1.Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China 2.School of Science, Inner Mongolia University of Science and Technology, Baotou 014010, China 3.State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51861030, 51571126) and the State Key Development Program for Basic Research of China (Grant No. 2016YFB0700900).
Received Date:14 March 2019
Accepted Date:07 June 2019
Available Online:01 September 2019
Published Online:05 September 2019
Abstract:The coercivities in rare earth permanent magnets even with the same intrinsic properties may differ largely. In this paper, what determines the coercivity is discussed via the investigation of thermal activation in Pr-Fe-B ribbons prepared by melt-spinning method. The thermal activation, resulting from thermal fluctuation overcoming the energy barrier under the applied field, is the critical behavior of magnetization reversal. The activation size is comparable to the theoretical domain wall size, implying that the magnetization reversal undergoes the nucleation of revered domain wall at grain outer-layer in Pr-Fe-B ribbons, and the defects near the grain boundary are critical for the magnetization reversal and coercivity. The exchange coupling between the defect region at grain outer-layer and the perfect region in the inside of grain promotes the nucleation of reversed domain and the depinning of domain wall motion. The reduced anisotropy of the defect region also contributes to the overcoming of energy barrier of magneto crystallie anisotropy in the inside of Pr-Fe-B grains by the coupling effect, so the nucleation field of reversed domain and coercivity decrease largely, and the domain wall size is a little larger than the theoretical value due to the coupling between the defect region at grain outer-layer and the perfect region in the inside of grain in the critical magnetization reversal. In Pr2Fe14B/α-Fe composite magnets, the exchange coupling between the soft and hard magnetic phase leads the domain wall size to increase in the critical magnetization reversal of thermal activation, and so the exchange energy plays a role in overcoming the energy barrier, resulting in the further decrease of coercivity. Via the addition of Ti and Nb element, the coercivity increases significantly. Based on the investigation of thermal activation, the size of defect region involved in the domain wall decreases, and the contribution of the anisotropy in the defect region and interface to the overcoming of energy barrier is weakened, so the applied magnetic field should be increased in the magnetization reversal. The coercivity can be enhanced by reducing the size of defect region at grain outer-layer and by making the anisotropy change abruptly at the interface between the hard and soft magnetic phase. Keywords:permanent magnets/ coercivity/ domain wall/ defect
虽然反磁化过程基本清晰, 但对晶粒边界和内部耦合是如何决定磁反转场和矫顽力问题还需进一步研究. 在外场小于磁反转场的情况下磁反转是可逆的, 当越过晶粒内部磁晶各向异性场势垒才能实现不可逆的反磁化[10,12]. 反磁化的热激活源于热扰动通过激活体积越过势垒的不可逆反磁化[15,16], 研究热激活可以探索反磁化临界过程的磁反转场和矫顽力. 图4为薄带在温度300 K磁场保持1200 s测量的热激活反磁化曲线. 首先, 将样品在正方向饱和磁化, 然后负方向加一约为矫顽力大小的磁场并保持1200 s. 由于热扰动, 即使外磁场并没有增加, 一些反转场稍高的磁矩会发生反转. 保持磁场1200 s之后, 以较慢速度10 Oe/s增加磁场, 这时磁体磁矩较稳定, 只有磁场升到一定值时才出现显著的磁反转, 这段磁场增加值就是热扰动的后效场, 也就是热扰动场. 如图4所示, 可通过对反磁化曲线做切线来获得热扰动场${H_{\rm{f}}}$[17]. 图 4 温度300 K磁场保持1200 s样品的热激活后的磁行为, 插图为热激活不可逆过程的激活尺寸和理论磁畴壁尺寸 Figure4. The magnetization behaviors of thermal activation for 1200 s of waiting time at temperature of 300 K, and the inset shows the activation size of thermal activation and the ideal domain wall size.
图 1 样品粉末的X射线衍射谱
图 2 样品在温度300 K的磁滞回线
图 3 样品的
图 4 温度300 K磁场保持1200 s样品的热激活后的磁行为, 插图为热激活不可逆过程的激活尺寸和理论磁畴壁尺寸
图 5 样品在温度300 K的
