Noncollinear Remanent Textures Induced by Surface Spin Flop in Synthetic Antiferromagnets with Perpendicular Anisotropy

The surface spin flop, observed in synthetic antiferromagnets (SAFs) with uniaxial anisotropy and strong antiferromagnetic (AF) interlayer exchange coupling, can be considered as a laterally homogeneous, vertical AF domain wall pushed into the SAF from either the top or the bottom in the presence of a strong external vertical magnetic field. As a result, the AF domain wall can be described as a one-dimensional entity. In this work, we present a concept to stabilize laterally homogeneous vertical AF domain walls by local variation of the perpendicular magnetic anisotropy in $\mathrm{Co}/\mathrm{Pt}$-based SAFs. Our approach not only allows the stabilization of the vertical AF domain wall in the absence of any external magnetic field, but furthermore enables a deterministic selection among four different remanent states, each one stable within a broad external magnetic field range of almost one tesla. We also demonstrate an extension to our concept by stabilizing two coexisting vertical AF domain walls, thus yielding a system with a total of six different selectable (and reprogrammable) remanent states. The controlled stabilization of noncollinear AF textures in the form of vertical AF domain walls at remanence could be used as an infrastructure for propagating spin waves within the AF domain wall itself, as well as for tuning the dynamic behavior of perpendicular standing spin wave modes existing vertically across the SAF.

Figure 1

Left: Enlarged views of the layering within the different sections of the multilayer. Center: True-to-scale sketch of the nominal layer stack. The thin purple layer at the bottom represents the $1.5\mathrm{nm}$ $\mathrm{Ta}$ adhesion layer, while the large gray blocks at the bottom and top represent the $20\mathrm{nm}$ $\mathrm{Pt}$ seed layer and the 3 nm $\mathrm{Pt}$ capping layer, respectively. The modified $\mathrm{Co}/\mathrm{Pt}$ blocks with reduced individual layer thicknesses are highlighted in red. Right: Cross-section TEM of the SDW sample. The top end was cut off during TEM sample preparation.

Figure 2

(a) Low-field regime of the OOP magnetization curves simulated by MuMax3 (blue), measured by SQUID-VSM (dark green) and MOKE (yellow), both at room temperature. The gray curve shows a minor loop from positive saturation to −0.4 T and back. For better visibility, only the ascending field branch, beginning at −0.3 T is displayed. The arrows indicate the direction of the field sweep. The bars at the top illustrate the stability ranges of the three states in the descending major and ascending minor loops as indicated by the labels (0 DW = uniform AF = UAF, 1 DW = single domain wall = SDW) and the arrows. (b) Reference SQUID-VSM OOP magnetization curve of the unmodified N = 20 sample extracted from Ref. [25]. (c) Full reversal view of the magnetization curves shown in (a) up to ±3 T. The MOKE data is scaled vertically to match to the SQUID-VSM data at high fields. (d) Left: Cross-section view of the magnetic states extracted from the simulation at the respectively numbered points in (a). (d) Right: Depth profile of the normalized PMA constant $\kappa$ averaged over the FM $\mathrm{Co}/\mathrm{Pt}$ subunits.

Figure 3

(a) Low-field regime of MuMax3 simulated major loop (blue), and minor loops between positive saturation and −0.25 T (gray) or −0.35 T (yellow) magnetization curves, respectively. (b) Low-field regime of SQUID-VSM measured major loop (dark green), and minor loops between positive saturation and −0.50 T (gray) or −0.57 T (yellow) magnetization curves, respectively. All three curves are measured at 4 K. The bars at the top illustrate the stability ranges of the three states in the descending major and ascending minor loops as indicated by the labels (0 DW = uniform AF = UAF, 1 DW = single-domain wall = SDW, 2 DW = double-domain wall = DDW) and the arrows. Arrows in (a) and (b) indicate the field sweep direction. (c) Reference room temperature SQUID-VSM loop of the unmodified N = 21 SAF extracted from Ref. [25]. The gray bars indicate the stability region for the two remanent AF states. (d) and (e) show full reversal views of (a) and (b), respectively. (f) Left: Cross-section view of the magnetic states extracted from the simulation at the respectively numbered points in (a). Numbers in (b) highlight the equivalent states from (a). (f) Right: Depth profile of the normalized PMA constant $\kappa$ averaged over the FM $\mathrm{Co}/\mathrm{Pt}$ blocks.