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Article Dans Une Revue Physical Review B Année : 2019

Quantitative imaging of hybrid chiral spin textures in magnetic multilayer systems by Lorentz microscopy

K. Fallon
  • Fonction : Auteur
S. Mcvitie
  • Fonction : Auteur
W. Legrand
  • Fonction : Auteur
F. Ajejas
  • Fonction : Auteur
D. Maccariello
  • Fonction : Auteur
S. Collin
  • Fonction : Auteur
V. Cros
  • Fonction : Auteur

Résumé

Chiral magnetic textures in ultrathin perpendicularly magnetised multilayer film stacks with an interfacial Dzyaloshinskii-Moriya interaction have been the focus of much research recently. The chirality associated with the broken inversion symmetry at the interface between an ultrathin ferromagnetic layer and a heavy metal with large spin-orbit coupling supports homochiral Néel domain walls and hedgehog (Néel) skyrmions. Under spin-orbit torques these Néel type magnetic structures are predicted, and have been measured, to move at high velocities. However recent studies have indicated that some multilayered systems may possess a more complex hybrid domain wall configuration, due to the competition between interfacial DMI and interlayer dipolar fields. These twisted textures are expected to have thickness dependent Néel and Bloch contributions to the domain or skyrmion walls. In this work, we use the methods of Lorentz microscopy to measure quantitatively for the first time experimentally both; i) the contributions of the Néel and Bloch contributions and ii) their spatial spin variation at high resolution. These are compared with modelled and simulated structures which are in excellent agreement with our experimental results. Our quantitative analysis provides powerful direct evidence of the Bloch wall component which exists in these hybrid walls and will be significant when exploiting such phenomena in spintronic applications. DOI: xxxxxxxxxx PACS numbers: 75.50.Ak, 75.60.Jk The Dzyaloshinskii-Moriya interaction (DMI) has proved to be of great interest in the study of magnetic materials whereby an antisymmetric exchange interaction causes rotation of neighbouring spins [1,2]. In magnetic thin film systems, this effect is prominent due to the strong spin-orbit coupling arising, in most cases, from the interfacial exchange interaction between the magnetic moments and neighbouring heavy metal atoms in a multi-layer system [3,4]. The rotation of spins is a result of the exchange Hamiltonian due to the DMI, given by H = Dij•(Si×Sj) where Dij is the DMI vector and Si and Sj are neighbouring spin vectors [5]. The interfacial DMI influences the chiral texture of domain walls in such systems so that a chiral Néel wall configuration is energetically favoured over the usual achiral divergence free Bloch rotation of the magnetisation expected in thin films [6,7]. The handedness of this Néel twisting of spins depends on the non-magnetic material (which determines the direction of D) as well as its position with respect to the magnetic layer, i.e. above or below [8,9]. In thin film systems layer combinations such as Pt|Co|Ir have been used to demonstrate chiral magnetic textures with a |D| value up to 2 mJ/m 2 [10]. By creating multilayer repeats of these structures, for specific conditions, homochiral walls can be obtained and skyrmions can be stabilised in all layers-due to the combination of DMI and the influence of dipolar interaction in such PMA systems. Indeed, these magnetic/metallic layer systems are likely to have potential applications in future spintronic based data storage and logic devices which would exploit the efficient manipulation of Néel type domain walls and skyrmions [11-16]. However, recent studies have found the existence of hybrid domain walls in multilayer systems in which the interlayer interactions overcome the DMI [17-19]. This type of wall has a three-dimensional structure where, in some cases, internal magnetic layer(s) have a Bloch wall configuration which is situated between Néel wall surface layer(s) with opposite chirality and results in a flux closure configuration through the thickness of the multilayer. This hybrid structure has been inferred by simulations and imaging of the sense of rotation (or effective chirality) in the top layers through circular dichroism (CD) in x-ray resonant arXiv: 1901.03652
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Dates et versions

hal-03008452 , version 1 (16-11-2020)

Identifiants

Citer

K. Fallon, S. Mcvitie, W. Legrand, F. Ajejas, D. Maccariello, et al.. Quantitative imaging of hybrid chiral spin textures in magnetic multilayer systems by Lorentz microscopy. Physical Review B, 2019, 100 (21), ⟨10.1103/PhysRevB.100.214431⟩. ⟨hal-03008452⟩
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