Identification of Néel Vector Orientation in Antiferromagnetic Domains Switched by Currents in $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ Thin Films

Understanding the electrical manipulation of the antiferromagnetic order is a crucial aspect to enable the design of antiferromagnetic devices working at THz frequencies. Focusing on collinear insulating antiferromagnetic $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ thin films as a materials platform, we identify the crystallographic orientation of the domains that can be switched by currents and quantify the Néel-vector direction changes. We demonstrate electrical switching between different T domains by current pulses, finding that the Néel-vector orientation in these domains is along [$\pm 5$ $\pm 5$ 19], different compared to the bulk $⟨112⟩$ directions. The final state of the in-plane component of the Néel vector ${\mathbf{n}}_{\mathrm{IP}}$ after switching by current pulses $\mathbf{j}$ along the $[1\pm 10]$ directions is ${\mathbf{n}}_{\mathrm{IP}}\parallel \mathbf{j}$. By comparing the observed Néel-vector orientation and the strain in the thin films, assuming that this variation arises solely from magnetoelastic effects, we quantify the order of magnitude of the magnetoelastic coupling coefficient as $b_0+2b_1=3\times {10}^7\mathrm{J}/{\mathrm{m}}^3$. This information is key for the understanding of current-induced switching in antiferromagnets and for the design and use of such devices as active elements in spintronic devices.

Figure 1

(a) X-ray absorption spectrum of $\mathrm{Mg}\mathrm{O}(001)//\mathrm{Ni}\mathrm{O}$(10 nm)/$\mathrm{Pt}$(2 nm) at the $\mathrm{Ni}$ $L_2$ edge for linear vertically (LV) and linear horizontally (LH) polarized light. (b) Device layout and pulsing scheme. The $\mathrm{Pt}$ cross is oriented along the [100] crystallographic axes. The angles defining the linear polarization vector and the Néel vector are defined with respect to the crystallographic axes. (c) The virgin state of the sample and (d) the three-level contrast after applying $j=8.0\times {10}^{11}\mathrm{A}{\mathrm{m}}^{-2}$ along the [100] direction for 1 ms.

Figure 2

(a) The three-level contrast for an incident angle of the beam of $\gamma =-{45}^{\circ }$ and a polarization $\omega =0^{\circ }$. Changing the polarization to (b) $\omega ={40}^{\circ }$ and (c) $\omega ={90}^{\circ }$ changes the relative contrast between the domains. The corresponding XMLD signal for (d) $\gamma ={27}^{\circ }$ and (e) $\gamma =-{45}^{\circ }$ can be fitted including a component stemming from the magnetic signal and an out-of-plane crystal field. (f) The observed domains are T domains along the four [$\pm 5\pm 5$ 19] directions.

Figure 3

XLD images taken at the O-K edge for an incident angle of the beam of $\gamma ={27}^{\circ }$ and (a) linear horizontally and (b) linear vertically polarized light. (c) XMLD image taken on the same sample device at the $\mathrm{Ni}$ $L_2$ edge for $\gamma ={27}^{\circ }$ and linear horizontally polarized light, showing the same domain structure.

Figure 4

The application of current pulses at the current threshold alternating between [100] and [010] (a)–(c) reveals reversible switching of small regions of all three domains. (d) Increasing the current density leads to increased switched regions after a [100] pulse. (e) A perpendicular pulse creates a monodomain state showing saturation, switching back to the three-level state after a pulse along [100] (f). All images are taken with linearly vertical polarized x rays and in all images a background intensity is subtracted.

Figure 5

(a) Device layout and pulsing scheme of the 5-nm $\mathrm{Ni}\mathrm{O}$ film with the $\mathrm{Pt}$ cross oriented 45° to the crystallographic axes. (b) Observed in-plane component of the Néel vector in the three domains of the $\mathrm{Ni}\mathrm{O}$(5 nm)/$\mathrm{Pt}$ sample and current pulse direction. (c) Domain structure of the virgin state for linear horizontally polarized light. The inset shows a small area of the sample at a different incident angle of the x rays and a different polarization angle $(\omega ={45}^{\circ },\gamma =0^{\circ })$, revealing a third small domain (green). (d) and (e) show the domain structure after 1 ms writing pulses of $j=1.0\times {10}^{12}\mathrm{A}{\mathrm{m}}^{-2}$ in two different directions as indicated by the arrows.

Figure 6

(a) 2θ-ω XRD scans of the $\mathrm{Ni}\mathrm{O}$(10 nm)/$\mathrm{Pt}$(2 nm) sample. (b) Symmetric reciprocal space-mapping data around the $\mathrm{Mg}\mathrm{O}$(002) peak. (c) Antisymmetric RSM around the $\mathrm{Mg}\mathrm{O}$(113) peak, showing the same h value for $\mathrm{Ni}\mathrm{O}$ and $\mathrm{Mg}\mathrm{O}$.

Figure 7

(a) Simulation of the XMLD signal for the three different domains shown in Fig. 2 of the main text, when considering only a magnetic contribution to the signal. Note that the crossing points are reproduced, but not the overall trend. (b) By including a crystal-field component, we obtain good agreement between the simulation and the XMLD data (dashed lines).

Figure 8

Applying current pulses in two orthogonal directions shows reproducible switching favoring the in-plane component of the Néel vector to be aligned parallel to the previously applied current pulse. All images are taken with linear horizontally polarized x rays under an azimuthal angle of γ = −45°.

Figure 9

In a $\mathrm{Ni}\mathrm{O}$(10 nm)/$\mathrm{Pt}$(2 nm) sample comprising larger domains one can see that the electric switching favors the in-plane component of the Néel vector to be aligned parallel to the applied pulse. Note that here we show images taken for linear vertically polarized x rays. The approximately horizontal contrast at the center is topographic contrast of nonmagnetic origin.

Figure 10

Equilibrium configuration of the Néel vector $n$ as a function of the magnetoelastic coefficients. (a) Polar angle of the Néel vector at equilibrium. (b) Out-of-plane component of the Néel vector at equilibrium.