An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions

The polarized optical spectra of the ions Ti3+, V3+, Cr3+, Mn3+, Co3+, and Ni3+ in corundum single crystals have been studied at temperatures from 4.2° to 1200°K. A theory of the band strength based on the point‐charge model and p‐d mixing has been developed and applied to the data with results in fair agreement with experiment. The effects of temperature show that the vibrational‐electronic contribution to band strength is quite small at low temperature but may be appreciable at high temperatures. The crystal‐field parameters have been calculated as convergent lattice sums. The observed trigonal‐field parameter has the opposite sign from that calculated by the point‐charge model if the impurity ion is assumed to occupy an Al3+ ion position in the lattice, but has the same sign as calculated for an ion 0.1 A displaced along the c3 axis toward the empty octahedral site. Details of the spectra have been interpreted as showing that the surroundings of an ion are distorted in some electronic states.

Optical spectra of transition-metal ions in corundum

Light-induced changes in the coercivity ΔHc are reported for cobalt ferrite nanoparticles. These changes arise from charge transfer initiated by optical absorption near 2 eV. An insulator-to-metal transition at 170 K is an upper limit for significant ΔHc. The larger ΔHc for smaller (17 nm) particles was correlated with a distortion about the Co2+ ions at B sites, which increases the absorption probability.

Photomagnetism and structure in cobalt ferrite nanoparticles

Simplified synthesis of single-crystalline magnetic CoFe2O4 nanorods by a surfactant-assisted hydrothermal process

Through a systematic investigation of the complete dielectric tensor, between 0.5 and 5.0 eV, of Fe 3 O 4 and of related spinel ferrites, i.e., MgFe 2 O 4 , Li 0.5 Fe 2.5 O 4 , NiFe 2 O 4 , and CoFe 2 O 4 , we have established that intervalence charge transfer and intersublattice charge transfer transitions dominate the optical and magneto-optical spectrum (between 0.5 and 5.0 eV) of all spinel ferrites of the general composition Me x Fe 3−x O 4 . In all cases examined the same set of intersublattice charge transfer transitions was observed. These are the only transitions observed in the cases where Me is a nonmagnetic ion ( Mg 2+ , Li + ). In the cases where Me is a magnetic ion ( Fe 2+ , Ni 2+ , Co 2+ ) additional intervalence charge transfer transitions are observed. CoFe 2 O 4 is the only spinel ferrite with a major contribution of crystal field transitions to the magneto-optical spectrum. The observed presence of only two intense crystal field transitions in specifically CoFe 2 O 4 is explained. The observed relative strengths of these two transitions in CoFe 2 O 4 , in which remarkably the upper transition at 1.82 eV is more intense than the lower transition at 0.83 eV is also explained in a crystal field analysis.

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J.W.D. Martens

Papers

Optical, magneto-optical and mössbauer spectroscopy on Co3+ substituted cobalt ferrite Co2+Fe2−xCo3+xO4(0 ⩽ x ⩽ 2)

An ellipsometric and magneto-optical study of cobalt ferrite single crystals