths, no hazardous elements
• Seems to be stable
+ Single crystals of sufficient sizes available!
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Crystal Structure of Mn4FeSi3 from literature
1∞[□(Mn,Fe)3]
Si
Mn/Fe
Fe
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Space group P63/mcm
a=b≈6.8Å, c ≈4.7Å
Fe
Mn/Fe
Si
1/3 2/3
0.2231 0.0
0.5929 0.0
0.0
0.25
0.25
1∞[FeSi3]
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Temperature dependent X-ray and
neutron powder diffraction
SPODI, MLZ Garching
20
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Temperature, K
Temperature dependent powder diffraction
470K
4K
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Temperature dependence of the lattice
parameter of MnFe4Si3
4,75
c lattice parameter (Å)
a lattice parameter (Å)
6,812
6,810
6,808
6,806
6,804
4,74
4,73
4,72
4,71
0
100
200
300
400
500
0
100
Temperature (K)
200
300
400
500
Temperature (K)
190,8
190,4
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3
Unit cell volume (Å )
190,6
190,2
190,0
189,8
189,6
189,4
189,2
189,0
0
100
200
300
400
500
Temperature (K)
22
Single crystal x-ray and neutron diffraction
Neutron scattering lengths:
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Mn -3.730
Fe 9.450
Si 4.149
Atomic scattering factor of Mn (grey) and Fe (red)
Neutrons:
X-rays:
Reflections h-hl: l=2n+1 are observed
Reflections h-hl: l=2n+1 are extinct
→ no c-glide plane
→ P63/mcm
What is the correct space group?
Crystal structure of MnFe4Si3
1/3Mn+2/3 Fe
0.37Mn+0.63Fe
0.28Mn+0.72 Fe
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Fe
P63/mcm
P-6
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Magnetic structure:
Single crystal diffraction data
@ 200K in ferromagnetic phase
Mn/Fe
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Fe
Magnetic space group Pm´
Mn/Fe-position: 1.5(2) μB
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Acknowledgements
HoNi2B2C
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Mn5-xFexSi3
Vladimir Hutanu
Martin Meven
Andrew Sazonov
Oksana Zaharko
Georg Roth
Karine Sparta
Eddy Lelievre-Berna
Günter Fuchs
Paul Hering
Thomas Brückel
Marcus Herlitschke
Jörg Voigt
Raphael Hermann
Anatoliy Shenyshin
Andrzej Grzechnik
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Crystallography matters!