Intrinsic Mixed Bloch–Néel Character and Chirality of Skyrmions in Asymmetric Epitaxial Trilayers

Abstract

Recent advances on the stabilization and manipulation of chiral magnetization configurations in systems consisting in alternating atomic layers of ferromagnetic and nonmagnetic materials hold promise of innovation in spintronics technology. The low dimensionality of the systems promotes spin orbit driven interfacial effects like antisymmetric Dzyaloshinskii-Moriya interactions (DMI) and surface magnetic anisotropy, whose relative strengths may be tuned to achieve stable nanometer sized magnetic objects with fixed chirality. While in most of the cases this is obtained by engineering complex multilayers stacks in which interlayer dipolar fields become important, we consider here a simple epitaxial trilayer in which a ferromagnet, with variable thickness, is embedded between a heavy metal and graphene. The latter enhances the perpendicular magnetic anisotropy of the system, promotes a Rashba-type DMI, and can sustain very long spin diffusion length. We use a layer-resolved micromagnetic model (LRM) to describe the magnetization textures and their chirality. Our results demonstrate that for Co thickness larger than 3.6 nm, a skyrmion having an intrinsic mixed Bloch-Néel character with counter-clock-wise chirality is stabilized in the entire (single) Co layer. Noteworthy, for thicknesses larger than 5.4 nm, the skyrmion switches its chirality, from counter-clock-wise to clock-wise.

Large Perpendicular Magnetic Anisotropy in Nanometer-Thick Epitaxial Graphene/Co/Heavy Metal Heterostructures for Spin–Orbitronics Devices

“…Concerning the shape anisotropy, as shown in SI Section 9, the SA of ideal (monodomain) fct Gr/Co n /HM films favors in-plane magnetization, that is, negative MAE values, with an energy contribution that evolves almost linearly with n and, therefore, its incidence on the total MAEs would be to even further reduce the PMA value. Nevertheless, we have excluded the SA term in Figure since, in real samples, the existence of multiple domains with different/opposite magnetization directions during magnetization reversal should reduce considerably its contribution.…”

Vortex propagation and phase transitions in a chiral antiferromagnetic nanostripe

“…Starting from the vortex core, the Néel vector goes towards the in-plane direction by rotating in the (13) as well as in the ( 23) plane. This results in a vortex configuration which is between Néel and Bloch near the vortex core, similarly to what happens with Dzyaloshinskii DWs [46] or skyrmions with intermediate chirality [47,48]. The (13) rotation is a consequence of the interfacial DMI, while the (23) rotation is a consequence of the boundary conditions which force the Néel vector to be oriented primarily along ê2 in the far field.…”

Origin of the Large Perpendicular Magnetic Anisotropy in Nanometer-thick Epitaxial Graphene/Co/Heavy Metal Heterostructures

“…Nevertheless, we have excluded the SA term in Fig. 3 since, in real samples, the existence of multiple domains with different/opposite magnetization directions during magnetization reversal 35 should reduce considerably its contribution.…”