Size dependence of the anisotropy constant in antiferromagnetic Cr2O3 nanoparticles

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

Buenos Aires, Argentina

Workshop; At the Frontiers of Condensed Matter IV; 2008

Surface and size effects in magnetic particles have been a subject of increasing interest in the past few decades. When the size of the particles is reduced to nanometric scale, the surface to volume ratio increases and as a result the surface effects become more and more important, affecting the internal magnetic order and the magnetic phase transitions. In a recent work [1] we have reported a study of the magnetic behaviour of Cr2O3 nanoparticles systems as a function of the particles size. We showed that the parameters that characterize the AFM system, as the exchange and anisotropy eld constants, are strongly modi ed when the size of the nanoparticles is reduced. In particular, we have observed a decrease of more than one order of magnitude in the anisotropy constant, from K=3.8 104 to 8.7 102 erg/g, when the sized was reduced from bulk to 30 nm. 2O3 nanoparticles systems as a function of the particles size. We showed that the parameters that characterize the AFM system, as the exchange and anisotropy eld constants, are strongly modi ed when the size of the nanoparticles is reduced. In particular, we have observed a decrease of more than one order of magnitude in the anisotropy constant, from K=3.8 104 to 8.7 102 erg/g, when the sized was reduced from bulk to 30 nm. In this work we continue with the study of the magnetic properties of the Cr2O3 system when the size of the nanoparticles is further reduced to ~ 8 nm. The smallest nanoparticles present superparamagnetic behaviour with a blocking temperature of TB  ~ 28 K. As a consequence of the size reduction we observed an  enhancement in the surface spin disorder manifested in the enlargement of the coercive field up to 1400 Oe. In addition we have observed an important reduction of the Néel temperature to ~ 270 K. We calculated the evolution of the anisotropy field as a function of the nanoparticle size. Finally we analyze which contribution, the magnetocrystalline or the surface anisotropy, governs the magnetic behaviour when the surface to volume ratio increases. [1] D. Tobia, E. Winkler, R.D. Zysler, M. Granada and H.E. Troiani, Phys. Rev. B 78, 104412 (2008)