Dysprosium doping induced shape and magnetic anisotropy of Fe3−xDyxO4 (x=0.01–0.1) nanoparticles
Introduction
Magnetite (Fe3O4) nanoparticles are very widely used ferromagnetic material due to their unique properties like high magnetization, high resistivity, non-linear optical behavior and biocompatibility [1], [2], [3], [4]. Efforts have been done to further improve on these properties by using techniques like doping. Improved magnetization properties with doping have been reported in cases of Zn, Ni, La, Dy, Eu, and Tb doped Fe3O4 [5], [6], [7], [8]. Fe3O4 possesses inverse spinel structure with eight Fe3+ and eight Fe2+ ions in octahedral sites [site-A] and eight Fe3+ ions in tetrahedral site [site-B] [9]. Due to symmetrically placed Fe3+ ions at the two sites with opposite spins, their spin interaction nullifies each other's effect and Fe2+ ions govern the magnetic behavior. However, substitution of other element ion in place of Fe3+/Fe2+ ion can result into modified magnetic behavior. It is proposed that dysprosium could prove to be a good candidate for such substitution due to its high magnetic moment (10.6 μB). There have been earlier reports on modification of magnetic and structural properties caused by dysprosium doping in Co–Zn ferrite, Ni ferrite and cobalt ferrite [10], [11], [12]. In this work we investigate the effect of Dy doping on the properties of magnetite.
Here, the co-precipitation method has been used for synthesis of Fe3−xDyxO4 particles with x varying from 0 to 0.1. The synthesized nanoparticles were characterized by using techniques like XRD, TEM, XPS and VSM. TEM images reveal round shaped particles of ~8–14 nm diameter in case of undoped Fe3O4 nanoparticles whereas rod like structures with aspect ratio in the range of 6–12 are visible for different Dy doping concentrations. The magnetic characterization of the oriented thin layers in parallel and perpendicular direction of applied magnetic field show anisotropic behavior emanating from the shape anisotropy. The saturation magnetization value shows gradual increase in the initial stages of doping with maximum value reaching for x=0.03. XRD indicates formation of dysprosium ferrite phase beyond x=0.03. To the best of our knowledge this is the first report on formation of nanorods in magnetite prompted by rare earth doping, where no surfactant or template was used. Such rod shaped particles can be used as catalysts and for biomedical application because the effect of functional materials attached to them will be over the large area as compared to spherical particles.
Section snippets
Synthesis
Fe3−xDyxO4 (with x=0, 0.01, 0.02, 0.025, 0.03, 0.06, 0.10) particles were synthesized using an inexpensive co-precipitation method. Stoichiometric amounts of FeCl3 and DyCl3 were dissolved in de-ionized water to form 0.2 M solution. NaOH solution was added in this mixture to maintain the pH value of 10. This solution [solution-A] was magnetically stirred at 80 °C for 2.5 h. Stoichiometric amount of 0.2 M FeSO4 and NaOH [solution-B] were ultrasonicated at room temperature for 5 min. After cooling
Morphological studies
The transmission electron micrographs of undoped and Dy-doped Fe3O4 (for x=0.0, 0.02, 0.06, 0.1) are shown in Fig. 2(a, b, c, d). It is observed that some rod like structures appear as the Dy concentration increases.
It is clear from the images that the undoped particles are near round shaped with 8–14 nm size. With dysprosium doping, we observe elongated rod like structures along with round shaped particles. The number and length of the rods is found to increase with dysprosium doping
Conclusion
The Fe3−xDyxO4 particles have been synthesized with x varying from 0 to 0.1 using co-precipitation method. In the undoped sample, the composition of iron valence (Fe2+/Fe3+) is found to be in the ratio 1:2.09 in conformity with the nominal valence expected for Fe3O4. The undoped particles exhibit round shape of 8–14 nm size whereas rod like structures with aspect ratio in the range of 6 to 12 emerge with Dy doping. The magnetic characterization of the oriented thin films in parallel and
Acknowledgments
The authors thank Prof. Annapoorni, Delhi University, Delhi and Dr. Subhalakshmi Lamba, IGNOU, New Delhi for fruitful discussions. The authors would also like to thank Mr. Rajan Goyal, Delhi University, Delhi for facilitating VSM measurements. The authors acknowledge the support of Dr. D.M. Phase, UGC-DAE- Consortium for Scientific Research, Indore, India for XPS investigations and Indian Institute of Technology, New Delhi for XRD characterization. The TEM characterization was carried out at
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