Paramagnetic-superparamagnetic transition in molecular-sieve-supported antiferromagnetic particles

Molecular sieves of the type CoAPO-5 have been studied by TEM and ac susceptibility measurements. In the as-synthesised state the material is paramagnetic due to the isolated cobalt ions and microstructurally it looks very homogeneous. After calcination the system develops very small cobalt-containing particles of A size between 1 and 3 nm, is paramagnetic at high temperature, superparamagnetic below about 20 K and starts 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 transition of antiferromagnetic particles. The susceptibility data have been used to estimate the degree of intraparticle spin uncompensation.