Hard/soft permanent magnets have attracted a lot of attention because of their rich magnetic properties and their potential for realizing giant energy products. However, energy products obtained by scientists in experiments are much smaller than the theoretical values, which has been studied by various analytical and numerical methods. The famous Stoner-Wohlfarth model(S-W model) is too simple to give the hysteresis loops whereas the intensively used variational method is too complicated to reveal the underlying mechanism in a simple form. The analytical model proposed in this paper maintains a balance between simplicity and precision, where the spins in the soft layer rotate fast and coherently with the applied field while those in the hard layer response to the applied field much slower but also coherent. An exchange coupling is provided to maintain the exchange spring which drags the spins in the hard layer to follow those in the soft layer. Similar to the more sophisticated model, the calculated hysteresis loops display three typical magnetic phases, i.e., the rigid composite magnet, the exchange spring and decoupled magnet, whereas the simple SW model can only give one single phase, i.e., the rigid composite one. In addition to the hysteresis loop, the energy product and the nucleation fields have been calculated and compared with those calculated by other methods, which justifies our model.Careful comparisons show that our calculations are in good agreement with the experimental results and other theoretical results, especially for the important coercivity value and the related mechanism.
Coercivity mechanism in permanent magnets has been debated for many years.In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast.Coherent rotation and curling models can reveal the underlying reversal mechanism clearly based on isolated grains with elliptic shapes.By contrast, the numerical methods consider inter-grain interactions while simulating the evolution of the spins and hysteresis loops with complicated shapes.However, an exact simulation of magnetic reversal in permanent nanomagnets requires many meshes to mimic the thin domain wall well.Nucleation and pinning are the two main coercivity mechanisms in permanent magnets.The former signifies the beginning of the magnetic reversal, whilst the latter completes it.Recently, it is proposed that the large difference between the intrinsic magnetic properties of the nucleation centers and those of the main phase can result in a large pinning field(self-pinning), which has the attributes of both traditional nucleation and pinning.Such a pinning explains the experimental data of permanent magnets very well, including the enhancement of the coercivity by the grain boundary pinning.
本文利用高温油相法制备出尺寸、形状均一的MnO纳米颗粒,X射线衍射图(XRD)和透射电子显微镜(TEM)照片清晰表明MnO纳米颗粒为单一的面心立方岩盐晶体结构,尺寸为15 nm,粒径分布很窄.通过零场冷却(ZFC)和带场冷却(FC)的磁滞回线发现MnO纳米颗粒具有明显的交换偏置效应,而且磁滞回线同时表现出横向和纵向偏移.横向偏移说明纳米颗粒中两相复合的存在,纵向偏移说明了存在自旋玻璃相或者超顺磁相.进而通过不同频率下随温度变化的交流磁化率的测定,根据Mydosh的经验数值确认MnO纳米颗粒表面层为自旋玻璃相,并得到MnO纳米颗粒表面自旋玻璃相的转变温度为TSG=32 K.
