* Corresponding author. Tel.: #54-2944-445158; fax #54- 2944-445299. E-mail address: zysler@cab.cnea.gov.ar (R.D. Zysler). Journal of Magnetism and Magnetic Materials 226}230 (2001) 1907}1909 Annealing e!ects on structural and magnetic properties of -Fe  O  nanoparticles M. Vasquez-Mansilla, R.D. Zysler*, C. Arciprete, M. Dimitrijewits, D. Rodriguez-Sierra, C. Saragovi Centro Ato & mico Bariloche and Instituto Balseiro, S. C. de Bariloche, RN, 8400 Argentina Departamento de Fn & sica, Centro Ato & mico Constituyentes, Av. Gral. Paz 1499, Bs. As. 1650, Argentina Abstract Thee!ectoftransformationsinducedbyannealingonthestructureandmagneticpropertiesof -Fe  O  antiferromag- netic nanoparticles synthesized by chemical route has been investigated by transmission electron microscopy, magnetiz- ation measurements and Mo K ssbauer spectroscopy. Annealing of these samples re-crystallize the nanoparticles without changing its mean size modifying the crystalline anisotropy energy of the nanoparticles leading to a change in the reversible}irreversible regime and in the Morin transition behaviour.  2001 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; Nanocrystallization; Hematite; Annealing e!ect Bulk -Fe  O  (hematite) besides the Ne H el temperature (¹  "960K) has a "rst-order magnetic transition at ¹  "263K, which is called the Morin transition. Below ¹  , the antiferromagnetically (AF) ordered spins are oriented along the c-axis while above ¹  spins lie AF in thebasalplaneofthecrystalwithaweakferromagnetism component [1,2]. In hematite nanoparticles, it has found that the Morin temperature is strongly dependent on particle size [3], strain and crystal defects (e.g. low cry- stallinity of the particles, vacancies) [4]. Hematite nanoparticles were prepared by a chemical route following the same procedure reported previously [5]. Thermal treatment was performed by heating the sample at a rate of 13C/min up to 6003C and after 4h, cooling the sample at a rate of 13C/min to room temper- ature. As-prepared sample: Bright-"eld TEM measurements showthattheas-preparedparticlesaresinglecrystalwith mainly a romboedrical shape (Fig. 1a). The bulk of the nanoparticles presents some "ssures and their surfaces show some roughness. The size distribution of the nanoparticles obtained by the micrographs "ts a log}normal distribution with a mean length of l  "38.8nm and a dispersion in the logarithm (l/l  ) of "0.26. The temperature dependence of M(¹) is reported in Fig. 2. The e!ective Morin transition temperature results ¹  "177K, derived from the temperature where the magnetization has its main in#ection point, half-way between the AF state and the WF state values of the M(¹) curve with a transition width, ¹  "70K. Mo K ssbauer measurements vs. ¹ were performed and the spectra were "tted with hyper"ne "eld distribution (HDF)usingtheDIST3EProgram[6].Fig.3ashowsthe hyper"ne "eld dependence vs. temperature. The values of H  shown correspond to the maxima of the correspond- ing HFD. The low-temperature saturation value of the fractional spectral area of the WF phase is 21%; ¹  +168K as obtained from the 50% fractional area. Annealed sample: The results presented here indicate that the annealing treatment strongly a!ects the micro- structure and the magnetic properties of hematite nanoparticles synthesized by this chemical route. After annealing, TEM micrograph exhibits changes in the hematite nanoparticles (Fig. 1b) showing that an- nealed nanoparticles have more spherical shape with 0304-8853/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00858-1