Structural and Magnetic Properties of Nanosized Barium Hexaferrite Powders Obtained by Microemulsion Technique

Thin hexagonal barium hexaferrite particles synthesized using the microemulsion technique were studied. A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and an aqueous phase were used. The microstructural and magnetic properties were investigated. The particles obtained were mono-domain with average particle size 280 nm. The magnetic properties of the powder were investigated at 4.2 K and at room temperature. The saturation magnetization was 48.86 emu/g and the coercivity, 2.4 x 10 A/m at room temperature. The anisotropy field Ha and magneto-crystalline anisotropy K1 were 1.4 x 10 A/m and 2.37 x 10 J/m, respectively. Introduction Barium hexaferrite particles are one of the most promising materials for high-density magnetic recording media due to their unique recording characteristics, namely, high coercivity, moderate magnetic moment, low or positive temperature coefficient of coercivity, and high chemical stability [1-3]. The M-type barium hexaferrite (BaFe12O19) is the hexaferrite family’s best known compound. Its crystal structure is the so-called magnetoplumbite structure that can be described as a stacking sequence of the basic S (spinel) and R (hexagonal) blocks [4, 5]. The magnetic ion (Fe) occupies five different interstitial positions in a ferrimagnetic order resulting in a net magnetic moment. Two of the possible 16 tetrahedral positions (4f1) and four of the possible octahedral positions (2a) are occupied by Fe in the S block. Fe in the R block occupies octahedral sites in the octahedra shared by common faces (12k), in octahedra at the interface of adjacent blocks (4f2), and trigonal bipyramidal sites (2b). The presence of magnetic Fe cations in these positions is responsible for the BaFe12O19 magnetic properties and for its magneto-crystalline anisotropy (K1 = 3.3 x 10 J/m) [6]. The physical properties of an inorganic microstructure are fundamentally related to the size, crystal structure and morphology, which can vary depending on the preparation route [7]. The traditional methods to prepare nanoparticles are rather complicated; they involve a number of different steps with multiple microstructural problems that may have a detrimental effect on the Solid State Phenomena Vol. 159 (2010) pp 57-62 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.159.57 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 195.96.237.59-09/12/09,14:04:42) magnetic performance. This is one of the reasons why researchers keep on looking for new routes of synthesis and improvement of the known ones. The most commonly used among these “new routes” is synthesis of nano-sized powders by using “wet chemistry”, often called the “chemical route”. It is known that the composition, shape and size of the precursor particles used for high-temperature synthesis affects the microstructural characteristics of the material produced. Co-precipitation is one of the techniques used frequently for preparation of nanosized particles. The co-precipitation allows one to vary the average size of nanoparticles by adjusting the pH and the temperature of the aqueous media, but one has only limited control over the particles size distribution [8]. Recent investigations demonstrated the possibility to prepare homogeneous nanosized magnetic oxide powders by applying the microemulsion process [9, 10]. A microemulsion system consists of an oil phase, a surfactant phase and an aqueous phase. The reverse microemulsion system exhibits a dynamic structure of nanosized aqueous droplets which are in constant deformation, breakdown, and coalescence. Each of the aqueous droplets can act as a nanosized reactor for forming nanosized precipitate particles [7]. One of the advantages of this technique is the preparation of very uniform precursors’ particles (< 10% variability) [8]. Since the condition of synthesis affect considerably the chemical, structural and physical properties, our attention was focused on investigating the microstructural and magnetic properties of BaFe12O19 powder synthesized by reverse microemulsion technique. Experiment A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB), (24 wt.%) as a cationic surfactant, n-butanol (16 wt.%) as ca o-surfactant, n-hexanol (20 wt.%) as a continuous oil phase, and an aqueous solution (40 wt.%) was used. The metallic ions (Ba and Fe) concentration in the aqueous phase was 0.44 M. The molar ratio of Ba to Fe was fixed at 1:10. In the first step of the synthesis procedure, the co-precipitation occurred when the microemulsion containing an aqueous solution of Ba(NO3)2 and FeCl3 was added to the microemulsion containing the precipitating agent NaOH. The amount of NaOH was set to a value that resulted in the final pH value after precipitation being 11. The precipitate obtained was separated in a centrifuge and was washed with water and solution of chloroform and methanol (50 v.% and 50 v.%) to remove the excess surfactant. The hydroxide precursor was dried and milled. In the second step of the synthesis the powder obtained was heated at 580°C for 4 h. After grinding, the powder was finally calcined at 900°C for 5 h to ensure complete conversion of the precursors into BaFe12O19. The barium hexaferrite powder was characterized using XRD analysis (TUR diffractometer with Bragg-Brentano geometry at room temperature using Cu-Kα radiation) and scanning electron microscopy (SEM, Philips ESEM XL30 FEG). The Mössbauer spectra were obtained by a conventional home-made spectrometer. A 50 mCi Co (Rh) source was used. The magnetic measurements were carried out at room temperature and at 4.2 K using a vibration sample magnetometer with a maximum magnetic field of 2.3 x 10 A/m. The high magnetic field measurements (up to 1 x 10 A/m) were performed on a homemade pulsed magnetometer [11]. The magnetic measurements were done on an unoriented random assembly of particles. Results and discussions The XRD spectrum of the synthesized BaFe12O19 powder is presented in Fig. 1. It shows the characteristic peaks corresponding to the barium hexaferrite structure. No other phases are detected. This confirms the complete conversion of the precursor powder into BaFe12O19. The lattice constants obtained from the XRD spectra are a = 0.584 nm and c = 2.341 nm. Fig. 2 shows the morphology of the calcined powder. It exhibits a narrower grain-size distribution, with the average particle size being 280 nm. Most of the particles have an almost perfect hexagonal shape. It can be seen that the smallest particles with a size of about 250 nm have 58 Nanostructured Materials, Thin Films and Hard Coatings for Advanced Applications