Magnetic properties of Mn-doped ZnO diluted magnetic semiconductors

A series of Mn-doped ZnO films have been prepared in different sputtering plasmas by using the inductively coupled plasma enhanced physical vapour deposition. The films show paramagnetic behaviour when they are deposited in an argon plasma. The Hall measurement indicates that ferromagnetism cannot be realized by increasing the electron concentration. However, the room-temperature ferromagnetism is obtained when the films are deposited in a mixed argon-nitrogen plasma. The first-principles calculations reveal that antiferromagnetic ordering is favoured in the case of the substitution of Mn^2＋ for Zn^2＋ without additional acceptor doping. The substitution of N for O （NO^-） is necessary to induce ferromagnetic couplings in the Zn-Mn-O system. The hybridization between N 2p and Mn 3d provides an empty orbit around the Fermi level. The hopping of Mn 3d electrons through the empty orbit can induce the ferromagnetic coupling. The ferromagnetism in the N-doped Zn-Mn-O system possibly originates from the charge transfer between Mn^2＋ and Mn^3＋ via NO^-, The key factor is the empty orbit provided by substituting N for O, rather than the conductivity type or the carrier concentration.     

稀磁半导体的制备与性质

报道了稀磁半导体的制备、性质与实验研究进展，介绍了稀磁半导体的应用及发展前景．     

Origin of ferromagnetism and nano-scale phase separations in diluted magnetic semiconductors

This paper reviews various origins of ferromagnetic response that has been detected in diluted magnetic semiconductors (DMS). Particular attention is paid to those ferromagnetic DMS in which no precipitation of other crystallographic phases has been observed. It is argued that these materials can be divided into three categories. The first consists of (Ga,Mn)As and related compounds. In these solid solutions the theory built on p–d Zener's model of hole-mediated ferromagnetism and the Kohn–Luttinger kp theory of semiconductors describes quantitatively thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods enabling magnetisation manipulation and switching. To the second group belong compounds, in which a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation that results in characteristics similar to colossal magnetoresistance oxides. Finally, in a number of compounds a chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent is present. It has recently been suggested that this spinodal decomposition can be controlled by the charge state of relevant magnetic impurities. This constitutes a new perspective method for 3D self-organised growth of coherent magnetic nanocrystals embedded by the semiconductor matrix.     

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn,Gd and Mn/Gd) nanocrystals

We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the A_x:Zn_1?xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd₂O₃ is observed. Due to the large mismatch of the ionic radii between Mn²⁺ and Gd³⁺ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn²⁺ and Gd³⁺ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.     

Polaron transport of amorphous semiconductors with embedded crystallites

Near-room-temperature (narrow-band) polaron transport of an amorphous semiconductor with embedded annealing-induced semiconducting crystallites is treated within an effective-medium approach. Carrier mobilities in the crystallites are assumed much larger than those of the amorphous phase. Nonetheless, crystallites act as macroscopic traps when their carriers’ energies lie below those in the amorphous phase. Then the mixture’s dc conductivity falls below that of the amorphous phase at low enough carrier concentrations. However, with increasing carrier concentration the shifting chemical potential diminishes this trapping effect, enabling crystallites’ larger mobilities to drive the mixtures’ electrical conductivity above that of the amorphous phase. Meanwhile the Seebeck coefficient remains insensitive to the annealing-induced introduction and growth of embedded crystallites. These features are qualitatively similar to those reported for an amorphous organic polymer FET with annealing-induced embedded crystallites.     

Pressure induced resistivity changes in diluted magnetic semiconductors of (Hg,Fe)Se and (Hg,Mn)Te

Abstract

Electrical resistance measurements have been performed on samples of Hg_1-xFe_xSe and Hg_1-xMn_xTe for × ≤ 0.30 as a function of pressure to 7.0 GPa. Both materials crystallize in the B3 structure at atmospheric pressure and room temperature and belong to the class of materials known as diluted magnetic semiconductors. At elevated pressures, HgTe undergoes a phase change from the B3 to B9 phase which is accompanied by an increase in the electrical resistivity of several orders of magnitude. The results reported here are that increasing × in both materials serves to increase the B3-B9 transition pressure. The implication being that partial replacement of the Hg-bonds by Fe-Se and Mn-Te, respectively, increases the stability range of the B3 phase     

TM掺杂的Ⅲ-Ⅴ族稀磁半导体电磁性质的第一原理计算

使用基于自旋局域密度泛函理论的第一性原理方法对3d过渡金属(TM=V，Cr，Mn，Fe，Co和Ni)掺杂的Ⅲ-Ⅴ族半导体(GaAs和GaP)的电磁性质进行了计算.结果发现：用V，Cr和Mn掺杂时体系将出现铁磁状态，而Fe掺杂时将出现反铁磁状态，Co和Ni掺杂时，其磁性则不稳定.其中，Cr掺杂的GaAs和GaP将可能是具有较高居里温度的稀磁半导体(DMS).在这些DMS系统中，V离子的磁矩大于理论期待值，Fe，Co和Ni离子的磁矩小于理论期待值，Cr和Mn离子的磁矩与期待值的差距取决于晶体的对称性以及磁性离子的能带分布.此外，使用Si和Mn共同对Ⅲ-Ⅴ族半导体进行掺杂，将有利于DMS表现为铁磁状态，并可以使体系的T_C进一步提高. 关键词： 稀磁半导体 过渡金属 掺杂 共掺杂     

Concentration Dependence of Room Temperature Magnetic Ordering in Dilute Fe: Ge1 − x Te x Alloy

Hyperfine interaction techniques like Mossbauer spectroscopy are very sensitive tools to study the local probe interactions in dilute magnetic semiconductors. We report here a Mossbauer study on the concentration dependence in Fe_0.008Ge_{1 − x} Te _x for x = 0, 0.008, 0.016, 0.03 and 0.05. At room temperature magnetic interactions were observed for all concentrations of Te and the population of magnetic site was found to increase gradually with the Te concentration. A constant magnetic hyperfine field of 136 KOe was found. A quadrupole doublet due to the FeTe₂ compound phase was also seen.     

A first-principles study of the magnetic properties in boron-doped ZnO

First-principles calculations based on density functional theory are performed to study the origin of ferromagnetism in boron-doped ZnO. It is found that boron atoms tend to reside at Zn sites. The induced Zn vacancy is a key factor for ferromagnetism in Zn1-xBxO (0相似文献   

Effect of bond angle and dihedral angle disorder on diamagnetic susceptibility of tetrahedrally coordinated amorphous semiconductors

We study the effect of bond angle and dihedral angle disorder on the diamagnetic susceptibility (χ) of a model amorphous semiconductor by adopting a linear combination of hybrids formalism. We have constructed orthormal basis states for the disorder network by introducing distortion in bond angles and dihedral angles. We have used the disorder basis states in the expression for χ and adopted suitable averaging techniques to obtain χ in terms of disorder parameters, which shows interesting results.      

退火氛围对掺杂ZnO薄膜磁性的影响

采用直流磁控反应共溅法制备了非磁性元素Al和磁性元素Co掺杂的ZnO薄膜, 样品原位真空退火后再空气退火处理. 利用X射线衍射仪(XRD) 和物理性能测量仪(PPMS) 对薄膜的结构和磁性进行了表征. XRD和PPMS结果表明, 不同的退火氛围对掺杂薄膜的结构和磁性有着很大的影响. 真空退火的Al掺杂ZnO薄膜没有观察到铁磁性, 而空气退火的样品却显示出明显的室温铁磁性, 铁磁性的来源与空气退火后导致Al和ZnO基体间电荷转移增强有关. 而对于Co掺杂ZnO薄膜, 真空退火后再空气退火, 室温铁磁性明显减弱. 其磁性变化与Co离子和ZnO基体间电荷转移导致磁性增强和间隙Co原子被氧化导致磁性减弱有关.     

First-principles study of B,C, N and F doped graphene-like MgO monolayer

Based on the first-principles calculations, we have investigated the stable geometries, electronic and magnetic properties of the graphene-like MgO monolayer with O atom substituted by B, C, N, and F atoms. The formation energy decreases in the order of B>C>N>F, which may be influenced by the different electronegativities. The band gaps of p-type doped MgO monolayers are tunable due to the emergence of impurity states within the band gap, while F-doped MgO monolayer realizes the transition from semiconductor to metal. The results show that p-type doped MgO monolayer exhibit magnetic behaviors due to polarizations of dopants and surrounding Mg or O atoms near the dopants, while no magnetism is observed in the case of F doped MgO monolayer. These results are potentially useful for spintronic applications and the development of magnetic nanostructures.     

Mn掺杂ZnO稀磁半导体材料的制备和磁性研究

采用共沉淀方法制备了名义组分为Zn_1-xMn_xO(x=0.001,0.005,0.007,0.01)的Mn掺杂的ZnO基稀磁半导体材料,并研究了在大气气氛下经过不同温度退火后样品的结构和磁性的变化.结果表明：样品在600℃的大气条件下退火后, 仍为单一的六方纤锌矿结构的ZnO颗粒材料;当样品经过800℃退火后,Mn掺杂量为0.007,0.01的样品中除了ZnO纤锌矿结构外还观察到ZnMnO₃第二相的存在.磁性测量表明,大气条件下600℃退火后的样品,呈现出室温铁磁性;而800℃退火后的样品,其室温铁磁性显著减弱,并表现为明显的顺磁性.结合对样品的光致发光谱的分析,认为合成样品的室温铁磁性是由于Mn离子对ZnO中的Zn离子的替代形成的. 关键词： ZnO 掺杂 稀磁半导体 铁磁性     

A Green＇s function model for ferromagnetism and spin excitations of （Ga,Mn）As diluted magnetic semiconductors

We study (Ga, Mn)As diluted magnetic semiconductors in terms of the Ruderman--Kittel--Kasuya--Yosida quantum spin model in Green's function approach. Random distributions of the magnetic atoms are treated by using an analytical average of magnetic configurations. Average magnetic moments and spin excitation spectra as functions of temperature can be obtained by solving self-consistent equations, and the Curie temperature T_C is given explicitly. T_C is proportional to magnetic atomic concentration, and there exists a maximum for T_C as a function of carrier concentration. Applied to (Ga, Mn)As, the theoretical results are consistent with experiment and the experimental T_C can be obtained with reasonable parameters. This modelling can also be applied to other diluted magnetic semiconductors.     

Magnetic field effects on excited states,charge transport,and electrical polarization in organic semiconductors in spin and orbital regimes

Magnetic field can influence photoluminescence, electroluminescence, photocurrent, injection current, and dielectric constant in organic materials, organic–inorganic hybrids, and nanoparticles at room temperature by re-distributing spin populations, generating emerging phenomena including magneto-photoluminescence, magneto-electroluminescence, magneto-photocurrent, magneto-electrical current, and magneto-dielectrics. These so-called intrinsic magnetic field effects (MFEs) can be observed in linear and non-linear regimes under one-photon and two-photon excitations in both low- and high-orbital materials. On the other hand, spin injection can be realized to influence spin-dependent excited states and electrical conduction via organic/ferromagnetic hybrid interface, leading to extrinsic MFEs. In last decades, MFEs have been serving as a unique experimental tool to reveal spin-dependent processes in excited states, electrical transport, and polarization in light-emitting diodes, solar cells, memories, field-effect transistors, and lasing devices. Very recently, they provide critical understanding on the operating mechanisms in advanced organic optoelectronic materials such as thermally activated delayed fluorescence light-emitting materials, non-fullerene photovoltaic bulk-heterojunctions, and organic–inorganic hybrid perovskites. While MFEs were initially realized by operating spin states in organic semiconducting materials with delocalized π electrons under negligible orbital momentum, recent studies indicate that MFEs can also be achieved under strong orbital momentum and Rashba effect in light emission, photovoltaics, and dielectric polarization. The transition of MFEs from the spin regime to the orbital regime creates new opportunities to versatilely control light-emitting, photovoltaic, lasing, and dielectric properties by using long-range Coulomb and short-range spin–spin interactions between orbitals. This article reviews recent progress on MFEs with the focus on elucidating fundamental mechanisms to control optical, electrical, optoelectronic, and polarization behaviors via spin-dependent excited states, electrical transport, and dielectric polarization. In this article both representative experimental results and mainstream theoretical models are presented to understand MFEs in the spin and orbital regimes for organic materials, nanoparticles, and organic–inorganic hybrids under linear and non-linear excitation regimes with emphasis on underlying spin-dependent processes.     

Magnetic and electrical transport properties of amorphous Zr-Fe alloys

Amorphous Zr_1?xFe_x samples were prepared in the composition range 0.2 ? x ? 0.9 either by means of vapour deposition or melt spinning. The electrical resistivity was determined in the range 4.2–300 K. Negative temperature coefficients were observed in the whole concentration range. The extended Ziman theory (diffraction model) was found to be able to explain these results only if the effective valence of the Fe atoms involves not only s electrons but also d electrons. The magnetic properties and the ⁵⁷Fe Mössbauer effect of the Zr_1?xFe_x alloys were studied in the range 4.2–300 K. The Fe-rich alloys are ferromagnetic. The Fe moment vanishes in alloys of an Fe concentration lower than about 50 at%. In most alloys (x ? 0.8) the Curie temperature is below room temperature and continuously decreases with Zr concentration. By means of Mössbauer spectroscopy and magnetic measurements it is shown that compositional short-range order (CSRO) is present to a higher degree in melt-spun alloys than in vapour-deposited alloys. The effect of sign and magnitude of the heat of solution on CSRO and the magnetic properties is discussed.     

First principles study on the structural, electronic and optical properties of diluted magnetic semiconductors Zn1-xCoxX （X=S, Se, Te）

The electronic and optical properties of zincblende ZnX（X=S, Se, Te） and ZnX：Co are studied from density functional theory （DFT） based first principles calculations. The local crystal structure changes around the Co atoms in the lattice are studied after Co atoms are doped. It is shown that the Co-doped materials have smaller lattice constant （about 0.6%-0.9%）. This is mainly due to the shortened Co-X bond length. The （partial） density of states （DOS） is calculated and differences between the pure and doped materials are studied. Results show that for the Co-doped materials, the valence bands are moving upward due to the existence of Co 3d electron states while the conductance bands are moving downward due to the reduced lattice constants. This results in the narrowed band gap of the doped materials. The complex dielectric indices and the absorption coefficients are calculated to examine the influences of the Co atoms on the optical properties. Results show that for the Co-doped materials, the absorption peaks in the high wavelength region are not as sharp and distinct as the undoped materials, and the absorption ranges are extended to even higher wavelength region.     

Structural stability at high pressure,electronic, and magnetic properties of BaFZnAs:A new candidate of host material of diluted magnetic semiconductors

The layered semiconductor BaFZnAs with the tetragonal ZrCuSiAs-type structure has been successfully synthesized.Both the in-situ high-pressure synchrotron x-ray diffraction and the high-pressure Raman scattering measurements demonstrate that the structure of BaFZnAs is stable under pressure up to 17.5 GPa at room temperature. The resistivity and the magnetic susceptibility data show that BaFZnAs is a non-magnetic semiconductor. BaFZnAs is recommended as a candidate of the host material of diluted magnetic semiconductor.     

二氧化铈基稀磁氧化物的第一性原理研究

基于第一性原理的计算方法研究了纯CeO_2、Co掺杂CeO_2和同时引入氧空位Vo和Co掺杂的CeO_2稀磁半导体体系.通过计算体系的能带结构和态密度,探讨了该体系磁性产生的机制.计算发现,纯CeO_2体系不具有磁性;没有氧空位Vo的Co掺杂CeO_2体系中,Co离子之间通过O原子发生超交换反铁磁耦合,体系无铁磁性;当氧空位Vo和Co离子同时存在于CeO_2体系中时,Co离子之间通过氧空位Vo发生铁磁耦合,该体系表现出铁磁性能.另外,由氧空位Vo诱导的Co离子之间的铁磁耦合不仅发生在紧邻的两个Co离子,而且可以扩展到几个原子距离的长度.计算结果证明了氧空位Vo诱导铁磁性耦合机制.本文工作将为CeO_2基稀磁半导体体系制备与磁学性质的研究提供支持.     

Growth and properties of self‐catalyzed (In,Mn)As nanowires

Mn‐assisted molecular beam epitaxy is used for the growth of (In,Mn)As nanowires (NWs) on GaAs(111)B. The transmission electron microscopy measurements revealed that despite the relatively high growth temperature regime this technique can be used to obtain (In,Mn)As NWs with high crystalline quality without any crystal defects, such as dislocations, stacking faults or precipitates inside the investigated NWs or on their side‐walls, although the growth processes of NWs were accompanied by the formation of MnAs precipitates between the NWs at the interface of the wetting layer. The results obtained are of importance for the realization of new spintronic nanostructured materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)