纯三元及含Zr, Ga元素磁粉的各向异性

未经均匀化热处理的纯三元及含Zr, Ga元素的SC合金铸片经优化的HDDR工艺处理都可以制备各向异性NdFeB磁粉。这表明：元素的添加及SC铸片是否进行了均匀化热处理都不是HDDR磁粉各向异性形成的必要条件。磁粉各向异性形成的关键因素在于HDDR工艺的调节,即适当地加快歧化反应过程,减缓脱氢再结合过程以及控制脱氢再结合时的合适氢气压强均有利于磁粉各向异性的形成。本文将为制备低成本高各向异性磁粉提供重要的指导。     

Co0.9Fe0.1薄膜面内元素分辨的磁各向异性

利用X射线磁性圆二色技术对Co0.9Fe0.1薄膜面内元素分辨的磁各向异性进行了研究,通过剩磁模式测量不同磁化方向的样品组分原子单位空穴磁矩的变化,发现除了在生长的磁诱导方向存在易磁化轴外,在与该轴垂直的方向还存在一个类似易轴的软磁化轴;面内的两个难磁化轴与易磁化轴取向大约成66°夹角,从而构成了面内双轴磁各向异性;对不同组分元素,其单位空穴磁矩随磁化方向的变化趋势基本相同,不同磁化方向Fe原子单位空穴的磁矩值约为Co的对应值的87%,反映了Fe原子和Co原子之间存在着强烈的铁磁性耦合.     

Co0.9Fe0.1薄膜面内元素分辨的磁各向异性

利用X射线磁性圆二色技术对Co_0.9Fe_0.1薄膜面内元素分辨的磁各向异性进行了研究，通过剩磁模式测量不同磁化方向的样品组分原子单位空穴磁矩的变化，发现除了在生长的磁诱导方向存在易磁化轴外，在与该轴垂直的方向还存在一个类似易轴的软磁化轴；面内的两个难磁化轴与易磁化轴取向大约成66°夹角，从而构成了面内双轴磁各向异性；对不同组分元素，其单位空穴磁矩随磁化方向的变化趋势基本相同，不同磁化方向Fe原子单位空穴的磁矩值约为Co的对应值的87%，反映了Fe原子和Co原子之间存在着强烈的铁磁性耦合. 关键词： 磁各向异性 X射线磁性圆二色 铁磁耦合 CoFe合金薄膜     

Cu元素对Cu(In,Ga)Se_2薄膜及太阳电池的影响

Cu元素成分对Cu(In,Ga)Se2(简称CIGS)薄膜材料的电学性质及其电池器件性能有很重要的影响.本文利用蒸发法制备了贫Cu和富Cu的CIGS吸收层(0.7Cu/(Ga+In)1.15)及相应的电池器件.扫描电镜和Hall测试发现,富Cu材料的结构特性(晶粒大、结晶状态好)和电学特性(电阻率低、迁移率高等)优于贫Cu材料,而性能测试表明贫Cu器件的效率优于富Cu器件.变温性能测试分析表明,贫Cu器件的主要复合路径是体复合,激活能与CIGS禁带宽度相当;富Cu器件的主要复合路径是界面复合,其激活能远小于CIGS禁带宽度,这大大降低了开路电压Voc,从而降低了电池效率.最后利用蒸发三步法制备了体材料稍富Cu表面贫Cu的CIGS吸收层,降低了短路电流和开路电压的损失,获得了超过15%的电池效率.     

三元层状陶瓷M-Al-N(M=Ti,Zr,Hf)的结构及力学性质的第一性原理模拟

基于第一性原理方法,探索M-Al-N（M=Ti,Zr,Hf）结构的稳定性,计算其力学性质.计算M-Al-N化合物的能量,发现除实验已知的结构Ti₂AlN和Ti₄AlN₃、Zr₂AlN、Hf₂AlN外,还存在两种新的热力学稳定结构Zr₄AlN₃、Hf₄AlN₃.弹性常数和声子谱的计算,表明这两个结构是力学稳定和晶格动力学稳定的.计算M₂AlN和M₄AlN₃的力学性质,发现它们具有高的体模量、剪切模量、弹性模量、维氏硬度等;分析其力学性质随组分比例、组成元素的变化规律,为该类材料的选择和应用提供理论依据.最后计算M₂AlN和M₄AlN₃的电子态密度和分态密度、电子密度分布、Mulliken群分析等.     

磁无序及合金化效应影响Co2CrZ(Z=Ga,Si,Ge)合金相稳定性和弹性常数的第一性原理研究

采用确切的Muffin-Tin轨道结合相干势近似方法,本文系统计算研究了0 K下,磁无序及合金化效应影响Co₂CrZ(Z=Ga,Si,Ge)合金L2₁和DO₂₂相稳定性的规律性及物理机理.研究结果表明,0 K下,L2₁相合金晶格常数、体弹性模量、磁矩和弹性常数均与理论和实验值基本吻合;铁磁下合金具有L2₁结构,随磁无序度(y)的增大,L2₁相能量相对逐渐增大,最终由低于转变到高于D0₂₂相,因此,当y≥0.1(0.2)时,Z=Si和Ge(Z=Ga)的合金具有DO₂₂相稳定结构;随y的增大,L2₁相的四方剪切弹性模量(C’=(C₁₁-C₁₂)/2)还不断软化,表明无论在能量还是力学角度上,磁无序都有利于3种合金发生四方晶格变形;磁无序影响L2₁和D0₂₂相相对稳定性的电子结构机理归因于Jahn-Teller...     

TEM studies of the effects of Zr additions on some HDDR-processed, high boron, NdFeB-type powders and hot-pressed magnets

The transmission electron microscope has been used to study the physical and magnetic microstructures of two HDDR-processed NdFeB-type alloys, one without Zr and the other containing 1.1 at% Zr. Studies were made of the as-produced powders and the solids produced following hot-pressing at 900°C. In the HDDR powders, the principal effects of adding Zr were to reduce the average grain size by and made the grain size distribution more uniform. In the hot-pressed samples, the effect of Zr was more dramatic in that grain growth was very significantly reduced. Zr-containing phases were identified and a simple model, due to Zener, used to provide a plausible explanation of how the small amount of Zr present could stabilise the grain size to ≈0.5 μm. The microstructural results correlated well with measured magnetic properties.     

Annealing effect on magnetic anisotropy in ultrathin（Ga,Mn）As

We investigated the effect of low temperature annealing on magnetic anisotropy in 7-nm ultrathin Ga0.94Mn0.06As devices by measuring the angle-dependent planar Hall resistance(PHR).Obvious hysteresis loops were observed during the magnetization reversal through the clockwise and counterclockwise rotations under low magnetic fields(below 1000 Gs,1 Gs = 10-4 T),which can be explained by competition between Zeeman energy and magnetic anisotropic energy.It is found that the uniaxial anisotropy is dominant in the whole measured ferromagnetic range for both the as-grown ultrathin Ga0.94Mn0.06As and the annealed one.The cubic anisotropy changes more than the uniaxial anisotropy in the measured temperature ranges after annealing.This gives a useful way to tune the magnetic anisotropy of ultrathin(Ga,Mn)As devices.     

Electronic structures,magnetic properties and half-metallicity in Heusler alloys Zr2IrZ (Z = Al,Ga, In)

The electronic structures, magnetic properties and half-metallicity in Zr₂IrZ (Z = Al, Ga, In) alloys with the Hg₂CuTi-type structure were systematically investigated by using the first-principle calculations. Zr₂IrZ (Z = Al, Ga, In) alloys are predicted to be half-metallic ferrimagnets which are quite robust against hydrostatic strain and tetragonal deformation. The total magnetic moment of Zr₂IrZ (Z = Al, Ga, In) alloys mainly originates from the 4d electrons of Zr atoms and follows the conventional Slater-Pauling rule: M_t = Z_t−18. (M_t is the total magnetic moment per unit cell and Z_t is the valence concentration). The origin of the band gap for Zr₂IrZ (Z = Al, Ga, In) alloys is also well studied. Unconventionally, Zr₂Ir-based alloys contain element with 5d valence electrons, which implies a wider field to search for new half-metallic materials.     

Anisotropy of elasticity and minimum thermal conductivity of monocrystal M4AlC3 （M = Ti,Zr, Hf）

The elastic constants, elastic anisotropy index, and anisotropic fractional ratios of Ti4AlC3, Zr4AlC3, and Hf4AlC3 are studied by using a plane wave method based on density functional theory. All compounds are characterized by the elastic anisotropy index. The bond length, population, and hardness of the three compounds are calculated. The degrees of hardness are then compared. The minimum thermal conductivity at high temperature limitation in the propagation direction of [0001](0001) is calculated by the acoustic wave velocity, which indicates that the thermal conductivity is also anisotropic. Finally, the electronic structures of the compounds are analyzed numerically. We show that the bonding of the M4AlC3 lattice exhibits mixed properties of covalent bonding, ionic bonding, and metallic bonding. Moreover, no energy gap is observed at the Fermi level, indicating that various compounds exhibit metallic conductivity at the ground state.     

Effect of microstructure on the coercivity of HDDR Nd-Fe-B permanent magnetic alloy

The effect of the grain boundary microstructure on the anisotropy and coercivity was investigated in an HDDR Nd-Fe-B permanent magnetic alloy. Considering the special microstructure of its magnetic powder grain, an anisotropic theoretical model influenced simultaneously by the structure defect at the grain boundary and the exchange coupling interaction was put forward. The variations of the structure defect factors based on the nucleation and pinning mechanism with 2r ₀/lex (where r ₀ and lex are the defect thickness and the length of exchange coupling, respectively) were calculated. The results show that the coercivity mechanism of an HDDR Nd-Fe-B permanent magnetic alloy is greatly related to its microstructure defect at the grain boundary. For a fixed lex, when 2r ₀/lex < 1.67, the coercivity is controlled by the pinning mechanism; when 2r ₀/lex > 1.67, it is determined by the nucleation mechanism. The coercivity reaches the maximum when 2r ₀/lex = 1.67. The calculation result is consistent well with the experimental result given by Morimoto et al. Supported by the National Natural Science Foundation of China (Grant No. 50671055)     

Origin of anisotropy for the HDDR Nd13.5Fe79.5B7 magnetic powders

For the HDDR Nd_13.5Fe_79.5B₇ magnetic powders, effects of disproportionation time and hydrogen pressure on the anisotropy were studied during the slow desorption stage. Studies showed that shorter disproportionation times caused the magnetic powders displaying higher anisotropy. With increasing disproportionation times, the degree of crystallographic alignment decreased. This in turn caused a drop in remanence and anisotropic character. Longer disporportionation times have also been correlated to a change in disproportionated microstructure from lamella to columnar. XRD (X-Ray Diffraction) studies showed that except NdH₂,α-Fe and Fe₂B, no other phases were included in the disproportionation mixture. This elucidated that the strong anisotropy is only related to a lamella disproportionation microstructure, which corresponds to a short disproportionation times. The lamella disproportionation microstructure may remain or inherit the alignment of original Nd₂Fe₁₄B grain, and may also be related to the alignment of the newly formed Nd₂Fe₁₄B grain. Thus, the anisotropic formation mechanism of ternary magnetic powders accords with “anisotropy-mediating phase” model. If the disproportionation mixture were carried out an optimum hydrogen pressure treatment during the HDDR process, the degree of crystallographic alignment can be further enhanced.     

Preparation,microstructure, and magnetic properties of a nanocrystalline Nd12Fe82B6 alloy by HDDR combined with mechanical milling

Nanocrystalline Nd₁₂Fe₈₂B₆ (atomic ratio) alloy powders with Nd₂Fe₁₄B/α-Fe two-phase structure were prepared by HDDR combined with mechanical milling. The as-cast Nd₁₂Fe₈₂B₆ alloy was disproportionated via ball milling in hydrogen, and desorption–recombination was then performed. The phase and structural change due to both the milling in hydrogen and the subsequent desorption–recombination treatment was characterized by X-ray diffraction (XRD). The desorption–recombination behavior of the as-disproportionated alloy was investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The morphology and microstructure of the final alloy powders subject to desorption–recombination treatment were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results showed that, by milling in hydrogen for 20 h, the matrix Nd₂Fe₁₄B phase of the alloy was fully disproportionated into a nano-structured mixture of Nd₂H₅, Fe₂B, and α-Fe phases with average size of about 8 nm, and that a subsequent desorption–recombination treatment at 760 °C for 30 min led to the formation of Nd₂Fe₁₄B/α-Fe two-phase nanocomposite powders with average crystallite size of 30 nm. The remanence B_r, coercivity H_c, and maximum energy product (BH)_max of such nanocrystalline Nd₁₂Fe₈₂B₆ alloy powders achieved 0.73 T, 610 kA/m, and 110.8 kJ/m³, respectively.     

高质量稀磁半导体(Ga,Mn)Sb单晶薄膜分子束外延生长

理论预言窄禁带稀磁半导体(Ga,Mn)Sb及其异质结构可能存在量子反常霍尔效应等新奇特性, 近年来受到了特别关注. 但是, 由于(Ga,Mn)Sb薄膜生长窗口窄, 纯相(Ga,Mn)Sb薄膜制备比较困难, 迄今关于这类材料的研究报道为数不多. 本文采用低温分子束外延的方法, 通过优化生长条件, 成功制备出厚度为10 nm, Mn含量在0.016至0.039之间的多组(Ga,Mn)Sb薄膜样品. 生长过程中反射式高能电子衍射原位监测和磁性测量都表明没有MnSb等杂相的偏析, 同时原子力显微镜图像表明其表面形貌平滑, 粗糙度小. 通过生长后退火处理, (Ga,Mn)Sb薄膜的最高居里温度达到30 K. 此外, 本文研究了霍尔电阻和薄膜电阻随磁场的变化关系, 在低温下观测到明显的反常霍尔效应.     

Structure, room-temperature magnetic and optical properties of Mn-doped TiO2 nano powders prepared by the sol-gel process

TiO 2 nano powders with Mn concentration of 0 at%-12 at% were synthesized by the sol-gel process,and were annealed at 500 C and 800 C in air for 2 hrs.X-ray diffraction (XRD) measurements indicate that the Mn-TiO 2 nano powders with Mn concentration of 1 at% and 2 at% annealed at 500 and 800 C are of pure anatase and rutile,respectively.The scanning electron microscope (SEM) observations reveal that the crystal grain size increases with the annealing temperature,and the high resolution transmission electron microscopy (HRTEM) investigations further indicate that the samples are well crystallized,confirming that Mn has doped into the TiO 2 crystal lattice effectively.The room temperature ferromagnetism,which could be explained within the scope of the bound magnetic polaron (BMP) theory,is detected in the Mn-TiO 2 samples with Mn concentration of 2 at%,and the magnetization of the powders annealed at 500 C is stronger than that of the sample treated at 800 C.The UV-VIS diffuse reflectance spectra results demonstrate that the absorption of the TiO 2 powders could be enlarged by the enhanced trapped electron absorption caused by Mn doping.