Chirality is a phenomenon constantly in front of us [1], as are our hands from which its name originates (χεíρ=hand). It is well known that when inversion symmetry is absent and a structure is non superimposable on its mirror image, a sample absorbs right and left circularly polarised light differently giving rise to natural circular dichroism (NCD). When a magnetised sample interacts with circularly polarised light, another variety of dichroism, magnetic circular dichroism (MCD), can be observed. A chiral magnetised object interacts in a different way with unpolarised light depending on the sample chirality, the magnetisation vector, and the direction of propagation of light. This non-reciprocal phenomenon called magneto-chiral dichroism (MχD) has been observed for the first time only in 1997 with visible light [2] and even more recently with X-rays [3]. This effect is in fact one of the few phenomena that is simultaneously parity odd and time-reversal odd. XMχD has been considered as a possible origin of the homochirality of life on earth, in alternative to the parity-violating electroweak nuclear interactions [1]. There are very few examples of MχD in literature, most of them reporting a very weak effect. Here we report on detection of very strong magneto-chiral dichroism in paramagnetic chiral complexes in the hard X-ray range.

View of the helix formed by cobalt(II)-hexafluroacetylacetonates and nitronyl-nitroxide radicals

Fig. 17: View of the helix formed by cobalt(II)-hexafluroacetylacetonates and nitronyl-nitroxide radicals (Co in blue; methyl groups, fluorine and hydrogen atoms omitted for clarity). The arrows indicate the non-collinear spin structure of the chain. The XANES and the three dichroic signals (XNCD, XMCD, and XMχD) normalised to the XANES intensity at the edge jump, are shown in the four panels respectively (in colour for the depicted helicity, in grey for the other enantiomer).

Two molecular helices crystallising in enantiopure trigonal crystals and comprising cobalt(II) or manganese(II) ions bridged by stable organic radicals (see Figure 17) have been investigated at the K-edge of the transition metal ions by exploiting the unique capabilities of the ID12 beamline. All three dichroic spectra were measured on pairs of opposite enantiomers, using single crystals with a volume of the order of mm3. The results for the cobalt derivatives are reported in Figure 17. By using both enantiomers, we were able to check the change of sign for XNCD and XMχD signals as well as the change in sign on reversal of the magnetic field for XMCD and XMχD. It appears clearly from Figure 17 that the spectral shapes of the three dichroic contributions are very different, the magneto-chiral dichroism being significant only where electric quadrupolar transitions from 1s to 3d orbitals of the cobalt atoms are involved. It is also interesting to notice that the intensity of the magneto-chiral dichroism is indeed larger than the magnetic one, in contrast to the assumption that MχD is weak being the product of two other weak effects (NCD and MCD).

The novelty of the investigation also resides on the possibility of investigating two isostructural helices differing for their magnetic properties. In fact, while the cobalt(II) ions have strong magnetic anisotropy due to the orbital magnetic moments resulting in a non-collinear magnetic helix, the manganese(II) derivative, comprising spin-only d5 ions, is a pure Heisenberg spin chain. For the latter, the same hard X-ray investigation revealed comparable natural and magnetic dichroisms, but a vanishingly small magneto-chiral contribution. The combination of magnetic anisotropy with helical structure resulting in a non-collinear spin arrangement seems to be the key for a strong X-ray MχD.

The XMχD signal detected at the K-edge of the cobalt atom provides information on the orbital toroidal currents (drawn in red), or orbital anapole, while XMCD is indicative of the orbital dipolar moment (in blue)

Fig. 18: The XMχD signal detected at the K-edge of the cobalt atom provides information on the orbital toroidal currents (drawn in red), or orbital anapole, while XMCD is indicative of the orbital dipolar moment (in blue).

The magneto-chiral dichroism has an unusual symmetry, which is odd with respect to both parity and time-reversal symmetry but invariant with respect to their product. It is closely related to the anapole moment, which in this particular case originates from the toroidal orbital currents of the cobalt atoms, as depicted in Figure 18. The more usual orbital currents generating the magnetic moment detected by XMCD are also shown. Orbital toroidal currents are of relevance for many phenomena, ranging from multiferroicity to superconductivity [4], while the combination of chirality and magnetism could be a viable route to store information in protected magnetic structures like skyrmions [5].

 

Principal publication and authors
R. Sessoli (a), M.-E. Boulon (a),  A. Caneschi (a), M. Mannini (a),  L. Poggini (a), F. Wilhelm (b) and  A. Rogalev (b), Nature Phys. 11, 69–74 (2015).
(a) Department of Chemistry ‘University of Florence (Italy)
(b) ESRF

References
[1] G.H. Wagnière, On Chiralty and the Universal Asymmetry. (Verlag Helvetica Chimica Acta, Zürich, 2007).
[2] G. Rikken and E. Raupach, Nature 390, 493-494 (1997).
[3] J. Goulon, A. Rogalev, F. Wilhelm,  C. Goulon-Ginet, P. Carra, D. Cabaret and C. Brouder, Phys. Rev. Lett. 88, 237401 (2002).
[4] V. Scagnoli, U. Staub, Y. Bodenthin, R.A. de Souza, M. García-Fernández,  M. Garganourakis, A.T. Boothroyd,  D. Prabhakaran and S.W. Lovesey, Science 332, 696-698 (2011).
[5] S. Mühlbauer, B. Binz, F. Jonietz,  C. Pfleiderer, A. Rosch, A. Neubauer,  R. Georgii and P. Böni, Science 323, 915-919 (2009).