Materials Science and Engineering B 190 (2014) 52–58 Contents lists available at ScienceDirect Materials Science and Engineering B jo ur nal home p age: www.elsevier.com/locate/mseb Structural and magnetic properties of CoFe 2-x Mo x O 4 nanocrystalline ferrites Mohamed Bakr Mohamed a , Adel Maher Wahba b,∗ , M. Yehia c a Ain Shams University, Faculty of Science, Physics Department, Cairo, Egypt b Tanta University, Faculty of Engineering, Department of Engineering Physics and Mathematics, Tanta, Egypt c Reactor Physics Department, Nuclear Research Center, Atomic Energy Authority, PO Box 13759 Cairo, Egypt a r t i c l e i n f o Article history: Received 4 May 2014 Received in revised form 10 August 2014 Accepted 2 September 2014 Available online 16 September 2014 Keywords: Cobalt ferrite Magnetic properties Cation distribution Mössbauer spectroscopy a b s t r a c t Structural and magnetic properties of CoFe 2-x Mo x O 4 (x — 0, 0.04, 0.08, 0.12, 0.16, 0.2, and 0.3) nano fer- rites synthesized by autocombustion method were investigated. X-ray powder diffraction patterns have confirmed the pure cubic crystalline phase of the synthesized nanoparticles. Magnetic properties were explored using vibrating sample magnetometry and Mössbauer spectroscopy. Both the crystallite size (D) and the saturation magnetization (M s ) decreased continuously with increasing Mo content. Bertaut method based on X-ray diffraction, infrared spectroscopy (IR) data, saturation magnetization and Möss- bauer spectroscopy were used to suggest the cation distribution in tetrahedral (A) and octahedral [B] sites for the whole samples. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The nano-size CoFe 2 O 4 is an important magnetic material for its wide range of applications including electronic and microwave devices [1], ferro-fluids [2], and high-density infor- mation storage [3]. Its excellent properties include high cubic magnetocrystalline anisotropy [4], high coercivity [5], reasonable saturation magnetization [6], and high Curie temperature T c [7]. Molybdenum-substituted ferrites have been investigated as an attempt to understand the correlation of the cation distribution with both electric and magnetic properties [8–10]. The cation distribution of such ferrites is characterized by the existence of mixed-valence states of Fe, Mo, and/or Co cations [11]. Dwivedi et al. [9] recorded a coexistence of both ferroelectricity and mag- netic ordering around room temperature in Mo-substituted cobalt ferrite. They suggested that Mo enters into the tetrahedral site where it transforms Fe 3+ into Fe 2+ cations to maintain the charge neutrality. They also argued that the observed giant dielectric- constant values are attributed to the Maxwell-Wagner relaxation process and that the origin of ferroelectricity is the presence of d 0 -ness of Mo 6+ ion. Well known is that nanoferrites can be easily oxidized between 100 and 500 ◦ C, while keeping its original struc- ture. This provides the possibility of the metal cations to appear in ∗ Corresponding author. Tel.: +020 1281138450; fax: +020 226842123. E-mail address: a m wahba@yahoo.co.uk (A.M. Wahba). more than single valence state; the fact that allows both electric and magnetic properties to be tailored for several applications, e.g. sensors and multiferroics [9,11–13]. In the present work, citrate-precursor autocombustion method was used to prepare Mo-substituted CoFe 2-x Mo x O 4 nanoferrites (x — — — — 0, 0.04, 0.08, 0.12, 0.16, 0.2 and 0.3), in which Fe 3+ was partially substituted by Mo 6+ . The latter is characterized by its high valence state and its d 0 electronic configuration. The effect of Mo substi- tution on the structural and magnetic properties of nano CoFe 2 O 4 was investigated using XRD, IR, Mössbauer spectroscopy and VSM magnetometery. The cation distribution suggested based on those measurements was compared to that obtained for the bulk Mo- substituted cobalt ferrites. 2. Experimental Nanocrystalline CoFe 2-x Mo x O 4 (x — — — — 0, 0.04, 0.08, 0.12, 0.16, 0.2, and 0.3) were prepared using citrate-precursor autocom- bustion method [14]. Analytical grade stoichiometric amounts of Co(NO 3 ) 2 ·6H 2 O, Fe(NO 3 ) 3 ·9H 2 O, (NH 4 ) 6 Mo 7 O 24 and dehydrated citric acid C 6 H 8 O 7 were used as starting materials. Metal com- pounds and citric acid were dissolved in minimum amounts of doubly distilled water. The citric to the all-metal molar ratio was chosen as 1:1. Metal solutions were mixed together and then the citric solution was added, followed by one-hour stirring using a magnetic stirrer. Ammonia solution (NH 4 OH) was then added drop by drop to the nitrate-citrate solution to adjust the pH value at 6.0; http://dx.doi.org/10.1016/j.mseb.2014.09.010 0921-5107/© 2014 Elsevier B.V. All rights reserved.