Microstructural and magnetic properties of Ni-Ce nanocomposite particles

A new type of magnetic core-shell Ni-Ce nanocomposite particles (15-50 nm) are presented. Scanning electron microscope (SEM) images and X-ray energy-dispersive analysis (EDX) spectra indicate that these nanoparticles are strongly magnetic interacting order with chain-like features. Typical HREM images show that many planar defects (i.e., nanotwins and stacking faults) exist in large Ni core zone (10-45 nm); the shell layers (3-5 nm) are consisted of innermost NiCe alloy and outermost NiO oxide. Selected area electron diffraction (SAED) patterns show an indication of well-defined spots characteristic of core-shell nanocomposite materials. Magnetization measurements as a function of magnetic fields and temperatures were performed in a SQUID magnetometer. Superparamagnetic behavior above average blocking temperature (T_B) 170 K was exhibited, this superparamagnetic relaxation behavior was found to be modified by interparticle interactions, which depend on the applied field and size distribution. In addition, antiferromagnetic order occurred with a Neél temperature T_N of about 11 K. A spin-flop transition was also observed below T_N at a certain applied filed. In particular, electron paramagnetic resonance (EPR) spectra at low and room temperature reflected this magnetic order nature associated with this type of core-shell microstructure, coupling with the strong interparticle interaction.