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 capped CoPt$_{3}$ single crystals with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths $\sigma$=0.15(1). In the entire temperature range from 5 K to 400 K, the DC-susceptibility $\chi$(T) displays significant deviations from ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, this is due to the (mean-field) type reduction of the ferromagnetic moments, while below the blocking temperature $T_{b}$, $\chi$(T) is suppressed by the presence energy barriers, the distributions of which scale with the particle sizes. Scaling analyses of the shape of the magnetic absorption $\chi$''(T,$\omega$) reveals distribution functions for the barriers consistent with those from transmission electron microscopy. Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior providing 2.5(1) $\mu_{B}$ per CoPt$_{3}$ in agreement with reports on bulk and thin film CoPt$_{3}$. The non-Langevin shape at lower temperatures is for the first time interpreted by taking into account an anisotropy energy of the nanoparticles $E_{A}$(T). Using the magnitude and temperature variation of $E_A(T)$, the mean barriers and 'unphysical' small switching times of the particles 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 and the conventional field dependence of $E_{A}$ into account.