Remanent magnetization in CoO antiferromagnetic nanoparticles.

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.