Methods of Test for Rare Earth Magnet Materials

(一)无闭合磁路的感应法测量磁化强度工业标准中规定的永磁体材料的测试方法是使用铁芯电磁铁的回路示踪剂的闭合磁路法。如果矫顽力超过电磁铁产生的最高磁场，则不能获得完整的磁滞曲线。在本工作中，高功率水冷磁体产生较高的磁场，并用开路磁路的感应法测量磁化强度。通过计算得到磁化强度的绝对值，并用纯镍样品进行标定。(2)由软磁材料的磁化强度决定的退磁系数在开路磁路法中，需要对退磁磁场进行校正。由于磁化强度的不均匀性，柱状样品的校正比较困难。然而，对于软磁材料，平均退磁因子可以从初始磁化曲线的梯度确定。我们使用不同尺寸比例的纯镍柱测定了这些因素，尽管这种修正是否适用于硬磁材料是值得怀疑的。(3)许多制造商提供了各种永磁材料和稀有稀土磁性材料的磁化测量，如烧结或粘结的Sm-Co和Nd-Fe-B。测量了直径为10 mm、长度为2 mm、3.5 mm、7 mm和14 mm的圆柱形样品的磁化强度。用镍样品确定的退磁因子进行校正后，不同尺寸比样品的磁化曲线基本一致。然而，应该强调的是，硬磁材料的退磁校正在原则上是不合理的，因此当所获得的数据被用作工业标准时，应具体说明样品的尺寸。

(1) Magnetization measurements by an induction method without a closed magnetic circuitThe methods for testing permanent magnet materials stipulated in the industrial standards are closed magnetic circuit methods which employ a loop tracer using an iron-core electromagnet. If the coercivity exceeds the highest magnetic field generated by the electromagnet, full hysteresis curves cannot be obtained. In the present work, higher magnetic fields were produced by a high-power water-cooled magnet, and the magnetization was measured by an induction method with an open magnetic circuit. The absolute value of magnetization was obtained by calculation, and calibrated by pure nickel samples.(2) Demagnetizing factors determined by the magnetization of soft magnetic materialsIn the open magnetic circuit method, the demagnetizing field should be corrected. The correction for cylindrical samples is rather difficult because of the non-uniformity of magnetization. For soft magnetic materials, however, mean demagnetizing factors can be determined from the gradient of initial magnetization curve. We determined these factors using pure nickel cylinders with various dimension rations, although the application of this correction to hard magnetic materials is doubtful.(3) Magnetization measurements for various permanent magnet materialsRare earth magnet materials such as sintered or bonded Sm-Co and Nd-Fe-B were provided by a number of manufacturers. Magnetizations of cylindrical samples with 10 mm in diameter and 2 mm, 3.5 mm, 7 mm or 14 mm in length were measured. The magnetization curves for the samples with different dimension ratios become almost identical after the correction using the demagnetizing factors determined by nickel samples. It should be emphasized, however, that the demagnetizing correction for hard magnetic materials is not justified in principle, and so the dimensions of the sample should be specified when the data obtained are used as industrial standards.