resumo
We report a study on the remanent magnetization M-r induced by field cooling across the ordering temperature T-N in antiferromagnetic CoO nanoparticles with different sizes. The nanoparticles are composed by a structurally and magnetically ordered core and a structurally ordered and magnetically disordered shell with a thickness of about 2 nm. The ordered core has cell parameters, moments direction, and modulus similar to those of bulk CoO. Mr is shown to be proportional to the cooling field H-cool. The low-temperature saturation values of M-r [M-r(0)] in the CoO nanoparticles are about two orders of magnitude higher than those found for bulk CoO. M-r/M-r(0) of CoO nanoparticles scales with temperature in a single curve, independently on the magnitude of Hcool and on nanoparticles size, except for temperatures near to T-N since T-N is size dependent.
palavras-chave
X-RAY-DIFFRACTION; EXCHANGE BIAS; PARTICLES; SIZE; TEMPERATURE; GAMMA-FE2O3; FERROMAGNET; ANISOTROPY; NEUTRON; SPINS
categoria
Physics
autores
Silva, NJO; Millan, A; Palacio, F; Martins, M; Trindade, T; Puente-Orench, I; Campo, J
nossos autores
agradecimentos
We acknowledge the collaboration of Thomas Hansen in the experiments at D20 and ILL for beamtime allocation. We also acknowledge R. Soares for the XRD patterns and A. Ferreira for enlightening comments about XRD instrumental contribution. The Aveiro-Zaragoza collaboration has been supported by the Integrated Spanish-Portuguese Action under Grant No. PT2009-0131. The work in Zaragoza has been supported by the research Grants No. MAT2007-61621, No. MAT2009-13977-C03-01, and No. CONSOLIDER CSD2007-00010 from the Ministry of Education. N.J.O.S. acknowledges CSIC for a I3P contract and FCT for Ciencia 2008 program.