Nanophase hard magnets

The effect of ratio z on the high temperature magnetic properties of Sm(Co, Fe, Cu, Zr)z magnets have been examined in a range of ratio z values from 6.7 to 9.1. Reasonably high coercivity at room temperature has been achieved in all the magnets. It was found that the lower the ratio z, the smaller the temperature coefficient of coercivity. When the ratio z=7.0, a temperature coefficient of coercivity of −0.03%/°C can be achieved, which is more than eight times smaller than the magnet with z=8.5. A coercivity of more than 10 kOe has been obtained at 773 K for the newly developed magnets, which is believed to be the highest coercivity at 773 K ever reported. Transmission electron microscope studies showed that the cell size decreases with the decrease of the ratio z, while the density of lamella phase remains almost the same. This suggests that smaller cell size leads to a smaller temperature coefficient of intrinsic coercivity.

Microstructure and high temperature magnetic properties of Sm(Co, Cu, Fe, Zr)z (z=6.7–9.1) permanent magnets

We have studied the magnetization reversal and magnetoresistance behavior in lithographically defined structures based on a 400 A Ni80Fe20 film The structures consist of square arrays of holes with size d in the range from 05 to 15 μm fabricated using electron beam lithography and an optimized pattern transfer process For the field applied along the intrinsic easy axis, a marked increase in the coercive field is observed as the hole size is decreased This has been attributed to the pinning of the domain walls in the vicinity of the holes However, for the field applied along the intrinsic hard axis direction, there is a marked increase in the remanence as the hole size is reduced due to the competition between the intrinsic uniaxial anisotropy field and the shape induced magnetic anisotropy field Unusual magnetoresistance effects are observed as a function of orientation of applied field in submicron structures

Magnetic properties of arrays of “holes” in Ni80Fe20films

In this study, we investigated intermediate-heat treatment (IHT) at temperatures between those of sintering and solution treatment, and evaluated the effects on the macrostructure, microstructure, and magnetic properties of Sm(Cobal.Fe0.35Cu0.06Zr0.018)7.8. We found that squareness was clearly improved by adopting the IHT, which promotes grain growth. These results indicate that reducing the fraction of grain boundaries by increasing the grain size affects the behavior of domain-wall motion. Magnetic properties of Mr 1.22 T, HcJ 1580 kA/m, and (BH)max 282 kJ/m3 (>35 MGOe) were obtained for Sm(Cobal.Fe0.35Cu0.06Zr0.018)7.8 subjected to IHT at 1453 K.

Influence of intermediate-heat treatment on the structure and magnetic properties of iron-rich Sm(CoFeCuZr)Z sintered magnets

We demonstrate a simple method to tailor the magnetization reversal mechanisms of Co/Pt multilayers by depositing them onto large area nanoporous anodized alumina (AAO) with various aspect ratios, A=pore depth/diameter. Magnetization reversal in composite (Co/Pt)/AAO films with large A is governed by strong domain-wall pinning, which gradually transforms into a rotation-dominated reversal for samples with smaller A, as investigated by a first-order reversal curve method in conjunction with analysis of the angular dependent switching fields. The change in the magnetization reversal mode is attributed to topographical changes induced by the aspect ratio of the AAO templates.

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Controlling magnetization reversal in Co/Pt nanostructures with perpendicular anisotropy

We have enlarged the theory of ferromagnetic domain wall pinning by defects to include large as well as small deviations of the magnetic anisotropy, magnetization, and/or magnetic exchange energy of the defect, from the host material for all values of the defect width. In particular, we obtain results for the coercive force due to such pinning as a function of two dimensionless constants characterizing the defect and of the defect width. We also obtain results for the maximum obtainable coercive force in terms of these parameters. We discuss these results in terms of applications to the Sm2(Co, Cu, Zr, Fe)17 hard magnetic materials.

D. I. Paul

Papers

Extended theory of the coercive force due to domain wall pinning