  Using microemulsion methods, CoO-Pt core-shell nanoparticles (NPs), with
diameters of nominally 4 nm, were synthesized and characterized by
high-resolution transmission electron microscopy (HRTEM) and a suite of x-ray
spectroscopies, including diffraction (XRD), absorption (XAS), absorption
near-edge structure (XANES), and extended absorption fine structure (EXAFS),
which confirmed the existence of CoO cores and pure Pt surface layers. Using a
commercial magnetometer, the ac and dc magnetic properties were investigated
over a range of temperature (2 K $\leq$ T $\leq$ 300 K), magnetic field ($\leq$
50 kOe), and frequency ($\leq$ 1 kHz). The data indicate the presence of two
different magnetic regimes whose onsets are identified by two maxima in the
magnetic signals, with a narrow maximum centered at 6 K and a large one
centered at 37 K. The magnetic responses in these two regimes exhibit different
frequency dependences, where the maximum at high temperature follows a
Vogel-Fulcher law, indicating a superparamagnetic (SPM) blocking of interacting
nanoparticle moments and the maximum at low temperature possesses a power law
response characteristic of a collective freezing of the nanoparticle moments in
a superspin glass (SSG) state. This co-existence of blocking and freezing
behaviors is consistent with the nanoparticles possessing an
antiferromagnetically ordered core, with an uncompensated magnetic moment, and
a magnetically disordered interlayer between CoO core and Pt shell.
