双层钙钛矿锰氧化物LaDy_(0.2)Sr_(1.8)Mn_2O_7的磁性和电性研究

双层钙钛矿锰氧化物LaDy_(0.2)Sr_(1.8)Mn_2O_7是采用传统固相反应法制备得到的多晶样品,通过测量样品的磁化强度与温度变化曲线(M-T)以及不同温度下磁化强度与外加磁场的变化曲线(M-H)对样品的磁性进行了研究.结果表明,在15K~375K的整个温度测量范围内,在类Griffiths相温度(T_G≈350K)以上,样品处于纯顺磁态;在奈尔温度(T_N≈200K)~T_G范围内,随温度的降低,样品的铁磁性逐渐增强;在T_N以下,随温度的降低,样品出现了团簇玻璃行为,反铁磁性增强,铁磁性减弱,系统处于反铁磁-铁磁共存态.另外,通过居里外斯拟合以及Griffiths相模型拟合,发现样品在T_G以下存在类Griffiths相.通过对样品LaDy_(0.2)Sr_(1.8)Mn_2O_7电性的分析发现,磁电阻会受到掺杂的影响而减小,由于掺杂量过大,使得金属-绝缘温度消失了,从而样品表现出了绝缘态的特性.     

面心立方双层铁磁薄膜中的二维布里渊区能带结构

利用二次量子化方法研究了由相同面心立方结构(100)材料构成的铁磁性双层薄膜,重点讨论了铁磁性界面交换作用(JAB>0)和反铁磁性界面交换作用(JAB<0)对体系二维布里渊区能带结构的影响.结果表明,当界面交换作用大于体交换作用时,在体模上方出现光学型界面模;当界面交换作用小于体交换作用时,在体模下方出现声学型界面模.特别的是,当铁磁性界面交换作用大小在零与体交换作用之间时,在面心立方结构中同样能出现声学型界面模,这与简立方结构有明显区别.并且讨论了不同界面交换作用时宇称对界面模的影响.     

Mn2NiSn掺杂Ni、Co体系的结构与磁性研究

本文通过在Heusler合金Mn2NiSn中掺杂Ni、Co元素对其结构与磁性交换作用变化进行了研究。结果表明,在Mn2-xNi1+xSn体系中,随着Ni逐渐替代Mn,合金结构会由立方XA逐渐向L21结构转变。由于Ni原子相对于Mn原子具有较小的原子半径,Ni2MnSn相对于Mn2NiSn体积大概收缩1.2%。Mn含量的减少使得最近邻的Mn(A)-Mn(B)间反铁磁交换作用减弱,导致居里温度由x = 0时的519 K接近线性的降低到x=1时的340 K。当用Co元素替换部分Mn元素时,即Mn2-yCoyNiSn中,复杂的原子占位使得Mn(A)-Mn(B)交换作用降低的同时伴随着Co-Mn之间铁磁交换作用的增强,两者交换作用的竞争导致居里温度先降低后升高,临界点出现在y = 0.3处。在Mn2Ni1-zCozSn体系中,当Co替代部分Ni时,由于Ni-Mn之间的交换作用小于Co-Mn之间的交换作用,交换作用的增强导致居里温度随着Co含量的增加逐渐升高。室温下的饱和磁化强度也随着掺杂量的增大逐渐增大。     

亚铁磁Heusler合金Mn2CoGa和Mn2CoAl掺杂Cr,Fe和Co的局域铁磁结构

通过实验和计算的方法研究了Mn₂CoM_xGa_1-x 和Mn₂CoM_xAl_1-x (M=Cr, Fe, Co)掺杂系列合金样品. 研究发现, 在共价作用的影响下, Fe和Co原子占A位, 使被取代的MnA (-2.1 μ_B)变成MnD (3.2 μ_B), 在最近邻的强交换作用下亚铁磁基体中形成了MnB-CoC-MnD局域铁磁性结构, 使分子磁矩的增量最高可达6.18 μ_B. Fe, Co 掺杂后建立同样的局域铁磁结构, 居里温度的变化趋势却不同. 实验观察到Mn₂Co_1+xAl_1-x中掺杂容忍度高达x=0.64, 远高于在Mn₂CoGa中(x=0.36)的结果; 以及随着Al的减少, 合金由B2有序向A2混乱转变等现象, 为共价作用对合金结构稳定的影响提供了证据. 磁测量中发现Cr掺杂后磁矩增量高达3.65 μ_B以及居里温度快速上升的反常现象, 意味着对占位规则的违背.     

Y2(Fe1-x-y,Coy,Crx)17化合物的结构及居里温度

通过x射线衍射及磁测量手段研究了Y2(Fe1-y-x,Coy,Crx)17化合物的结构及居里温度.研究结果表明Y2(Fe1-y-x,Coy,Crx)17化合物具有六角相的Th2Ni17型结构.随着x的增加,Y2(Fe1-y-x,Coy,Crx)17化合物的单胞体积呈现先小后大的非线性减小.通过Y2(Fe1-y-x,Coy,Crx)17化合物居里温度的变化,研究了Y2Fe17化合物中哑铃晶位上的Fe原子对之间的交换作用,从而在实验上证明了在Y2Fe17化合物中,哑铃晶位上的Fe原子对之间存在反铁磁交换作用,研究结果表明在Y2Fe17化合物中,Cr,Co同时少量替代Fe时,化合物的居里温度有显著提高.     

La_(0.4)Ca_(0.6)MnO_3中Mn-位Fe和Cr掺杂对磁性质的影响

研究了Fe和Cr掺杂对La0.4Ca0.6Mn O3中电荷有序反铁磁基态的调控作用.磁性质的测量结果表明,两种离子掺杂均能有效抑制原型样品中的长程电荷有序相,但是Fe离子掺杂样品均具有反铁磁的基态,而Cr掺杂样品中则出现了显著的铁磁性.结合电输运测量结果显示,Cr掺杂引起的铁磁态同时具有金属性,表明其中是电子双交换作用占主导.对比两种掺杂离子的电子结构发现,Cr离子空的e g电子轨道促进了电子双交换作用,而Fe掺杂则只是引入了不同的自旋交换作用,导致自旋无序.     

La_(2/3)Ca_(1/3)Mn_(1-x)Fe_xO_3电磁输运性质和电子自旋共振谱的研究

用固相合成法制备了La2/3Ca1/3Mn1-xFexO3(x=0、0.1、0.2)材料,通过X射线衍射、磁化强度-温度曲线、电子自旋共振谱线,研究了Fe替代部分的Mn对La2/3Ca1/3Mn1-xFexO3电磁性质的影响.结果表明:Fe离子掺杂对晶体结构影响较小;对电磁输运性质和磁结构影响较大,体系在低温区域较宽的温度范围内显示出巨磁电阻效应;ESR的测量结果也表明Fe离子掺杂形成反铁磁的交换作用,阻塞了铁磁Mn3+-O-Mn4+双交换通道,降低了体系的铁磁性.     

La$lt;sub$gt;0.4$lt;/sub$gt;Ca$lt;sub$gt;0.6$lt;/sub$gt;MnO$lt;sub$gt;3$lt;/sub$gt;中Mn-位Fe和Cr掺杂对磁性质的影响

研究了Fe和Cr掺杂对La_0.4Ca_0.6MnO₃ 中电荷有序反铁磁基态的调控作用. 磁性质的测量结果表明, 两种离子掺杂均能有效抑制原型样品中的长程电荷有序相, 但是Fe离子掺杂样品均具有反铁磁的基态, 而Cr掺杂样品中则出现了显著的铁磁性. 结合电输运测量结果显示, Cr掺杂引起的铁磁态同时具有金属性, 表明其中是电子双交换作用占主导. 对比两种掺杂离子的电子结构发现, Cr离子空的e_g电子轨道促进了电子双交换作用, 而Fe掺杂则只是引入了不同的自旋交换作用, 导致自旋无序. 关键词： 磁性氧化物 反铁磁     

溶胶凝胶合成锰掺杂ZnO的室温磁性行为

通过溶胶凝胶自燃法合成锰掺杂氧化锌纳米晶体, 研究了Mn掺杂ZnO稀磁半导体(简称DMS)的性质．X射线衍射光谱表明,锰掺杂氧化锌保留纤锌矿型状氧化锌六角晶体结构．采用能量色散X射线能谱和扫描电子显微镜分别对成分和形态进行研究．温度依赖的电阻率显示了DMS的半导体材料行为．振动样品磁强计测定的室温磁性行为,揭示了锰掺杂氧化锌的铁磁性和反磁性特性.     

钙钛矿锰氧化物Lao.8-x Eux Sro.2MnO3(x=0,0.05)的磁性和磁熵变研究

本篇文章主要研究钙钛矿氧化物La_(0.8-x)Eu_xSr_(0.2)MnO_3(x=0,0.05)中A位掺杂铕(Eu)后对样品的磁性和磁熵变的影响.采用传统的固相反应法制备多晶样品,根据数据拟合得到XRD图像和晶格参数,通过对两样品的M-T曲线和M-H曲线研究发现:x=0和x=0.05两样品在高温区均表现出顺磁性,居里温度T_c分别为283 K(x=0)和284 K(x=0.05),且在居里温度附近表现出铁磁性.随着掺杂量增加,样品的居里外斯温度降低(θ_(x=0)=322 K、θ_(x=0.05)=304 K),表明Eu~(3+)掺杂改变了系统内的铁磁耦合.在7 T磁场下磁熵变的最大值分别为2.73 J/kg·K和4.19 J/kg·K,表明Eu~(3+)掺杂使得最大磁熵变值增大.对比制冷效率,发现该系列样品具有作为磁制冷材料的潜质.     

Magnetic properties of epitaxial oxide heterostructures

Diluted Magnetic Semiconductors (DMS) are of great interest as injection sources for spin-polarized currents into semiconductors. Epitaxial devices have been synthesized with an intermediate spacer layer of the same semiconductor (zinc oxide, ZnO) used to produce the DMS material (ZnCoO) ensuring a homoepitaxial junction to help reduce the interface states and conduction mismatch. We observe a large magnetoresistance of about 32% in the devices at low temperatures. The present work suggests that spin polarized transport could be achieved with DMS materials acting as the source of injected spins into a non-magnetic host.     

Electronic and magnetic properties of a new 2D diluted magnetic semiconductor La1 − x Ba x Zn1 − x Mn x AsO from Ab initio calculations

Very recently, on the example of hole- and spin-doped semiconductor LaZnAsO, quite an unexpected area of potential applications of quasi-two-dimensional 1111-like phases was proposed (C. Ding et al., Phys. Rev. B 88, 041102R (2013)) as a promising platform for searching for new diluted magnetic semiconductors (DMSs). In this work, by means of the ab initio calculations, we have examined in detail the electronic and magnetic properties of LaZnAsO alloyed with Ba and Mn. Our results demonstrate that Ba or Mn doping transforms the parent non-magnetic semiconductor LaZnAsO into a non-magnetic metal or a magnetic semiconductor, respectively. On the other hand, the joint effect of these dopants (i.e., co-doping Ba + Mn) leads to transition of La_0.89Ba_0.11Zn_0.89Mn_0.11AsO into the state of magnetic metal, which is formed by alternately stacked semiconducting non-magnetic blocks [La_0.89Ba_0.11O] and metallic-like magnetic blocks [Zn_0.89Mn_0.11As].     

First-principles studies of interlayer exchange coupling in (Ga,Co)N-based diluted magnetic semiconductor multilayers

Interlayer exchange coupling (IEC) in a series model diluted magnetic semiconductor (DMS) multilayer consisting of two magnetic (Ga, Co)N layers separated by non-doped or Mg-doped GaN non-magnetic spacers has been studied by first-principles calculations. The effects of the spacer thickness and hole doping to the IEC were studied systematically. It is observed that (1) for the GaN spacers without Mg doping, the IEC between two magnetic (Ga, Co)N layers is always ferromagnetic, which is clarified as an intrinsic character of the Ruderman–Kittle–Kasuya–Yoshida (RKKY) interaction based on a two-band model for a gapped system; and (2) for the Mg-doped GaN spacers, the IEC is tunable from ferromagnetic to antiferromagnetic by varying the spacer’s thickness and the dopant’s site.     

3d过渡金属掺杂对Cd12O12纳米线电子和磁性能的影响

采用基于密度泛函理论的第一性原理计算方法,系统研究了3d过渡金属元素(Sc、Ti、Cr、Mn、Co、Cu和Zn)掺杂Cd12O12纳米线的几何结构,电子结构和磁性。结果表明：所有掺杂体系均是热力学稳定的;掺杂Ti或Zn时体系保留了原有的非磁半导体特性,掺杂Mn、Co或Cu时能够实现磁性半导体态,而在掺杂Sc（Cr）时体系转变为非磁性金属态（磁性金属态）。研究结果表明,掺杂3d过渡金属元素的Cd12O12纳米线在电子、光电和自旋电子学领域具有潜在的应用价值。     

Polarization selective magneto-optical study on the coupled quantum dots using resonant excitation

We have studied a double-layer self-assembled quantum dot (QD) structures consisting of non-magnetic CdSe and magnetic CdMnSe. Transmission electron microscopy image shows that QDs are formed within the CdSe and CdMnSe layers, and they are vertically correlated in the system. The strong interband ground state transition was observed in magneto-photoluminescence (PL) experiments. In contrast to a typical behavior for many low-dimensional systems involving diluted magnetic semiconductors (DMSs), where PL signal dramatically increases when an external magnetic field is applied, we have observed a significant decrease of the PL intensity as a function of magnetic field in the double-layer structures where the alternating QD layers contain the DMS and non-DMS QDs. We attribute such effect to carrier transfer from non-magnetic CdSe dots to magnetic CdMnSe dots due to the large Zeeman shift of the band edges of DMS QDs in magnetic field. Since the band alignment of QD structure strongly depends on the spin states of system, we performed polarization-selective PL measurement to identify spin-dependent carrier tunneling in this coupled system.     

Spin-dependent shot noise in diluted magnetic semiconductor/semiconductor heterostructures with a nonmagnetic barrier

We investigate quantum size effect on the spin-dependent shot noise in the diluted magnetic semiconductor (DMS)/semiconductor heterostructure with a nonmagnetic semiconductor (NMS) barrier in the presence of external magnetic and electric fields. The results demonstrate that the NMS barrier plays a quite different role from the DMS layer in the electron transport process. It is found that spin-down shot noise shows relatively regular oscillations as the width of DMS layer increases, while the spin-up shot noise deceases monotonically. However, as the width of NMS layer increases, the spin-down shot noise displays irregular oscillations at first and then decreases while the spin-up shot noise decreases at a quite different rate. The results indicate that the shot noise can be used as a sensitive probe in detecting material type and its size.     

Transport properties and electronic structure of epitaxial tunnel junctions

We present ab initio calculations for the electronic ground-state and transport properties of epitaxial Fe/semiconductor/Fe (0 0 1) tunnel junctions. The ground state properties are determined by the ab initio Screened KKR Green's function method and the transport properties by a Green's function formulation of the Landauer–Büttiker formalism. We focus on tunnel junctions with a semiconducting ZnSe barrier and compare them to results for junctions with Si and GaAs barriers. We comment on the presence of metal-induced gap states (MIGS) in the semiconductor, the spin polarization of which strongly depends on the nature of the barrier. We investigate furthermore the influence of one atomic layer at the interface of a non-magnetic metal (Cu, Ag, Al) and of a magnetic 3d transition metal.     

Manipulating electronic and magnetic properties of black phosphorene with 4d series transition metal adsorption

We carry out first-principles calculations within density-functional theory to investigate the structural, electronic and magnetic properties of 4d transition metal (TM) decorated monolayer black phosphorene (BP). The results indicate that the TM adsorption on BP can have dilute magnetic semiconductor (DMS) properties. The spin polarized semiconducting state is realized in BP by adsorption of Y, Nb and Ru, while a half-metal state is obtained by Tc adsorption. In the case of two same types of TMs adsorption on BP, only [email protected] shows DMS state. In particular, two different types of TMs decorated BP can induce magnetic moments, localized mainly on the 4d TMs and the neighboring P atoms. Furthermore, the 4d TMs may enrich the electronic properties of BP, such as half-metallic, metallic and semiconducting features. These findings suggest that the 4d TM adsorbed BP can be used as a potential next-generation spintronics and magnetic storage material.     

Spatially resolved inhomogeneous ferromagnetism in (Ga,Mn)as diluted magnetic semiconductors: a microscopic study by muon spin relaxation

Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-microSR) as well as by transport and magnetization measurement techniques. LE-microSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.     

Dilute magnetic semiconductor nanowires

Semiconductor materials form the basis of modern electronics, communication, data storage and computing technologies. One of today’s challenges for the development of future technologies is the realization of devices that control not only the electron charge, as in present electronics, but also its spin, setting the basis for future spintronics. Spintronics represents the concept of the synergetic and multifunctional use of charge and spin dynamics of electrons, aiming to go beyond the traditional dichotomy of semiconductor electronics and magnetic storage technology. The most direct method to induce spin-polarized electrons into a semiconductor is by introducing appropriate transition-metal or rare-earth dopants producing a dilute magnetic semiconductor (DMS). At the same time the seamless integration of future spintronic devices into nanodevices would require the fabrication of one-dimensional DMS nanostructures in well-defined architectures. In this review we focus on recent advances in the synthesis of DMS nanowires as well discussing the structural, optical and magnetic properties of these materials. PACS 75.75.+a; 81.07.Vb; 68.65.La