摘要
寻找和制备能有效吸收雷达波的新型隐身材料,是世界各国军事研究领域的重点攻关课题之一。从实用性考虑,新型吸波隐身材料必须要具有:高频性能优越且阻抗匹配、质轻、耐高温、耐腐蚀等特点。铁氧体薄膜是重要的备选材料之一。另外,随着电子设备向小型化、轻量化、薄形化、高频化、低损耗和高可靠性的发展,对电子元件及材料的要求是朝着进一步的小型化、集成化方向压缩,部分器件将由三维的体材料向二维的薄膜材料方向过渡。而微波及毫米波器件、可调滤波器、高密度大容量的薄膜磁记录介质、薄膜型磁头、磁传感器、薄膜电感器和生物医学中用做药物和探测试剂载体等,都需要膜厚均匀、无缺陷、具有合适电磁性能的铁氧体薄膜。
铁氧体薄膜以其优异的高频电磁特性、良好的机械耐磨性和稳定的化学性能而成为颇具应用价值的材料引起人们的极大关注。由于传统的制备铁氧体的工艺都要经过高温热处理工序,而高质量Mn-Zn系列软磁铁氧体薄膜对热处理环境敏感,不易获得。因此,目前的研究热点集中在块体、纳米颗粒铁氧体材料的动态磁化机理等方面,而对于软磁铁氧体薄膜的制备和高频性能的研究才刚刚起步,尚处于初期实验阶段。
本论文分别采用了低温水溶液中的化学镀工艺和分层交替溅射法制备了不同Zn含量、不同厚度的Mn_(1-x)Zn_xFe_2O_4 (0.1≤x≤0.9)铁氧体纳米晶薄膜。利用X射线衍射仪(XRD)、扫描电镜(SEM)、振动样品磁强计(VSM)、矢量网络分析仪(PNA 8363B)和背散射穆斯堡尔谱仪(CEMS),并结合其它的一些仪器设备,研究了Mn-Zn软磁铁氧体薄膜样品的结构与磁性,探讨了微结构与磁性之间的关系,与块体材料进行了对比,并对不同方法制备得到的相同成分的样品性能进行了比较;特别是研究了不同方法制备薄膜样品的高频动态磁特性。得到的主要结果如下:
1.成功地摸索和掌握了水溶液化学镀制备纳米颗粒与薄膜的工艺,自行设计加工了整套实验装置。首次利用水溶液化学镀工艺在玻璃基底上成功地制备了不同Zn含量、不同厚度的Mn_(1-x)Zn_xFe_2O_4 (0.1≤x≤0.9)铁氧体纳米晶薄膜由于整个反应是在90℃的水浴环境中进行,薄膜样品不需经过高温热处理工序,从而很好地避免了Mn、Fe元素的氧化变价、析出和Zn元素的挥发问题,制得了高质量的Mn-Zn铁氧体纳米晶薄膜。
2.通过化学镀工艺制备的不同Zn含量的Mn_(1-x)Zn_xFe_2O_4薄膜样品的饱和磁化强度M_s的变化趋势和块体类似,都是先增大,在x=0.5左右达到最大,然后开始减小。这种变化趋势完全符合含Zn铁氧体的超交换相互作用理论。而矫顽力H_c的变化则和块体完全不同,它先减小,在x=0.5左右达到最小,然后开始异常增大。这种变化趋势上的差别是由于:矫顽力和薄膜样品的表面平整度有强烈的关联作用,表面起伏越大矫顽力也越大,这种矫顽力增大主要来源于缺陷的增多和由于晶界体积增大所产生的“钉扎”效应对磁畴翻转的阻碍作用。
3.通过化学镀工艺制备的不同Zn含量的Mn_(1-x)Zn_xFe_2O_4薄膜样品,由于Zn~(2+)对于A位有着强烈的化学亲和性而优先占据A位,随着Zn含量的增加致使Fe~(3+)不断的从A位向B位迁移。
4.多种测量结果均显示,通过化学镀和射频溅射制备的Mn_(1-x)Zn_xFe_2O_4铁氧体薄膜的金属离子分布和块体不同,其占据A位的Fe~(3+)离子数目远高于块体材料,并由此导致反铁磁结构A,B位抵消后的有效磁矩减小,从而M_s明显地小于相应成份的块体值。
5.通过单层交替溅射法制得的薄膜样品的矫顽力H_c大于相同成分通过化学镀工艺制备的薄膜样品值,更是远大于块体Mn-Zn铁氧体的数值。这主要是由于溅射法制备铁氧体薄膜经过了热处理工序,晶粒比低温水溶液化学镀工艺制备的薄膜样品大的原因。
6.室温背散射穆斯堡尔谱测量证明,通过化学镀和射频溅射制备的300 nm厚Mn_(1-x)Zn_xFe_2O_4铁氧体薄膜在形状各向异性作用下,磁矩近似的垂直于γ射线的入射方向,都在薄膜平面内部排列,这种易面各向异性的产生是突破Snoek极限、使薄膜样品的共振频率向高频方向大幅提升的根本原因。而高频磁谱的测量表明,两种方法制得的Mn-Zn系列铁氧体薄膜其自然共振频率f_r都集中在400-900 MHz (远高于根据μ′=44的Mn-Zn铁氧体块状样品的斯诺克极限所估算的f_r=130 MHz),在共振频率附近,磁导率虚部μ″达到最大值(≥15),磁导率实部μ′为15~35。
7.高频磁谱的实验数据和拟合数据的吻合表明:通过化学镀和射频溅射制备的系列Mn-Zn铁氧体薄膜,其磁矩的有阻尼进动可以用经典的Landau-Lifshitz-Gilbert方程来描述,吸收高频电磁波的机理为自然共振。
Stealth using radar absorbing materials enables the control or reduction of the signatures of weapon systems. National security is enhanced by new combat aircraft that use stealth materials to help control their signature, so scientists from various countries focus to develop the radar absorbing materials. The ferrite thin films have been considered as good candidates from some practical considerations such as high permeability, impedance matching, light quality, high temperature resistant and corrosion resistant. In recent years, as electronic devices have become more miniaturized, offering increasing high levels of performance, the study of electronic devices under much higher-signal frequencies has become a trend. The growth of thin layers of magnetically soft ferrites such as Mn-Zn or Ni-Zn ferrite has been studied by various groups. This trend and interest are motivated by potential applications such as magnetic thin-film read heads, cladding layers for thin film inductors and microwave acoustic devices.
Owing to their large resistivities, low power losses, and high permeabilities, ferrites have become very important in high frequency soft magnetic applications. Although Mn-Zn ferrite films have such good properties including high resistivity, good quality at high frequency, and good performance of soft magnets, during their preparation there are many factors that are not easy to control. Mn-Zn ferrite is sensitive to its fabrication environment, especially at high annealing temperature. So the study of soft ferrite thin films such as Mn-Zn and Ni-Zn ferrite thin films are underway and interesting.
We have prepared Mn_(1-x)Zn_xFe_2O_4(0.1≤x≤0.9) thin films of different thickness and Zn concentration by electroless plating method in aqueous solution and rf sputter deposited method, respectively. With XRD, SEM, VSM, CEMS and an Agilent vector network analyzer (PNA 8363B). the crystallographic structure, morphology of the films. the high-frequency performance. the macroscopic magnetic properties and their correlations are characterized and discussed. The main conclusions are as follow:
1. We have designed and fabricated the equipment of electroless plating. Mn_(1-x)Zn_xFe_2O_4 (0.1≤x≤0.9) thin films of good quality with different thickness and Zn concentration were prepared on glass substrates firstly without heat treatment by electroless plating in aqueous solution at 90℃.
2. The change of the coercivity of the plated Mn_(1-x)Zn_xFe_2O_4 thin films is not consistent with that of the bulk materials. As the Zn content in the films increases, the value of H_c decreases firstly, and then increases. At x=0.5, H_c of Mn_(1-x)Zn_xFe_2O_4 thin film shows a minimum of 3.7 kA/m and M_s shows a maximum of 419.6 kA/m. Here, M_s is much smaller and H_c is much larger than those of the bulk materials, which may be due to the following factor: the defects in the film microstructure can affect the M_s and H_c obviously: and the grain size of our samples is much smaller than that of bulk materials, which induces an increase in the number of grain boundaries, and then acts as pinning sites for domain walls , so H_c of Mn_(1-x)Zn_xFe_2O_4 films plated is much larger than the value of the bulk materials.
3. For the electroless plated Mn-Zn ferrite thin films with different Zn concentration, the Fe~(3+) ions transfer from A sites to B sites and the hyperfine magnetic field (HF) reduces with the increase of Zn content, meaning that Zn~(2+) ions have strong chemical affinity towards the A sites.
4. The CEMS results indicate that the cation distribution of Mn_(1-x)Zn_xFe_2O_4 ferrite thin films fabricated by electroless plating method and rf sputter deposited method is different from the bulk materials' and a great quantity of Fe~(3+) ions are still present on A sites, which reduces the number of unpaired spins between the A sites and B sites, so M_s of Mn_(1-x)Zn_xFe_2O_4 thin films plated is much smaller than the value of the bulk materials.
5. For the rf sputter deposited Mn-Zn ferrite thin films, H_c is much larger than those of the electroless plated thin films with same Zn content the and bulk materials.
6. For the Mn_(1-x)Zn_xFe_2O_4 ferrite thin films fabricated by electroless plating method and rf sputter deposited method, the distribution of the magnetic moments in the sample is perpendicular to the incident direction ofγ-rays. In another word, the array of magnetic moments lies in the plane thin film. For the electroless plated Mn-Zn ferrite thin films and the rf sputter deposited Mn-Zn ferrite thin films, the real partμ' of samples keep from 15 to 35 and the peak frequency f_r keep from 400 to 900 MHz corresponds to a natural resonance frequency. The peak frequency f_r is much higher than that of the bulk materials.
7. For the Mn_(1-x)Zn_xFe_2O_4 ferrite thin films fabricated by electroless plating method and rf sputter deposited method, the spectra of real and imaginary permeabilities accord with fitting the data. Damped precession of the magnetic moments corresponds to the classical Landau-Lifshitz-Gilbert formula, and the mechanism of the absorption of the high electromagnetic waves is relevant to the natural resonance theory.
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