Defects enhanced ferromagnetism in Cu-doped ZnO thin films

We present here new evidences of point defects enhanced ferromagnetism in Cu-doped ZnO thin films by different characterization methods. Cu-doped ZnO thin films with Cu concentrations ranging from 0.05 to 5 at.% were prepared by an inductively coupled plasma enhanced physical vapor deposition system. Room-temperature ferromagnetism is observed in all the films. The saturation magnetization shows an increasing trend with the increase of Cu concentration except a slight decrease for the 1 at.% Cu-doping. Further study performed by Raman spectra, X-ray absorption spectra and extended X-ray absorption fine structure indicate the existence of Cu²⁺ ions and point defects in all the films. The local structural characterization and magnetic properties reveal that the sample with larger saturation magnetization has a higher concentration of point defects.     

Ferromagnetism in defective yttria-stabilized zirconia

Significant research attention has been devoted to identifying and synthesizing new magnetic materials via doping of non-magnetic materials. The material defects offer an approach to stabilize ferromagnetism in non-magnetic materials such as oxygen-deficient HfO₂ and oxygen-deficient ZrO₂. In this study, we demonstrated room-temperature ferromagnetism via nitrogen ion implantation on yttria-stabilized zirconia (YSZ) single crystals. The results of structural and chemical analyses indicate the formation of a distinct surface layer through the implantation of nitrogen ions and potential oxygen vacancies. The lattice constant in this surface layer increased by 0.6% compared to the bulk value. Nitrogen ions were observed in this region, and their concentration was estimated to be 2.32 atoms per unit cell. In contrast to the lack of magnetic hysteresis in a YSZ single crystal, ferromagnetic hysteresis was observed in the ion-implanted YSZ crystals, owing to defects—nitrogen ions and oxygen vacancies in the surface layer.     

Copper-doped AlN polycrystalline powders: A class of room-temperature ferromagnetic materials

Cu-doped AlN polycrystalline samples were synthesized by a solid-state reaction. X-ray diffraction analysis revealed the hexagonal structure of the doped samples. Photoluminescence measurements reveal the substitution of Cu for Al in the AlN lattice. Clear hysteresis loops are observed in the M-H curves for the samples at 300 K, revealing room temperature ferromagnetism of the samples. Our experimental results verify that room temperature ferromagnetism is an intrinsic property of Cu-doped AlN.     

Ferromagnetic properties of Cu-doped ZnS: A density functional theory study

Using plane-wave pseudopotential (PWPP) method, the magnetism and spin-resolved electronic properties of Cu-doped ZnS system are studied. Our calculations indicate that ferromagnetic (FM) state is ground state in Cu-doped ZnS. The FM coupling strength in ZnS doping with Cu fluctuates with the variation of distance between two dopants and the fluctuation gets larger with increase in distance. Room temperature ferromagnetism can be observed in Cu-doped ZnS with high dopant concentration. Formation energy calculation implies that the clustering effect is not obvious in Cu-doped ZnS. Thus, Cu-doped ZnS can be a promising dilute magnetic semiconductor (DMS), which promises to be free of magnetic precipitates.     

Defect types and room-temperature ferromagnetism in undoped rutile TiO2 single crystals

Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO2 single crystals when a magnetic field is applied parallel to the sample plane.By combining X-ray absorption near the edge structure spectrum and positron annihilation lifetime spectroscopy,Ti3+-V O defect complexes(or clusters) have been identified in annealed crystals at a high vacuum.We elucidate that the unpaired 3d electrons in Ti3+ ions provide the observed room-temperature ferromagnetism.In addition,excess oxygen ions in the TiO2 lattice could induce a number of Ti vacancies which obviously increase magnetic moments.     

N空位对Cu掺杂AlN电磁性质的影响的第一性原理研究

利用基于密度泛函理论的平面波超软赝势法研究了N空位对Cu掺杂AlN的电子结构和磁学性质的影响。结果表明,与Cu最近邻的N原子更易失去形成N空位。N空位的引入减小了Cu掺杂AlN体系的半金属能隙;减弱了Cu及其近邻N原子的自旋极化的强度以及Cu3d与N2p轨道间的杂化,因而减小了体系的半金属铁磁性。因此,制备Cu掺杂AlN稀磁半导体时应尽可能地避免产生N空位。     

Defects in room-temperature ferromagnetic Cu-doped ZnO films probed by x-ray absorption spectroscopy

We report a comprehensive study of the defects in room-temperature ferromagnetic (RTFM) Cu-doped ZnO thin films using x-ray absorption spectroscopy. The films are doped with 2 at.% Cu, and are prepared by reactive magnetron sputtering (RMS) and pulsed laser deposition (PLD), respectively. The results reveal unambiguously that atomic point defects exist in these RTFM thin films. The valence states of the Cu ions in both films are 2(+). In the film prepared by PLD, the oxygen vacancies (V(O)) form around both Zn ions and Cu ions in the hexagonal wurtzite structure. Upon annealing of the film in O(2), the V(O) population reduces and so does the RTFM. In the film prepared by RMS, the V(O)s around Cu ions are not detected, and the V(O) population around Zn ions is also smaller than in the PLD-prepared film. However, zinc vacancies (V(Zn)) are evidenced. Given the low doping level of spin-carrying Cu ions, these results provide strong support for defect-mediated ferromagnetism in Cu-doped ZnO thin films.     

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.     

Ferromagnetism in Cu-doped ZnSe semiconducting quantum dots

The projection of integrating optical, magnetic and electronic functionalities into a single material have aggravated passionate attention in mounting wide band gap diluted magnetic semiconductor (DMS) in the midst of room temperature ferromagnetism. We report the evidence of ferromagnetism in Cu-doped ZnSe quantum dots (QDs) below room temperature, grown from a single source precursor by lyothermal method with the sizes of approximately 3.2–5.14 nm. QDs mainly exhibit paramagnetic behavior between 80 and 300 K, with a weak ferromagnetic/anti-ferromagnetic exchange at lower temperature as observed by superconducting quantum interference device (SQUID) magnetometer. From the Curie–Weiss behavior of the susceptibility, Curie temperature (T _c) of Cu-doped ZnSe sample has been evaluated. From EPR, we obtain the Lande-g factor in the Zeeman interaction term as 2.060. Photoluminescence and EPR measurements support and confirm the view that Cu²⁺ substitutes for Zn²⁺ in Cu-doped ZnSe quantum dots.     

First-principles study on electronic and magnetic properties of Cu-doped CdS

Based on the full-potential linearized augmented plane wave (FLAPW) method, the electronic structures and magnetic properties in Cu-doped CdS diluted magnetic semiconductors (DMSs) have been investigated. The results indicate that Cu-doped CdS systems show half-metallic character with a total magnetic moment of 1.0 μ_B per supercell. In the case of two Cu atoms substituting for Cd atoms, the long-range ferromagnetism is observed, which results from Cu(3d)-S(3p)-Cd-S(3p)-Cu(3d) coupling chain. The estimated Curie temperature of Cu-doped CdS is predicted to be 400 K, higher than room temperature. These results suggest that Cu-doped CdS may be a promising half-metallic ferromagnetic material for practical applications in electronics and spintronics.     

Enhanced ferromagnetic properties of ZnO: Cu tuned by HPHT

A series of Cu-doped ZnO ceramics were synthesized from pre-prepared Cu-doped ZnO nanocrystals using a cubic anvil high pressure apparatus under various pressure and temperature conditions. The structures, vibrational spectra and magnetic properties were investigated in detail. The results indicated that the enhancement of ferromagnetism had been achieved for the sample tuned by proper pressure and temperature (5 GPa, 500 °C). The ferromagnetism returned again to the primary state once the tuning temperature was raised up to 800 °C. Compared with those of the samples prepared at ambient pressure, the magnetic properties of the samples tuned by HPHT method had been improved greatly.     

The magnetic phase transition in Cu-doped ZnO: From bulk to nanocluster

Using the first-principles calculations based on the density functional theory, we have investigated the magnetic properties of Cu-doped ZnO both in bulk and nanocluster. The calculated results show that the substitutional Cu ions are spin polarized and have a tendency to assemble. It is found that the ground state has shown a change from ferromagnetic phase to antiferromagnetic phase as the size for the doping system decreases from bulk to nanocluster. In bulk ZnO, the ferromagnetism is attributed to the strong hybridization between Cu 3d and O 2p states. In ZnO nanocluster, however, the antiferromagnetic exchange interaction is dominant because of the very close Cu–Cu distance.     

Cu、Ag和Au掺杂AlN的电磁性质的第一性原理研究

采用基于密度泛函理论（DFT）的平面波超软赝势法,研究了Cu、Ag、Au掺杂AlN的晶格常数、磁矩、能带结构和态密度。电子结构表明,Cu、Ag、Au的掺杂使在带隙中引入了由杂质原子的d态与近邻N原子的2p态杂化而成的杂质带,都为p型掺杂,增强了体系的导电性。Cu掺杂AlN具有半金属铁磁性,半金属能隙为0.442eV,理论上可实现100%的自旋载流子注入;Ag掺杂AlN具有很弱的半金属铁磁性;而Au掺杂AlN不具有半金属铁磁性。因此,与Ag、Au相比,Cu更适合用来制作AlN基稀磁半导体。     

Electronic structures and magnetic properties in Cu-doped two-dimensional dichalcogenides

We explore the electronic structures and magnetic properties in Cu-doped MX₂ (=MoS₂, MoSe₂, MoTe₂, and WS₂) based on density functional theory. A Cu dopant leads to a net moment of 5.0 or 1.0 μ_B in MX₂, which mainly depend on the size of crystal-field splitting relative to that of the spin splitting. No magnetism is observed in Cu-doped MoTe₂. The local distortion around the Cu atom reduces the total magnetic moment in two-Cu-doped MX₂. The magnetic coupling between the nearest neighboring Cu atoms is ferromagnetic for all the cases, but they demonstrate various magnetic ground states with the increasing distance between Cu atoms: the Cu-doped MoS₂ and WS₂ exhibit anti-ferromagnetic and nonmagnetic ground state, respectively. A long-range ferromagnetic or ferrimagnetic coupling is attributed to double-exchange interaction in Cu-doped MoSe₂. Half-metallic ferromagnetism with Curie temperature above room temperature in Cu-doped MoSe₂ provides a useful guidance to engineer the magnetic properties of MoSe₂ in experiments.     

Investigation of morphology evolution of Cu–ZnO nanorod arrays and enhancement of ferromagnetism by codoping with N

Cu-doped ZnO nanorod arrays have been synthesized by a facile electrochemical deposition method. It has been found that the morphology transition from conical tops to flat tops is extremely suppressed in unstirred growth environment, so we can get nanorods with conical top much easier. In addition, the effect of nitrogen on the magnetic properties of Cu-doped ZnO has been investigated. It has been found that incorporation of N leads to an apparent enhancement of the ferromagnetism in Cu-doped ZnO films. The ferromagnetic behavior is a compensated result in the doped ZnO system.     

Magnetic and optical properties of epitaxial n-type Cu-doped ZnO thin films deposited on sapphire substrates

In this paper, we report on pulsed laser deposition of n-type Cu-doped ZnO thin films on c-plane sapphire substrates at 700°C. XRD and HRTEM were employed to study the epitaxial growth relationship between the Zn_1−x Cu _x O film and sapphire substrate. Absorption measurements showed excitonic nature of the thin films and a decrease in the bandgap energy with increased Cu concentration was observed. Such as-deposited films showed room temperature ferromagnetism with Curie temperature (T _c ) at around 320 K. The moment per Cu atom decreases as the Cu concentration increases. The largest magnetic moment about 0.81μ _B /Cu atom was observed for Zn_0.95Cu_0.05O thin films. The presence of any magnetic second phase was ruled out and the ferromagnetism was attributed to Cu ions substituted directly into the ZnO lattice.     

Half-metallic ferromagnetism in Cu-doped zinc-blende ZnO from first principles study

Electronic structures and magnetism of Cu-doped zinc-blende ZnO have been investigated by the first-principle method based on density functional theory (DFT). The results show that Cu can induce stable ferromagnetic ground state. The magnetic moment of supercell including single Cu atom is 1.0 μ_B. Electronic structure shows that Cu-doped zinc-blende ZnO is a p-type half-metallic ferromagnet. The half-metal property is mainly attribute to the crystal field splitting of Cu 3d orbital, and the ferromagnetism is dominated by the hole-mediated double exchange mechanism. Therefore, Cu-doped zinc-blende ZnO should be useful in semiconductor spintronics and other applications.     

First-principles study on the origin of ferromagnetism in n-type Cu-doped ZnO

Based on the density functional calculations with the GGA+U correction, we elucidate the origin of the experimentally reported ferromagnetism in n-type Cu-doped ZnO. Pure Cu-doped ZnO shows the unoccupied 3d states in the gap introduced by Cu, resulting in the insulating ground state and weak magnetic exchange interactions, in contrast to the half-metallic ground state and high ferromagnetic stability predicted by the calculations without U correction. However, the electron traps induced by Cu in n-type Cu-doped ZnO may lead to the partial occupancy of the Cu gap states, which stabilize the ferromagnetic ordering between two Cu atoms.     

Effects of the 3d transition metal doping on the structural,electronic, and magnetic properties of BeO nanotubes

First-principles calculations have been performed on the structural, electronic, and magnetic properties of seven 3d transition-metal(TM) impurities(V, Cr, Mn, Fe, Co, Ni, and Cu) doped armchair(5,5) and zigzag(8,0) beryllium oxide nanotubes(BeONTs). The results show that there exists a structural distortion around the 3d TM impurities with respect to the pristine BeONTs. The magnetic moment increases for V- and Cr-doped BeONTs and reaches a maximum for Mn-doped BeONT, and then decreases for Fe-, Co-, Ni-, and Cu-doped BeONTs successively, consistent with the predicted trend of Hund’s rule to maximize the magnetic moments of the doped TM ions. However, the values of the magnetic moments are smaller than the predicted values of Hund’s rule due to the strong hybridization between the 2p orbitals of the near O and Be ions of BeONTs and the 3d orbitals of the TM ions. Furthermore, the V-, Co-, and Ni-doped(5,5) and(8,0) BeONTs with half-metal ferromagnetism and thus 100% spin polarization character are good candidates for spintronic applications.     

过渡金属与F共掺杂ZnO薄膜结构及磁、光特性

采用溶胶-凝胶法在玻璃衬底上制备了过渡金属元素与F共掺杂Zn_0.98-xTM_xF_0.02O (TM_x=Cu_0.02, Ni_0.01, Mn_0.05, Fe_0.02, Co_0.05)薄膜, 进而利用X射线衍射仪、扫描电子显微镜、紫外-可见透过谱、光致发光及振动样品磁强计等研究了薄膜的表面形貌、微结构、禁带宽度及光致发光(PL)和室温磁学特性. 研究表明: 掺杂离子都以替位的方式进入了ZnO晶格, 掺杂不会破坏ZnO的纤锌矿结构. 其中Zn_0.93Co_0.05F_0.02O薄膜样品的颗粒尺寸最大, 薄膜的结晶度最好且c轴择优取向明显; Zn_0.93Mn_0.05F_0.02O薄膜样品的颗粒尺寸最小, 薄膜结晶度最差且无明显的c轴择优取; Cu, Ni, Fe与F共掺杂样品的颗粒尺寸大小几乎相同. TM掺杂样品均表现出很高的透过率, 同时掺杂后的薄膜样品的禁带宽度都有不同程度的红移. PL谱观察到Zn_0.98-xTM_xF_0.02O薄膜的发射峰主要由较强的紫外发射峰和较弱的蓝光发射峰组成. Zn_0.93Mn_0.05F_0.02O薄膜样品的紫外发光峰最弱, 蓝光发射最强, 饱和磁化强度最大; 与之相反的是Zn_0.96Cu_0.02F_0.02O薄膜, 其紫外发光峰最强, 蓝光发射最弱, 饱和磁化强度最小. 结合微结构和光学性质对Zn_0.98-xTM_xF_0.02O薄膜的磁学性质进行了讨论.