Exchange spring like magnetic behavior in cobalt ferrite nanoparticles

Cobalt ferrite nanoparticles were prepared by sol-gel technique and were annealed at 900 degrees C in air for 2 h. Structural properties were studied by X-ray diffraction, Raman spectroscopy and Fourier transformed infrared spectroscopy. Scanning electron microscopy and transmission electron microscopy studies show presence of mostly two different sizes of grains in these samples. Magnetization value of 58.36 emu/g was observed at 300 K for the as prepared sample and an enhanced magnetization close to the bulk value of 80.59 emu/g was observed for the annealed sample. At 10 K a two stepped hysteresis loop showing exchange spring magnetic behavior was observed accompanied by very high values of coercivity and remanence. Two clear peaks were observed in the derivative of demagnetization curve in the as prepared sample where as two partially overlapped peaks were observed in the annealed sample. The observed magnetic properties can be understood on the basis of the grain size and their distribution leading to the different types of intergranular interactions in these nanoparticles. (C) 2015 Elsevier B.V. All rights reserved

Structural and magnetic properties of ZnXCo1-XFe2O4 nanoparticles: Nonsaturation of magnetization

ZnXCo1-XFe2O4 nanoparticles were synthesized by sol-gel method and were annealed at two different temperatures; 500 degrees C and 900 degrees C in air for 2 h. Structural studies were carried out by X-ray diffraction and Fourier transformed infrared spectroscopy. The crystallite size didn't show any variation with the increase in Zn2+ concentration and was increased after annealing. The magnetization value at 300 K for the as-prepared samples increased from 53 emu/g to 60 emu/g when Zn2+ concentration increased from x= 0 to 0.2 and then it decreased to 11 emu/g for x=1. Similar magnetic behavior was also observed for the annealed samples with a peak at x=0.2. A very high magnetization value of 116 emu/g at 60 K was observed for the 900 degrees C annealed sample with x=0.4. The coercivity decreased monotonically with the increase in the Zn2+ concentration for both the as-prepared and the annealed samples. The magnetization and coercivity values were observed to be enhanced with the decrease in measurement temperature. The nonsaturation behavior of the magnetic hysteresis loops of these nanoparticle samples observed for all compositions and temperatures was studied by the method of approach to saturation by fitting M(H)=M(infinity) [ 1-(H*/H) (1/2)] to the high field data of the initial curve from 20 kOe to 30 kOe. It was observed that H* value which is the measure of the nonsaturation increased with the increase in the Zn2+ concentration. The observed magnetic properties in these nanoparticle samples can be ascribed to the changed cation distribution in the spinel structure and to the decrease of Co2+ concentration

Tailoring magnetic properties of cobalt ferrite nanoparticles by different divalent cation substitution

Different divalent cation substituted Co-ferrite (MXCo1-XFe2O4, where M = Mg2+, Ni2+, Cu2+, Zn2+, with x = 0.20 and 0.75) nanoparticles were synthesized by sol-gel method and were annealed at 900 A degrees C in air. After annealing, grain growth was observed for all the samples. With the substitution of Mg2+, Ni2+ and Cu2+ with x = 0.20, the magnetization of the as-prepared and the annealed samples was decreased from that of the Co-ferrite whereas Zn2+ substitution enhanced the magnetization. The highest magnetization values of 79.9 and 92.9 emu/g at 300 and 60 K respectively were observed for the Zn2+ substituted annealed sample with x = 0.20. For higher concentration of x = 0.75, the magnetization value was further decreased in all the samples and the lowest magnetization value of 5.1 emu/g was observed in the Zn2+ substituted annealed sample with x = 0.75 at 300 K. The coercivity was reduced in the samples except for the Cu2+ substituted sample. In the Cu2+ substituted sample with x = 0.75, the highest coercivity of 1.43 kOe at 300 K was observed after annealing. The changed cation distribution in the spinel structure, ionic magnetic moment and anisotropy compared to the Co2+ in these nanomaterials can explain the observed magnetic properties