Dong Hyun An

Superparamagnetic relaxation in CoFe2O4 nanoparticles

Abstract CoFe 2 O 4 nanoparticles synthesized by a microemulsion method have been studied by X-ray diffractometry, transmission electron microscopy, SQUID magnetometry and Mossbauer spectroscopy. All peaks of X-ray diffraction patterns can be attributed to a cubic spinel structure with the lattice constant a 0 =8.39 A. The average size of the particles, determined by transmission electron microscopy, is 7.8xa0nm. Superparamagnetic behavior of the particles is confirmed by the coincidence of plots of the magnetization versus field divided by temperature. As the temperature increases toward the Neel point, Mossbauer line broadening and a pronounced central doublet appear, suggesting superparamagnetic relaxation. As the temperature increases, the relaxation rate increases rapidly as the seventh power of the absolute temperature.

Mössbauer study of Zn0.4Cu0.6Fe1.2Cr0.8O4

Abstract Zn 0.4 Cu 0.6 Fe 1.2 Cr 0.8 O 4 has been studied by Mossbauer spectroscopy, SQUID magnetometry, and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a 0 =8.3667 A . The iron ions are in ferric states and occupy both the tetrahedral (A) and octahedral (B) sites; the fractions of the iron ions at the A-sites and B-sites are 0.52 and 0.34, respectively. While spin orderings are collinear at higher temperatures, spin canting begins to appear around 25 K and increases with decreasing temperature; the canting angle at 4.7 K reaches up to 27°. Debye temperatures of the tetrahedral and octahedral sites are determined to be 339 and 335 K, respectively.

Mössbauer study of CoFeRhO4

Abstract CoFeRhO 4 has been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a 0 =8.451±0.005xa0A. The iron ions are in ferric states. The temperature dependence of the magnetic hyperfine field is analyzed by the Neel theory of ferrimagnetism. The intersublattice superexchange interaction is antiferromagnetic and strong with a strength of J AB =−12.39k B while the intrasublattice superexchange interactions are weak with strengths of J AA =−4.96k B and J BB =6.20k B . As the temperature increases toward the Neel temperature T N , a systematic line broadening effect in the Mossbauer spectrum is observed and interpreted to originate from different temperature dependences of the magnetic hyperfine fields at various iron sites.