Effect of microstructure on magnetic properties and anisotropy distributions in Co/Pd thin films and nanostructures

The structure of Co/Pd multilayers has a strong effect on the localized anisotropy distribution within a film and on the resulting switching properties of nanostructures fabricated from identical material. By varying the underlying seed layer in sputtered films, the microstructure can be controlled from being highly (111) textured to having a random grain orientation. We find a strong correlation between the lateral homogeneity of grain orientations and the localized anisotropy distribution in the material. X-ray diffraction and reflectivity indicate that the interface is better defined and more uniform in the textured case, consistent with the presence of a strong interfacelike anisotropy.

Figure 1

(Color online) (a) SEM micrograph of the $t_{\text{Co}}=0.29\text{nm}$, Pd seed sample showing nominal 50-nm-diameter nanostructures. (b) Overlaid hysteresis and remanent hysteresis curves showing identical results with either technique. (c) Hysteresis curves and resulting differential curves for various diameter structures. The lines through the differential data are Gaussian distribution fits used to extract values of $H_{sf}$ and $\sigma$.

Figure 2

Cross-section HR-TEM micrographs of: (a) Pd seed and (b) Ta seed multilayer film. Higher magnification images are included (right) as well as the corresponding Fourier transforms of the micrographs (insets).

Figure 3

(Color online) Specular XRD spectra and best fits for: (a) Pd seed and (b) Ta seed multilayer films. The ideal locations of the (111) Pd and (111) Co peaks are indicated by the vertical lines.

Figure 4

(Color online) AFM images of: [(a) and (b)] Ta seed and [(c) and (d)] Pd seed multilayer films. The (e) grain size and (f) rms roughness are also plotted as a function of Co layer thickness.

Figure 5

(Color online) XRR spectra and best fits for: (a) Pd seed and (b) Ta seed multilayer films. The scattering profile generated by the best fits are included as insets to the respective spectra.

Figure 6

(Color online) Typical SQUID in-plane magnetization curve showing how the saturation moment and effective anisotropy field are determined.

Figure 7

(Color online) (a) Saturation magnetization as a function of inverse Co layer thickness for Pd seed (○) and Ta seed (▲) samples. A linear fit to the combined data is included which was used to determine the amount of Pd magnetic polarization. An ideal (b), and more realistic (c), schematics of the magnetic moment distribution with magnetic polarization of the interfacial Pd atoms are also shown.

Figure 8

(Color online) Plot of anisotropy energy density as a function of the inverse Co layer thickness for Pd seed (○) and Ta seed (▲) samples. A linear fit to the Ta data is included, which is used to separate the $K_{bulk}$ and $K_{int}$. The three values of $t_{\text{Co}}$ that were patterned into nanostructures are indicated in the figure by vertical lines.

Figure 9

(Color online) Plots of $H_{sf}$ as a function of nominal nanostructure diameter for (a) $t_{\text{Co}}=0.29\text{nm}$, (b) $t_{\text{Co}}=0.33\text{nm}$, and (c) $t_{\text{Co}}=0.45\text{nm}$ samples, as well as plots of SFD as a function of nominal nanostructure diameter for (d) $t_{\text{Co}}=0.29\text{nm}$, (e) $t_{\text{Co}}=0.33\text{nm}$, and (f) $t_{\text{Co}}=0.45\text{nm}$ samples with both Pd seed (○) and Ta seed (▲).