1 research outputs found
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 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
zero-field cooled susceptibility chi(T) 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(T) is suppressed by the
presence of energy barriers, the distributions of which scale with the particle
volumes obtained from transmission electron microscopy (TEM). 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 being also consistent with the volume distributions observed by TEM.
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 of the magnetization curves at lower
temperatures is for the first time interpreted as anisotropic
superparamagnetism 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 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.Comment: 12 pages with 10 figures and 1 table. Version accepted for
publication in Phys. Rev. B . Two-column layou