Magnetic anisotropy of ${\mathrm{Co}}_x{\mathrm{Pt}}_{1-x}$ clusters embedded in a matrix: Influences of the cluster chemical composition and the matrix nature

We report on the magnetic properties of ${\mathrm{Co}}_x{\mathrm{Pt}}_{1-x}$ clusters embedded in various matrices. Using a careful analysis of magnetization curves and zero field cooled susceptibility measurements, we determine the clusters magnetic anisotropy energy (MAE) and separate the surface and volume contributions. By comparing different chemical compositions, we show that a small amount of Pt (15%) induces an important increase in the volume anisotropy with respect to pure Co clusters, even in chemically disordered fcc clusters. Comparing the measurements of clusters embedded in Nb and MgO matrices, we show that the oxide matrix induces an important increase of the surface MAE attributed to the formation of an antiferromagnetic CoO shell around the clusters.

Figure 1

TEM micrograph of the ${\mathrm{Co}}_{85}{\mathrm{Pt}}_{15}$ clusters deposited on an amorphous carbon grid. Inset: diameter distribution. The histogram corresponds to the TEM measurements and the continuous line corresponds to a lognormal fit.

Figure 2

Normalized hysteresis loops measured at $T=10\mathrm{K}$ on the samples with Co, ${\mathrm{Co}}_{85}{\mathrm{Pt}}_{15}$, and ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters. The dark lines correspond to the measurements in the MgO matrix and the gray lines correspond to the measurements in the Nb matrix.

Figure 3

ZFC-FC curves measured on the Co, ${\mathrm{Co}}_{85}{\mathrm{Pt}}_{15}$, and ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters embedded in MgO and Nb matrices. The curves were recorded in a small field of $5\mathrm{mT}$ (Co:MgO and ${\mathrm{Co}}_{85}{\mathrm{Pt}}_{15}:\mathrm{Mg}\mathrm{O}$) or $10\mathrm{mT}$ (Co:Nb, ${\mathrm{Co}}_{85}{\mathrm{Pt}}_{15}:\mathrm{Nb}$, ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}:\mathrm{Nb}$, and ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}:\mathrm{Mg}\mathrm{O}$). The dots correspond to the measurements and the continuous lines correspond to the fits using the model described below.

Figure 4

Superparamagnetic $M\left(H\right)$ curves measured on the Co:MgO sample at $100\mathrm{K}$, $200\mathrm{K}$, and $300\mathrm{K}$. The two measures at the highest temperatures are fitted using the convolution of a Langevin function with lognormal distribution of the magnetic diameter. The curve recorded at $100\mathrm{K}$ cannot be fitted with the same parameters due to a non-negligible MAE.