Contrasted role of disorder for magnetic properties in an original mixed-valency iron phosphate

Abstract

We have measured the magnetic properties of a mixed-valency iron phosphate. It presents an original structure with crossed chains containing ${\text{Fe}}^{\text{II}}$ and orthogonal to the longest direction of the crystallites. Microstructural investigations using electron microscopy show the presence of random nanotwinning. The ac susceptibility measurements demonstrate similarities with the kinetics of a disordered magnetic, spin-glasslike state but are shown to be essentially due to this peculiar disorder. Scaling properties are characteristics of three-dimensional second-order transition implying that this disorder at a small scale does not influence significantly long-range magnetic ordering. At low temperature, a decrease in the spontaneous magnetization and an irreversible metamagnetic transition is observed, and is attributed to a canting of the spins in the iron chain.