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ARTIFICIAL DOUBLE-HELIX FOR GEOMETRICAL CONTROL OF MAGNETIC CHIRALITY

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A study published in ACS Nano demonstrated the imprinting of complex 3D chirality at the nanoscale using state-of-the-art fabrication techniques and magnetic microscopy at MISTRAL beamline of the ALBA Synchrotron. The results prove the possible control of the magnetic configuration with geometrical morphologies displaying 3D chirality and open a new avenue on applied nanomagnetism. The research was the result of a multiple collaboration of scientists from Cambridge, Glasgow and Zaragoza Universities, the ALBA Synchrotron and the Lawrence Berkeley Laboratory.

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Figure: a) 3D-printing of a cobalt nano-helix by FEBID. After injection of Co2(CO)8 into the chamber of a scanning electron microscope (SEM) using a gas injection system (GIS), the focused electron beam (in green and magenta) alternatively exposes the two helix strands. b) Coloured SEM image of the nanostructure under investigation, consisting of two double-helices of opposite chirality joined at the tendril perversion marked *. Scale bar 250nm, c) XMCD image of the double helix studied, which changes geometric chirality at *. Image at zero field, after application of a saturating field H along the axis as indicated. D) XMCD image of the double-helix under study in the as-grown state. Scale bars in c) and d) 200 nm.



Cerdanyola del Vallès, 8th July 2020
  An object is chiral if its image in a mirror cannot bring to coincide with itself as our right and left hands. Chirality plays a major role in nature, for example DNA double helix is a chiral right-handed structure. In magnetism, interactions between spins which are sensitive to chirality generate, in 2D structures with engineered interfaces, complex magnetic configurations as skyrmions that may be of future use in spintronics. In this study, researchers demonstrate the imprinting of complex 3D chirality at the nanoscale using state-of-the-art fabrication techniques and magnetic microscopy at MISTRAL. By fabricating a double helix ferromagnetic structure, magnetic domains were created having the same chirality of the double helix. Moreover, if the geometrical chirality was inverted in the course of the fabrication of the strand, then, the chirality of the magnetic domains was also inverted. At the location were both magnetic chiralities meet, a confined 3D magnetization was evidenced. The ability to create chiral 3D structures with nano patterning enables the control of complex topological magnetic states that might be important for future materials in which chirality provides a specific functionality.

Careful control of the deposition process allowed to fabricate a magnetic double-helix formed of two twisted and overlapping cylindrical nanowires. Figure a) schematically shows the growth of a double helix made of cobalt (Co) nanowires from gas phase Co2(CO)8 molecules (dicobalt octacarbonyl) using focused electron beam induced deposition (FEBID). The chirality of the double helix can be manipulated as shown at the central part of panel b), where the chirality of the helix is inverted at the position marked (*). The magnetic configuration of the double helix is the result of dipolar interaction favouring antiparallel alignment of the magnetization of the wires, exchange interaction which favours parallel alignment and coupling via the geometrically induced chirality.

At MISTRAL beamline, using circularly polarized X-rays and magnetic dichroism at the Co L3 edge, the magnetic configuration of the helix was determined by acquiring images at different orientations to take advantage of the angular dependence of the dichroism.

Figure c) shows the experimentally determined magnetization of the double helix. Each individual wire had a diameter of 85 nm and the length of the strand was 850 nm. Red and blue colours refer to inward and outward magnetization respectively. The image evidences that the magnetization displays the helical structure of the strand and the change of chirality at the location *. This magnetic configuration was obtained at remanence after applying a field along the axial direction. Alternatively, if the images were acquired as grown without any applied field, the magnetic configuration was different. Both wires displayed antiparallel magnetizations as depicted in panel d) indicating the dominating effect of the dipolar interaction.

The results demonstrate the possible control of the magnetic configuration with geometrical morphologies displaying 3D chirality and open a new avenue on applied nanomagnetism.

 

Reference: D. Sanz-Hernández, A. Hierro-Rodríguez, C. Donnelly, J. Pablo-Navarro, A. Sorrentino, E. Pereiro, C. Margén, S. McVitie, J.M de Teresa, S. Ferrer, P. Fischer, A. Fernández-Pacheco. Artificial double-helix for geometrical control of magnetic chirality. ACS Nano 2020. DOI: 10.1021/acsnano.0c00720



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