L Journal of Alloys and Compounds 265 (1998) 87–92 Magnetic studies of nanosized nickel ferrite particles synthesized by the citrate precursor technique * Seema Prasad, N.S. Gajbhiye Department of Chemistry, Indian Institute of Technology, Kanpur 208016 ( U.P), India Received 23 December 1996; received in revised form 21 July 1997 Abstract Nanometer size NiFe O particles were synthesized by the Citrate precursor technique and control of solid state reactivity yields the 2 4 monophasic ultrafine NiFe O which were characterized by XRD, SEM and BET surface area measurements. Interestingly, single domain 2 4 particles were found to form the linear chain like clusters because of strong magnetic dipolar interactions. The low saturation magnetization values were attributed to the spin noncollinearity predominantly at the surface. The Hopkinson effect is exhibited by an assembly of noninteracting single domain particles and is explained within the mathematical formalism given by the Stoner and Wohlfarth model.  1998 Elsevier Science S.A. Keywords: Citrate; Precursor; Reactivity; Ultrafine; Ferrite 1. Introduction systems as Nd Fe B , Nd Fe B, BaFe O [15–17]. 16 76 8 2 14 12 19 Popov et al. has explained this effect within the mathemati- Nanosized spinel ferrite particles have attracted consid- cal formalism given by Stoner and Wohlfarth [18] contrary erable attention and efforts continue to investigate them for to the domain wall bowing and pinning mechanism their technological importance to the microwave industries, [19,20]. The Hopkinson effect in the noninteracting single high speed digital tape or disk recording, repulsive suspen- domain particles of NiFe O and their spin structure are 2 4 sion for use in levitated railway systems, ferrofluids, explored in detail in this study. catalysis and magnetic refrigeration systems [1–3]. Large scale applications of ferrites with small particles have prompted the development of various chemical methods 2. Experimental details which includes hydrothermal, co-precipitation, freeze dry- ing, spray drying, precursor and sol-gel for the prepara- 2.1. Preparation of nanosized NiFe O particles by 2 4 tions of the stoichiometric and chemically pure spinel precursor technique ferrites [4–8]. In particular, the citrate precursor technique has been shown to have great potential in the preparation A 0.2M solution of Nickel nitrate, Citric acid (AnalaR, of rare earth iron garnets and ferrites [9–13]. The present S.d.fine Chem) and Ferric nitrate (GR, LOBA Chemie) investigation deals with the synthesis of the fine particles were mixed in the molar ratio of Ni:Fe:Citric acid5 of NiFe O particles by the Citrate Precursor technique. 1:2:2.7. The pH value was maintained in the range 1.25– 2 4 Solid state reactivity controls the formation of monophasic 2.5. The resultant homogeneous solution was refluxed for fine grained stoichiometric NiFe O particles. The size 15 h at 363 K which was slowly evaporated on a water 2 4 dependent magnetic behaviour of single domain NiFe O bath to form a viscous liquid (gel). Further drying was 2 4 particles show the Hopkinson effect [14] where the emer- carried out at 373 K in an oven for 5 h to remove adsorbed gence of a maximum in magnetization occurs just below water. During the process of drying, the gel swells into the Curie temperature T in the presence of small magnetic fluffy mass which eventually breaks into the brittle flakes. c field. This effect has been reported in the literature for the The precursor was found to be X-ray amorphous and assembly of noninteracting single domain particles in the hygroscopic in nature. The degree of hydration was found to be dependent on the atmospheric humidity. Wet chemi- * Corresponding author. cal analyses of nickel iron citrate precursor gave Ni5 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388(97)00431-3