Effect Of Microstructure On The Effective Anisotropy And Coercivity In Nanometer Permanent Magnetic Materials

Permanent functional devices have been the central functional devices in the fields of compute, network, communication, spaceflight, traffic, office automatization et al. With the devices become smaller and smaller, the product energy of permanent materials is required more higher. Due to the exchange-coupling interaction between magnetically soft and hard grains, nanocomposite permanent magnetic materials can simultaneously keep both the high saturation magnetization of soft phase and the high coercivity of hard phase, and can have the high energy product. However, up to date now, the experimental energy product of nanocomposite magnetic materials is still far below the theoretical value. This is caused by the severe decrease of the coercivity despite the obvious increase of the remanence. Therefore, the application of the material is limited.Microstructure is an important factor influencing magnetic properties. According to the traditional ferromagnetic theory, the intrinsic magnetic parameter---magnetic crystalline anisotropy is determined by the chemical content of material, have no relationship with the microstructure. In fact, the anisotropy of nanometer grain varies due to the exchange-coupling interaction. This denotes the anisotropy in nanocomposite magnetic materials is determined not only by element but also by micorstructure. Random anisotropy model (RAM) was used to explain the optical magnetic properties of soft magnetic materials by Herzer. Based on the RAM, Arcas 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(inner part without exchange-coupling interaction (uncoupled) and interfacial part with exchange-coupling interaction (coupled)).Both of them pointed the anisotropy of coupled part is smaller than that uncoupled part. But they thought the anisotropy of coupled part is a constant, which is too simple.Based on the analysis above, we investigated the effect of microstructure on the exchange-coupling interaction, effective anisotropy and coercivity in nanometer single phase hard magnetic materials and nanocomposite magnetic materials by adopting RAM. The main contents and important results are following:1. Effecitive anisotropy and coercivity in nanometer single phase hard magnetic materialsTaking Nd₂Fe₁₄B nanometer material for example, the effects of grain size and its distribution and intergranular phase on effective anisotropy K_eff and coercivity H_c have been investigated. The results showed that K_eff and H_c decrease with grain size, and the nonideal distribution of grain size decrease them further. The decrease of coercivity in nanometer single phase hard magnetic materials is mainly caused by the decrease of anisotropy. In order to obtain higher values of K_eff and H_c, grain size should be larger than 15nm, and the grain size distribution should be as central as possible. The existence of intergranular phase weakens the exchange-coupling interaction. And the existence of intergranular phase with certain thickness may result in the increase of coercivity.2. Anisotropy at nanometer grain boundaryAn expression K₁^ij(r) = K₁ⁱ -â–³K^ij((L_ex^ij -2r)/L_ex^ij3/2 (i,j = s,h) has been given to describe the anisotropy at different grain boundary andâ–³K^ij = K₁ⁱ-K₁^ij(0) , where K₁ⁱ describe the anisotropy constant of i phase grain,K₁^ij(r) denotes the anisotropy at i phase grain boundary and r is the distance to grain surface.â–³K^ij and L_ex^ij have the similar physical meanings as to K₁^ij(r). And the anisotropy at grain surface is determined by the degree of exchange-coupling and the microstructure of grain boundary. Based on the expression the average anisotropy of hard-hard and hard-soft grains and the effective anisotropy of hard-soft grains have been calculated. The main results are following:For different values of anisotropy at hard-hard grains interface K^hh(0), hh> increases entirely with increasing D, and the increase of hh> becomes more rapid with reducing K^hh(0). When K^hh(0) ranges from 0.3K₁^h to 0.7K₁^h, the variations of hh> with D are similar to that of coercivity calculated by Kronmuller et al. The variation of average anisotropy of hard-soft grains sh> is influenced by the anisotropy at soft-hard grain surface K₁^sh(0). When K₁^sh(0) is smaller( equal to n(K₁^sK₁^h)^1/2 or smaller than 0.7(K₁^s+K₁^h)/2), sh> increases monotonously with the increase of D .When K₁^sh(0) is in the range of 0.7(K₁^s+K₁^h)/2-(K₁^s+K₁^h)/2, sh> reaches maximum at certain value of grain size, which is consistent well with the relative results.The effective anisotropy K_eff^sh between hard and soft grains is influenced by thegrain sizes D_s, D_h of soft, hard grains. For the given D_s, K_eff^sh increases rapidly firstly, then reaches a relative stable value with the increase of hard grain D_h. The value of D_h corresponding to the stable value of K_eff^sh enhances with increasing D_s. For the givenvalue of D_h, K_eff^sh decreases with increasing D_s, and the decrease becomes slow with increasing D_h.3. Exchange-coupling interaction and effective anisotropy in nanocomposite magnetic materialsThe degree of exchange-coupling interaction is different between different magnetic grains. The fractions and distributions of soft and hard phases lead to the different degree of exchange-coupling interactions.The effective anisotropy K_eff in nanocomposite magnetic materials is determined by the average anisotropy of grains in different coupling states and the corresponding probability. When the grain sizes of soft and hard grains are identical, K_eff appears the maximum with the increase of grain size. When the soft grain size D_s is given and the volume fraction of hard phase is higher, K_eff obtains the maximum at certain hard grain size, and increases with the volume fraction of hard phase. When the hard grain size D_h is given, K_eff increases with increasing D_s, and decreases with increasing the volume fraction of soft phase. The results we obtained is very similar to that of Sun et al., when the volume fraction of soft phase is in certain range, both of them reach maximum. The expression we gave describes the anisotropy at grain boundary very well. The decrease of coercivity in nanocomposite magnetic materials is mainly caused by the decrease of anisotropy.