Effects of Mg substitution on the structural and magnetic properties of Co0.5Ni（0.5-x）MgxFe2O4 nanoparticle ferrites

In this study, nanocrystalline Co-Ni-Mg ferrite powders with composition Coo.5Nio.5-xMgxFe2O4 are successfully synthesized by the co-precipitation method. A systematic investigation on the structural, morphological and magnetic properties of un-doped and Mg-doped Co-Ni ferrite nanoparticles is carried out. The prepared samples are characterized using x-ray diffraction （XRD） analysis, Fourier transform infrared spectroscopy （FTIR）, field emission scanning electron microscopy （FESEM）, and vibrating sample magnetometry （VSM）. The XRD analyses of the synthesized samples confirm the formation of single-phase cubic spinel structures with crystallite sizes in a range of - 32 nm to - 36 nm. The lat- tice constant increases with increasing Mg content. FESEM images show that the synthesized samples are homogeneous with a uniformly distributed grain. The results of IR spectroscopy analysis indicate the formation of functional groups of spinel ferrite in the co-precipitation process. By increasing Mg2- substitution, room temperature magnetic measurement shows that maximum magnetization and coercivity increase from - 57.35 emu/g to - 61.49 emu/g and - 603.26 Oe to 684.11 Oe （l Oe = 79.5775 A.m-l）, respectively. The higher values of magnetization Ms and Mr suggest that the opti- mum composition is Co0.5Ni0.4Mg0.1Fe204 that can be applied to high-density recording media and microwave devices.

Optical behavior of self-assembled high-density Ge nanoislands embedded in SiO_2

The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2 sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2 sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011 cm-2, respectively. Atomic force microscopy （AFM） reveals root mean square （RMS） roughness sensitivity as the SiO2 sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2 and Ge. The Ge nanoisland polyorientation on SiO2 （50-nm thickness） is revealed by X-ray diffraction （XRD） patterns. Photoluminescence （PL） peaks of 2.9 and 1.65 eV observed at room temperature （RT） are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2 and Si02/Si interfaces, respectively. The mean island sizes are determined by fitting the experimental Raman profile to two models, namely, the phonon confinement model and the size distribution combined with phonon confinement model. The latter model yields the best fit to the experimental data. We confirm that SiO2 matrix thickness variations play a significant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies. OCIS codes： 250.5230, 170.5660.