Magnetic instability of iron oxide thin films

The coercivity Hand coercive squareness S* of γ - FeO films made by oxidation of reactive-sputtered, Os-doped Fe0, 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 Hand an S* of 0.85 to 0.95, corresponding to a global anisotropy in the range of 5 × 10⁴erg/cm³. Reversible changes happen so rapidly that one can see large differences on consecutive VSM loops. These films also exhibit very high susceptibility (x > 10^-3G/Oe) and rotational hysteresis (W> 10⁵erg/cm³) 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 decreases initially 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 reactive-sputtered Fe0, whether undoped or Co- or Os-doped, show both large W and global anisotropy reversible at room temperature. In fact these features are already present in as-deposited, undoped FeO. © 1989 IEEE