Journal of Materials Synthesis and Processing, Vol. 8, No. 2, 2000 1064-7562 / 00 / 0300-0067$18.00 / 0  2000 Plenum Publishing Corporation 67 Collapse of the Magnetic Hyperfine Structure of Barium Hexaferrite by Mechanochemical Activation Monica Sorescu, 1,5 A. Grabias, 1,3 L. Diamandescu, 2,4 and D. Tarabasanu 2 The collapse of the magnetic hyperfine structure of barium hexaferrite as an effect of mechanochemical activation was investigated. The various inequivalent sites and phases present in the milled material were studied by transmission M¨ ossbauer spectroscopy. Barium hexaferrite nanoparticles were obtained after only 2 h of milling time and were found to coexist with iron oxide particles for longer times of exposure to mechanochemical activation. The milled samples were then subjected to thermal annealing, in order to test the reversibility of the collapse of the hyperfine structure. The thermal annealing performed (700 C, 1 h) was partly successful in restoring the magnetic hyperfine structure of milled hexaferrite. Complementary information was obtained using transmission electron microscopy (TEM) and electron diffraction (ED). KEY WORDS: Barium hexaferrite; mechanochemical activation; M¨ ossbauer spectroscopy; hyperfine structure; nanoparticles. 1. INTRODUCTION Barium hexaferrite is a hard magnetic material with large crystalline anisotropy and high intrinsic coercivity. It has already found applications in permanent magnets or microwave devices. Fine particles of BaFe 12 O 19 are used in high density perpendicular magnetic recording media. The particle size factor plays a significant role in the macroscopic magnetic properties of dispersed sys- tems. The size of the particles can be controlled dur- ing preparation, depending on the production method, or after preparation, by mechanical [1, 2] or thermal treat- ments [3–5]. 1 Duquesne University, Bayer School of Natural and Environmental Sciences, Physics Department, Pittsburgh, Pennsylvania 15282. 2 Institute of Atomic Physics, National Institute for Materials Physics, 76900 Bucharest, Romania. 3 Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warszawa, Poland. 4 Universita “Ca Foscari,” Dipartamento Di Chimica Fisica, Calle Larga S. Marta 2137, 30123 Venezia, Italy. 5 To whom all correspondence should be addressed at e-mail: sorescu@ duq.edu In this paper we present results for the BaFe 12 O 19 powder, which was the subject of mechanical grinding using a high-energy ball mill. Phase transformations of the BaFe 12 O 19 powder were studied by M¨ ossbauer spec- troscopy, transmission electron microscopy (TEM), and electron diffraction (ED). These measurements allowed the characterization of the structural and magnetic prop- erties of the barium hexaferrite particles as a function of ball milling time. Thermal treatment was performed for the as-milled samples. This was recognized to have a great influence on recovering both structural and mag- netic properties of fine particles [4, 5]. 2. EXPERIMENTAL The barium hexaferrite (BaFe 12 O 19 ) was synthe- sized by the hydrothermal process [6]. A vibratory Spex 8000 mixer mill with steel vial and balls (type 440, four of 0.25-in diameter, and two of 0.5-in diameter) was used to process the starting barium hexaferrite. The milling process was performed in air for milling times up to 60 h. The samples were collected after 2, 27, 41, and