Using microemulsion methods, CoO-Pt core-shell nanoparticles, with diameters of nominally 4 nm, were synthesized and characterized by high-resolution transmission electron microscopy and a suite of x-ray spectroscopies, including diffraction, absorption, absorption near-edge structure, and extended absorption fine structure, 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 $\le T\le$ 300 K), magnetic field ($\le 50$ kOe), and frequency ($\le 1$ 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 dependencies, where the maximum at high temperature follows a Vogel-Fulcher law, indicating a superparamagnetic 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 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 the CoO core and the Pt shell.

• Received 5 November 2013
• Revised 13 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.104417