The Size Effect Of Nanometer Permanent Magnetic Materials

Due to the exchange-coupling interaction between magnetically soft and hard phases, the remanence enhancement is exhibited remarkably in nanocomposite permanent magnetic materials. The theoretical results indicate that the exchange-coupling interaction among magnetic films 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/m³. However, experimental results show that the enhancement of remanent magnetization is obvious, but the coercivity decreases seriously and the energy product of nanocomposite magnetic material is far lower than its theoretical value with the reduction of magnetically soft grains size.In order to lessen the gap between the theoretical and experimental coercivty and investigate of thickness effect on nanoscaled permanent magnetic films, lots of work has been devoted to explore the thickness-dependent demagnetization and coercivity mechanism. Unfortunately, the detail is still unclear. This result is mainly attributed to the following two aspects: one is the oversimplified theoretical models; the other is that the nanocrystalline magnet with fully density and uniform microstructure can not be realized in experiment.In this thesis, the effects of thickness in double-layer permanent magnets have been investigated in detail. Taking single phased Nd₂Fe₁₄B and two phased Nd₂Fe₁₄B/α-Fe films as typical samples. We firstly calculated the dependences of nucleation field on thicknesses of layers. Secondly, We investigated the thickness effects on remanence, coercivity and hysteresis loop, and compared with experimental results. The main conclusions are listed as following:1. The actual nucleation mode corresponds to the smallest nucleation field. By numerical calculating and comparing the Brown's micromagnetic incoherent rotation and coherent rotation model, it implies that the real nucleation mode of permanent magnet changes from the coherent mode to the incoherent one with the reduction of the misaligned/soft phased thickness, and the average effective anisotropy can only be used in limited cases(at very small grains size).2. There exist exchange-coupling interactions in single phased double-layer nano-structured hard magnetic materials with perpendicular easy axes. The reversal mechanism of the system changes from nucleation to pinning with the increasing of misaligned layer thickness L. For a given thickness L'(the aligned layer thickness), the coercivity hc decreases firstly and subsequently keeps at a constant C with the increasing of L and the constant C saturates at 0.414 as the aligned phase thickness L' is positive infinitive. Meanwhile, the hysteresis loop of single phased double-layer permanent magnets changes from"s"curve to ellipse with the reduction of aligned layer thickness L.3. The dependences of magnetic characteristic on soft and hard layer thicknesses and angle in nano-composite permanent magnet(Nd₂Fe₁₄B/α-Fe) are obtained systematically. For a given hard phase thickness Lh, the coercivity decreases firstly as increases of soft phase thickness Ls, and then converges a constant value C' with a infinitive Ls. For sufficiently large Ls, C' is equal to 540 (Oe)/-540 (Oe) in aligned composite magnetic layers and perpendicular anisotropy layers respectively. Our results are basically in accordance with the experimental results and other coercivity theories, and our study will shed light on the nanocomposite material design.