【作者】 刘忻；

【导师】 李发伸；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2010， 博士

【摘要】 传统的高频磁性材料的初始磁导率和共振频率的乘积遵从Snoek极限,逐渐难以满足更高频率下对材料的要求。本文提出具有平面各向异性的稀土-3d过渡金属间化合物颗粒有望突破Snoek极限,具有高面外各向异性场和较低的面内各向异性场的稀土-3d金属间化合物可以大幅度提高材料的磁导率的同时,使共振频率保持在较高的范围。以V为稳定元素,Co为替代元素,采用真空电弧熔炼和真空热处理制备了不同Co替代含量的1:12型稀土铁基金属间化合物样品。然后利用室温粉末样品的X射线衍射、振动样品磁强计等实验方法对这些化合物的结构与磁性进行了研究。对用石蜡混合粉末制成的复合材料样品,使用同轴线方法对其高频性能进行了测量与分析。分析了Co元素对Fe元素的替代对金属间化合物的结构和磁性,以及磁晶各向异性和高频磁性能的影响,并讨论了各向异性对高频磁性的影响和高频复数磁导率的机制。得到以下结论：(1)对该化合物的研究发现,在x=0-0.5的范围内,可以得到单一的ThMn12型结构的纯相。随着Co原子的替代量的增加,Nd(Fe1-xCox)10V2的晶格常数单调减小,晶胞体积收缩,这是由于Co原子半径小于Fe原子半径导致的。(2)随着Co原子的替代量的增加,Nd(Fe1-xCox)10V2的饱和磁化强度由115emu/g (x=0)增加到122 emu/g (x=0.2),而后减小到96 emu/g(x=0.5);其矫顽力则先减小后增大；样品的易磁化方向由平行于c轴(x=0)转变为平行于a-b平面(x=0.2),当Co原子含量进一步增加时,易磁化方向再次趋向平行于c轴(x=0.5)。(3)随着Co原子的替代量的增加,Nd(Fe1-xCox)10V2的居里温度由570 K(x=0),单调增加到676 K(x=0.4),但是当Co含量继续增加时,居里温度下降到662 K(x=0.5)。这是由于Co原子先占据原子间距较小的8f和8j位置,使得Fe-Fe负相互作用减弱,3d次晶格整体的相互作用增强,从而提高了居里温度。(4)随着Co原子的替代量的增加,Nd(Fe1-xCox)10V2的自旋重取向温度也发生了变化,其室温以下的自旋重取向温度由120 K(x=0)逐渐增加到280K(x=0.4)。在室温以上,当没有Co原子的掺入时(x=0),测量温度直到居里转变点都没有自旋重取向现象。但当Co原子掺入后,出现了自旋重取向现象,并且自旋重取向的温度随着Co含量的增加不断升高,直到x=0.5时,自旋重取向温度降低,但是自旋重取向发生时易磁化方向的转变已经与前面的变化不同。这说明稀土次晶格在与Fe次晶格的竞争中逐渐占优,Co原子的替代使得Fe次晶格的各向异性减弱。(5)通过穆斯堡尔谱的测量,可以看到V原子优先占据品格中的8i位置。但Co原子替代Fe原子进入晶格后,Nd(Fe1-xCox)10V2中的Co原子表现出明显的择优占据,当Co元素刚开始进入晶格中时,由于原子间距的不同和熵的原因,Co原子优先进入8f和8j位置。随着Co原子的替代,8i和8j位置上的同质异能随着Co原子含量的增加不断增大,但在8f位置上其基本保持不变；四极矩劈裂在x=0和x=0.15之间以及x=0.4和x=0.5之间发生了突变,说明了存在易磁化方向的转变。(6)随着Co原子的替代量的增加,体积比为35%的Nd(Fe1-xCox)10V2粉末/石蜡复合样品的复介电常数并没有随着频率发生明显的变化,并且对于不同Co含量的样品,复介电常数值也没有较大的变化。但是复数磁导率随着Co原子的替代量的不同发生了明显的变化：对x=0的样品,其低频磁导率为2.2,当x=0.2时,其低频磁导率升高为3.2,当x=0.5时,其低频磁导率又降为2.3;其磁导率虚部在x=0和x=0.5时,整个测量范围内没有共振峰出现,并且磁损耗主要是由涡流损耗贡献的,但是对于x=0.2-0.4等样品,磁导率虚部出现了明显的共振吸收峰,其损耗中主要由平面各向异性导致的自然共振贡献的。这是因为其高频磁性与磁晶各向异性有很大的关系,当各向异性发生变化时,磁导率也会随之变化。从上面的实验结果可以看出,具有平面各向异性的Nd(Fe1-xCox)10V2样品,具有更好的高频磁性。(7)随着Co原子含量的增加导致的Nd(Fe1-xCox)10V2样品的磁各向异性的转变,使得样品的微波吸收性能也发生了明显的变化,根据测量得到的磁导率和介电常数计算出了不同易磁化方向样品的反射损耗,对于面各向异性的样品(x=0.2)具有比单轴各向异性的样品(x=0)优异的微波吸收性能。更多还原

【Abstract】 The property of traditional high frequency magnetic materials obeys the Snoek limit, thus it is hard to improve. This study is based on principle of the rare earth-3d intermetallic compounds particles with planar magnetocrystalline anisotropy can exceed the limitation of the Snoek limit. Under the common acting of a small in plane anisotropic field and a rather large out of plane anisotropic field, which can get a higher permeability without a badly descent of resonance frequency.Using the Vanadium as a stabilizing element, and using the Cobalt to substitute the iron, The ThMn12-type rare earth-3d intermetallic compounds of Nd(Fe1-xCox)10V2 were prepared by means of vacuum arc melting, subsequent vacuum annealing, then they were ball milling for 12 hours. The influence of the substitution of Cobalt for iron on the structure and statistic magnetic properties of these compounds is investigated by X-ray powder diffraction and magnetic measurements. The paraffin composites were prepared by mixing paraffin with compounds particles and pressing into toroidal shape, so that the relative complex permeability of the composites has been measured in the range of 0.1-10 GHz. The mechanism of relative complex permeability affected by the change of anisotropy in high frequency has been discussed. The main research contents and results are shown as follows:(1) The structure of Nd(Fe1-xCox)10V2 compounds is a single phase with tetragonal ThMn12-type structure in a range of x=0-0.5. The lattice constants and unit-cell volume decrease monotonically with increasing of Co concentration, due to Cobalt atom radius is smaller than iron.(2) The saturation magnetization of Nd(Fe1-xCox)10V2 compounds increase from 115 emu/g (x=0) to 122 emu/g (x=0.2) at first and then decrease to 96 emu/g (x=0.5) with increasing Co concentration while the coercivity of the compounds are decreasing firstly and then keep constant, then it increased abruptly. The easy magnitization direction of the compounds also changed with the Co content from c-axial (x=0) to a-b plane(x=0.2), and return to c-axial when x=0.5.(3) The Curie temperature (Tc) of Nd(Fe1-xCox)10V2 compounds increase monotonically from 570 K (x=0) to 676 K (x=0.4) with increasing of Co concentration, but it decreased to 662 K (x=0.5)when further increase the Co content. The decrease of interatomic distances and a preferential substitution of Co for Fe in regions where there is negative exchange could account for the strengthening of exchange interaction and thus for an enhancement of Tc,(4) The M-T curves show that the temperature of spin reorientation transition (SRT) ascent quickly with increasing the Co concentration:below the room temperature, the temperature of SRT increased from 120 K (x=0) to 280 K (x=0.4); Above the room temperature, there was no SRT when x=0, while the SRT appeared after Co enter the compounds, and its temperature increased with increasing of Co content till x=0.4, when x=0.5 the temperature of SRT decreased and the transition is different with before. It is account for the Co substitution weakens the anisotropy of Fe sublattice.(5) By the Mossbauer measurement, the V atom preferes to occupy the 8i sites. When Co atom substituted the Fe atom, the Co appeared to occupy the 8f and 8j sites, this is account for the 8f and 8j sites have smaller interatomic distances and this occupation made the system with low enthalpy. The Isomer Shift (IS) of 8i and 8j sites increased obviously while that of 8f sites was almost constant. The Quadrupole Splitting (QS) value changed abruptly in 8f sites between x=0 and x=0.15, as well as in 8i sits between x=0.4 and x=0.5, which means the transition of the easy magnetization direction exists in these ranges.(6) For the paraffin composites with 35 vol.% of Nd(Fe1-xCox)10V2, both of real and imagine part of the permittivity was almost constant in our measurement frequency range, and for different x, the value did not change a lot neither, however the complex permeability varied with the increasing of Co concentration, the real part of permeability at low frequency was 2.2 for x=0, and then it ascend to 3.2 for x=0.2, when x=0.5, it drops to 2.3; the imagine part of permeability has no resonance peak for x=0 and x=0.5, and the eddy current loss is the main loss, but there was a resonance peak for x=0.15-0.4, and the loss was caused by nature resonance, that is in respect that the permeability has a relationship with anisotropy, when anisotropy changed, the permeability would change too.(7) The absorbing properties of the Nd(Fe1-xCox)10V2 with plane anisotropy were much better than the ones with uniaxial anisotropy.更多还原