Fabrication of Zinc Ferrite Nanocrystals by Sonochemical Emulsification and Evaporation: Observation of Magnetization and Its Relaxation at Low Temperature Manickam Sivakumar,* †,‡ Tsuyoshi Takami, § Hiroshi Ikuta, § Atsuya Towata, † Kyuichi Yasui, † Toru Tuziuti, † Teruyuki Kozuka, † Dipten Bhattacharya, | and Yasuo Iida † Ultrasonic Processing Group, AdVanced Manufacturing Research Institute (AMRI), National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan, Department of Pharmaceutical Engineering & Technology, Bharathidasan Institute of Technology, Bharathidasan UniVersity, Tiruchirappalli 620024, India, Department of Crystalline Materials Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, and Electroceramics DiVision, Central Glass and Ceramic Research Institute, Calcutta 700 032, India ReceiVed: September 6, 2005; In Final Form: June 3, 2006 A new ultrasound assisted emulsion (consisting of rapeseed oil and aqueous solution of Zn 2+ and Fe 2+ acetates) and evaporation protocol has been developed for the synthesis of zinc ferrite (ZnFe 2 O 4 ) nanoparticles with narrow size distribution. The as-synthesized sample consisted of crystalline zinc ferrite particles with an average diameter of ∼4 nm, whereas the average size of the heat-treated ferrite particles increases to ∼12 nm. To remove the small amount of oil present on the surface of the as-synthesized ferrite sample, heat treatment was carried out at 350 °C for 3 h. The as-synthesized and heat-treated ferrites were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM), and energy dispersion X-ray spectroscopy (EDS) techniques. Magnetic measurements show that the nanocrystalline ZnFe 2 O 4 , prepared through this technique, is either at par with those obtained in other cases or even more improved. Both the as-synthesized and heat-treated samples reveal relaxation of magnetization. Our study also shows that one can tailor the magnetization and relaxation pattern by suitably controlling the particle size of the nanocrystalline ZnFe 2 O 4 . The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods, (b) the usage of necessary additive components (stabilizers or surfactants, precipitants), and (c) calcination requirements. In addition, rapeseed oil has replaced organic nonpolar solvents used in earlier studies. As a whole, this simple straightforward sonochemical approach results in a better pure phase system of nanoferrite with improved magnetic properties. Introduction In recent years, there has been tremendous activity for the preparation of transition metal ferrites with the molecular formula MFe 2 O 4 , as they represent an important class of technological materials. Although these types of ferrites are traditionally prepared in bulk, the miniaturization of magnetic and electronic devices demands advanced materials with smaller sizes and new forms and shapes, such as nanoparticles. 1 Particularly, zinc ferrite (ZnFe 2 O 4 ) nanoparticles have generated a lot of interest owing to their potential applications in gas sensor and semiconductor photocatalysis as their magnetic properties differ markedly from those of their bulk counterpart. Zinc ferrite has a normal spinel structure with tetrahedral A-sites occupied by Zn 2+ ions and octahedral B-sites by Fe 3+ ions. 2 It has been indicated that with the change in particle size ZnFe 2 O 4 exhibits improved properties. 3-7 The ferrites with normal and inverse spinel structure exhibit a variety of magnetic order and properties depending on the choice of the tetrahedral A-site ion. For the nonmagnetic Zn ion at the tetrahedral site, the magnetic interaction takes place only within octahedral B sites. It has been observed that bulk ZnFe 2 O 4 depicts a long range order below 10 K as well as a short range order at higher temperature. 8 On the other hand, (Ni,Zn)Fe 2 O 4 exhibits ferrimagnetic order, as a result of competition among A-A, A-B, and B-B exchange interac- tions. The saturation magnetization (M s ) for the bulk (Ni,Zn)- Fe 2 O 4 system is found to be ∼119 emu/g at ∼10 K and ∼70.3 emu/g at ∼300 K. 9 The size reduction down to nanometer scale can give rise to novel magnetic properties. 10 Several studies 1,11-15 have been carried out on nanocrystalline ferrites, both with magnetic and nonmagnetic ions at the A site, prepared by different techniques like sol-gel, coprecipitation, microemul- sion, normal and reverse micelle, microwave, plasma, etc. In the case of nonmagnetic ion, Zn at the A site, it has been observed that for the particles of sizes of 3-6 nm the saturation magnetization at 3-10 K varies between 10 and 30 emu/g, whereas the coercive field (H c ) varies over 310-650 Oe. These observations have motivated us to carry out the magnetic hysteresis measurements at different temperatures between 5 and 300 K as well as field-cooled (f c ) and zero-field cooled (zfc) magnetization vs temperature measurements under different fields on as-synthesized and heat-treated nanocrystals of ZnFe 2 O 4 prepared by the present sonochemical technique. These data help in understanding and comparing the nature of magnetic order in the as-synthesized nanocrystalline ZnFe 2 O 4 and heat-treated system. In addition, we have studied the relaxation patterns of * To whom correspondence should be addressed. E-mail: manickam-sivakumar@aist.go.jp. Fax: +81-52-7367400. † Advanced Manufacturing Research Institute (AMRI). ‡ Bharathidasan University. § Nagoya University. | Central Glass and Ceramic Research Institute. 15234 J. Phys. Chem. B 2006, 110, 15234-15243 10.1021/jp055024c CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006