Processing and Hard Magnetic Properties of Nanocrystalline Sm(Co,Zr) 7 Magnet Powders
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Processing and Hard Magnetic Properties of Nanocrystalline Sm(Co,Zr)7 Magnet Powders H. Tang1, 3, J. Zhou1, 3, Y. Liu2, 3, and D.J. Sellmyer1, 3 1 Behlen Laboratory of Physics, 2 Department of Mechanical Engineering, 3 Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588-0113 ABSTRACT Isotropic nanocrystalline Sm(Co,Zr)7 permanent magnet powders of the TbCu7 type structure and with high coercivity and enhanced remanent magnetization has been synthesized by mechanically milling Sm12.5Co87.5-xZrx alloys (x = 0, 1, 2, 3) alloys and subsequently appropriate annealing. The mechanical-milling process of the alloys and the formation of nanostructured Sm(Co,Zr)7 magnet powders have been investigated with respect to hard magnetic properties. Hard magnetic properties are found to be strongly dependent upon the processing condition (like milling time, annealing temperature, etc.). Optimal coercivity Hci of value above 21 kOe has been obtained in Sm12.5Co85.5Zr2 magnet powders subjected to milling for 5 hr and annealing at 600°C for 20min. Optimal remanent magnetization Mr of 73.4 emu/g, remanence ratio Mr/Ms of 0.71, and maximum energy products (BH)max of over 13 MGOe have been realized in Sm12.5Co87.5-xZrx (x = 1, 2) magnet powders with grain size of 15-20 nm. The hard magnetic properties are ascribed to the nanosized Sm(Co,Zr)7 phase of the TbCu7 type structure with grain size of 10-20 nm. The enhancement of remanent magnetization may be contributed from the enhanced exchange-coupling interaction between nanosized grains.
INTRODUCTION Sm-Co based rare earth permanent magnet materials have attracted much attentions [1,2] during the recent search for high temperature permanent magnets, due to their high Curie temperature, huge anisotropy and good temperature stableness of magnetic properties [3]. Many attempts have been focused on the nanostructured Co-based rare earth permanent magnets [1,2, 4-7] to obtain high coercivity with much enhancement of remanent magnetization. Owing to the capability of formation of amorphous alloys (generally the precursor of nanostructured alloys), Co-based nanostructured alloys are fabricated by means of MM or MA techniques [4,5,8]. Meanwhile, Sm(Co,Zr) (with a small amount of Zr addition) alloys of the TbCu7 type structure can be stabilized by introducing a small amount of transition metal elements like Zr, Ti, and is reported to have enhanced huge uniaxial magnetocrystalline anisotropy compared with that of their Sm2Co17 counterpart with the Th2Zn17 type structure [9]. Previous investigations [4,5,8] indicate that the obtained hard magnetic properties are dependent upon the processing condition: like milling time, annealing temperature, etc. In the present investigation, we will systematically study the processing (mechanical milling time) of nanocrystalline Sm(Co,Zr) hard magnets with the TbCu7 type structure. The hard magnetic properties will be discussed with respect to the nanostructure developed during the processing and the underlying intergrain interacti
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