Gd和Ti共掺杂对BiFeO_3矫顽力和剩余磁矩的影响(英文)

通过快速液相烧结法制备了BiFeO3、Bi0.9Gd0.1FeO3和Bi0.9Gd0.1Fe0.95Ti0.05O3(BGFTO)块材.x射线衍射和拉曼散射光谱表明Gd和Ti共掺杂后的BGFTO样品产生结构相变.共掺杂样品BGFTO室温下的铁磁性得到显著提高,矫顽力达到1.51T,剩余磁矩为0.175emu/g.磁性得到显著提高的根本原因是共掺杂引起元素替换导致了体系中的螺旋磁结构被抑制.A位和B位共掺杂是引发BiFeO3自发磁化的最有效的方法.     

溶胶-凝胶法制备的高压电常数（Bi0.5Na0.5）1－xBaxTiO3系无铅压电陶瓷

测量了使用溶胶-凝胶工艺制备的(Bi0.5Na0.5)1-xBaxTiO3(x=0.00，0.04，0.06，0.08,0.12)系陶瓷的介电、压电、铁电和热释电性能.由于使用了溶胶-凝胶工艺制备的粉料，因此所有样品的压电性能都得到了较大提高.其中(Bi0.5Na0.5)0.94Ba0.06TiO.3系陶瓷具有该系列最大的压电常数，d33=173×10-12C/N，与传统工艺相比，d33提高了近40%.同时，在一定范围内，随Ba含量的增加，材料的剩余极化Pr和矫顽场Ec逐渐减小，退极化温度逐渐降低.对于(Bi0.5Na0.5)0.94Ba0.06TiO.3系陶瓷，剩余极化和矫顽场分别为25μC/cm2和28kV/cm，退极化温度约为80℃. 关键词： 溶胶-凝胶 压电常数 剩余极化 矫顽场     

溶胶-凝胶法制备的高压电常数(Bi_(0.5)Na_(0.5))_(1-x)Ba_xTiO_3系无铅压电陶瓷

测量了使用溶胶 凝胶工艺制备的 (Bi0 5Na0 5) 1 -xBaxTiO3(x =0 0 0 ,0 0 4 ,0 0 6 ,0 0 8,0 12 )系陶瓷的介电、压电、铁电和热释电性能 .由于使用了溶胶 凝胶工艺制备的粉料 ,因此所有样品的压电性能都得到了较大提高 .其中(Bi0 5Na0 5) 0 94 Ba0 0 6 TiO3系陶瓷具有该系列最大的压电常数 ,d33=173× 10 - 1 2 C N ,与传统工艺相比 ,d33提高了近4 0 % .同时 ,在一定范围内 ,随Ba含量的增加 ,材料的剩余极化Pr 和矫顽场Ec 逐渐减小 ,退极化温度逐渐降低 .对于 (Bi0 5Na0 5) 0 94 Ba0 0 6 TiO3系陶瓷 ,剩余极化和矫顽场分别为 2 5 μC cm2 和 2 8kV cm ,退极化温度约为 80℃ .     

ErFeO3单晶反常负磁化和补偿温度点性研究

通过光学浮区法生长了直径为6mm,长度约为75mm的晶体,结构分析表明实验样品呈单相的正交钙钛矿结构,并通过劳厄定向仪对ErFeO3晶体进行了定向.系统测量了ErFeO3晶体a,c方向的热磁曲线和不同温度下的磁化曲线.结果表明,随着温度的降低,在98～86K范围该体系经历了从Г4(Gx,Fz)相到Г2(Gz,Fx)相的自旋重取向转变.在ErFeO3体系中,由于Er离子磁矩反平行于Fe离子磁矩分子场,在46K附近,在a方向上Er的反向磁矩大于Fe离子磁矩,导致出现负磁化和磁化补偿点现象.随着外加测量场的增强,a轴向上的负磁化被抑制,补偿温度点向高温移动.     

Tb1-xGdxFe2的磁矩和矫顽力

测量了赝二元立方Laves相化合物TbGdFe₂(0≤x≤1)的磁化曲线、居里温度和矫顽力随温度的变化。发现化合物由TbFe₂向GdFe₂过渡时,样品的饱和磁化强度几乎直线下降。在所有成分下,Fe原子的磁矩都是常数(μ_Fe=1.60±0.04μ_B),但Tb原子的磁矩,由于受晶场影响,却小于自由离子的值(gJ=9.Oμ_B)。矫顽力由两部分组成:一为畴壁受Peierls势垒和稀土 关键词：     

Dy_(0.9)Sm_(0.1)FeO_3单晶中的自旋组态和磁化翻转效应

我们用光学浮区法生长了稀土正铁氧体Dy_(0.9)Sm_(0.1)FeO_3(DSFO)单晶,并通过劳厄衍射法确定了单晶样品的方向.对DSFO单晶的磁性研究表明:在DyFeO_3中掺杂10%的Sm后,体系的自旋重取向转变温度从51 K(DyFeO_3)降低到9 K,而且相变过程变成了由Γ_4到Γ_(42)再到Γ_1.在12—300 K区间DSFO处于Γ_4组态,沿c方向Fe离子磁晶格有一个净的磁矩和一个由它诱导的稀土离子的净磁矩.稀土磁矩和Fe磁矩之间本征地反平行耦合,但一个较小的磁场可以改变反平行耦合到平行耦合.在场冷降温模式下的磁化-温度曲线测试中,我们发现在不同的磁场下磁化曲线会在不同的位置发生跳变;这一现象由稀土磁矩和Fe磁矩之间的平行反平行转变导致,有望作为特定条件下的自旋开关而应用于自旋电子学器件中.     

铁磁/反铁磁双层膜中交换偏置及矫顽场的温度特性研究

讨论了铁磁-反铁磁双层膜中交换偏置和矫顽场随温度变化的关系。在本模型中,温度的依赖性来源于系统态的热激发以及相关磁学参量的温度依赖性。数值结果显示:低温下,交换偏置和矫顽场随温度的升高而减少,但是随着界面的交换耦合的增强或铁磁层各向异性的减少,其交换偏置变得平坦。随着温度的升高,交换偏置减少直至零;而矫顽场却达到峰值后再减为零。这些结果与实验结果定性一致。根据数值计算结果,可以预见软的铁磁层耦合上硬的反铁磁层,在恰当的交换耦合强度下,可构建具有大的交换偏置、小矫顽场;并在某温度区几乎不随温度变化的磁存贮器件.     

铋层化合物Sr1+xBi4-xTi4-xTaxO15(x=0—1)陶瓷的介电和铁电特性

研究了铋层化合物Sr1+xBi4-xTi4-xTaxO15(x=0—1)陶瓷的介电和铁电特性.研究发现随着x值的增大,材料的居里点由540℃降到30℃左右,但材料始终具有铁电性.对于SrBi4Ti4O15陶瓷施加120kVcm的电场仍无法得到饱和的回线,此时的剩余极化和矫顽场分别约为7μCcm2和73kVcm.对于x=04—05的材料由于它们同时具有较高的居里温度300℃—360℃,较大的剩余极化7—8μCcm2和较小的矫顽场37—47kVcm,因此是一类性能较为优良的铁电材料. 关键词： 居里温度 剩余极化 矫顽场     

Gd2Co2Al电子结构和磁性的第一性原理研究

从第一性原理出发,在局域自旋密度近似(LSDA)和LSDA+U(在位库仑能)近似下,采用FPLAPW密度泛函能带计算方法研究了Gd2Co2Al的电子结构和磁性.从平均场近似出发,估算了体系的居里温度,并分析了导致体系居里温度偏低的原因.研究结果显示Gd2Co2Al为金属导体,其强的铁磁性的提供者主要是Gd,且Co的局域铁磁性是不稳定的.基于LSDA近似的计算表明Gd2Co2Al呈现亚铁磁性,因为Co与Gd两者磁矩反平行排列.考虑在位库仑能修正的LSDA+U方法则发现一个适当的在位库仑能(U=3.0eV)使体系从亚铁磁态转变为铁磁态,此时Co原子磁矩基本为零与实验结果更为相符.在位库仑能的变化对Co原子磁矩以及磁性原子的能级分布影响较大,但对Gd的磁性基本无影响.由于体系5d-3d态杂化和在位库仑排斥作用竞争使得Co原子磁矩呈现出波动性的特性.     

利用［Fe/Pt］n多层膜降低L10-FePt有序化温度

采用直流磁控溅射方法制备了Fe/Pt多层膜和FePt单层薄膜，再经不同温度真空热处理得到 了有序相L10₀-FePt薄膜.通过x射线衍射谱和磁性研究表明，FePt单层薄膜需 要在500℃ 以上热处理，才能开始有序化转变，而Fe/Pt多层膜可以降低FePt薄膜有序化温度.［Fe(1 5nm)/Pt(15nm)］13₁₃薄膜在350℃热处理后，有序度已经增加到 06，相应矫 顽力达到了501kA/m.多层膜化促进有序化在较低的温度下进行，这是由于热处理过程中多 关键词： 0-FePt有序相')" href="#">L10₀-FePt有序相 磁控溅射 有序度 Fe/Pt多层膜     

Growth of well-aligned Bi nanowire on Ag(1 1 1)

We report the fabrication of one-dimensional (1D) Bi nanowires grown on Ag(1 1 1) with average lateral width from 9 to 20 nm by physical vapor deposition in ultra high vacuum conditions. In situ low-temperature scanning tunneling microscopy analyses reveal that the preferred growth of 1D Bi nanostructures is driven by the highly anisotropic bonding in the crystallographic structure of the Bi(1 1 0) plane. The Bi nanowires grow along direction and align with the directions on Ag(1 1 1). The growth of the Bi nanowires proceeds in a bilayer growth mode resulting from the layer pairing in Bi(1 1 0) which saturates the dangling bonds and lowers the total energy.     

Magnetic characterization of Bi(Fe1 − xMnx)O3

Bi(Fe_1 − xMn_x)O₃ ceramics (x up to 0.3) were prepared by rapid sintering. Weak ferromagnetism with two magnetic anomalies at low temperatures was observed for Bi(Fe_0.95Mn_0.05)O₃ and Bi(Fe_0.9Mn_0.1)O₃. From temperature-dependent magnetic relaxation measurements, the anomalies at 20 K and 100 K are related to the freezing of cluster spin glass.     

Dielectric, magnetic and magnetoelectric properties of La and Nb codoped bismuth ferrite

Single-phase Bi(0.80)La(0.20)FeO(3) (BLFO) and Bi(0.80)La(0.20)Fe(1-x)Nb(x)O(3) (BLFNO) samples were prepared in order to study the dielectric, magnetic and magnetoelectric properties of La and Nb codoped BiFeO3. Rietveld refinement of x-ray diffraction patterns of La and Nb codoped samples has been performed using the R3c space group. Magnetic hysteresis loops revealed that codoping can effectively increase the spontaneous magnetization due to change in the bond angle of Fe-O-Fe as a result of distortion created by the Nb5+ doping. Magnetic field-induced relative change of the dielectric constant for BLFO and BLFNO samples is a signature of magnetoelectric coupling.     

Improved multiferroic properties of La-doped 0.6BiFeO₃-0.4SrTiO₃ solid solution ceramics

The 0.6(Bi1-xLax)FeO 3-0.4SrTiO 3(x = 0,0.1) multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization(Pr) and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6(Bi0.9La0.1)FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

Surface alloying and anomalous diffusion of Bi on Cu(1 1 1)

We have used Low-Energy Electron Microscopy (LEEM) to study the formation and structure of the surface alloy of Bi on the Cu(1 1 1) surface. As a function of Bi coverage we find a progression from a dilute surface alloy to a structure, which coexists with a Bi overlayer when the Bi coverage exceeds 1/3 ML. At a temperature of 401 K, an unusual formation of alloy domains is observed. The origin of this effect is traced down to the mobility of Bi adatoms, which is found to strongly depend on the Bi coverage of the surface alloy on which they reside.     

Interdiffusion of adsorbed Pb and Bi on Cu(1 0 0)

The impingement and interdiffusion of adsorbed Pb and Bi layers spreading from separated 3D pure bulk sources on Cu(1 0 0) has been studied, at T = 513 K, by in situ scanning Auger microscopy. When the leading edges of the pure Pb and Bi diffusion profiles impinge, they both consist of low-coverage lattice gas surface alloyed phases. In these low-coverage phases, Pb displaces surface alloyed Bi and the point of intersection of the profiles drifts towards the Bi source. These features lead to the conclusion that Pb atoms are more strongly bound at surface alloyed sites in Cu(1 0 0) than Bi atoms. Once the total coverage (Pb + Bi) on the substrate reaches about one monolayer, Pb and Bi are dealloyed from the substrate, and the interdiffusion profiles become essentially symmetric. Pb and Bi mix in all proportions, with an interdiffusion coefficient of ∼10⁻¹³ m²/s. This is considerably smaller than the self-diffusion coefficients previously observed for pure Pb and Bi in their respective high-coverage phases, indicating that the mechanism of interdiffusion is different from that of self-diffusion. As interdiffusion proceeds, the point of intersection of the Pb and Bi profiles reverses its drift direction, leading to the conclusion that binding of Bi atoms to the Cu(1 0 0) substrate is stronger than that of Pb atoms in the highest-coverage surface dealloyed layers.     

FeO(1 1 1) formation by exposure of Fe(1 1 0) to atomic and molecular oxygen

The growth, structure, and morphology of ultrathin iron oxide layers formed on a Fe(1 1 0) single crystal surface are investigated by Auger electron spectroscopy, low energy electron diffraction, and grazing ion scattering. For Fe oxidation by atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude because surface sticking and dissociation are not limiting the growth process. A well-ordered FeO(1 1 1) film with low defect density is only obtained with atomic oxygen. Compared to the bulk, the FeO lattice is laterally compressed by about 5-6% resulting in an in-plane oxygen (Fe) nearest-neighbor distance of 2.87 Å. Independent of the preparation method, long-range structural order is poor if the oxide film thickness is increased to 3-5 layers. This is attributed to the relatively large lattice mismatch between FeO(1 1 1) and Fe(1 1 0).     

The Fe/ZnO(0 0 0 1) interface: Formation and thermal stability

The room temperature growth mode and the interface reaction of Fe films on single crystalline ZnO(0 0 0 1) substrates prepared in ultra high vacuum (UHV) has been investigated by means of X-ray photoelectron and Auger electron spectroscopy (XPS, AES), low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The results show that Fe grows in the pseudo layer-by-layer mode. At ambient temperature the deposited Fe film reduces the underlying ZnO single crystal resulting in FeO at the interface and metallic Zn, which partially diffuses into the remaining Fe overlayer. Annealing leads to a stepwise oxidation of the Fe to FeO (670 K) and Fe₂O₃ (820 K). The Fe₂O₃ mixes with the substrate resulting in two (1 1 1) oriented textures of a spinel phase found by electron backscatter diffraction analysis (EBSD). Fe-based spin-injection may play a vital role for ZnO-based spintronic devices.     

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2)R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2)R45° phase by a first order transition, whereas the transition from c(9√2 × √2)R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step-step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step-step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu-Bi alloys is temperature dependent.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.