Mitglied der Helmholtz-Gemeinschaft Crystallographic studies on correlated electron systems Tbilisi, July 8th 2014 Karen Friese 1

Mitglied der Helmholtz-Gemeinschaft 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

Mitglied der Helmholtz-Gemeinschaft Examples Superconductors: HoNi 2 B 2 C Magnetocalorics: Mn 5-x Fe x Si 3

Mitglied der Helmholtz-Gemeinschaft 4 Example 1 HoNi 2 B 2 C

Mitglied der Helmholtz-Gemeinschaft Superconductivity and Magnetism in HoNi 2 B 2 C Structure Properties 300 K I4/mmm paramagnetic ~ 8 K superconductingco-existence of commens. AFM Phase + incommen. spiral state ~ 6.25 K2nd 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, (2006), Lynn et.al., Phys. Rev. B55, 6585 (1997)

Mitglied der Helmholtz-Gemeinschaft Open Questions? based on Schneider et. al, Phys. Rev. B74, (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?

Mitglied der Helmholtz-Gemeinschaft HEIDI Single crystal diffractometer on hot source Heinz Maier-Leibnitz Zentrum, Garching Local Contacts: Martin Meven, Andrew Sazonov λ=0.79Å, 1036 reflections, room temperature and 2K

Mitglied der Helmholtz-Gemeinschaft Ho C Ni B I4/mmm Structure determination combining neutron and x-ray data I4mm a=3.5177(1) Å c= (3) Å

Mitglied der Helmholtz-Gemeinschaft I4/mmm I4mm Structure is polar! Structure determination combining neutron and x-ray data Ho C Ni B a=3.5177(1) Å c= (3) Å

Mitglied der Helmholtz-Gemeinschaft 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 HoNi 2 B 2 C Extinction rules: 2K No extinctions: Primitive lattice Ambient temperature I-centered: h+k+l=2n

Mitglied der Helmholtz-Gemeinschaft Magnetic all nuclear magnetic reflections NP Direction of Symmetrymagnetic moments P[I]4nc / Mz C[B]mc / Mx=My P[I]mn / My Refinement of the magnetic structure Primitive nuclear structure → structural phase transition all h+k+l=2n h+k+l=2n+1 Cm´m2´ / Mx=My Pm´m2´ / My P2´ / Mx,My Nuclear structure stays I-centered tetragonal

Mitglied der Helmholtz-Gemeinschaft Cmm´2´ Pmm´2´ Magnetic models for HoNi 2 B 2 C 6.71% 7.28% Z=1.0 Z=0.5 Z=0.0

Mitglied der Helmholtz-Gemeinschaft Spherical Neutron Polarimetry POLI Heinz Maier-Leibnitz Zentrum,Garching Local Contact: Vladimir Hutanu Direction of magnetic moments Volume fractions of different domains

Mitglied der Helmholtz-Gemeinschaft Models Cmm2 : Single domain with magnetic moment along [110]: Single domain with magnetic moment along [-110]: distribution of domains 50%/50% Terms depending on the domain population are off- diagonal Models Pmm2: Single domain with magnetic moment along [100]: Single domain with magnetic moment along [010]: distribution of domains 50%/50% 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 domain population: 0.45:0.55 structure refinement: 0.40:0.60

Mitglied der Helmholtz-Gemeinschaft Magnetic structure of HoNi 2 B 2 C at 2.2 K Cmm´2´ a=b=5.497Å c= Å refined magnetic moments on Ho: 7.98(10) μ B 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

Mitglied der Helmholtz-Gemeinschaft Example 2 Mn 5-x Fe x Si 3

Mitglied der Helmholtz-Gemeinschaft 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

Mitglied der Helmholtz-Gemeinschaft 18 System Mn 5-x Fe x Si 3 After Songlin et al., J. Alloys Compds 334 (2002) ] x Modestly large MCE ≈ 2.9J/Kg K at ΔB=0-2T T c =299.6(1.0) K No rare earths, no hazardous elements Seems to be stable + Single crystals of sufficient sizes available! For x=4

Mitglied der Helmholtz-Gemeinschaft Crystal Structure of Mn 4 FeSi 3 from literature Space group P6 3 /mcm a=b≈6.8Å, c ≈4.7Å Fe Mn/Fe Si 1∞[FeSi 3 ]1∞[ □ (Mn,Fe) 3 ] Fe 1/3 2/3 0.0 Mn/Fe Si

Mitglied der Helmholtz-Gemeinschaft 20 Temperature dependent X-ray and neutron powder diffraction SPODI, MLZ Garching

Mitglied der Helmholtz-Gemeinschaft 21 Temperature dependent powder diffraction Temperature, K 4K 470K

Mitglied der Helmholtz-Gemeinschaft 22 Temperature dependence of the lattice parameter of MnFe 4 Si 3

Mitglied der Helmholtz-Gemeinschaft Atomic scattering factor of Mn (grey) and Fe (red) Neutron scattering lengths: Mn Fe Si Single crystal x-ray and neutron diffraction X-rays: Reflections h-hl: l=2n+1 are extinct → P6 3 /mcm Neutrons: Reflections h-hl: l=2n+1 are observed → no c-glide plane What is the correct space group?

Mitglied der Helmholtz-Gemeinschaft 24 Crystal structure of MnFe 4 Si 3 P6 3 /mcm P-6 1/3Mn+2/3 Fe Fe 0.37Mn+0.63Fe 0.28Mn+0.72 Fe

Mitglied der Helmholtz-Gemeinschaft 25 Magnetic structure: Single crystal diffraction 200K in ferromagnetic phase Mn/Fe Fe Magnetic space group Pm´ Mn/Fe-position: 1.5(2) μ B

Mitglied der Helmholtz-Gemeinschaft Acknowledgements 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 a Jülich Center for Neutron Science, Research Center Jülich, Germany b Institute for Crystallography, RWTH Aachen University, JCNS Outstation at FRM II, Garching, Germany c Paul Scherrer Institute, Villingen, Switzerland HoNi 2 B 2 C Mn 5-x Fe x Si 3

Mitglied der Helmholtz-Gemeinschaft Crystallography matters!