Synthesis and magnetic investigation of ordered mesoporous two-line ferrihydrite.

We report here for the first time on the synthesis and characterization of ordered mesoporous two-line ferrihydrite. Ordered mesoporous ferrihydrite has been prepared via the nanocasting route. Two types of ordered mesoporous silica, two-dimensional hexagonal SBA-15 and three-dimensional cubic KIT-6, were employed as hard templates. The magnetic behavior of ferrihydrite replicas with an average diameter of about 7 nm was investigated by direct current magnetometry. The temperature dependence of magnetization shows a superparamagnetic transition around 70 K. Field-induced changes in the low-field behavior of the magnetization were observed below 30 K. The results are explained assuming a spin-glass-like state of the surface spins. The 2D hexagonal two-line ferrihydrite showed very large coercivity up to 1.6 kOe at 5 K.