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

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The saturation magnetostriction constant (λ s ) 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 λ s and increases the coercive force. As Co is added to γ-Fe 2 O 3 , λ s becomes more negative and the stress demagnetization is greater; when Co is added to Fe 3 O 4 , λ s is less positive and the stress demagnetization becomes smaller. Impact-induced losses of up to 30% of the remanence are measured on Co doped γ-Fe 2 O 3 tapes with \lambda_{s} = -15 \times 10^{-6} ; this compares with a 1% loss in CrO 2 tapes where \lambda_{s} \sim 1 \times 10^{-6} . \lambda_{s} is 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.