IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009 2601 Conversion of Conventionally Synthesized Strontium Hexaferrite Powder Into a Nano Size Powder With Enhanced Coercivity Using GTMR Method S. A. Seyyed Ebrahimi, R. Dehghan, H. R. Koohdar, and A. Yourdkhani Center of Excellence in Magnetic Materials, School of Metallurgy and Materials, University of Tehran, Tehran, Iran Strontium hexaferrite nanocrystalline powder was prepared by , CO and gas heat treatment and recalcination of the con- ventionally synthesized powder. The conventionally synthesized strontium hexaferrite was heated in hydrogen, carbon monoxide and methane dynamic atmospheres. This resulted in a change in magnetic nature of the material from hard to soft by decomposition of hexaferrite and reduction of the resultant iron oxide to the ultrafine grains of iron. Then the resultant powders calcined at 1000 C for 1 hour to form the single phase strontium hexaferrite again but with a nanocrystalline structure due to the formation of hexaferrite crystallites on the ultrafine grains of iron. In this work, the comparative effects of these processes on the phase evolution, morphology and size of the crystallites and magnetic properties were investigated by X-Ray Diffraction (XRD), Scanning and Transmission Electron Microscopy (SEM and TEM) and Vibration Sample Magnetometery (VSM) techniques. The results show a good enhancement in the magnetic properties particularly coercivity by applying this new method on the hexaferrite powder. Finally, the effect of milling process before recalcination on the magnetic properties has been investigated which shows a more significant improvement in the intrinsic coer- civity of the initial material. Index Terms—Gas treatment, milling, nano size powder, strontium hexaferrite. I. INTRODUCTION M -TYPE hexaferrites have been widely used as perma- nent magnets due to their low cost of production, high uniaxial magnetic anisotropy and excellent corrosion resistivity [1]. These ferrites show promising properties in microwave devices like thick films in microwave monolithic integrated circuits and recording media such as credit cards [2], [3]. The conventional method of production of this material is the solid-state reaction between and at tempera- tures higher than 1100 [4]. For preparation of strontium hexaferrite nanoparticles, different production methods such as mechanical alloying [5], hydrothermal [6], co-precipitation [7], sol-gel [8] and sol-gel auto combustion synthesis [9]–[11] have been employed in the last decade. A novel magnetic properties modification technique in- volving the heat treatment of the conventionally synthesized Sr-hexaferrite powder in the static atmosphere of hydrogen and nitrogen has also been reported in the last few years [12]–[15]. The effect of the hydrogen and nitrogen is the decomposition of the Sr-hexaferrite into and , with a marked reduction of the resultant to Fe. This divides the initial coarse particles of the hexaferrite into ultrafine particles of iron and introduces the powder as a soft magnetic material with a low coercivity and high saturation magnetization. However, after a recalcination process, the hexaferrite phase re-forms again but with a nanocrystalline structure due to the re-formation of hexaferrite crystallites on the surfaces of ultra- fine grains of iron which results in a good enhancement particu- larly in the coercivity of the material. This improvement is more significant in the atmosphere rather than nitrogen. Manuscript received October 11, 2008. Current version published May 20, 2009. Corresponding author: S. A. S. Ebrahimi (e-mail: saseyyed@ut.ac.ir). Digital Object Identifier 10.1109/TMAG.2009.2018910 In the current research, the effects of dynamic atmosphere of , CO and and then recalcination on the phase evolution, morphology and size of the crystallites and magnetic properties of the conventionally synthesized Sr-hexaferrite were investi- gated. The effect of low energy milling before recalcination on the properties was also studied. II. EXPERIMENTAL PROCEDURE M-type strontium hexaferrite was produced conventionally by calcination of hematite and strontium carbonate without using any additives at 1100 for 1 h in air. Gas heat treating was carried out at dynamic atmosphere of hydrogen , carbon monoxide (CO) and methane using a tube furnace with quartz reactor. Subsequent calcination process was carried out on the optimized samples in a regular mufle type furnace at 1000 for 1 hour in air. Then the milling process was performed wet on the hydrogen treated sample, be- cause of its better resulting properties rather than two other sam- ples, in a low energy ball mill and methanol was used as milling medium. The comparative effects of these processes on the phase evo- lution, morphology and magnetic properties of the product were investigated by XRD, SEM, TEM and VSM techniques. The VSM samples were mounted in molten wax but were not sub- jected to a magnetic alignment field and so were magnetically isotropic. The crystallite size of the powders was also calculated from the X-ray diffraction patterns using the Scherrer formula. III. RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffraction pattern of the initial con- ventionally synthesized powder. The main peaks in the figure are the main peaks of strontium hexaferrite and no noticeable peaks of the other phases could be detected. Fig. 2 shows the XRD patterns of the optimized powders heat treated in a dynamic atmosphere of , CO and . 0018-9464/$25.00 © 2009 IEEE