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Chirality magnetic windings observed

Chirality is one of the fascinating properties of nature, discovered in 1848 by Pasteur in molecules. An object is chiral when it cannot be superimposed over its own image in a mirror, like our hands which are a perfect example at human scale. At atomic scale, chirality generally arises from symmetry breaking, a situation commonly encountered at surfaces and interfaces. As a consequence, in ultrathin films where the surfaces contribution dominates over the volume one, an increased effect can be expected.

Recently, ultrathin ferromagnetic films deposited on high spin-orbit layers have indirectly shown chiral effects : experiments on domain wall dynamics could only be reproduced and explained via the chiral interaction of Dzyaloshinskii-Moriya. However, a more perfect proof requires an image of the domain walls, which should be chiral (i.e. with a unique sense of rotation). This is a real challenge as it requires a resolution better than 10 nm so that up to now, only model systems (monocristalline samples at low temperature) different from those used in dynamics experiments could be observed with existing techniques. Our study, involving several groups of university Paris-Sud (LPS, Laboratoire Aimé Cotton and Institut d’Electronique Fondamentale), from Grenoble (Spintec, CEA) and an industrial partner (Singulus Technology AG), has shown the ability of a new imaging technique, without imposing strong constrains on the sample quality.

Recently, a collaboration with the research group of Vincent Jacques from Laboratoire Aimé Cotton, allowed the development of a new imaging technique based on scanning probe magnetometry, via the photoluminescence of atomic defects (ia nitrogen atom adjacent to a vacancy) in nano diamonds. The idea is to move at a given height the diamond attached to the tip of an atomic force microscope and measure the magnetic field arising from the sample. The high sensitivity (10 µT.Hz-1/2) and excellent spatial resolution (the « field sensor » consists of two atoms only) of this technique are key advantages to study narrow domain walls. Comparing experimental field maps with calculations, we have tested several structures of domain walls. Depending on the sample, we have shown the absence of chirality in Ta/CoFeB (1 nm thickness)/MgO and a strongly pronounced chirality in Pt/Co (0.6 mn thickness)/Al2O3, which indicates a stronger chiral interaction for magnetic layers in contact with platinum than with tantalum. These results are in perfect agreement with dynamic measurements and with the first « ab-initio » calculations on the interface induced Dzyaloshinskii-Moriya interaction.

Figure : Image of a chiral domain wall in cobalt ultrathin film (0.6 nm thick) sandwiched between platinum and alumina layers. (a) Atomic force microscope (AFM) image of the nanowire at the domain wall position. (b)Magnetic field map (proportional to the Zeeman shift) in the vicinity of the domain wall. (c) Comparison between measurements and simulations (non chiral Bloch domain wall, left and right chiral Néel domain wall).

Reference :
The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry
J.-P. Tetienne, T. Hingant, L.J. Martinez, S. Rohart, A. Thiaville, L. Herrera Diez, K. Garcia, J.-P. Adam, J.-V. Kim, J.-F. Roch, I.M. Miron, G. Gaudin, L. Vila, B. Ocker, D. Ravelosona et V. Jacques
Nature Communications 6, 6733 (2015).

Contact :
Stanislas Rohart