Effect of grain size on the Néel temperature of nanocrystalline nickel ferrite
Introduction
The synthesis of nanosized magnetic oxide particles, such as spinel nanoferrites of the type MFe2O4 (M is a divalent metal cation), is intensively investigated in terms of their applications in high-density magnetic recording media and magnetic fluids. These materials are also largely used in electric and electronic devices and in catalysis. In order to improve sinterability and magnetic properties, the investigation of alternative, nonconventional synthesis methods to obtain ferrites in the form of nanostructured powders is the current subject [1]. It is well known that the method of preparation plays a very vital role in determining the chemical, structural and magnetic properties of spinel ferrites [2], [3], [4], [5].
Nickel ferrite (NiFe2O4) is an important member of the spinel family and it is found to be the most versatile technological materials suited for high-frequency applications due to its high resistivity [6]. In the bulk state, this material possesses an inverse spinel structure, in which tetrahedral (A) sites are occupied by Fe3+ ions and octahedral [B] sites by Fe3+ and Ni2+ ions. It exhibits ferrimagnetism that originates from the antiparallel orientation of spins on (A) and [B] sites.
In our previous work [7], nanocrystalline NiFe2O4 was synthesized by sol–gel auto-ignition method. In the present study, we are reporting on its structural and magnetic behavior with the change in calcination temperature. The nanocrystalline NiFe2O4 was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The Néel temperatures of NiFe2O4 samples were determined using vibrating sample magnetometer (VSM) with a small applied field of 10 Oe.
Section snippets
Experimental
Nanocrystalline powders of NiFe2O4 were prepared by sol–gel auto-ignition method [7]. The a.r. grade citric acid (C6H8O7 H2O), nickel nitrate (Ni(NO3)2 6H2O), ferric nitrate (Fe(NO3)3 9H2O) from Sigma Aldrich were used as starting materials. The synthesis technique is described in detail elsewhere [7]. To study the response of NiFe2O4 nanoparticles to changes in temperature, the as-prepared powders were heat treated separately at different temperatures ranging from 500 to 1000 °C for 4 h. The
Grain size and lattice parameters
Fig.1 shows the XRD pattern of NiFe2O4 samples taken after their heat treatment at various temperatures ranging from 500 to 1000 °C. As seen, NiFe2O4 with the spinel structure has been formed already at 500 °C. It was noted that a small amount of α–Fe2O3 phase had also been formed, which disappeared at calcination temperature above 700 °C (Fig. 1). Several authors [2], [3], [4], [5] have observed the hematite phase even up to the calcination temperature of 900 °C. It is clearly seen in Fig. 1 that
Conclusions
The grain size decreases with increasing calcination temperature. The IR analysis supported the formation of spinel ferrite phase with the presence of high-frequency metal-oxygen bands corresponding to intrinsic stretching vibrations of tetrahedral and octahedral sites. From the size dependent studies of the calcined samples we conclude that the Néel temperatures of nanocrystalline NiFe2O4 are found to decrease with decreasing grain size and are in accordance with finite size scaling.
Acknowledgements
One of the authors (A.T.R) thanks “The National Foundation for Science, Higher Education and Technological Development of the Republic of Croatia” for awarding Postdoctoral Fellowship under Brain Gain for foreign researchers. Special thanks are due to Prof. M. Cindric and Mr. M. Mustapic (University of Zagreb) for providing the instrument for the synthesis.
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