Fully Compensated Synthetic Antiferromagnets with Pronounced Anomalous Hall and Magneto-Optical Responses

We report the realization of fully compensated synthetic antiferromagnets (SAFs) in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}/{\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ multilayers with perpendicular magnetic anisotropy, pronounced anomalous Hall resistances and magneto-optical responses. In particular, the properties of SAFs are systematically investigated through optimizing the thicknesses of the $\mathrm{Co}$ layer, the $\mathrm{Ru}$ spacer, and the concentration (x) of the ferrimagnet (FIM) ${\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ layers. The incorporation of the FIM ${\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ films into SAFs accelerates the search for a magnetic multilayer with fully compensated magnetism and exhibiting pronounced anomalous Hall resistance and magneto-optical signals. These advantages enable spin-orbit-torque switching and magneto-optical Kerr effect imaging experiments on the fully compensated SAFs to be readily investigated. Through incorporating FIMs into SAFs, we believe that the present material system could provide a promising platform not only for adequately addressing the fully compensated spin dynamics of SAFs at room temperature, but also for developing intriguing SAF-based spin-orbitronic devices.

Figure 1

(a) Schematic illustration of the film stack of SAFs: $\mathrm{FM}/\mathrm{Ru}/\mathrm{FM}$ and $\mathrm{FM}/\mathrm{Ru}/\mathrm{FIM}$ multilayers. Top FM layer will be replaced by a FIM layer in the present study (lower panel). Magnetic hysteresis loops (top panels) and AHE loops (bottom panels) of $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}(3)$ multilayer (b), $\mathrm{Ru}(3)/(\mathrm{Fe}$-dominated)${\mathrm{Fe}}_{0.82}{\mathrm{Tb}}_{0.18}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ multilayer (c), and $\mathrm{Ru}(3)/(\mathrm{Tb}$-dominated)${\mathrm{Fe}}_{0.61}{\mathrm{Tb}}_{0.39}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ multilayer (d). Blue arrow denotes the magnetization orientation of the $\mathrm{Co}$ layer (${\overrightarrow{M}}_{\mathrm{Co}}$). Dark- and light-green arrows represent the antiparallel magnetization orientation of $\mathrm{Tb}$ (${\overrightarrow{M}}_{\mathrm{Tb}}$) and $\mathrm{Fe}$ (${\overrightarrow{M}}_{\mathrm{Fe}}$) elements, respectively.

Figure 2

Magnetic hysteresis loops (top panels) and anomalous Hall effect loops (bottom panels) of $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}($$t_{\mathrm{Ru}}$)/${\mathrm{Fe}}_{0.61}{\mathrm{Tb}}_{0.39}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ samples with three different $\mathrm{Ru}$ spacer thicknesses ($t_{\mathrm{Ru}}$) of 0.7 nm (a), 1.1 nm (b), and 2.0 nm (c). Dark and light colors represent ascending and descending branches, respectively, of the whole sweeping loop, which is indicated by the dotted line arrows in the top panel of (b). Four different magnetization states are also schematically marked in the bottom panel of (b). Blue arrows denote the magnetizations, ${\overrightarrow{M}}_{\mathrm{Co}}$, of the $\mathrm{Co}$ layer. Dark- and light-green arrows represent antiparallel magnetizations, ${\overrightarrow{M}}_{\mathrm{Tb}}$ and ${\overrightarrow{M}}_{\mathrm{Fe}}$, of the FIM ${\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x$ layers, respectively. (d) Antiferromagnetic exchange-coupling field, $H_{\mathrm{AFEC}}$, as a function of $t_{\mathrm{Ru}}$.

Figure 3

(a) Hysteresis loops (top panels) and AHE loops (bottom panels) of the $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}(1.1)/{\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ SAFs with varying $\mathrm{Tb}$ concentration (x = 0.39, 0.35, 0.31, and 0.26). (b) Net magnetization, ${\overrightarrow{M}}_{\mathrm{net}}$, at zero external field as a function of $\mathrm{Tb}$ concentration (x). Dotted line is zero net magnetization of the fully compensated SAF (${\overrightarrow{M}}_{\mathrm{net}}=0$). (c) Evolution of the antiferromagnetic exchange-coupling fields, $H_{\mathrm{AFEC}}$, as a function of $\mathrm{Tb}$ concentration (x) for $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}(1.1)/{\mathrm{Fe}}_{1-x}{\mathrm{Tb}}_x(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ SAFs.

Figure 4

Perpendicular magnetic hysteresis loops of $\mathrm{Pt}(3)/\mathrm{Co}(t_{\mathrm{Co}})/\mathrm{Ru}(1.1)/{\mathrm{Fe}}_{0.61}{\mathrm{Tb}}_{0.39}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ SAFs with varying $\mathrm{Co}$-layer thickness ($t_{\mathrm{Co}}$). Notably, $\mathrm{Co}$ layer of 1.4 nm exhibits weak PMA compared with $\mathrm{Co}$ layers of 0.8, 1.0, and 1.2 nm. Net magnetization of $\mathrm{Pt}(3)/\mathrm{Co}(1.2)/\mathrm{Ru}(1.1)/{\mathrm{Fe}}_{0.61}{\mathrm{Tb}}_{0.39}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$ SAF vanishes and the fully compensated SAF can be achieved through optimizing the thickness of the $\mathrm{Co}$ layer.

Figure 5

Hysteresis loops from polar MOKE magnetometry, in which Kerr rotation, ${\theta }_K$, as a function of perpendicular magnetic field ($H_z$) is measured in multilayers of (a) $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}(3)$, (b) $\mathrm{Ru}(3)/{\mathrm{Fe}}_{0.61}{\mathrm{Tb}}_{0.39}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$, and (c) $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}(1.1)/{\mathrm{Fe}}_{0.61}{\mathrm{Tb}}_{0.39}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$.

Figure 6

(a) Optical image of the Hall bar device made from the fully compensated SAF of stacking order $\mathrm{Pt}(3)/\mathrm{Co}(1)/\mathrm{Ru}(1.1)/{\mathrm{Fe}}_{0.65}{\mathrm{Tb}}_{0.35}(6.5)/{\mathrm{Si}}_3{\mathrm{N}}_4(5)$. (b) Current-induced SOT-switching measurements performed with in-plane fields of $\pm 1\mathrm{kOe}$. Two switched states are two fully compensated antiferromagnetic configurations, as schematically shown in Fig. 2. (c) Switching phase diagram acquired at different in-plane fields, $H_x$. (d) Polar MOKE images at subsequent current pulses during SOT switching with in-plane field of $-1\mathrm{kOe}$ (along the current direction). Isolated SAF magnetic bubbles are indicated by yellow dotted circles in (d).