Time-dependent magnetic behaviour of Ba ferrite particles at low temperature

The time dependence of the magnetization of pure and Co/Ti-doped Ba ferrite particles is analysed at low temperatures and compared with the dependence at room temperature. In undoped particles the variation of the magnetization and the magnetic viscosity are smaller at low than at room temperature, as expected from the increase of thermal stability of the magnetization with the decrease of temperature. On the contrary, in Co/Ti-doped particles for magnetic recording media, we observe an anomalous increase of the magnetization variation during the time and of magnetic viscosity when the temperature decreases. This behaviour is interpreted on the basis of the evolution of magnetic anisotropy with the temperature in doped Ba ferrite particles, where the magnetocrystalline anisotropy at low temperature becomes comparable with the shape anisotropy.