EXAFS analysis of cations distribution in structure of Co1−xNixFe2O4 nanoparticles obtained by hydrothermal method in aloe vera extract solution

- Magnetic behavior of Co1−xNixFe2O4 NPs depends on Ni content and size of NPs.
- Distribution of Co2+ and Ni2+ ions in the structure results in the increase of Ms.
- Superparamagnetic behavior is observed with increasing of the aspect ratio.
- Ms is increased by a factor 1.4 to a value of 57.57 emu/g in Co0.25Ni0.75Fe2O4.
- Hc is decreased by a factor 20 to a value of 63.15 Oe in Co0.25Ni0.75Fe2O4.

Effect of cations distribution upon EXAFS analysis on magnetic properties of Co1−xNixFe2O4 (x = 0, 0.25, 0.50, 0.75 and 1.0) nanoparticles prepared by the hydrothermal method in aloe vera extract solution were studied. XRD analysis confirmed a pure phase of cubic spinel ferrite of all samples. Changes in lattice parameter and particle size depended on the Ni content with partial substitution and site distributions of Co2+, Ni2+ ions of different ionic radii at both tetrahedral and octahedral sites in the crystal structure. Particle sizes of samples estimated by TEM images were found to be in the range of 10.87-62.50 nm. The VSM results at room temperature indicated the ferrimagnetic behavior of all samples. Superparamagnetic behavior was observed in NiFe2O4 sample. The coercivity (Hc) and remanance (Mr) values were related to the particle sizes of samples. The saturation magnetization (Ms) was increased by a factor of 1.4 to a value of 57.57 emu/g, whereas the coercivity (Hc) was decreased by a factor of 20 to a value of 63.15 Oe for a sample with x = 0.75. In addition to the cations distribution, the increase of aspect ratio (surface to volume ratio) due to the decrease of particle size could significantly affect the magnetic properties of the materials.

It is obvious from the M-H curves at room temperature of Co1−xNixFe2O4 (x = 0, 0.25, 0.50, 0.75 and 1.0) nanoparticles that partially substitution of the lower Bohr magneton (2 μB) and smaller atomic radii (0.55 Å at A site, 0.69 Å at B site) of Ni2+ ions on the higher Bohr magneton (3 μB) and larger atomic radii (0.58 Å at A site, 0.74 Å at B site) of Co2+ ions can increase the saturation magnetization (Ms) of sample with x = 0.75 to approximately 1.4 times of sample with x = 0, due to the increase of the aspect ratio (surface to volume) of nanoparticles, as a result of particle size decreasing from 37.03 to 12.63 nm. In addition to this, the ferrimagnetic behavior of CoFe2O4 has been changed to superparamagnetic behavior with the dramatic decrease of the coercivity from 1365.60 to 63.15 Oe.116