PARAMAGNETIC-SUPERPARAMAGNETIC TRANSITION IN MOLECULAR-SIEVE-SUPPORTED ANTIFERROMAGNETIC PARTICLES

Molecular sieves of the type CoAPO-5 have been studied by TEM and ac s usceptibility measurements. In the as-synthesised state the material i s paramagnetic due to the isolated cobalt ions and microstructurally i t looks very homogeneous. After calcination the system develops very s mall cobalt-containing particles of a size between 1 and 3 nm, is para magnetic at high temperature, superparamagnetic below about 20 K and s tarts to experience particle blocking below about 5 K. The tendency of cobalt to produce oxides and hydroxides of antiferromagnetic nature, together with the observed reduction of the effective moment below 20 K suggests that this is indeed a paramagnetic-superparamagnetic transi tion of antiferromagnetic particles. The susceptibility data have been used to estimate the degree of intraparticle spin uncompensation.