In some magnetic materials under certain conditions, a stable twisted or whirling configuration of local spins can form. These magnetic configurations are known as chiral spin textures and are of scientific interest both as manifestations of unique topological properties and for their potential use in spintronic devices. When these twisted spin textures form at the surface of a magnetic material as a 2D structure, then they are called skyrmions, named after the theoretical physicist Tony Skyrme. These chiral spin textures can also form in the bulk of a material, where the 3D spin structures are known as Hopfions, named after the German physicist Heinz Hopf for his contributions to the field of topology.
Chiral spin textures do not seem to form in every magnetic material, but are
frequently observed in systems that exhibit the Dzyaloshinskii-Moriya inter-
action (DMI), an anisotropic superexchange interaction present in some an-
tiferromagnets that causes canting of the magnetic moments away from the
Néel vector and manifests as a weak ferromagnetic moment perpendicular to
the Néel vector [1, 2] . Phenomenologically the DMI can be expressed as an
⃗⃗⃗energy term in the spin Hamiltonian as D · (Si × Sj ) [3, 4]. The DMI, and
the accompanying chiral spin textures that often form, make materials that exhibit this interaction especially interesting for magnetic studies. One such material is the antiferromagnet NiF2 [5, 6].
Interestingly, while NiF2 exhibits the DMI, the other antiferromagnetic transi- tion metal fluorides with the same crystal structure as NiF2 (specifically MnF2, CoF2, and FeF2) do not exhibit the DMI. This is a result of the magnetic anisotropy of NiF2, which has a Néel vector that points along either the [100] or [010] crystallographic directions, while the other transition metal fluorides named have a Néel vector that points along the [001] [5]. The presence of the DMI in NiF2 makes it an especially interesting material to study, both alone and in alloys, for the potential to host chiral spin textures such as skyrmions or Hopfions.
[1] A. Fert, N. Reyren, V. Cros, Nature Reviews Materials 2017, 2, Number: 7 Publisher: Nature Publishing Group, 1–15.
[2] N. Kent, N. Reynolds, D. Raftrey, I. T. G. Campbell, S. Virasawmy, S. Dhuey, R. V. Chopdekar, A. Hierro-Rodriguez, A. Sorrentino, E. Pereiro, S. Ferrer, F. Hellman, P. Sutcliffe, P. Fischer, Nature Communications 2021, 12, Number: 1 Publisher: Nature Publishing Group, 1562.
[3] I. Dzyaloshinsky, Journal of Physics and Chemistry of Solids 1958, 4, 241–255.
[4] T. Moriya, Physical Review 1960, 120, Publisher: American Physical Society, 91–98.
[5] T. Moriya, Physical Review 1960, 117, Publisher: American Physical Society, 635–647.
[6] A. Borovik-Romanov, A. Bazhan, N. Kreines, Journal of Experimental and Theoretical Physics 1973, 37, 695–702.
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