Perpendicular magnetic anisotropy in granular multilayers of CoPd alloyed nanoparticles

Co-Pd multilayers obtained by Pd capping of pre-deposited Co nanoparticles on amorphous alumina are systematically studied by means of high-resolution transmission electron microscopy, x-ray diffraction, extended x-ray absorption fine structure, SQUID-based magnetometry, and x-ray magnetic circular dichroism. The films are formed by CoPd alloyed nanoparticles self-organized across the layers, with the interspace between the nanoparticles filled by the non-alloyed Pd metal. The nanoparticles show atomic arrangements compatible with short-range chemical order of $\mathrm{L}{1}_0$ strucure type. The collective magnetic behavior is that of ferromagnetically coupled particles with perpendicular magnetic anisotropy, irrespective of the amount of deposited Pd. For increasing temperature three magnetic phases are identified: hard ferromagnetic with strong coercive field, soft-ferromagnetic as in an amorphous asperomagnet, and superparamagnetic. Increasing the amount of Pd in the system leads to both magnetic hardness increment and higher transition temperatures. Magnetic total moments of 1.77(4) ${\mu }_B$ and 0.45(4) ${\mu }_B$ are found at Co and Pd sites, respectively, where the orbital moment of Co, 0.40(2) ${\mu }_B$, is high, while that of Pd is negligible. The effective magnetic anisotropy is the largest in the capping metal series (Pd, Pt, W, Cu, Ag, Au), which is attributed to the interparticle interaction between de nanoparticles, in addition to the intraparticle anisotropy arising from hybridization between the $3d–4d$ bands associated to the Co and Pd chemical arrangement in a $\mathrm{L}{1}_0$ structure type.

Figure 1

(a) HRTEM images in cross section configuration for the sample with $t_{\mathrm{Co}}=$ 0.7 nm and $t_{\mathrm{Pd}}=$ 0.6 nm. (a) The arrangement of the NPs within layers is highlighted. Inset at bottom right corner: a selected zone with its corresponding FFT with the lines indicating (111) planes compatible with ${\mathrm{Co}}_x{\mathrm{Pd}}_{1-x}$ alloys. Right: interplanar crystalline distances of a selected zone. (b) HAADF image and EDS maps for both Pd and Co in the highlighted zone (insets). (c) Zoom of the highlighted zone of the HAADF image in (b) (left) and corresponding EELS Co chemical map (right).

Figure 2

HRTEM images in cross section configuration for samples with $t_{\mathrm{Co}}=$ 0.7 nm, and (a) $t_{\mathrm{Pd}}=$ 1.5 nm, (b) $t_{\mathrm{Pd}}=$ 4.5 nm and (c) $t_{\mathrm{Pd}}=$ 6.0 nm. Selected zones with the corresponding FFT are highlighted in all cases, with the spots indexed according to the interplanar distances of an fcc Pd cell. Evidence of twining and dislocations are also highlighted in (c).

Figure 3

(a) X-ray diffraction patterns of the Co-Pd granular films on glass ($t_{\mathrm{Pd}}=$ 0.6, 4.5, and 6.0 nm) and on crystal silicon ($t_{\mathrm{Pd}}=$ 1.5 nm). Reference crystalline diffractions positions of Co-Pd alloy [12, 34], fcc Pd [33], and Co [37] are marked by dashed vertical lines. The corresponding fit for sample $t_{\mathrm{Pd}}$ = 1.5 nm is indicated (see Supplemental Material for additional information). Inset: zoom of the XRD patterns for the samples with $t_{\mathrm{Pd}}=$ 6.0 nm. (b) Bragg positions as a function of the deposited Pd thickness.

Figure 4

(a) XANES spectra at the Co $K$ edge for the CoPd NPs and those of the Co foil and ${\mathrm{Co}}_{50}{\mathrm{Pd}}_{50}$ bulk alloy, plotted for reference. Inset: area difference of peaks I and II with respect to Co bare NPs, as a function of the Pd concentration. (b) First derivative plots of Co $K$ edge XANES spectra shown in (a). The meaning of the vertical arrow is explained in the main text.

Figure 5

(a) FT of the Co $K$ edge EXAFS signal of the Co-Pd samples and Co bare NPs with $t_{\mathrm{Co}}=0.7$ nm included for comparison. (b) Comparison of the FT of the EXAFS signal of ${\mathrm{Co}}_3\mathrm{Pd}$, CoPd, and ${\mathrm{CoPd}}_3$ bulk reference samples and Co foil.

Figure 6

Contribution of the first coordination shell to the EXAFS signal at the Co $K$ edge for the CoPd NPs with the best fits plotted in (a) $R$ space and (b) as filtered EXFS signal. Symbols represent the experimental data and solid lines, the best fits. Curves have been vertically shifted for clarity.

Figure 7

(a) XANES spectra at the Pd $K$ edge for the Co-Pd granular multilayer samples and Pd foil sample, plotted for reference. The spectra are shifted in the intensity scale to ease comparison. Inset: the spectra after the absorption edge jump in the 24375–24450 eV range without shifts in the intensity scale. (b) First derivative plots of Pd $K$ edge XANES spectra presented in (a).

Figure 8

$M\left(T\right)$ curves measured under an applied field $H_{\mathrm{dc}}$ = 0.1 kOe parallel ($\parallel$) and perpendicular ($\perp$) to the substrate plane for the samples with (a) $t_{\mathrm{Pd}}=1.5$ nm, and (b) $t_{\mathrm{Pd}}=6.0$ nm. $M\left(T\right)$ curves measured at different applied fields values, $H_{\mathrm{dc}}$ indicated beside each curve, in the (c) $\perp$ and (d) $\parallel$ direction for the sample with $t_{\mathrm{Pd}}=1.5$ nm. Dashed lines $(--)$ show the dependence of $T_1$ and $T_f$ with $H_{\mathrm{dc}}$.

Figure 9

Magnetization curves measured for the sample with $t_{\mathrm{Pd}}=1.5$ nm under both parallel and perpendicular applied field at (a) 5 K, (b) 250 K, and (c) 350 K.

Figure 10

(a) Temperature dependence of the coercive field when the magnetic field is perpendicular to the substrate plane for all the samples. (b) Magnetic phase diagram of the CoPd NPs with $t_{\mathrm{Pd}}=1.5$ nm.

Figure 11

Co $K$-edge XMCD signal measured in the CoPd NPs and comparison to that of CoPt NPs from Ref. [21] and of bare Co NPs with the same $t_{\mathrm{Co}}$ from Ref. [20]. Curves were shifted vertically for the sake of clarity.

Figure 12

Co $L_{2,3}$ edge spectra of the sample with $t_{\mathrm{Co}}=0.7$ nm and $t_{\mathrm{Pd}}=1.5$ nm. (a) Normalized average XAS spectrum (solid line), double-step function used, and integrated white line (dashed line). (b) Normalized XMCD spectrum (solid line) and its integrated area (dashed line). Arrows mark the end energy limits of the integrals $p$ and $q$ used in the sum rules.

Figure 13

(a) Normalized XAS and (b) XMCD spectrum at the Pd $L_{2,3}$ edges in the sample with $t_{\mathrm{Co}}=0.7$ nm and $t_{\mathrm{Pd}}=1.5$ nm. The XAS spectrum of Ag $L_{2,3}$ edges is shown in (a) for comparison (dashed line). In (b), the XMCD integrated area is plotted as a dashed line and the end energy limits for the integrals $p$ and $q$ are marked by arrows.

Figure 14

Experimental XMCD ($•$) and XAS (solid line) signals at the Co $K$ edge for CoPd NPs. Calculated XMCD signals at the Co $K$ edge of nanoparticles with the $\mathrm{L}{1}_0$ ($\square$) and A1 ($▵$) structures.