Perpendicular Interlayer Coupling in ${\mathrm{N}\mathrm{i}}_{80}{\mathrm{F}\mathrm{e}}_{20}/\mathrm{N}\mathrm{i}\mathrm{O}/\mathrm{C}\mathrm{o}$ Trilayers

An in-plane perpendicular magnetic coupling between ${\mathrm{N}\mathrm{i}}_{80}{\mathrm{F}\mathrm{e}}_{20}$ and Co has been found in $\mathrm{N}\mathrm{i}\mathrm{F}\mathrm{e}/\mathrm{N}\mathrm{i}\mathrm{O}/\mathrm{C}\mathrm{o}$ trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field $H_{\mathrm{C}}$ increases when the thickness of the NiO layer $t_{\mathrm{N}\mathrm{i}\mathrm{O}}$ increases. Because of the coupling, $H_{\mathrm{C}}$ is always larger than for $\mathrm{N}\mathrm{i}\mathrm{O}/\mathrm{C}\mathrm{o}$ bilayers with the same thicknesses. The saturation field of the NiFe layer $H_{\mathrm{S}}$ decreases when $t_{\mathrm{N}\mathrm{i}\mathrm{O}}$ increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed $90°$ coupling.

Figure 1

Room temperature MOKE magnetization curves $M(H)$ of a 10 nm $\mathrm{N}\mathrm{i}\mathrm{F}\mathrm{e}/8\mathrm{n}\mathrm{m}\mathrm{N}\mathrm{i}\mathrm{O}\mathrm{/}\mathrm{2}\mathrm{n}\mathrm{m}\mathrm{C}\mathrm{o}$ trilayer film with $H$ parallel to the Co easy axis. (a) $M_{\mathrm{\parallel }}$ major (solid line) and minor loops (symbols), (b) $M_{\mathrm{\parallel }}$ minor loops, (c) $M_{\mathrm{\perp }}$. The inset shows the overall geometry for the growth process. The NiO plane of incidence (gray plane) is $55°$ off with respect to the normal sample surface (dark gray). The arrow indicates the field direction in the present experiment.

Figure 2

(a) Layer selective hysteresis curves obtained with XMCD for Co (open circles, right-hand y axis) and NiFe (filled squares, left-hand y axis) with the field applied parallel (a) and perpendicular (b) to the Co easy-axis for a 10 nm $\mathrm{N}\mathrm{i}\mathrm{F}\mathrm{e}/8\mathrm{n}\mathrm{m}\mathrm{N}\mathrm{i}\mathrm{O}/2\mathrm{n}\mathrm{m}\mathrm{C}\mathrm{o}$ trilayer.

Figure 3

NiO thickness dependence of the Co coercive field $H_{\mathrm{C}}$ (circles, left-hand y axis) and NiFe saturation field $H_{\mathrm{S}}$ (squares, right-hand y axis). The data are taken from $M(H)$ curves with the field applied parallel to the Co easy-axis. The dotted line is a guide for the eyes in $H_{\mathrm{C}}(H)$ whereas the solid line represents a fit to the $H_{\mathrm{S}}(H)$ data with the inverse of the NiO thickness (see text).

Figure 4

Cross-section views of the simulated spin configurations of a $\mathrm{F}\mathrm{e}\mathrm{N}\mathrm{i}/\mathrm{N}\mathrm{i}\mathrm{O}/\mathrm{C}\mathrm{o}$ trilayer film at saturation (a) and zero field (b). The right-hand panels are magnetizations of the marked areas. The parameters in the simulation are $K_{\mathrm{N}\mathrm{i}\mathrm{F}\mathrm{e}}=0$, $K_{\mathrm{C}\mathrm{o}}=10\times K_{\mathrm{N}\mathrm{i}\mathrm{O}}$ (see text). The gray scale denotes the angle of the spins with respect to the field direction, from $0°$ (white) to $\pm 90°$ (black). Note that both AF spin sublattices are shown.