Magnetic properties of Sm(Co/sub bal/Fe/sub 0.31/Zr/sub 0.05/Cu/sub 0.04/B/sub x/)/sub z/ alloys and their melt-spun materials (x=0.02--0.04,z=7.5--12)
Cast alloys and melt-spun ribbons with nominal compositions of Sm(Co/sub bal/Fe/sub 0.31/Zr/sub 0.05/Cu/sub 0.04/B/sub x/)/sub z/ (x=0.02-0.04,z=7.5-12) have been synthesized and characterized in a temperature range of 10-1273 K and at fields up to 5T. The main phase in the as-cast alloys exhibited a Th/sub 2/Ni/sub 17/ type structure, with a strong uniaxial anisotropy. Minor phases with a TbCu/sub 7/ and/or CaCu/sub 5/ structure emerged as z (3d/R) decreased. As a result, the anisotropy field (H/sub A/) increased from 67 to 120 kOe, while 4/spl pi/M/sub s/ fell from 12.8 to 10.5 kG at 300 K when z was decreased from 12 to 7.5. For melt-spun ribbons, they are nano-structured in nature and magnetically hard, even in the as-spun state. By lowering the value of z(3d/R) and raising the B content, a finer microstructure and a higher H/sub ci/ were obtained. Hard magnetic properties of H/sub ci/=4.9-12 kOe, 4/spl pi/M/sub s/=9.0-12.0 kG at 300 K have been obtained from ribbon samples. Among them, Sm(Co/sub bal/Fe/sub 0.31/Zr/sub 0.05/Cu/sub 0.04/B/sub 0.02/)/sub 10/ ribbon showed the highest (BH)/sub max/ of 10.8 MGOe at 300 K. A Henkel plot analysis suggested the existence of exchange-coupling interaction between the magnetically hard and soft phases in the ribbon materials.
High coercivity in boron substituted Sm-Co melt-spun magnets
The structural and magnetic properties of nanocomposite melt-spun Sm(Co/sub 0.74-x/Fe/sub 0.1/Cu/sub 0.12/Zr/sub 0.04/B/sub x/)/sub 7.5/ magnets have been investigated as a function of boron content (x = 0.005 0.05), wheel speed and annealing conditions. The as-spun ribbons are nanocrystalline with fine microstructure and average grain size of 60-100 nm. X-ray diffraction indicates that the as-spun samples have the metastable hexagonal TbCu/sub 7/-type structure phase and fcc-Co as a secondary soft phase. Magnetization at nonsaturating 5 T field is 45-72 emu/g and the reduced remanence (Mr/Ms) is above 0.8. The loop shape exhibits a characteristic step due to the soft magnetic phase. At room temperature, Hc values of 20-28 kOe are obtained for as spun samples, with a record value of 38.5 kOe for x = 0.04. At 380/spl deg/C Hc values higher than 5 kOe are observed. Coercivity and loop shape are strongly dependent on annealing conditions.
Towards a more analytic understanding of the pinning of domain walls in high-temperature permanent magnets
We discuss analytic results for the depinning of domain walls based on different models for wall pinning in permanent magnets. We propose an explanation for the different temperature dependences of the coercive field for repulsive and attractive pinning potential when the pinning boundary forms an array of pyramids and suggest that the switching dynamics are different in the two regimes. This may be tested experimentally by magnetic viscosity measurements on samples with different Cu contents. We also found analytically that when the pinning boundary forms an array of cubes, the coercive field always decreases no matter whether the pinning potential is repulsive or attractive. This is confirmed by Monte Carlo simulations.