X-ray resonant magnetic scattering study of magnetization reversals in a nanoscale spin-valve array

We present an x-ray resonant magnetic scattering study that uses the periodicity of a patterned array of trilayer $(\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Ni}\mathrm{Fe})$ elements to determine not only layer-dependent magnetic hysteresis, but, more importantly, to extract the magnetization reversal in different sections of the picture-frame-shaped structure. Spatially resolved and layer-resolved magnetization measurements have revealed that magnetic switching mechanism is very distinct in different regions of the structure and results from a balancing of the shape anisotropy and strong interlayer dipolar coupling. These results demonstrate how spatially averaged measurements are not sufficient to resolve the nature of the reversal mechanism within the structure.

Figure 1

(Color online) (a) Measured (circles) and calculated (solid lines) diffraction intensities of the sample rocking scan along the $q_x$ direction at a fixed detector position $(q_z\simeq 0.904{\mathrm{nm}}^{-1})$ and the $\mathrm{Ni}{L}_3$ edge. Insets: SEM micrograph and schematics of the experimental setup. Circularly polarized (CP) x rays were used to probe the sample magnetization along the applied field $H$. (b) The differences $(I^{+}-I^{-})$ of the specularly reflected intensities at the zeroth order measured at the Co and $\mathrm{Ni}{L}_3$ edges. The difference intensities are normalized by the saturation amplitudes.

Figure 2

(Color online) The differences $(I^{+}-I^{-})$ of the diffraction intensities measured at the different diffraction peaks (indicated by the numbers in Fig. 1) and at the (a) Co and (b) $\mathrm{Ni}{L}_3$ edges. Inset: Calculated diffraction amplitudes at the first (red line) and second (blue line) diffraction orders are superimposed over the parallel [red (dark gray)] and perpendicular [blue (light gray)] sides of the picture frame with respect to the beam direction, which is the same as the field direction here.

Figure 3

(Color online) [(a)–(c)] Magnetization reversal behaviors for the three subcells extracted from the hysteresis loops measured at three diffraction orders. The Co and NiFe layer magnetization reversal behaviors are shown separately. (d) The calculated magnetization loops obtained by averaging all extracted loops of the three subcells are shown and in agreement with the loops measured by the specular reflectivity in Fig. 1b.

Figure 4

(Color online) Magnetization reversal mechanism in the ${\mathrm{M}}_2$ region (the sides of the picture frame perpendicular to the field direction). The negative-to-positive curves of the magnetization reversals extracted for the perpendicular sides (${\mathrm{M}}_2$ subcell in Fig. 3) of the picture frames with respect to the field direction are shown for the Co (black line) and NiFe (red line) layers. The NiFe curve shows an inverted loop. The insets show schematics of how the dipolar field (broken lines with arrows) from the Co layer acts on the NiFe moments through the sides of the picture frames. The solid-line arrows in the insets represent the Co (black arrows) and NiFe (red arrows) moments, respectively.

Figure 5

(Color online) (a) The net field $H_{\text{net}}^{\mathrm{Ni}\mathrm{Fe}}$ (blue line) for the NiFe layer is obtained by subtracting the demagnetization field $H_{\mathrm{demag}}$ (red line), calculated from the magnetization of the Co layer, from the applied field $H_{\text{applied}}$. (b) The actual magnetization reversal curve under the net magnetic field for the NiFe layer in the ${\mathrm{M}}_2$ region is shown. This simple S-shaped loop can be fitted by the Stoner-Wohlfarth coherent rotation model (blue line).