| Summary: | 博士 === 國立中正大學 === 物理所 === 97 === The magnetic properties, phase evolution, and exchange coupling effect of melt spun FexPt100-x (x = 50 - 70), (FePt)100-xAux (x = 0 - 40), (FexPt1-x)100-yBy (x = 0.6 - 0.7; y = 12 - 20), Fe56.7Pt25.3M2B16 (M = none, Ta, Nb, Ti and Zr), and [(Fe1-xCox)1-yPty]100-zBz (x = 0 - 0.5; y = 0.325 - 0.275; z = 18 - 14) ribbons have been investigated.
For binary FexPt100-x ribbons, even though an ordered L10-FePt phase can be formed for x ? 65 after optimized thermal treatment, the coercivity is below 2.1 kOe due to the coarser microstructure in the as-quenched state. For the Au added, FePtAu ribbons, L10-FePt granular phase is isolated by Au-rich phase. The higher volume fraction of Au addition seems not to modify the composition of the L10-FePt phase. With the increase of Au content, grain size becomes finer, leading to a remarkable enhancement in coercivity from 2.1 kOe for Au-free ribbon to 19.5 kOe for 40 at% Au addition.
For FePtB ribbons, the addition of sufficient boron not only promotes ordering transformation but also refines grain size of each phase. Multiple phases, namely ordered L10-FePt, Fe2B and Fe3B, with nanoscaled grain size are easily formed in the ribbons after proper annealing. Strong exchange-coupling between magnetically hard and soft phases in FePtB nanocomposite ribbons gives rise to an enhancement in permanent magnetic properties. An attractive magnetic properties of Br = 9.4 kG, iHc = 7.5 kOe, and (BH)max = 14.0 MGOe have been successfully developed in (Fe0.675Pt0.325)84B16 ribbons. On the other hand, the substitution of refractory elements on the Fe56.7Pt25.3M2B16 ribbons tends to impede the ordering transformation. A higher annealing temperature is necessary for the formation of ordered L10-FePt phase, giving rise to the formation of coarse or/and inhomogeneous grains, which result in deteriorated of magnetic properties.
The substitution of Co for Fe in FePtB ribbons can effectively enhance the coervicity due to the formation of ordered L10-(Fe,Co)Pt phase with higher anisotropy field. The [(Fe0.7Co0.3)0.675Pt0.325]84B16 ribbon having the highest intrinsic coercivity of iHc=10.0 kOe exhibits minimum value in activation volume V = 12.71×10-19 cm3. However, for excess Co substitution, longer annealing time is required for ordering transformation, leading to nonuniform and coarser microstructure and the deterioration of magnetic properties of the ribbons. In this study, even though the activation energy for ordering transformation is raised, the best permanent magnetic properties of Br = 10.1 kG, iHc = 5.4 kOe, and (BH)max = 15.7 MGOe can be achieved in boron-less and Pt-lean [(Fe0.7Co0.3)0.725Pt0.275]85B15 ribbons after optimal the
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