Virginie Simonet Institut Néel, CNRS & Université Grenoble Alpes, Grenoble, France

1. Neutrons and chirality Virginie Simonet Institut Néel, CNRS & Université Grenoble Alpes, Grenoble, France

2. Introduction to chirality  Casualdefinition of chirality (handedness): what distinguishes a phenomenon from its materialization in a mirror (or through an inversion center) Χειρ Hand in Greek • Called « dissymetry » by Pasteur (1848) • He succeeded in separating the two enantiomorphs of paratartaric acid • Word « chirality » proposed by lord Kelvin (1904)

3. Introduction to chirality •  Chirality concept pervades in modern science • Physics of elementary particle • Chemistry • Homochirality of life • Biological forms • Pharmacology … Nobel prize in physics 1957 (chiral weak interaction) Nobel prize in chemistry 2001 (chiral catalysis) Canonical example : helix Reversal of the screw sense Chiral molecules Neutrinos with left helicity

4. Introduction to chirality  Structural chirality : some crystals without inversion center or mirror. Two enantiomorphs give different signature (anomalous x-ray scattering, optics)

5. Introduction to chirality • Extended definition of chirality in Magnetism • Sense of rotation of non collinear spins along an orientated line Introduced by Jacques Villain 1977 (CEA-Grenoble)

6. Introduction to chirality Complex forms of magnetic states due to frustrated magnetic interactions  magnetic chirality • Example of a triangle of magnetic moments antiferromagnetically interacting • Example of complex chiral magnetic textures: • skyrmions • Helical arrangement

7. Introduction to chirality Physics of chirality in spin networks  Potential roads toward renewal of technology of information • Multiferroics: electric polarization P related to magnetic chirality • Chirality domains and electric polarization switchable by electric/magnetic fields • Multifunctional materials • Skyrmions: new media for encoding information P P

8. Neutrons probe of magnetism and chirality Neutron probe of magnetism • Antiferromagnetism Predicted by Louis Néel in 1936 (Nobel Prize) • Demonstrated by neutron diffraction in MnO • Clifford Shull et al. 1951 (Nobel Prize) Transition temperature TN=116 K T<TN T>TN antiparallel magnetic moments

9. Neutrons probe of magnetism and chirality Neutron probe of magnetism Magnetic scattering thanks to the spin (S) of the neutron  Interference phenomena with (diffraction) or without (inelastic scattering) conservation of energy

10. Neutrons probe of magnetism and chirality Neutron probe of chirality Using polarized neutron beam (with P=<S> ≠ 0) ou SF NSF Pi Pf

11. Neutrons probe of magnetism and chirality Neutron probe of chirality Using polarized neutron beam (with P=<S> ≠ 0) Blume-Maleyev equations: probe chiral scattering Dedicated instrumentation CRYOPAD

12. Neutrons probe of magnetism and chirality Neutron probe of magnetic excitations Magnetic excitations (spin waves) provide crucial information on the microscopic parameters driving the magnetic properties of a system Dispersion relation Energy Spin waves in antiferromagnet reciprocal space

13. A case study, the Fe langasite: Structure Crystal growth Ba3NbFe3Si2O14, non-centrosymmetric chiral structure Fe3+ magnetic ions

14. A case study, the Fe langasite: Magnetic structure Neutron diffraction @ILL on powder on crystal and usingpolarized neutrons D1B, D15, D23, IN22

15. A case study, the Fe langasite: Magnetic structure Neutron diffraction @ILL on powder on crystal and usingpolarized neutrons D1B, D15, D23, IN22 • Magnetic order below transition temperature TN=28 K • Triangles of magnetic moments in plane • Magnetic helices propagating along c

16. A case study, the Fe langasite: Magnetic structure 4 possible chirality states: Polarized neutrons Unique magnetic chirality = unique sense of rotation of the magnetic moments

17. A case study, the Fe langasite: Magnetic excitations Inelastic neutron scattering@ILLon crystaland usingpolarized neutrons, IN5, IN12, IN20, IN22 Dispersion relation (a) Experiment Energy [meV] [0 -1 ]

18. A case study, the Fe langasite: Magnetic excitations Inelastic neutron scattering@ILLon crystaland usingpolarized neutrons, IN5, IN12, IN20, IN22 Dispersion relation (a) Experiment Calculation Energy [meV] Energy [meV] [0 -1 ] [0 -1 ] Identification of the ingredients producing the unique chiral properties and of the relation between structure and magnetism Chiral spin waves: dynamical fingerprint of chirality

19. A case study, the Fe langasite: chiral phase transition TN Temperature Antiferromagnetism Paramagnetism

20. A case study, the Fe langasite: chiral phase transition TN Temperature Antiferromagnetism Paramagnetism Chiral phase transition in magnetic systems predicted by Kawamura (1998) The chiral spatial and temporal fluctuations diverge at the phase transition towards the ordered state Phase transition driven by the magnetic chirality

21. Conclusion • Non-centrosymmetricBa3NbFe3Si2O14langasite, • a model system • remarkable chiral static and dynamical properties: unique sense of rotation of the magnetic moments • Giving rise to interesting properties (multiferroicity, magnetoelectrics) • First direct evidence of phase transition driven by the magnetic chirality • Probed by neutron scattering (elastic, inelastic, using polarized neutrons) • = unique tool for understanding of magnetic chirality

22. Collaborations: K. Marty, M. Loire, L. Chaix, E. Constable, R. Ballou, P. Bordet, S. deBrion, P. Lejay, C. V. Colin (NEEL), • E. Ressouche, L.-P. Regnault (INAC-MEM-MDN, GEA-Grenoble), • S. Petit (LLB, CEA-Saclay), • J. Ollivier, M. Enderle, P. Steffens, L. C. Chapon (ILL), • V. Scagnoli (PSI/ETH, Switzerland), A. Zorko (Jozef Stefan Institute, Slovenia), • Cano (ICMCB, Bordeaux) • Instruments at the ILL: D1B, D23, D15, IN5, IN12, IN20, IN22