Plate domain structure of sintered NdFeB magnets and dependence on compacting mode
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Ye Qing He Beijing Zhong Ke San Huan High-Tech Co., Ltd., Beijing 100080, People’s Republic of China
Shou Zeng Zhou State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China (Received 26 February 2001; accepted 2 August 2001)
The alignment degree of sintered Nd–Fe–B magnets and its dependence on applied field and compacting mode were studied by magnetic force microscopy. By analyzing the magnetic force images to illustrate the magnetic-domain structure, an experimental method for quantitatively evaluating the alignment degree of sintered Nd–Fe–B magnets was given. The results show that if the compacting mode is the same, the alignment of magnets will be better as field increases. Under the same field, the alignment degree for rubber isostatic pressing with vibration is better than that for nonmagnetic metal die pressing. However, if the sample is compacted by rubber isostatic pressing without vibration, the alignment degree decreases significantly.
I. INTRODUCTION
To achieve high magnetic properties, alignment degree of permanent magnets should be as high as possible, i.e., the easy magnetization axes of grains in magnets need to be oriented along the same direction.1–3 For sintered Nd– Fe–B magnets, alignment degree depends on compacting mode, strength and direction of the applied field, and sintering process. Several methods have been used to quantitatively evaluate the alignment degree of sintered Nd–Fe–B magnets. The traditional one is to measure the ratio of magnetic remanence Br to saturation polarization Js.3,4 But the accurate measuring of Js requires a very high external magnetic field (greater than the anisotropic field of Nd2Fe14B, about 8 T). The alignment degree has been also quantified by the relative intensity ratio of I(105) /I(006) in the x-ray diffraction (XRD) spectrum of Nd–Fe–B sintered magnets5,6 but it is inaccurate because (105) is only one of the (hkl) planes that deviate from the (00n) plane and the (006) plane is also one of the (00n) planes that are perpendicular to the alignment direction. In fact, the intuitive method for showing alignment degree is to detect crystal grains and their domain structures in sintered Nd–Fe–B magnets. By using magnetic force microscopy (MFM) to measure magnetic force images, we can clearly observe grains and grain boundaries, nonmagnetic phases that exist in the boundary regions and plate domains and their orientation in grains. These are 2992
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J. Mater. Res., Vol. 16, No. 10, Oct 2001 Downloaded: 14 Mar 2015
the main factors that affect the hard magnetic properties of sintered Nd–Fe–B magnets.7 Therefore, alignment degree can be intuitively demonstrated by MFM measurement. In this article, the plate domain structure of sintered Nd–Fe–B magnets and its dependence on compacting mode and applied field during aligning were investigated using MFM. II. EXPERIMENTS
The (Nd0.95Dy0.05)15.5(Fe0.99Al0.01)78.0B6.5 powders with average particle size
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