Magnetic instability of iron oxide thin films

The coercivity H_c and coercive squareness S* of γ-Fe₂O₃ films made by oxidation of reactively sputtered, Os-doped Fe₃O₄, are shown to change by as much as 30 and 50% respectively in one day at room temperature. On initial measurement the films are isotropic in the plane and have S* values of 0.5 to 0.7. Along the direction of remanence an easy axis develops with a 10 to 30% higher H_c and an S* of 0.85 to 0.95, corresponding to global anisotropy in the range of 5×10⁴ erg/cm³. Reversible changes happen so rapidly that large differences can be seen on consecutive vibrating sample magnetometer loops. These films also exhibit very high susceptibility (χ>10^-3 G/Oe) and rotational hysteresis (W_r>10⁵ erg/cm ³) at 16 kOe, suggesting random local anisotropy with components in excess of 50 kOe. Rotational hysteresis has a strong dependence on the rate of field rotation. In a stationary field the torque initially decreases logarithmically with time, yielding activation energies varying from 0.6 to 0.16 eV as the temperature increases from 24 to 120°C. All iron oxide films made by oxidation of reactively sputtered Fe₃O₄, whether undoped or Co- or Os-doped, show both larger W_r and global anisotropy reversible at room temperature