不同形状的纳米磁性金属及合金颗粒的微波磁性研究
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摘要
传统的高频磁性材料的性能遵从Snoek极限，难以满足日益发展的高频应用材料性能的需要。本论文的研究思路是纳米磁性金属或合金粉体材料由于表面各向异性远远高于体磁晶各向异性而有望突破Snoek常数，可能在高频微波波段具有优异的性能，成为有效的微波吸收剂。
根据这一想法，本工作考察了三类纳米磁性金属和合金材料：球形纳米粉体(羰基铁粉)，线状纳米粉体(镍纳米线)，和片状纳米粉体(FeCuNbSiB)。固态羰基铁粉是用热分解液态羰基铁的方法获得，使用阳极氧化铝模板电化学沉积法制备镍纳米线和使用高能行星式球磨制备片状磁性纳米晶FeCuNbSiB粉体。将上述粉末样品与石蜡均匀混合，制备成高电阻率的复合材料，使用网络分析仪同轴法测量它们在0．1-18 GHz频段的复数介电常数和复数磁导率，讨论了它们的微波吸收性质，系统研究了材料的高频复数磁导率机制。主要结果如下：
(1)．对球形羰基铁，微波介电谱和磁谱表明，直径20～30 nm纳米羰基铁有比微米羰基铁高得多得介电常数虚部，但磁导率虚部比微米羰基铁低，由于磁损耗是纳米和微米羰基铁复合材料中的主要损耗类型。总的微波吸收性质(反射损耗系数和带宽)纳米羰基铁并不比微米羰基铁复合材料占优；
(2)．通过分析纳米羰基铁颗粒不同体积分数复合材料的初始磁导率和共振频率，可以得到直径20～30 nm纳米羰基铁颗粒的本征磁导率为7，本征共振频率为11．5 GHz，微米羰基铁颗粒的本征磁导率为23，本征共振频率为3．4 GHz；
(3)．比较微米羰基铁的情况，分析纳米羰基铁本征初始磁导率和共振频率可以知道，两者都落在Snoek极限中。这与20～30 nm的铁纳米颗粒中表面各向异性对体各向异性占有支配性的优势有关；
(4)．镍纳米线共振峰的位置主要由形状各向异性决定，自然共振模式和交换共振模式两者导致了直径100 nm镍纳米线有较宽的磁谱共振峰；
(5)．由于交换共振模式对边界条件非常敏感，我们可以利用这一性质分析磁性镍纳米线的表面磁矩束缚状况，分析结果表明镍纳米线的表面各向异性等效场非常小，不足以束缚表面原子磁矩；
(6)．镍纳米线自然共振和交换共振模式对偶极场的响应不一样。自然共振模式频率对偶极场的响应是线性的，这与Kittel公式吻合；交换共振模式频率的响应是非线性的。此外偶极场会抑制交换共振模式的强度；
(7)．根据镍纳米线复合材料初始磁导率和共振频率的实验值，估算得到的Snoek常数值的大小在Snoek极限附近，由此可见磁性纳米线也很难突破Snoek极限；
(8)．根据对片状和不规则块状FeCuNbSiB纳米晶软磁粉体的研究，片状颗粒样品由于引入形状各向异性，将自然共振频率提高到GHz频段，使得片状颗粒在GHz具有较高的复磁导率实部和虚部；
(9)．考虑磁矩在空间方向的随机分布，利用Landau-Lifshitz-Gilbert (LLG)方程和Bruggeman有效介质理论可以很好的拟合片状颗粒的磁谱，由此可见磁共振的类型主要是LLG方程描述的自然共振，自然共振阻尼因子的大小主要源自与趋肤效应有关的磁振子-传导电子耦合；
(10)．FeCuNbSiB片状粉体实验的Snoek常数比理论估算的Sonek极限略高些，这可能与片状磁性粉体具有平面磁各向异性有关。
The high frequency magnetic materials obey the Snoek limit, thus it is difficult toimprove. This study bases on principle that the ferromagnetic metal and alloynanoparticles of different shapes can exceed the Snoek's constant by the large surfaceanisotropy and may have potential for microwave application. As the permeability andresonance frequency can be increased at the same time. The ferromagnetic metal andalloy nanoparticles of different shapes may have good microwave property in the GHzfrequency range and to be good microwave absorption filler.
In this work, nano spheroid (carbonyl iron), wire (nickel nanowire), flake-shaped(FeCuNbSiB) particles have been synthesized and investigated. The composite sampleswere prepared by mixing the metal and alloy particles with paraffin wax with differentvolume concentration of the particles. The complex permeability of the compositesamples was obtained using an Agilent E8363B vector network analyzer in the 0.1-18GHz range. The main results are shown as follows:
(1). For the spheroid carbonyl iron, the imaginary part of permittivity of nanoparticleis much higher than the microparticle, but the imaginary part of permeability is lower. Asthe magnetic loss is the main loss of carbonyl iron composition, the microwave propertyof nanoparticles (Reflection Loss and Absorption Width) is not better thanmicroparticles;
(2). For the composites with different volume concentration of the particles, we canget the intrinsic permeability of 20～30 nm carbonyl iron nanoparticle at low frequency is7, the intrinsic resonance frequency is 11.5 GHz. At the same time, the intrinsicpermeability of carbonyl iron microparticle is 23, the intrinsic resonance frequency is 3.4GHz;
(3) According to the intrinsic permeability and resonance frequency, both of them arenot beyond the Snoek's limitation, this can be ascribed the easily domination of surfaceanisotropy;
(4). The value of the Ni nanowires' resonance peak can be explained by the shapeanisotropy. The natural resonance mode and the exchange resonance mode lead a wideimaginary part of permeability in Ni nanowires with diameter 100 nm;
(5). As the exchange mode is sensitive to the condition of the boundary, it can be usedto analyzes the surface anisotropy. The large surface anisotropy means large pinning, andthe small surface anisotropy means small pinning. The fit to the experimental data is poorfor large pinning, thus implying that the surface anisotropies in the Ni nanowires aresmall;
(6). The natural resonance mode and exchange resonance mode of Ni nanowires havedifferent responses to the dipolar field, the natural resonance frequency is linearity to thedipolar field by Kittel's equation; the exchange resonance mode is nonlinearity to thedipolar field. At the same time, the dipolar field will suppress the intensity of exchangeresonance mode;
(7). According to the intrinsic permeability and resonance frequency of Ni nanowire,the product of them is not beyond the Snoek's limitation;
(8). As the use of the shape anisotropy, the flake soft magnetic particles FeCuNbSiBovercome the difficulty of relatively small intrinsic anisotropies and increase the naturalresonant frequencies to GHz range so as to lead the higher real part and the imaginarypart of the permeability;
(9). The resonance peak of flake particles was simulated by using the combination ofthe Landau-Lifshitz-Gilbert equation and Bruggeman's effective medium theory,considering a random spatial distribution of magnetic easy axes. The magnetic lossmechanism in flake particles is mainly caused by the natural resonance. The dampingfactor mainly comes from the coupling of magnon and conduct electron by skin effect;
(10). The product of initial permeability and resonance frequency of FeCuNbSiB isbeyond the Snoek's limitation, this may results from the planar magnetic anisotropy.
根据这一想法，本工作考察了三类纳米磁性金属和合金材料：球形纳米粉体(羰基铁粉)，线状纳米粉体(镍纳米线)，和片状纳米粉体(FeCuNbSiB)。固态羰基铁粉是用热分解液态羰基铁的方法获得，使用阳极氧化铝模板电化学沉积法制备镍纳米线和使用高能行星式球磨制备片状磁性纳米晶FeCuNbSiB粉体。将上述粉末样品与石蜡均匀混合，制备成高电阻率的复合材料，使用网络分析仪同轴法测量它们在0．1-18 GHz频段的复数介电常数和复数磁导率，讨论了它们的微波吸收性质，系统研究了材料的高频复数磁导率机制。主要结果如下：
(1)．对球形羰基铁，微波介电谱和磁谱表明，直径20～30 nm纳米羰基铁有比微米羰基铁高得多得介电常数虚部，但磁导率虚部比微米羰基铁低，由于磁损耗是纳米和微米羰基铁复合材料中的主要损耗类型。总的微波吸收性质(反射损耗系数和带宽)纳米羰基铁并不比微米羰基铁复合材料占优；
(2)．通过分析纳米羰基铁颗粒不同体积分数复合材料的初始磁导率和共振频率，可以得到直径20～30 nm纳米羰基铁颗粒的本征磁导率为7，本征共振频率为11．5 GHz，微米羰基铁颗粒的本征磁导率为23，本征共振频率为3．4 GHz；
(3)．比较微米羰基铁的情况，分析纳米羰基铁本征初始磁导率和共振频率可以知道，两者都落在Snoek极限中。这与20～30 nm的铁纳米颗粒中表面各向异性对体各向异性占有支配性的优势有关；
(4)．镍纳米线共振峰的位置主要由形状各向异性决定，自然共振模式和交换共振模式两者导致了直径100 nm镍纳米线有较宽的磁谱共振峰；
(5)．由于交换共振模式对边界条件非常敏感，我们可以利用这一性质分析磁性镍纳米线的表面磁矩束缚状况，分析结果表明镍纳米线的表面各向异性等效场非常小，不足以束缚表面原子磁矩；
(6)．镍纳米线自然共振和交换共振模式对偶极场的响应不一样。自然共振模式频率对偶极场的响应是线性的，这与Kittel公式吻合；交换共振模式频率的响应是非线性的。此外偶极场会抑制交换共振模式的强度；
(7)．根据镍纳米线复合材料初始磁导率和共振频率的实验值，估算得到的Snoek常数值的大小在Snoek极限附近，由此可见磁性纳米线也很难突破Snoek极限；
(8)．根据对片状和不规则块状FeCuNbSiB纳米晶软磁粉体的研究，片状颗粒样品由于引入形状各向异性，将自然共振频率提高到GHz频段，使得片状颗粒在GHz具有较高的复磁导率实部和虚部；
(9)．考虑磁矩在空间方向的随机分布，利用Landau-Lifshitz-Gilbert (LLG)方程和Bruggeman有效介质理论可以很好的拟合片状颗粒的磁谱，由此可见磁共振的类型主要是LLG方程描述的自然共振，自然共振阻尼因子的大小主要源自与趋肤效应有关的磁振子-传导电子耦合；
(10)．FeCuNbSiB片状粉体实验的Snoek常数比理论估算的Sonek极限略高些，这可能与片状磁性粉体具有平面磁各向异性有关。
The high frequency magnetic materials obey the Snoek limit, thus it is difficult toimprove. This study bases on principle that the ferromagnetic metal and alloynanoparticles of different shapes can exceed the Snoek's constant by the large surfaceanisotropy and may have potential for microwave application. As the permeability andresonance frequency can be increased at the same time. The ferromagnetic metal andalloy nanoparticles of different shapes may have good microwave property in the GHzfrequency range and to be good microwave absorption filler.
In this work, nano spheroid (carbonyl iron), wire (nickel nanowire), flake-shaped(FeCuNbSiB) particles have been synthesized and investigated. The composite sampleswere prepared by mixing the metal and alloy particles with paraffin wax with differentvolume concentration of the particles. The complex permeability of the compositesamples was obtained using an Agilent E8363B vector network analyzer in the 0.1-18GHz range. The main results are shown as follows:
(1). For the spheroid carbonyl iron, the imaginary part of permittivity of nanoparticleis much higher than the microparticle, but the imaginary part of permeability is lower. Asthe magnetic loss is the main loss of carbonyl iron composition, the microwave propertyof nanoparticles (Reflection Loss and Absorption Width) is not better thanmicroparticles;
(2). For the composites with different volume concentration of the particles, we canget the intrinsic permeability of 20～30 nm carbonyl iron nanoparticle at low frequency is7, the intrinsic resonance frequency is 11.5 GHz. At the same time, the intrinsicpermeability of carbonyl iron microparticle is 23, the intrinsic resonance frequency is 3.4GHz;
(3) According to the intrinsic permeability and resonance frequency, both of them arenot beyond the Snoek's limitation, this can be ascribed the easily domination of surfaceanisotropy;
(4). The value of the Ni nanowires' resonance peak can be explained by the shapeanisotropy. The natural resonance mode and the exchange resonance mode lead a wideimaginary part of permeability in Ni nanowires with diameter 100 nm;
(5). As the exchange mode is sensitive to the condition of the boundary, it can be usedto analyzes the surface anisotropy. The large surface anisotropy means large pinning, andthe small surface anisotropy means small pinning. The fit to the experimental data is poorfor large pinning, thus implying that the surface anisotropies in the Ni nanowires aresmall;
(6). The natural resonance mode and exchange resonance mode of Ni nanowires havedifferent responses to the dipolar field, the natural resonance frequency is linearity to thedipolar field by Kittel's equation; the exchange resonance mode is nonlinearity to thedipolar field. At the same time, the dipolar field will suppress the intensity of exchangeresonance mode;
(7). According to the intrinsic permeability and resonance frequency of Ni nanowire,the product of them is not beyond the Snoek's limitation;
(8). As the use of the shape anisotropy, the flake soft magnetic particles FeCuNbSiBovercome the difficulty of relatively small intrinsic anisotropies and increase the naturalresonant frequencies to GHz range so as to lead the higher real part and the imaginarypart of the permeability;
(9). The resonance peak of flake particles was simulated by using the combination ofthe Landau-Lifshitz-Gilbert equation and Bruggeman's effective medium theory,considering a random spatial distribution of magnetic easy axes. The magnetic lossmechanism in flake particles is mainly caused by the natural resonance. The dampingfactor mainly comes from the coupling of magnon and conduct electron by skin effect;
(10). The product of initial permeability and resonance frequency of FeCuNbSiB isbeyond the Snoek's limitation, this may results from the planar magnetic anisotropy.
引文
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