Magnetic transitions in (Co, Fe)-doped ZnO nanoparticles: A Mossbauer and critical exponents study

Zn1-2zFezCozO nanoparticles were synthesized by a lyophilization process followed by thermal annealing. These samples were structurally and magnetically characterized to detail the magnetic ordering at room temperature in magnetic-doped semiconductors. X-ray diffraction results showed that the samples were monophasic for z <= 0.03 whereas Fe-57 Mossbauer spectroscopy displayed two groups of iron ions substitutional to zinc cations in the wurtzite matrix: isolated iron and iron ions with magnetic neighbors. Magnetic characterization was supported by critical transition analyses and revealed that the samples were paramagnetic at room temperature and transited to a spin-glass-like state at low temperatures after crossing a temperature region with competing ferromagnetic and antiferromagnetic interactions. The study based on critical exponents could be applied successfully for other 3d-doped oxide nanoparticle systems.