| Nanocomposite permanent magnetic material is composed of nano-structured soft and hard magnetic phases. Due to the exchange-coupling interaction between magnetically soft and hard grains, the enhancement of remanence and single ferromagnetic characteristic are exhibited simultaneously in nanocomposite magnets. Theoretical results indicate that the exchange-coupling interactions among magnetic grains can make nanocomposite magnet keep both the high saturation magnetization of soft phase and the high coercivity of hard phase, and the theoretical magnetic energy product of aligned nanocomposite magnet can achieve as high as 1MJ·m-3 . However, experimental results show that the enhancement of remanent magnetization is obvious, but coercivity decreases seriously and energy product of nanocomposite magnet is far lower than its theoretical value. This result is attributed to the following two aspects: one is the oversimplified theoretical models; the other is that the bulk nanocrystalline magnet with fully density and uniform microstructure can not be realized in experiment. The great contrast between experiment and theory leads to the shrink of investigation on the nanocomposite permanent magnetic material at the end of the last century. However, the report that the nanocomposite magnet with magnetic energy product as high as 45MGOe is prepared using hot press and hot deformation by S. Liu et al. evokes the great interest again since the last two years.According to the traditional ferromagnetic theory, the magnetic grain is superparamagnetic when the size is smaller than the critical dimension of single domain, which does not explain the dependence of the magnetic properties on grain size in nanocomposite magnets. Experimental investigations show that the remanence increases monotonously with reducing grain size, but the coercivity does not. For different phase compositions, the dependences of coercivity on grain size show the dissimilarity. (The coercivity of single hard phase decreases monotonously, but that of nanocomposites increases or shows the maximum withreducing the grain size)In order to make the grain size effect of nanocomposite magnet known, much research have been developed using many physical models and calculate methods by researchers. Thereinto, the coercivity mechanism is the focus of theoretical studies. Based on the traditional expression of coercivity in sintered magnet, Kronmuller et al. and Zhang et al. inserted an exchange-coupling coefficient αex in order to describe the effect of exchange-coupling interaction on the coercivity. Thus, the expression of coercivity in nanocomposite magnet would contain the factor of grain size. In facts, the magnetic properties of nanocomposite permanent material not only depend on the grain sizes and two phase fractions but also the phase distribution. So, we think the expression of coercivity not only include the influences of grain size and phase fraction, but also the phase distribution.The random anisotropy model (RAM) which was put forward by Alben et al. and applied to explain the excellent soft magnetic properties by Herzer indicates that two kinds of correlation lengths coexist in the nanocomposite system, i.e., the structural correlation lengths and magnetic correlation, which correspond to the average grain size D and ferromagnetic exchange length Lex, respectively. Their competition leads to the variation of exchange-coupling interactions. Based on the RAM, Areas et al. put forward the partial exchange-coupling model, i.e., when grain size is lager than ferromagnetic exchange length, there exists partial exchange-coupling interaction among grains.We deem that the magnetic grains situated at the full exchange-coupling state and that situated at the partial exchange-coupling state coexist in nanocomposite permanent magnetic materials. The intrinsic magnetic parameters, grain size and the magnetic properties of neighbor grain determine the exchange-coupling state of individual grain together. The fractions and distributions of soft and hard phases lead to the fluctuations of exchange-coupling interactions.Based on the above analyses, taking Nd2Fe14B/α-Fe as a typical sample, adopting the simple cubic model and assuming the different distributions of soft and hard phases, we firstly calculated the dependences of effective anisotropy and coercivity on grain size in single hard magnets, and compared which with experimental results. Secondly, we calculated the effects of phase distribution on the effective anisotropy and coercivity, and compared with experimental results. Thirdly, we discussed the differences and sameness between our viewpoints and the others. At last, we analyzed the developmental trend of the investigations of nanocomposite permanent magnet. The main conclusions are listed as following:1. There exist exchange-coupling interactions in single phase nano-structured hard magnetic material. The intergrain exchange-coupling length Lex is near to the thickness of domain wall of Nd2Fe14B magnets, being about 4.2nm. When grain size D>Lex, the anisotropy constant increases with the law of r3/2, where r is the distance to the grain surface. When D, of magnetically soft and hard grains, theexpression of which is Keff = (i,j=s, h), where fij is the proportionsof the interface area of soft-soft, hard-hard, soft-hard and hard-soft grains. The values of fij are determined by the distributions of magnetically soft and hard phases. The coercivity of nanocomposite magnet can be expressed by Hc = pc , where v and J denote the volume faction andsaturation polarization, respectively, s and h is the logograms of soft and hard phase.When the grain sizes of the soft and hard phases are identical and the volume fraction of the soft phase is given, the coercivity, Hc, of nanocomposite Nd2Fe14B/α-Fe magnet decreases with the reduction of grain size, and rapidly decreases while the grain size is less than 20nm. For the given grain sizes and volume fractions of soft phase, Hc shows a peak value as a function of the hard grain size, which derives from the random distribution of magnetically soft and hard phases. The nanocomposite Nd2Fe14B/α-Fe magnets with the multilayer structure of soft and hard grains can possess the higher coercivity than that with the random distribution. It is a realistic possibility that the high coercivity is obtained by designing the ideal distribution of magnetically soft and hard phases.3. Our results are basically in accordance with the experimental results and other coercivity theories, but some slight differences also exist. The traditional coercivity theories of NdFeB magnets include nucleation and pinning mechanisms. In order to describe the grain size effects, the expression oftraditional theories, Hc = is revised according to theexperimental results. Kronmuller et al. inserted an exchange-coupling coefficient αex into the above equation. Zhang et al. replace the exchange-coupling coefficient aex by 1/(1+6βLex/d). Zhou et al. deemed that the exchange-coupling pinning field is related to the r0-1, where r0 is the radius of magnetically soft grain. Based on the random anisotropy of magnetic grains and considering the distribution of soft and hard magnetic phases, we put forward the expression of nanocomposite permanent magnetic materials as above expatiated. By comparing, we conclude that the exchange-coupling coefficient, aex, expresses essentially the reduction of effective anisotropy. The decrement of coercivity for nanocomposite permanent materials is mainly duo to the reduction of effective anisotropy.
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