Chirality switching of an antiferromagnetic spiral wall and its effect on magnetic anisotropy

An antiferromagnetic NiO spiral wall in $\mathrm{Fe}/\mathrm{NiO}/\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}/\mathrm{vicinal}$ Ag(001) was created by rotating Fe magnetization and investigated using x-ray magnetic linear dichroism (XMLD). Different from the Mauri's 180° spiral wall, we find that the NiO spiral wall always switches its chirality at $\sim {90}^{\circ }$ rotation of the Fe magnetization, and unwinds the spiral wall back to a single domain with a further rotation of the Fe magnetization from 90° to 180°. The effect of this chirality switching on the magnetic anisotropy was studied using rotational magneto-optic Kerr effect (ROTMOKE) on $\mathrm{Py}/\mathrm{NiO}/\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}/\mathrm{vicinal}$ Ag(001). We find that the original Mauri's model has to be corrected by an energy folding due to the chirality switching, which consequently converts the exchange bias from the Mauri's 180° spiral wall into a uniaxial anisotropy and a negative fourfold anisotropy.

Figure 1

(a) Schematic drawing of the sample and XMLD measurement. (b) LEED patterns of Fe(1.5 nm)/NiO(10 nm)/$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$(8 nm)/vicinal Ag(001). XAS from Fe(1.5 nm)/$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$(8 nm)/vicinal Ag(001) with the x-ray polarization parallel ($\overleftrightarrow{E}//x$) and perpendicular to steps ($\overleftrightarrow{E}//y$) taken from (c) and (d) the Co $L_3$ edge and (e) and (f) the Ni $L_2$ edge. (g) Both the Co $L_3$ ratio and the Ni $L_2$ ratio exhibit ${cos}^2\varphi$ dependence on the x-ray polarization angle ($\varphi$) at $T=300$ K, showing that both Co and Ni spins in $\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$ are along the $y$ axis. (h) Both Co $L_3$ ratio and Ni $L_2$ ratio differences vanish at 430 K, showing $T_{\mathrm{N}}=430$ K for the 8 nm $\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$ film.

Figure 2

Result from Fe(1.5 nm)/NiO($d_{\mathrm{NiO}}$)/$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$(8 nm)/vicinal Ag(001). Fe hysteresis loops for (a) and (g) $\vec{H}//x$ and (d) and (i) $\vec{H}//y$ show that the Fe and NiO spins are coupled perpendicularly. By aligning the Fe magnetization along the $x$ and $y$ axis, (b) and (e) Co XAS and (c) and (f) Ni XAS show that both the $\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$ and the NiO AFM spins in $d_{\mathrm{NiO}}=3$ nm samples are frozen to the $y$ axis. (h) and (j) Ni XAS from the $d_{\mathrm{NiO}}=16.5$ nm sample show that the NiO AFM spins are twisted into a spiral wall after the rotation of Fe magnetization from the $x$ axis to the $y$ axis.

Figure 3

(a) Schematic drawing of the XMLD measurement. (b)–(e) NiO $\mathrm{\Delta }{R}_{L2}$ as a function of the Fe magnetization rotation angle (${\phi }_{\mathrm{FM}}$). The solid and dashed lines are calculated XMLD for right-handed and left-handed spiral walls, respectively. (f) Schematic drawing of the NiO AFM spiral walls in three $d_{\mathrm{NiO}}$ regions. (g) NiO spiral angle as a function of $d_{\mathrm{NiO}}$, where ${\phi }_{\mathrm{AF}}^{\max }$ is the NiO spin rotation angle at Fe/NiO interface at ${\phi }_{\mathrm{FM}}={90}^{\circ }$.

Figure 4

(a) Experiment geometry for ROTMOKE measurement. (b) Magnetic torque as a function of Py magnetization angle (${\phi }_{\mathrm{FM}}$) from Py(25 nm)/NiO($d_{\mathrm{NiO}}$)/$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}$(8 nm)/vicinal Ag(001). Solid lines are fitting results using Eq. (2). (c) Py uniaxial and fourfold anisotropies as a function of $d_{\mathrm{NiO}}$.