To reduce the irreversible demagnetization of Nd-Fe-B sintered magnets at elevated temperatures, it is required to discover the method of improving the squareness ratio (Hk/HcJ), where Hk and HcJ are the knee field and coercivity, respectively. It is pointed out that the intrinsic nucleation field (HN) and instability field of uniaxial single-domain particles are determined the effects of the second anisotropy constant (K2) and oblique magnetic fields. HN is influenced by the misaligned grains which deteriorate Hk/HcJ. Based on the results, we studied mainly the effects of K2 to the first anisotropy constant (K1) on Hk/HcJ for Nd-Fe-B sintered magnets at a temperature range from 298 to 473 K. It has also been examined the values of limiting condition (4K2/K1 <; 1) for preventing from the deterioration of Hk/HcJ. The samples used were Nd-Fe-B sintered magnets with HcJ of 0.99 and 2.73 MA/m. For a magnet with lower HcJ above approximately 400 K, the value of Hk/HcJ decreases rapidly, whereas K2/K1 increases. For a magnet with higher HcJ, Hk/HcJ slightly decreases and K2/K1 gradually increases as the temperature increases. However, the temperature changes in the values of Hk/HcJ and K2/K1 for a magnet with the higher HcJ are considerably smaller than those of a magnet with the lower HcJ. The temperature dependence of Hk/HcJ and K2/K1 shows the opposite trends regardless of HcJ. The value of 4 K2/K1 shows the opposite trend of Hk/HcJ also. According to a simulation of the torque curve, the direction of the easy axis was widely distributed in the case of a very large value for K2/K1. The excessive K2/K1 contributes to spread out the region of easy axis, that is, it will hinder the uniaxial anisotropy which is essential for anisotropic permanent magnets. Most of these may be attributed to the effect of large K1 of indispensable Dy for a very high HcJ magnet.

中文翻译：

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磁各向异性对不同矫顽力钕铁硼烧结磁体矩形比的影响


为了减少Nd-Fe-B烧结磁体在高温下的不可逆退磁，需要找到提高矩形比（Hk/HcJ）的方法，其中Hk和HcJ分别是拐点磁场和矫顽力。指出单轴单畴粒子的本征成核场(HN)和不稳定场是由第二各向异性常数(K2)和倾斜磁场的影响决定的。 HN 受到未对准晶粒的影响，导致 Hk/HcJ 恶化。基于该结果，我们主要研究了在 298 至 473 K 温度范围内 K2 和第一各向异性常数 (K1) 对 Nd-Fe-B 烧结磁体 Hk/HcJ 的影响。防止Hk/HcJ恶化的限制条件(4K2/K1<；1)。使用的样品是 Nd-Fe-B 烧结磁体，HcJ 分别为 0.99 和 2.73 MA/m。对于 HcJ 较低的磁体，在大约 400 K 以上时，Hk/HcJ 值迅速下降，而 K2/K1 则增加。对于HcJ较高的磁体，随着温度的升高，Hk/HcJ略有下降，而K2/K1逐渐增加。然而，具有较高HcJ的磁体的Hk/HcJ和K2/K1值的温度变化明显小于具有较低HcJ的磁体的温度变化。无论 HcJ 为何，Hk/HcJ 和 K2/K1 的温度依赖性显示出相反的趋势。 4 K2/K1 的值也显示了与 HK/HcJ 相反的趋势。根据扭矩曲线的模拟，在K2/K1值很大的情况下，易轴方向分布广泛。过高的K2/K1有助于扩展易轴区域，也就是说，它将阻碍各向异性永磁体所必需的单轴各向异性。 其中大部分可能归因于对于非常高 HcJ 磁体而言必不可少的 Dy 的大 K1 的影响。