Study of superparamagnetic clusters in Co2+-exchanged NaY zeolite

Magnetic properties of Co2+-exchanged NaY zeolite have been investigated using a dc magnetization technique. In the presence of low applied magnetic fields, a difference between zero-field-cooled and field-cooled magnetizations is observed below ∼3.4K. At T>5K, the M vs H∕T curves superimpose and these curves could be fitted well using the Langevin function, which yields a saturation moment of 3.2μB per Co2+ ion and a cluster moment of 6.66μB. This indicates that there are superparamagnetic clusters which are formed due to a positive exchange coupling among the Co2+ ions. For T<5K, the M vs H∕T curves do not superimpose on the curves for T>5K. The analysis of the low-temperature M vs H data suggests the building up of weak antiferromagnetic-type correlation between the superparamagnetic clusters at T<5K.