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Crystallographic studies on correlated electron systems

Tbilisi, July 8th 2014 Karen Friese

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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

Structure - Property Relationships

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Examples

Superconductors: HoNi2B2C

Magnetocalorics: Mn5-xFexSi3

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Example 1

HoNi2B2C

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Superconductivity and Magnetism in HoNi2B2C

Structure Properties

300 K I4/mmm paramagnetic

~ 8 K superconducting co-existence of commens. AFM Phase

+ incommen. spiral state

~ 6.25 K 2nd incommens. phase

~ 5 K reenters normal state incommens. phase suppressed;

orthorhombic? commens. AFM phase persists

~ 2-4 K superconducting

based on Schneider et. al, Phys. Rev. B74, 104426(2006), Lynn et.al., Phys. Rev. B55, 6585 (1997)

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Open Questions?

based on Schneider et. al, Phys. Rev. B74, 104426(2006), Lynn et.al., Phys. Rev. B55, 6585 (1997)

Symmetry of the nuclear structure in the commensurate antiferromagnetic phase?

Direction of the magnetic moments?

Do both Ho and Ni contribute to the magnetism?

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HEIDISingle crystal diffractometer on hot source

Heinz Maier-Leibnitz Zentrum, GarchingLocal Contacts: Martin Meven, Andrew Sazonov

λ=0.79Å, 1036 reflections, room temperature and 2K

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Ho

C

Ni

B

I4/mmm

Structure determination combining neutron and x-ray data

I4mm

a=3.5177(1) Åc=10.5278(3) Å

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I4/mmm I4mmStructure is polar!

Structure determination combining neutron and x-ray data

Ho

C

Ni

B

a=3.5177(1) Åc=10.5278(3) Å

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Two possible interpretations:

1. Contribution to new reflections is exclusively magnetic

2. New reflections have also a contribution from the nuclear structure → structural phase transition

Low temperature structure of HoNi2B2C

Extinction rules:

2K No extinctions: Primitive lattice

Ambient temperature I-centered: h+k+l=2n

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Magnetic all nuclear magnetic reflections NP Direction of Symmetry magnetic moments

P[I]4nc 16.11 6.85 39.64 717/718 13 MzC[B]mc21 8.28 6.67 12.64 1200/1208 14 Mx=MyP[I]mn21 8.81 7.36 12.42 1148/1152 16 My

Refinement of the magnetic structure

Primitive nuclear structure → structural phase transition

all h+k+l=2n h+k+l=2n+1Cm´m2´ 6.71 6.83 6.45 1200/1208 23 Mx=MyPm´m2´ 7.28 6.90 8.13 1148/1152 26 MyP2´ 6.62 6.04 8.13 1846/1865 33 Mx,My

Nuclear structure stays I-centered tetragonal

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Cmm´2´ Pmm´2´

Magnetic models for HoNi2B2C

6.71% 7.28%

Z=1.0

Z=0.5

Z=0.0

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Spherical Neutron PolarimetryPOLI

Heinz Maier-Leibnitz Zentrum,GarchingLocal Contact: Vladimir Hutanu

• Direction of magnetic moments• Volume fractions of different domains

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Models Cmm2 :

Single domain with magnetic moment along [110]:1 0 00 0 10 1 0Single domain with magnetic moment along [-110]:-1 0 00 0 -10 -1 0distribution of domains 50%/50%-1 0 00 0 00 0 0

Terms depending on the domain population are off- diagonal

Models Pmm2:

Single domain with magnetic moment along [100]:-1 0 00 1 00 0 -1Single domain with magnetic moment along [010]:-1 0 00 -1 00 0 1distribution of domains 50%/50%-1 0 00 0 00 0 0 Terms depending on the domain population are on the diagonal

Simulation of polarization matrices

Calculated polarization matrix for the magnetic reflection (00l)

-1,00(3) 0.01(1) 0.03(1) -0,01(1) -0,01(1) 0.13(1) 0,04(1) 0,11(1) 0,03(1)

Reflection 0 0 3

domain population: 0.45:0.55

structure refinement:0.40:0.60

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Magnetic structure of HoNi2B2C at 2.2 K

Cmm´2´a=b=5.497Åc=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

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Magnetocaloric Effect and Magnetocaloric Cooling

[modified from: O. Tegus et. al., Let. Nat. (2002)]

Magnetic field changes lead to

→ changes of the isothermal magnetic Entropy

→ changes of the adiabatic temperature

→ 20-30% higher efficiency potential compared to vapor cycle refrigeration

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System Mn5-xFexSi3

After Songlin et al., J. Alloys Compds 334 (2002) 249-252]

x

• 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!

For x=4

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Crystal Structure of Mn4FeSi3 from literature

Space group P63/mcma=b≈6.8Å, c ≈4.7Å

Fe

Mn/FeSi

1∞[FeSi3]1∞[□(Mn,Fe)3]

Fe 1/3 2/3 0.0 Mn/Fe 0.2231 0.0 0.25Si 0.5929 0.0 0.25

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Temperature dependent X-ray and

neutron powder diffraction

SPODI, MLZ Garching

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Temperature dependent powder diffraction Te

mpe

ratu

re, K

4K

470K

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Temperature dependence of the lattice

parameter of MnFe4Si3

0 100 200 300 400 5006,804

6,806

6,808

6,810

6,812

a la

ttic

e p

ara

me

ter

(Å)

Temperature (K)

0 100 200 300 400 5004,71

4,72

4,73

4,74

4,75

c la

ttic

e p

ara

me

ter

(Å)

Temperature (K)

0 100 200 300 400 500

189,0

189,2

189,4

189,6

189,8

190,0

190,2

190,4

190,6

190,8

Un

it ce

ll vo

lum

e (

Å3)

Temperature (K)

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Atomic scattering factor of Mn (grey) and Fe (red)

Neutron scattering lengths:

Mn -3.730Fe 9.450Si 4.149

Single crystal x-ray and neutron diffraction

X-rays:Reflections h-hl: l=2n+1 are extinct→ P63/mcm

Neutrons:Reflections h-hl: l=2n+1 are observed→ no c-glide plane

What is the correct space group?

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Crystal structure of MnFe4Si3

P63/mcm P-6

1/3Mn+2/3 Fe

Fe

0.37Mn+0.63Fe 0.28Mn+0.72 Fe

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Magnetic structure:

Single crystal diffraction data@ 200K in ferromagnetic phase

Mn/Fe

Fe

Magnetic space group Pm´Mn/Fe-position: 1.5(2) μB

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Acknowledgements

Vladimir Hutanu Martin Meven 

Andrew Sazonov 

Oksana ZaharkoGeorg Roth

Karine SpartaEddy Lelievre-Berna

Günter Fuchs

Paul HeringThomas Brückel

Marcus HerlitschkeJörg Voigt

Raphael HermannAnatoliy ShenyshinAndrzej Grzechnik

a Jülich Center for Neutron Science, Research Center Jülich, Germanyb Institute for Crystallography, RWTH Aachen University, JCNS Outstation at FRM II, Garching,

Germany c Paul Scherrer Institute, Villingen, Switzerland

HoNi2B2C

Mn5-xFexSi3

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Crystallography matters!