Structural and magnetic properties of Mn-doped ZnO powders

A mixture of bi(acetylacetonato) zinc(II)hydrate and tris(acetylacetonato) manganese(III) complexes was thermally co-dissociated to synthesize Mn-doped ZnO powders. In order to examine the effect of oxygen vacancies on the ferromagnetic coupling of Mn ions, two preparation routes were used: in route (I) the preparation was done in an open environment, whereas in route (II) the preparation was done in a closed environment. The X-ray diffraction (XRD) and the X-ray fluorescence (XRF) measurements indicate that the Mn content in the three samples are 3.9% (I), 3.3% (II) and 4.2% (II). The XRD results showed that the Mn ions were incorporated in the ZnO crystal and that a Zn_1−xMn_xO solid solution has formed. The magnetic characterization indicated that only samples prepared via route (II) exhibited a room temperature ferromagnetic component of magnetization. Furthermore, magnetic analysis showed that the magnetic moment per dopant ion in the samples examined was in the range of 4.2-6.1 μ_B/Mn. The percentages of coupled Mn atoms to the total number of Mn atoms were found to be extremely small (less than 0.1%), which by itself cannot explain the observed RT hysteresis loops. Thus, in order to produce long-range ferromagnetic order in these samples, the FM coupling has to be mediated via defects. The observed FM in this study may be attributed to the presence of oxygen vacancies, which mediate the ferromagnetic exchange between the coupled Mn ions. This is consistent with the bound magnetic polarons (BMP) model where defects like oxygen vacancies cause the polarons to overlap and give rise to a long-range ferromagnetic order in dilute magnetic semiconductors (DMS).     

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.     

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     

Influence of Fe-doping on the structural,optical and magnetic properties of ZnO nanoparticles

Zn_1–xFe_xO (x=0–0.05) nanoparticles were synthesized without a catalyst by a two-step method. Fe was doped into ZnO by a source of metallic Fe sheets in a solid–liquid system at 80 °C, and the Zn_1−xFe_xO nanoparticles were obtained by annealing at 300 °C. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy were used to characterize the structural properties of the as-grown Zn_1−xFe_xO. The optical properties were determined by Infrared and Ultraviolet–visible spectroscopy. The results confirm that the crystallinity of the ZnO is deteriorated due to Fe-doping. XPS results show that there is a mixture of Fe⁰⁺ and the Fe³⁺ in the representative Zn_0.95Fe_0.05O sample. The optical band gap of Zn_1−xFe_xO is enhanced with increasing of Fe-doping. Room temperature ferromagnetism was observed in all the Fe-doped ZnO samples.     

The structure and magnetic properties of Cu-doped ZnO prepared by sol-gel method

The Cu-doped ZnO and pure ZnO powders were synthesized by sol-gel method. The structural properties of the samples were investigated by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. All the results confirmed that copper ions were well incorporated into the ZnO lattices by substituting Zn sites without changing the wurtzite structure and no secondary phase existed in Cu-doped ZnO nanoparticles. The Zn_0.97Cu_0.03O nanoparticles exhibited ferromagnetism at room temperature, as established by the vibrating sample magnetometer analysis.     

Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films

XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 °C, while reduction of Ni²⁺ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 °C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 °C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni²⁺ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M_s) in the films was observed after annealing in air (M_s = 0.036 μ_B/Ni) or Ar (M_s = 0.033 μ_B/Ni) at 600 °C compared to that in as-deposited film (M_s = 0.017 μ_B/Ni). An even higher M_s value was observed in the film annealed in Ar at 800 °C (M_s = 0.055 μ_B/Ni) compared to that at 600 °C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 °C, which was also attributed to the diffusion of Ni.     

Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe³⁺ nature of the iron atom in ZnO matrix.     

Effect of nickel doping concentration on structural and magnetic properties of ultrafine diluted magnetic semiconductor ZnO nanoparticles

The ZnO:Ni²⁺ nanoparticles of mean size 2-12 nm were synthesized at room temperature by the simple co-precipitation method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Ni doping concentration and an additional NiO-associated diffraction peak was observed above 15% of Ni²⁺ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Ni²⁺ doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially, these nanoparticles showed strong ferromagnetic behavior, however, at higher doping percentage of Ni²⁺, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Ni-Ni ions suppressed the ferromagnetism at higher doping concentrations of Ni²⁺.     

Structural, optical and magnetic properties of Cr doped ZnO microrods prepared by spray pyrolysis method

A series of Cr-doped ZnO micro-rod arrays were fabricated by a spray pyrolysis method. X-ray diffraction patterns of the samples showed that the undoped and Cr-doped ZnO microrods exhibit hexagonal crystal structure. Surface morphology analysis of the samples has revealed that pure ZnO sample has a hexagonal microrod morphology. From X-ray photoelectron spectroscopy studies, the Cr 2p3/2 binding energy is found to be 577.3 eV indicating that the electron binding energy of the Cr in ZnO is almost the same as the binding energy of Cr³⁺ states in Cr₂O₃. The optical band gap E_g decreases slightly from 3.26 to 3.15 eV with the increase of actual Cr molar fraction from x = 0.00 to 0.046 in ZnO. Photoluminescence studies at 10 K show that the incorporation of chromium leads to a relative increase of deep level band intensity. It was also observed that Cr doped samples clearly showed ferromagnetic behavior; however, 2.5 at.% Cr doped ZnO showed remnant magnetization higher than that of 1.1 at.% and 4.6 at.% Cr doped samples, while 4.6 at.% Cr doped ZnO samples had a coercive field higher than the other dopings.     

Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu nanorods

Copper doped ZnO nanoparticles were synthesized by the chemical technique based on the hydrothermal method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) for different doping percentages of Cu²⁺ (1-10%). TEM/SEM images showed formation of uniform nanorods, the aspect ratio of which varied with doping percentage of Cu²⁺. The wurtzite structure of ZnO gradually degrades with the increasing Cu²⁺ doping concentration and an additional CuO associated diffraction peak was observed above 8% of Cu²⁺ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Cu²⁺ doping concentrations was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced.     

本征空位缺陷对ZnO:Mn体系电子特性及磁性的影响

本文采用基于密度泛函理论的第一性原理赝势法,分析了ZnO:Mn掺杂体系中本征空位缺陷V_(Zn)和V_O分别出现在相对Mn为近邻、次近邻、远次近邻位置时体系的晶体结构、能带分布、态密度和磁性.结果表明:ZnO:Mn体系中V_(Zn)比V_O更容易产生,且两种缺陷均更容易在Mn的近邻位置形成.其中V_(Zn)的出现没有明显改变ZnO:Mn体系的带隙,然而会使体系的导电性增加,且V_(Zn)与Mn的距离越远,导电性越强.同时,V_(Zn)减弱了体系的磁性,但与V_(Zn)的位置无关. V_O的出现会使体系带隙变宽,且电导率显著低于无缺陷ZnO:Mn体系,但是其导电性会随着V_O与Mn的距离变远而增强.同时,V_O的出现不会影响体系原来的磁性.     

Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO

The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations.It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO.The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom.The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms.     

不同位置的Mn掺杂对ZnO量子点的电磁特性影响

采用基于态密度泛函理论的第一性原理赝势法,分析了直径为1.2nm的ZnO量子点体系(Zn45O45H72,并经H钝化)在中心、中间、表面三种不同位置Mn掺杂情况下的晶体结构、能带结构、态密度分布和磁性.模拟结果表明：中间位置掺杂时,体系的结合能最低,同时导电性能最好;中心位置掺杂略劣于中间位置掺杂;而表面位置掺杂时,结合能明显偏高,同时导电性能明显减弱.在磁性方面,随着掺杂位置变化,体系中各原子的总磁矩和磁矩分布会发生明显变化,但体系的总磁矩基本不变.     

Effect of Mn doping on the nanostructure and optical properties of ZnO films synthesized by magnetron sputtering

We investigated the microstructure and optical properties of Zn_1−xMn_xO films synthesized by the magnetron sputtering technique. Structural analyses suggest that Mn occupied the Zn sites successfully and did not change the wurtzite structure of ZnO. In addition, nanoscale columnar grain arrays were found in the Mn-doped ZnO films. The experimental results indicate that moderate Mn doping could enhance the photoluminescence emission of ZnO. The possible origin of the emissions from our samples was also explored.     

脉冲磁场对水热法制备Mn掺杂ZnO稀磁半导体的影响

本文以Zn(CH₃COO)₂·2H₂O, Mn(CH₃COO)₂·4H₂O和氨水缓冲溶液为原料, 在4 T脉冲磁场下利用水热法制备了Mn掺杂ZnO稀磁半导体晶体, 通过X射线衍射、 扫描电子显微镜、透射电子显微镜、拉曼光谱、荧光分光光度计及振动样品磁强计等对样品的微观结构及磁性能等进行了表征, 结果表明: Mn掺杂ZnO稀磁半导体晶体仍保持ZnO六方纤锌矿结构, 4 T脉冲磁场下合成的Mn掺杂ZnO稀磁半导体晶体具有明显的室温铁磁性, 其饱和磁化强度(M_s)为0.028 emu/g, 比无脉冲磁场下制备的样品提高一倍以上, 且4 T 脉冲磁场将样品的居里温度提高了15 K.     

Influence of background carriers on magnetic properties of Mn-doped dilute magnetic Si

Using hybrid exchange density functional calculations we show that the type of background carriers has profound effects on magnetic interactions in Mn doped dilute magnetic Si. The p- and n-type Si were simulated by introducing an extra hole and an extra electron, respectively in the 64 atoms Si supercell. In case of p-type Si compensated by a homogeneous background potential and 1.6% Mn, the ground state is ferromagnetic, whereas other conditions remaining the same, the ground state becomes antiferromagnetic for the n-type Si. The exchange energies in Mn-doped extrinsic Si are higher by about 1 eV/Mn atom compared to the Mn doped intrinsic Si. Calculated electronic structures reveal that in p-type Si:Mn the hole localises over Mn and the short range magnetic coupling increases. Our calculations indicate that localisation of magnetic polarons at the Mn site is likely, which in turn enhances long range magnetic interaction between Mn ions and responsible for FM stabilisation. On the other hand, in the n-type host electron–electron repulsion increases within Mn–Si impurity band and the short range coupling decreases, which destroys the long range spin polarisation. These calculations explain the observed ferromagnetism in the p-type Si:Mn at higher temperatures than in the n-type Si:Mn and the magnetic moments of the systems compare well with experiments.     

Effects of doping concentration on properties of Mn-doped ZnO thin films

This paper reports that the radio frequency magnetron sputtering is used to fabricate ZnO and Mn-doped ZnO thin films on glass substrates at 500~°C. The Mn-doped ZnO thin films present wurtzite structure of ZnO and have a smoother surface, better conductivity but no ferromagnetism. The x-ray photoelectron spectroscopy results show that the binding energy of Mn_2p3 / 2 increases with increasing Mn content slightly, and the state of Mn in the Mn-doped ZnO thin films is divalent. The chemisorbed oxygen in the Mn-doped ZnO thin films increases with increasing Mn doping concentration. The photoluminescence spectra of ZnO and Mn-doped ZnO thin films have a similar ultraviolet emission. The yellow green emissions of 4~wt.% and 10~wt.% Mn-doped thin films are quenched, whereas the yellow green emission occurs because of abundant oxygen vacancies in the Mn-doped ZnO thin films after 20~wt.% Mn doping. Compared with pure ZnO thin film, the bandgap of the Mn-doped ZnO thin films increases with increasing Mn content.     

Effect of carbon co-doping on the magnetic properties of bulk Mn doped ZnO

We report the systematic study on the effect of C-doping and annealing atmosphere (air, O₂, N₂ and Ar) on the magnetic properties of pure ZnO as well as Zn_0.98Mn_0.02O (ZMO). We found diamagnetic behavior for pure ZnO samples in all the annealing atmospheres whereas all ZMO samples are paramagnetic except the one, which is annealed in argon. While maintaining the same annealing atmospheres and varying the C content we found that on increasing the C-doping concentration the ferromagnetic fraction in the sample increases. The observed RTFM in C-doped ZMO is possibly due to carriers introduced into the samples due to oxygen vacancies and the substitution of C at O sites.     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

Structural, magnetic and transport properties of Ni-doped ZnO films

In this work, Ni-doped ZnO (Zn_1−xNi_xO, x=0, 0.03, 0.06, 0.11) films were prepared using magnetron sputtering. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), temperature dependence electrical resistance, Hall and magnetic measurements were utilized in order to study the properties of the Ni-doped ZnO films. XRD and XAS results indicate that all the samples have a ZnO wurtzite structure and Ni atoms incorporated into ZnO host matrix without forming any secondary phase. The Hall and electrical resistance measurements revealed that the resistivity increased by Ni doping, and all the Ni-doped ZnO films exhibited n-type semiconducting behavior. The magnetic measurements showed that for the samples with x=0.06 and 0.11 are room-temperature ferromagnetic having a saturation magnetization of 0.33 and 0.39 μ_B/Ni, respectively. The bound-magnetic-polaron mediated exchange is proposed to be the possible mechanism for the room-temperature ferromagnetism in this work.