Impact-induced demagnetization, magnetostriction and coercive force in co-doped iron oxide recording tapes

The saturation magnetostriction constant (λs) determines the remanence loss of a recording tape when an impact is applied normal to the plane of the tape. The addition of Co to iron oxide particles alters λsand increases the coercive force. As Co is added to γ-Fe2O3, λsbecomes more negative and the stress demagnetization is greater; when Co is added to Fe3O4, λsis less positive and the stress demagnetization becomes smaller. Impact-induced losses of up to 30% of the remanence are measured on Co doped γ-Fe2O3tapes withλₛ = -15 × 10⁻⁶; this compares with a 1% loss in CrO2tapes whereλₛ ~ 1 × 10⁻⁶ . λₛis measured on particle compacts using a strain gage; its value can also be estimated from the coercive force change produced when applying uniaxial tension to a tape.