SIZE AND SURFACE EFFECTS IN Cr2O3 ANTIFERROMAGNETIC NANOPARTICLES

DINA TOBIA; ELIN WINKLER; ROBERTO ZYSLER; MARA GRANADA; HORACIO TROIANI

Bahía Blanca, Buenos Aires, Argentina

Congreso; Cuarta Reunión de la Asociación Argentina de Cristalografía; 2008

The ultrafine particles have been an object of increasing interest in the past few years due to the new physical properties that they present when their size is reduced to nanometric scale. In the special case of the antiferromagnetic (AFM) nanoparticles a significant increase in the magnetic moment is observed due to the spin non-compensation at the surface as the particle size is reduced.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 at low temperature in the bulk system. We have found that when the dimensions of the material are 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 nanoparticles systems as a function of the particles size. The study was performed by magnetization measurements and electron spin resonance (ESR) experiments in the temperature range of 5 K – 700 K. The nanoparticles were synthesized by calcinating the precursor Cr(OH)3 in air and in O2 atmospheres at different synthesis temperatures between 800 K and 1700 K. The precursor was obtained by chemical route. The crystalline structure was investigated by X-ray diffraction and the morphological characterization was made by transmission electron microscopy (TEM). The X-ray diffraction patterns of the powders confirm the R(-3)c Cr2O3 phase for all samples. From these measurements it can be observed a clear broadening of the diffraction peaks and a shift towards higher angles for the lowest synthesis temperature. The evolution of the lattice parameters and the cell volume was calculated by means of the Rietveld method. We found that the a lattice parameter grows with the synthesis temperature and approaches the bulk value for the higher temperature. Instead the c parameter remains almost unchanged in the studied samples. The calculated cell volume grows from 288.22 (7) Å^ 3 to 289.40 (5) Å^3 when the synthesis temperature increases; this expansion is related to the relaxation of the crystalline structure when the synthesis is performed at higher temperature. TEM images show that the particles present ellipsoidal shape with a major axis of ~ 170 nm and a minor axis that 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 particles with the minor axis of 30 nm. On the other hand, the smallest nanoparticles present nearly round shape (mean diameter ~ 7.8 nm) and a superparamagnetic behaviour with a blocking temperature TB = 30 K when the measurement is performed with H = 50 Oe. We analyse these results as a function of the surface disorder and the spin canting generated by the reduction of the particle size to nanometric scale.