Magnetic behavior of field induced spin-clusters in amorphous Fe₂O₃

The magnetic properties of bulk amorphous Fe₂O₃ have been investigated, using a dc magnetization technique, as a function of magnetic field and temperature. Evidence of four magnetic regimes: pure paramagnetic at T ≳ 110K at low applied field, field induced spin clusters (superparamagnetic like) at higher applied fields over the same temperature range, starting from ∼110 to 325 K (the highest measuring temperature), interacting spin clusters with progressive freezing over 35 ≲ T ≲ 110 K and collective freezing below ∼35 K has been found. In the field induced spin-clusters regime (T > 110 K), the magnetization data have been analyzed using a Langevin function with a log-normal distribution of the size of the spin-clusters. A mean diameter of ∼30 Å for the spin-clusters has been found. With increasing applied field, the freezing-temperature T_g(H) of spin-clusters decreases and follows the well-known de Almeida–Thouless (AT) line T_g(H) ∝ H^2/3 for low fields. The extrapolation of the AT line gives T_g(H=0) ≈ 35K