Anisotropic superparamagnetism of monodispersive cobalt-platinum nanocrystals

Based on the high-temperature organometallic route [Sun et al. Science 287, 1989 (2000)], we have synthesized powders containing ${\mathrm{CoPt}}_3$ single crystals with mean diameters of 3.3(2) and 6.0(2) nm and small log-normal widths $\sigma =0.15\mathrm{}\left(1\right).$ In the entire temperature range from 5 to 400 K, the zero-field-cooled susceptibility $\chi \mathrm{}\left(T\right)\mathrm{}$ displays significant deviations from ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the deviations arise from the mean-field-type reduction of the ferromagnetic moments, while below the blocking temperature $T_b,\chi \mathrm{}\left(T\right)\mathrm{}$ is suppressed by the presence of energy barriers, the distributions of which scale with the particle volumes obtained from transmission electron microscopy. This indication for volume anisotropy is supported by scaling analyses of the shape of the magnetic absorption ${\chi }^{″}(T,\omega )$ which reveal distribution functions for the barriers also being consistent with the volume distributions observed by TEM. Above 200 K, the magnetization isotherms $M(H,T)\mathrm{}$ display Langevin behavior providing $2.5\left(1\right)\mathrm{}{\mu }_B$ per ${\mathrm{CoPt}}_3,$ in agreement with reports on bulk and thin-film ${\mathrm{CoPt}}_3.$ The non-Langevin shape of the magnetization curves at lower temperatures is interpreted as anisotropic superparamagnetism by taking into account an anisotropy energy of the nanoparticles $E_A\mathrm{}\left(T\right).$ Using the magnitude and temperature variation of $E_A\mathrm{}\left(T\right),$ the mean energy barriers and ‘unphysical’ small switching times of the particles obtained from the analyses of ${\chi }^{″}(T,\omega )$ are explained. Below $T_b$ hysteresis loops appear and are quantitatively described by a blocking model, which also ignores particle interactions, but takes the size distributions from TEM and the conventional field dependence of $E_A$ into account.