Magnetic instability of iron oxide thin films
Abstract
The coercivity H<inf>c</inf>and coercive squareness S* of γ - Fe<inf>2</inf>O<inf>3</inf> films made by oxidation of reactive-sputtered, Os-doped Fe<inf>3</inf>0<inf>4</inf>, 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<inf>c</inf>and an S* of 0.85 to 0.95, corresponding to a global anisotropy in the range of 5 × 10<sup>4</sup>erg/cm<sup>3</sup>. Reversible changes happen so rapidly that one can see large differences on consecutive VSM loops. These films also exhibit very high susceptibility (x > 10<sup>-3</sup>G/Oe) and rotational hysteresis (W<inf>r</inf>> 10<sup>5</sup>erg/cm<sup>3</sup>) 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 Fe<inf>3</inf>0<inf>4</inf>, whether undoped or Co- or Os-doped, show both large W<inf>r</inf> and global anisotropy reversible at room temperature. In fact these features are already present in as-deposited, undoped Fe<inf>3</inf>O<inf>4</inf>. © 1989 IEEE