Magnetic ordering dependence of antiferromagnetic Cr2O3 nanoparticles with particle size

D. TOBIA; E. WINKLER; R. D. ZYSLER; M. GRANADA; H. E. TROIANI

Río de Janeiro, Brasil

Conferencia; 9th International Conference on Nanostructured Materials NANO 08; 2008

In this work we present a study of the magnetic properties of antiferromagnetic (AFM) Cr2O3 nanoparticles as a function of the nanoparticle size. Cr2O3 presents an antiferromagnetic order below the Néel temperature TN (Bulk) = 307.8 K. In the AFM ordered phase the four chromium spins in the unit cell are alligned along the threefold axis of the crystal. When a high magnetic field is applied to this system the chromium spins flop in the basal plane, maintaining the antiferromagnetic order. Our samples were synthesized by chemical precipitation followed by calcination at different temperatures, from 900 K to 1700 K. TEM images show that the synthesized nanoparticles present an ellipsoidal shape with the major axis of approximately 170 nm and the minor axis that increases with the synthesis temperature from 30 nm to 70 nm. When the dimensions of the material are reduced, the magnetic properties are highly modified due to the increase in the surface to volume ratio. Electron paramagnetic resonance (EPR) experiments together with magnetization measurements allowed us to perform a thorough characterization of the AFM nanoparticles system. We have found that the AFM order temperature and the spin-flop temperature are strongly dependent on the surface effects; they both increase with the particle size and approach the bulk values for the larger nanoparticles.2O3 nanoparticles as a function of the nanoparticle size. Cr2O3 presents an antiferromagnetic order below the Néel temperature TN (Bulk) = 307.8 K. In the AFM ordered phase the four chromium spins in the unit cell are alligned along the threefold axis of the crystal. When a high magnetic field is applied to this system the chromium spins flop in the basal plane, maintaining the antiferromagnetic order. Our samples were synthesized by chemical precipitation followed by calcination at different temperatures, from 900 K to 1700 K. TEM images show that the synthesized nanoparticles present an ellipsoidal shape with the major axis of approximately 170 nm and the minor axis that increases with the synthesis temperature from 30 nm to 70 nm. When the dimensions of the material are reduced, the magnetic properties are highly modified due to the increase in the surface to volume ratio. Electron paramagnetic resonance (EPR) experiments together with magnetization measurements allowed us to perform a thorough characterization of the AFM nanoparticles system. We have found that the AFM order temperature and the spin-flop temperature are strongly dependent on the surface effects; they both increase with the particle size and approach the bulk values for the larger nanoparticles.