Size effects on Cr2O3 AFM nanoparticles

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

Dublin

Congreso; IV Joint European Magnetic Symposia (JEMS´08); 2008

JEMS

The Cr2O3 crystallizes with the corundum structure presenting a unique threefold axis along the (111) direction. Below the antiferromagnetic order transition TN =308 K, at zero magnetic field, the Cr3+ order according to the array +-+- for the bacd Cr-sites along the (111) easy axis. When the magnetic field increases the spins show a reorientation in the basal plane maintaining the AFM array. This spin flop transition develops at HSF ~ 60 kOe in the bulk system. We have found that, when the dimension of the material is reduced to nanometric scale, both magnetic transitions are strongly dependent on the surface to volume ratio.In this work we report a study of the magnetic behaviour of Cr2O3 nanoparticle system as a function of the particle size. The study was performed by magnetization measurements and electron spin resonance (ESR) experiments in the temperature range of 5 K ? 800 K. The nanoparticles were synthesized by calcinating the precursor Cr(OH)3 in air atmosphere at different synthesis temperatures between 800 K and 1700 K. The precursor was obtained by chemical precipitation. The X-ray diffraction pattern of the powder confirms the R3c Cr2O3 phase. Transmission electron microscopy images show that the particles present ellipsoidal shape where the major axis is ~ 170 nm and the minor axis increases from 30 nm to 70 nm with the calcination temperature. From the magnetic measurements and ESR we observed a decrease of the magnetic order temperature, from 308 K to 270 K, when the nanoparticle size is reduced. The size effect is also manifested in the spin flop transition which shifts to HSF ~ 10 kOe for the smaller particles. On the other hand, the smallest nanoparticles present superparamangetic behaviour with a blocking temperature TB=30 K when the measurement is performed at H=50 Oe. We analyze these results as a function of the surface disorder and the spin canting generated by the reduction of the particle size to nanometric scale.