Size and crystallinity-dependent magnetic properties of CoFe 2 O 4 nanocrystals C.H. Chia a, * , S. Zakaria a , M. Yusoff a , S.C. Goh a , C.Y. Haw a , Sh. Ahmadi a , N.M. Huang b , H.N. Lim c a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b Solid State Physics Research Laboratory, Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia c Chemistry Department, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Received 17 July 2009; received in revised form 3 August 2009; accepted 23 September 2009 Available online 29 October 2009 Abstract CoFe 2 O 4 nanocrystals were synthesized by a wet chemical coprecipitation approach. In order to investigate the effect of degree of crystallinity and mean crystallite size of CoFe 2 O 4 nanocrystals on the magnetic properties, a series of CoFe 2 O 4 samples with different degree of crystallinity and mean crystallite size were produced by varying the synthesis and subsequent calcination temperatures. The higher synthesis and subsequent calcination temperatures have resulted in greater degree of crystallinity and bigger mean crystallite size of CoFe 2 O 4 nanocrystals. The VSM studies showed that the saturation magnetization (M s ) and coercivity (H c ) of the CoFe 2 O 4 nanocrystals possessed a linear relationship with the mean crystallite size. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Powders: chemical preparation; C. Magnetic properties; D. Ferrites; D. Spinels 1. Introduction Nano-sized transition metal oxides with spinel structure have recently received wide attention in several scientific and technological fields [1,2]. Spinel ferrites with cubic unit cell have a formula of M X Fe 3X O 4 , where the divalent metallic ions (M 2+ ) can be either Fe, Mn, Co, Ni, Cu or Zn. Cobalt ferrite (CoFe 2 O 4 ), one of the well known hard magnetic materials, has been widely used as high-density recording medium due to its strong magnetic anisotropy, moderate magnetization, and high coercivity at room temperature [3]. There are a number of methods which have been reported previously for the preparation of CoFe 2 O 4 nanocrystals, including microemulsion [4–6], sol–gel techniques [7], hydrothermal synthesis [8,9], coprecipitation [10–14] and electrochemical synthesis [15]. The chemical coprecipitation is the most widely used method due to its high yield and simplicity in producing ultrafine magnetic nanocrystals. To date, there are two different chemical coprecipitation methods that have been used to produce CoFe 2 O 4 nanocrystals, i.e., with and without oxidizing agent. Previous studies reported the precipitation of CoFe 2 O 4 nanocrystals by mixing Fe 2+ , Co 2+ and NaOH in the presence of oxidizing agent such as H 2 O 2 [16] and KNO 3 [3,17]. The oxidizing agent is required for the formation of Fe 3+ from Fe 2+ as precursor of the formation of CoFe 2 O 4 . However, the coexistence of Fe 2+ and Fe 3+ ions has resulted in the formation of magnetite (Fe 3 O 4 ) and CoFe 2 O 4 which will subsequently affect the purity of the products. In order to avoid this adverse effect, precipitation has been carried out without using oxidizing agent [18,19]. Previous studies have found that the synthesis temperature plays an important role in controlling the size of the CoFe 2 O 4 which will significantly influence the magnetic properties of the products [11,18]. Besides, a number of attempts have also been made to improve the magnetic properties of the CoFe 2 O 4 nanocrystals by subsequent calcination of the as-precipitated CoFe 2 O 4 nanocrystals [16,20,21]. In the present study, CoFe 2 O 4 nanocrystals are produced using a wet chemical method. The aim of this study is to investigate the effect of crystallite size and degree of crystallinity www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 36 (2010) 605–609 * Corresponding author. Tel.: +60 3 8921 5473; fax: +60 3 8921 3777. E-mail address: chia@ukm.my (C.H. Chia). 0272-8842/$36.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.10.001