伸缩-剪切模式自偏置铌酸锂基复合材料的磁电性能和高频谐振响应

选用多种切型铌酸锂(LiNbO_3)单晶,研究了铁基非晶合金(Metglas)/LiNbO_3叠层复合材料基于伸缩-剪切模式的磁电耦合性能,揭示了铌酸锂单晶压电系数与复合材料剪切磁电耦合系数的对应关系,在使用铌酸锂xzt/30~?切型时得到了最优化剪切磁电系数.通过SrFe12O19薄磁带提供偏置磁场,Metglas/LiNbO_3磁电复合材料可在没有外加直流磁场时实现剪切磁电响应,并在0.991 MHz和3.51 MHz频率时分别测出了谐振磁电系数,有望将铌酸锂基剪切磁电复合材料用于高频磁场探测.     

Structural,magnetic, and dielectric properties of Ni–Zn ferrite and Bi_2O_3 nanocomposites prepared by the sol-gel method

Ni–Zn ferrite and Bi_2O_3 composites were developed by the sol-gel method. The structural, magnetic, and dielectric properties were studied for all the prepared samples. X-ray diffraction(XRD) was performed to study the crystal structure.The results of field emission scanning electron microscopy(FE-SEM) showed that the addition of Bi_2O_3 can increase the grain size of the Ni–Zn ferrite. Magnetic properties were analyzed by a hysteresis loop test and it was found that the saturation magnetization and coercivity decreased with the increase of Bi_2O_3 ratio. In addition, the dielectric properties of the Ni–Zn ferrite were also improved with the addition of Bi_2O_3.     

纳米微粒BaFe12O19掺杂对单畴超导块材GdBa2Cu3O7-δ性能的影响

磁通钉扎性能对GdBa₂Cu₃O_7-δ超导块材的实际应用具有重要的影响, 而引入合适的第二相粒子可以改善GdBa₂Cu₃O_7-δ 超导块材的磁通钉扎性能.本文采用顶部籽晶熔融织构法成功地制备出纳米微粒BaFe₁₂O₁₉(<100 nm)掺杂的超导块材, 样品的最终组分为Gd123+ 0.4 Gd211+ x BaFe₁₂O₁₉ (x=0, 0.2 mol%, 0.4 mol%, 0.8 mol%)+ 10 wt%Ag₂O+ 0.5 wt%Pt. 通过研究不同掺杂量的BaFe₁₂O₁₉微粒对GdBa₂Cu₃O_7-δ 超导块材微观结构和超导性能的影响, 结果表明当掺杂量为0.2 mol%时, 样品的临界电流密度几乎在整个外加磁场下都有明显的提高.在零场下, 临界电流密度达到5.5× 10⁴ A/cm². 纳米微粒BaFe₁₂O₁₉不仅可以保持掺杂前的化学组成, 作为有效的钉扎中心存在于超导块材中, 并且能够改善Gd₂BaCuO₅粒子的分布和细化Gd₂BaCuO₅粒子, 使Gd₂BaCuO₅粒子的平均粒径由未掺杂时的1.4 μ m减小到掺杂后的0.79 μ m, 进而提高了超导块材的临界电流密度和俘获磁场, 明显提高了GdBa₂Cu₃O_7-δ 超导块材的超导性能.临界温度T_C也有所提升, 并能够维持在92.5 K左右. 该结果为进一步研究纳米磁通钉扎中心的引入并改善GdBa₂Cu₃O_7-δ 超导块材的性能有着重要的意义.     

Enhancement of microwave absorption of nanocomposite BaFe_(12)O_(19)/α-Fe microfibers

Nanocomposite BaFe12O19/α-Fe microfibers with diameters of about 1-5 μm are prepared by the organic gelthermal selective reduction process. The binary phase of BaFe12O19 and α-Fe is formed after reduction of the precursor BaFe12O19/α-Fe2O3 microfibers at 350℃ for 1 h. These nanocomposite microfibers are fabricated from α-Fe (16-22 nm in diameter) and BaFe12O19 particles (36-42 nm in diameter) and basically exhibit a single-phase-like magnetization behavior, with a high saturation magnetization and coercive force arising from the exchange-coupling interactions of soft α-Fe and hard BaFe12O19 . The microwave absorption characteristics in a 2-18 GHz frequency range of the nanocomposite BaFe12O19/α-Fe microfibers are mainly influenced by their mass ratio of α-Fe/BaFe12O19 and specimen thickness. It is found that the nanocomposite BaFe12O19/α-Fe microfibers with a mass ratio of 1:6 and specimen thickness of 2.5 mm show an optimal reflection loss (RL) of 29.7 dB at 13.5 GHz and the bandwidth with RL exceeding 10 dB covers the whole Ku-band (12.4-18.0 GHz). This enhancement of microwave absorption can be attributed to the heterostructure of soft, nano, conducting α-Fe particles embedded in hard, nano, semiconducting barium ferrite, which improves the dipolar polarization, interfacial polarization, exchange-coupling interaction, and anisotropic energy in the nanocomposite BaFe12O19/α-Fe microfibers.     

Observation of giant magnetocaloric effect under low magnetic fields in EuTi_(1-x)Co_xO_3

The magnetic properties and magnetocaloric effect(MCE) in EuTi_(1-x)Co_xO_3(x = 0, 0.025, 0.05, 0.075, 0.1) compounds have been investigated. When the Ti~(4+) ions were substituted by Co2+ions, the delicate balance was changed between antiferromagnetic(AFM) and ferromagnetic(FM) phases in the EuTiO_3 compound. In EuTi_(1-x)Co_xO_3 system, a giant reversible MCE and large refrigerant capacity(RC) were observed without hysteresis. The values of -?S_M~(max) were evaluated to be around 10 J·kg~(-1)·K~(-1) for EuTi_(0.95)Co_(0.05)O_3 under a magnetic field change of 10 kOe. The giant reversible MCE and large RC suggests that EuTi_(1-x)Co_xO_3 series could be considered as good candidate materials for low-temperature and low-field magnetic refrigerant.     

Tb掺杂双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7的磁熵变和电输运性质

研究了Tb掺杂对双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7磁熵变和电输运性质的影响.样品采用传统固相反应法制备,两样品的名义组分可以表示为(La_(1-x)Tb_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.025),磁场为7 T时的最大磁熵变?S_M分别为-4.60 J/(kg·K)和-4.18 J/(kg·K).比较后发现,Tb元素的掺杂使得最大磁熵变值减小,但同时增大了相对制冷温区.电性测量结果表明,x=0.025的样品在高温区的导电机制可以用小极化子模型解释,与母体三维变程跳跃模型不同;当温度降低至三维长程铁磁有序温度(T_c~(3D))附近时,掺杂样品发生金属绝缘相变;掺杂后样品在T_c~(3D)附近,磁电阻取得极大值(约为56%),表明是本征磁电阻效应.     

Fe_3O_4单晶薄膜磁性电场调控的微磁学仿真研究

磁性薄膜磁学特性电场调控的相关研究对开发新型低功耗磁信息器件具有突出意义.本文基于电场调控磁性的基本理论,以OOMMF(Object Oriented Micro-Magnetic Frame)微磁学仿真软件为工具,研究了电场对生长于PZN-PT单晶衬底上Fe_3O_4单晶薄膜磁学特性的调控.研究结果显示:无外加电场时,薄膜表现出典型的软磁特性;沿衬底[001]方向施加的外加电场可以使得薄膜矫顽力、矩形比等磁学特性发生显著改变:当外加磁场沿[100]([010])时,施加正值(负值)电场强度可以显著增大薄膜的矫顽力与矩形比,当电场强度不小于0.6 MV/m时薄膜矩形比达到1.这是因为外加电场导致薄膜产生单轴应力各向异性,使得薄膜的等效磁各向异性发生了从无外电场下的面内四重磁晶各向异性向高电场下的近似单轴磁各向异性的过渡.外加1 MV/m与-1 MV/m的电场时等效易磁化轴分别沿[100]与[010]方向.另外,外加1 MV/m(-1 MV/m)的电场强度可以使得铁磁共振的频率增大(减小)接近1 GHz.     

The magnetic properties of aligned M hexa-ferrite fibres

Aligned and random fibres of strontium hexaferrite (SrM, SrFe₁₂O₁₉) and barium hexaferrite (BaM, BaFe₁₂O₁₉) were manufactured by blow spinning from an aqueous inorganic sol–gel precursor, which was then fired to give the hexagonal ferrite fibre. Their magnetic properties were studied by VSM, investigating the evolution of these properties with firing and measurement temperature, and in particular the effects of fibre alignment. It has been predicted that aligned ferrite fibres will demonstrate an enhanced magnetisation along the axis of alignment with respect to perpendicular to the axis, and this has been demonstrated here for the first time. The optimum firing temperature was 1000 °C, at which point they still had submicron grains. In BaM random fibres M_s=63.8 emu g⁻¹ and H_c=428.1 kA m⁻¹, and in SrM random fibres M_s=63.3 emu g⁻¹ and H_c=452.8 kA m⁻¹, high values for polycrystalline materials. Fibres aligned parallel to the applied field had saturation magnetisation (M_s) values equal to those of the random fibres, whilst fibres aligned perpendicular to the field had M_s values 62% and 75% lower, for BaM and SrM, respectively. There was no change in coercivity (H_c) between random or aligned fibres of any orientation, and fibres aligned 45° and parallel to H appeared identical. Therefore, properties along the axis of alignment were superior when compared to measurements perpendicular to the axis of alignment, giving a directionality to the magnetisation in an otherwise randomly oriented ferrite material.     

Evolution of structure and physical properties in Al-substituted Ba-hexaferrites

The investigations of the crystal and magnetic structures of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions have been performed with powder neutron diffractometry. Magnetic properties of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions have been measured by vibration sample magnetometry at different temperatures under different magnetic fields.The atomic coordinates and lattice parameters have been Rietveld refined. The invar effect is observed in low temperature range(from 4.2 K to 150 K). It is explained by the thermal oscillation anharmonicity of atoms. The increase of microstress with decreasing temperature is found from Rietveld refinement. The Curie temperature and the change of total magnetic moment per formula unit are found for all compositions of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.     

Magnetic phase transition in (Ba, Sr)Fe12O19 single crystals

The critical exponents β, γ and δ of single crystals of hexagonal ferrites BaFe₁₂O₁₉ and SrFe₁₂O₁₉ were determined from modified Arrott plots, scaling plots, ln(J) versus ln(μ₀H) plots and the method of Kouvel and Fisher. The results resemble those of the strong anisotropic ferromagnet Nd₂Fe₁₄B. Crossover effects are discussed by estimating the ranges within which dipolar and anisotropic interactions dominate, and by analyzing the temperature-dependent exponents β(T) and γ(T). In addition, the λ-line was determined from measurements in the hard direction, and the critical exponent ω was fitted.     

Tuning of magnetic properties of aluminium-doped strontium hexaferrite powders

M-type Al-doped strontium ferrite powders(Sr Alx Fe_(2n-x) O_(19), n = 5.9) with nominal Al content of x = 0–2.0 are prepared by traditional ceramic technology. The phase identification of the powders, performed using x-ray diffraction,shows the presence of purity hexaferrite structure and absence of any secondary phase. The lattice parameters decrease with increasing x. The average grain size of the powders is about 300 nm–400 nm at Al~(3+)ion content x = 0–2.0. The roomtemperature hysteresis loops of the powders, measured by using vibrating sample magnetometer, show that the specific saturation magnetization(σ_s) value continuously decreases while the coercivity(Hc) value increases with increasing x, and Hc reaches to 9759 Oe(1 Oe = 79.5775 A/m) at x = 2.0. According to the law of approach saturation, Hc value increases with increasing Al~(3+)ion content, which is attributed to the saturation magnetization(Ms) decreasing more rapidly than the magnetic anisotropy constant(K_1) obtained by numerical fitting of the hysteresis loops. The distribution of Al~(3+)ions in the hexaferrite structure of Sr Alx Fe_(2n-x) O_(19) is investigated by using 57 Co Mssbauer spectroscopy. The effect of Al~(3+)doping on static magnetic properties contributes to the improvement of magnetic anisotropy field.     

Dy,Co共掺杂对BiFeO3陶瓷磁特性和磁相变温度Tc的影响

采用快速液相烧结法制备BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0, 0.05, 0.1, 0.15)陶瓷样品. 实验结果表明: 所有样品的主衍射峰与纯相BiFeO₃相符合且具有良好的晶体结构, 随着Co³⁺掺杂量的增大, Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品的主 衍射峰由双峰(104)与(110)逐渐重叠为单峰(110), 当掺杂量x>0.05时, 样品呈现正方晶系结构; SEM形貌分析可知: Dy³⁺, Co³⁺共掺杂使BiFeO₃晶粒尺度由原来的3—5 μ减小到约1 μ. 室温下, BiFeO₃样品表现出较弱的铁磁性, 随着Dy³⁺和Co³⁺掺杂, BiFeO₃样品的铁磁性显著提高. 在外加磁场为30 kOe的作用下, Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0.05, 0.1, 0.15)的M_r分别为0.43, 0.489, 0.973 emu/g; M_S分别为0.77, 1.65, 3.08 emu/g. BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品磁矩M随着温度T的升高而逐渐减小, Dy掺杂使BiFeO₃样品的T_N由644 K升高到648 K, 而T_C基本没有变化. Dy和Co共掺杂导致BiFeO₃样品磁相变温度T_C由870 K降低到780 K, 其T_C变化主要取决于Fe-O-Fe反铁磁超交换作用的强弱和磁结构的相对稳定性. 关键词： 铁磁电材料 磁滞回线 磁相变温度     

Effects of pH and citric acid contents on characteristics of ester-derived BaFe12O19 powder

The single domain size of BaFe₁₂O₁₉ powder with crystallite sizes less than 200 nm was produced using a citric acid precursor method. Fe³⁺ and Ba²⁺, in a molar ratio of 12, were chelated by COOH⁻ in an aqueous solution. After ethylene glycol additions, esterification, dehydration, and calcination led to the formation of ester-derived BaFe₁₂O₁₉ powder. High pH and/or high citric acid contents in the starting solution are required to complete chelate metallic ions in the solution and to form pure barium ferrite powder at 1073 K. Pure single magnetic domain BaFe₁₂O₁₉ particles of M(30 kOe)≈54 emu/g, M_r≈28 emu/g, and H_c≈3.7 kOe were produced using [citric acid]/[metallic ions]=1.5 and pH7.     

Magnetic and electromagnetic properties of RuZn and RuCo substituted BaFe12O19

Polycrystalline samples of M-type hexaferrites BaFe_12−2xRu_xZn_xO₁₉ and BaFe_12−2xRu_xCo_xO₁₉ with 0x0.45 have been prepared by a classical sintering method. The evolutions with x of the cell parameters, the saturation magnetization and the magnetic transition temperature have been measured; in this range of small doping ratios, saturation magnetization and Curie temperature of substituted hexaferrites remain close to those of the undoped BaFe₁₂O₁₉. X-ray diffraction measurements on oriented powders show that a change of magnetocrystalline anisotropy from axial to planar occurs in both cases for a small substituting ratio x_c=0.375. Microwave electromagnetic characteristics have been studied on the ceramic samples from 0.1 to 10 GHz. The behaviour of the magnetic losses (μ″) corroborates the anisotropy change when doping; a convolution of the dissipation mechanisms (domain wall motions and gyromagnetism) is obtained for x_c. The level of the magnetic losses is discussed in relation with others substituted Ba-hexaferrites.     

Strange Quark Matter and Strangelets in a Strong Magnetic Field

We study the stability properties of magnetized strange quark matter and strangelets under a strong magnetic field in the MIT bag model. The free energy per baryon of strange quark matter feels a great influence from the magnetic field. At the field strength about 10¹⁷G, the magnetized strange quark matter becomes more stable. Considering the finite size effect, the magnetic influence on strangelets becomes complicated. For a given magnetic field, there exists a critical baryon number, below which the magnetized strangelets have lower energy than the non-magnetized strangelets. For the field strength of 5× 10¹⁷G, the critical baryon number is A_c ～ 100. Generally, the critical baryon number increases with the decreasing external magnetic field. When the field strength is smaller than 10¹⁷G, the critical baryon number goes up to A_c～ 10⁵. The stable radius, electric charge, and quark flavor fractions of magnetized strangelets are shown.     

Dy3Al5O12磁热性质研究

用量子理论计算了Dy₃Al₅O₁₂的晶场能谱、Zeeman劈裂能级和波函数. 在外磁场H_e为0<H_e<9 T, 温度为3<T<42 K 范围内, 计算了该晶体的磁矩、磁熵变, 计算结果与相关实验数据吻合较好. 该计算结果表明, Dy₃Al₅O₁₂内磁性离子间的交换作用非常微弱, 可以忽略. 从理论上给出了绝热退磁过程中温度变化ΔT与T的关系, 并与Gd₃Ga₅O₁₂晶体进行了比较, 发现不同外磁场下, Dy₃Al₅O₁₂和Gd₃Ga₅O₁₂的低温制冷性能在不同温区有差别. 在进行低温(T<10 K)制冷时, 若外磁场较低, 选择Dy₃Al₅O₁₂作为磁制冷材料较好; 若外磁场较高, 选择Gd₃Ga₅O₁₂作为磁制冷材料较好.     

Dielectric and magnetic properties of xNiFe2O4+(1−x)Ba0.9Sr0.1TiO3 composites

Magnetoelectric composites, namely, xNiFe₂O₄+(1−x)Ba_0.9Sr_0.1TiO₃ were prepared by standard double sintering ceramic method. The X-ray diffraction analysis was carried out to check the phases formed during sintering and to calculate the lattice parameters. Scanning electron microscope (SEM) micrographs were taken to understand the microstructure of the samples. The dielectric constant (ε′) and loss tangent (tan δ) were measured as a function of frequency in the frequency range 100 Hz–1 MHz. Variation of dielectric constant and loss tangent with temperature and composition has been reported. The hysteresis measurements were done to determine saturation magnetization (M_S) and coersivity (H_c). The variation of saturation magnetization and magnetic moment is interpreted in terms of composition.     

Growth and magnetism of Ni nanoparticles in Ni/Al2O3/Si or Si3N4 multilayers

We report on the fabrication of Ni/Al₂O₃/Si and textured Ni/Al₂O₃/Si₃N₄ multilayers containing Ni nanoparticles that exhibit significantly improved results. The secondary phases arising from thermal reaction between Ni and Si can be remarkably suppressed with increasing layers of Al₂O₃ and deposition of Ni/Al₂O₃ multilayers on Si₃N₄ substrates. Atomic force microscopy shows the formation of large as well as nanoclusters of Ni when grown on Si, whereas textured Ni nanoparticles are formed on Si₃N₄ substrates. The magnetization measurements on Ni/Al₂O₃/Si containing a single buffer layer of Al₂O₃ shows higher coercivity field with magnetic nanowire-like behavior, whereas with several Al₂O₃ alternate layers almost a superparamagnetic-like behavior is observed. However, significantly improved magnetic hysteresis was observed in textured Ni/Al₂O₃/Si₃N₄ multilayers due to preferred alignment of Ni nanocrystallites.     

Microstructure and magnetocaloric properties in melt-spun and high-pressure hydrogenated La0.5Pr0.5Fe11.4Si1.6 ribbons

The effects of wheel speeds and high-pressure hydrogen treatment on phase evolution, microstructure, and magnetocaloric properties in La_0.5Pr_0.5Fe_11.4Si_1.6 melt-spun ribbons are studied in this work. The results reveal that the increase of wheel speed is beneficial to the formation of cubic NaZn₁₃-type phase and the grain refinement. The optimized wheel speed for microstructural and magnetocaloric properties is 30 m/s. The largest entropy change of 18.1 J/kg·K at 190 K under a magnetic field change of 0 T-5 T is obtained in La_0.5Pr_0.5Fe_11.4Si_1.6 ribbons melt-spun at 30 m/s. After a high-pressure hydrogen treatment of 50 MPa, the Curie temperature of the ribbons prepared at 30 m/s is adjusted to about 314 K and the large -ΔS_M of 17.9 J/kg·K under a magnetic field change of 0 T-5 T is achieved at room temperature with almost none hysteresis loss. The small thermal and magnetic hysteresis and the large -ΔS_M make the La_0.5Pr_0.5Fe_11.4Si_1.6 hydride ribbons appropriate for magnetic refrigerant applications around room temperature.     

AC and DC magnetic studies of critical currents in ceramics of the Ho1−xPrxBa2Cu3O7−δ system

AC magnetic susceptibility measurements were performed for ceramics of the Ho_1−xPr_xBa₂Cu₃O_7−δ system as a function of temperature, frequency, field amplitude and static magnetic field. From the measurements, temperature dependences of intergranular critical currents and corresponding pinning energies at grain boundaries were determined for different Pr concentrations. These critical currents strongly decrease with Pr doping. They are limited by superconductor–insulator–superconductor or superconductor–normal metal–superconductor junctions and for higher Pr concentrations additionally suppressed by flux creep. Also intragranular critical current at 4.3 K was determined as a function of Pr concentration from magnetic hysteresis loops. It is a monotonically decreasing function of Pr doping.