Magnetic instability of iron oxide thin films

The coercivity H/sub c/ and coercive squareness S* of gamma -Fe/sub 2/O/sub 3/ films made by oxidation of reactively sputtered, Os-doped Fe/sub 3/O/sub 4/, 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/sub c/ and an S* of 0.85 to 0.95, corresponding to global anisotropy in the range of 5*10/sup 4/ erg/cm/sup 3/. Reversible changes happen so rapidly that large differences can be seen on consecutive vibrating sample magnetometer loops. These films also exhibit very high susceptibility ( chi >10/sup -3/ G/Oe) and rotational hysteresis (W/sub r/>10/sup 5/ erg/cm/sup 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 degrees C. All iron oxide films made by oxidation of reactively sputtered Fe/sub 3/O/sub 4/, whether undoped or Co- or Os-doped, show both larger W/sub r/ and global anisotropy reversible at room temperature.<
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