Structural, compositional and magnetic characterization of bulk V2O5 doped ZnO system

This paper investigates the structural, compositional and magnetic properties of vanadium doped ZnO bulk samples prepared by solid state reaction technique. The Rietveld refinement analysis for XRD results of samples showed small change in lattice parameters for 3 and 5% vanadium doped ZnO samples indicating the substitution of Zn²⁺ ions by vanadium ions in ZnO lattice. Raman spectroscopy reveals the change in ZnO modes positions due to vanadium doping. The appearance of E₁ and E₂ modes showed that the wurtzite structure of ZnO is still maintained after doping of vanadium oxide. XPS analysis confirms the presence of the different elements and oxidation states of vanadium ions. M-H curves obtained from VSM showed weak ferromagnetism in the samples. The observation of ferromagnetic behavior indicates the formation of ZnVO phase with V²⁺ ion substitution in the ZnO lattice. XPS scans of the etched bulk samples confirmed the 2+ oxidation state of vanadium ions in our samples explaining the origin of ferromagnetism.     

Ferromagnetism in Mn-doped ZnO due to impurity bands

We have prepared quasi-homoepitaxial Zn_0.95Mn_0.05O-films on ZnO substrates by pulsed laser deposition. The thin films were characterized by in situ RHEED during growth, X-ray diffraction and reflectometry, as well as transmission electron microscopy (TEM). The magnetic properties have been analyzed by SQUID magnetometry and by measuring the anomalous Hall effect. By varying the growth conditions (substrate temperature: 200–600 ^∘C, deposition atmospheres: Ar, O₂) a different concentration of impurities has been generated. Our results show that the films are electron doped and exhibit a ferromagnetic coupling that depends on the impurity concentration.     

Synthesis of ZnFe2O4 nanomagnets by Fe-ion implantation into ZnO and post-annealing

Fe ions of dose 8 × 10¹⁶ cm^-2 are implanted into a ZnO single crystal at 180 keV. Annealing at 1073 K leads to the formation of zinc ferrite (ZnFe₂O₄), which is verified by synchrotron radiation X-ray diffraction (SR-XRD) and X-ray photoelectron spectroscopy (XPS). The crystallographically oriented ZnFe₂O₄ is formed inside the ZnO with the orientation relationship of ZnFe₂O₄ (111)//ZnO (0001). Superconducting quantum interference device (SQUID) measurements show that the as-implanted and post-annealing samples are both ferromagnetic at 5 K. The synthesized ZnFe₂O₄ is superparamagnetic, with a blocking temperature (T_B = 25 K), indicated by zero field cooling and field cooling (ZFC/FC) measurements.     

First-principles prediction of the electronic and magnetic properties of nitrogen-doped ZnO nanosheets

The electronic and magnetic properties of N-doped ZnO nanosheets are investigated by density functional theory using local spin density approximation. The results show that in an isolated N-doped ZnO nanosheet, there is a clear spontaneous polarization of N 2p state with a magnetic moment 1.0 μ_B/N. We also find that the doped nitrogen atoms in ZnO nanosheets have a clustering tendency with ferromagnetic coupling between them, and thus a high room-temperature ferromagnetic nature is expected. The ferromagnetic coupling in N-doped ZnO nanosheets can be attributed to the hole-mediated double-exchange mechanism through strong p–d interaction between nitrogen and zinc atoms.     

MBE growth and magnetotransport studies of ferromagnetic Ga1−xMnxSb semiconductor layers on hybrid ZnTe/GaAs substrates

Narrow-gap Ga_1−xMn_xSb layers grown on hybrid ZnTe/GaAs substrates are observed to be ferromagnetic by SQUID magnetization and anomalous Hall effect measurements. The layers display an easy axis of magnetization perpendicular to the layer plane, in contrast to in-plane easy axis orientation observed in Ga_1−xMn_xSb grown on GaSb substrates. Resistivity measured in the Ga_1−xMn_xSb/ZnTe/GaAs system shows a well-defined maximum at temperatures close to the ferromagnetic/paramagnetic transition. We determined the spontaneous resistivity anisotropy in Ga_0.98Mn_0.02Sb grown on hybrid ZnTe/GaAs substrates and compared it to that observed on Ga_0.98Mn_0.02Sb grown on a GaSb buffer. These results should provide a valuable test for future theories of transport in ferromagnetic semiconductors.     

Influence of annealing temperature on structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by sol-gel method

Thin films of Zn_1−xMn_xO (x=0.01) diluted magnetic semiconductor were prepared on Si (1 0 0) substrates by the sol-gel method. The influence of annealing temperature on the structural, optical and magnetic properties was studied by X-ray diffraction (XRD), atom force microscopy (AFM), photoluminescence (PL) and SQUID magnetometer (MPMS, Quantum Design). The XRD spectrum shows that all the films are single crystalline with (0 0 2) preferential orientation along c-axis, indicating there are not any secondary phases. The atomic force microscopy images show the surfaces morphologies change greatly with an increase in annealing temperature. PL spectra reveal that the films marginally shift the near band-edge (NBE) position due to stress. The magnetic measurements of the films using SQUID clearly indicate the room temperature ferromagnetic behavior, and the Curie temperature of the samples is above room temperature. X-ray photoelectron spectroscopy (XPS) patterns suggest that Mn²⁺ ions were successfully incorporated into the lattice position of Zn²⁺ ions in ZnO host. It is also found that the post-annealing treatment can affect the ferromagnetic behavior of the films effectively.     

Room-Temperature Ferromagnetic ZnMnO Thin Films Synthesized by Plasma Enhanced Chemical Vapour Deposition Method

Room-temperature ferromagnetic Mn-doped ZnO films are grown on Si （001） substrates by plasma enhanced chemical vapour deposition （PECVD）. X-ray diffraction measurements reveal that the Znl-xMn.O films have the single-phase wurtzite structure. X-ray photoelectron spectroscopy indicates the existence of Mn^2＋ ions in Mndoped ZnO films. Furthermore, the decreasing additional Raman peak with increasing Mn-doping is considered to relate to the substitution of Mn ions for the Zn ions in ZnO lattice. Superconducting quantum interference device （SQUID） measurements demonstrate that Mn-doped ZnO films have ferromagnetic behaviour at room temperature.     

Mn和N共掺ZnO稀磁半导体薄膜的研究

使用对Zn₂N₃:Mn薄膜热氧化的方法成功制备了高含N量的Mn和N共掺ZnO的稀磁半导体薄膜.在没有N离子共掺的情况下,ZnO:Mn薄膜的铁磁性非常微弱;如果进行N离子的共掺杂,就会发现ZnO:Mn薄膜在室温下表现出非常明显的铁磁性,饱和离子磁矩为0.23 μ_B—0.61 μ_B.这说明N的共掺激发了ZnO:Mn薄膜中的室温铁磁性,也就是受主的共掺引起的空穴有利于ZnO中二价Mn离子的铁磁性耦合,这和最近的相关理论研究符合很好. 关键词： 磁性半导体 受主掺杂 空穴媒介的铁磁性     

Electronic structure and magnetic properties of Cr monodoped and (Cr, Al) codoped ZnO

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of Cr monodoped and (Cr, Al) codoped in ZnO. The results indicate that Cr monodoped in ZnO favors a spin-polarized state with a total magnetic moment of 7.50μ_B per supercell and the magnetic moment mainly comes from the unpaired 3d electrons of Cr atoms. In addition, it was found that the ferromagnetic exchange interaction between Cr atoms is short-ranged in Cr monodoped ZnO. Interestingly, the ferromagnetic stability can be enhanced significantly by codoping Al_Zn. We think that the enhancement of ferromagnetic stability should be attributed to the additional electrons introduced by Al_Zn codoping.     

Synthesis and characterization of Ni-doped ZnO: A transparent magnetic semiconductor

We report synthesis of a transparent magnetic semiconductor by incorporating Ni in zinc oxide (ZnO) matrix. ZnO and nickel-doped zinc oxide (ZnO:Ni) thin films (∼60 nm) are prepared by fast atom beam (FAB) sputtering. Both undoped and doped films show the presence of ZnO phase only. The Ni concentration (in at%) as determined by energy dispersive X-ray (EDX) technique is ∼12±2%. Magnetisation measurement using a SQUID magnetometer shows that the Ni-doped films are ferromagnetic, having coercivity (H_c) values 192, 310 and 100 Oe and saturation magnetization (M_s) values of 6.22, 5.32 and 4.73 emu/g at 5, 15 and 300 K, respectively. The Ni-doped film is transparent (>80%) across visible wavelength range. Resistivity of the ZnO:Ni film is ∼2.5×10⁻³ Ω cm, which is almost two orders of magnitude lower than the resistivity (∼4.5×10⁻¹ Ω cm) of its undoped counterpart. Impurity d-band splitting is considered to be the cause of increase in conductivity. Interaction between free charges generated by doping and localized d spins of Ni is discussed as the reason for ferromagnetism in the ZnO:Ni film.     

Grain boundary layers in nanocrystalline ferromagnetic zinc oxide

The complete solubility of an impurity in a polycrystal increases with decreasing grain size, because the impurity dissolves not only in the crystallite bulk but also on the grain boundaries. This effect is especially strong when the adsorption layers (or the grain boundary phases) are multilayer. For example, the Mn solubility in the nanocrystalline films (where the size of grains is ∼20 nm) is more than three times greater than that in the ZnO single crystals. The thin nanocrystalline Mn-doped ZnO films in the Mn concentration range 0.1–47 at % have been obtained from organic precursors (butanoates) by the “liquid ceramic” method. They have ferromagnetic properties, because the specific area of the grain boundaries in them is greater than the critical value [B.B. Straumal et al., Phys. Rev. B 79, 205206 (2009)]. The high-resolution electron transmission microscopy studies show that the ZnO nanocrystalline grains with the wurtzite lattice are separated by amorphous layers whose thickness increases with the Mn concentration. The morphology of these layers differs greatly from the structure of the amorphous prewetting films on the grain boundaries in the ZnO:Bi₂O₃ system.     

Zn-catalyzed growth processes and ferromagnetism of Mn-doped ZnO nanorods on Si substrate

Well-aligned ZnO nanorods and Mn-doped ZnO nanorods are fabricated on Si (1 0 0) substrate according to the contribution of Zn metal catalysts. Scanning electron microscopy and high-resolution transmission electron microscopy images indicate that the influence of Zn catalyst on the properties of ZnO can be excluded and the growth of ZnO nanorods follows a vapor-liquid-solid and self-catalyzed model. Mn-doped ZnO nanorods show a typical room temperature ferromagnetic characteristic with a saturation magnetization (M_S) of 0.273μ_B/Mn. Cathodoluminescence suggests that the ferromagnetism of Mn-doped ZnO nanorods originates from the Mn²⁺-Mn²⁺ ferromagnetic coupling mediated by oxygen vacancies. This technique provides exciting prospect for the integration of next generation Si-technology-based ZnO spintronic devices.     

Structural and Magnetic Properties of Codoped ZnO based Diluted Magnetic Semiconductors

Zn1-xCoxO （x = 0.01, 0.02, 0.05, 0.10 and 0.20） diluted magnetic semiconductors are prepared by the sol-gel method. The structural and magnetic properties of the samples are studied using x-ray diffraction （XRD）, extended x-ray absorption fine structure （EXAFS） and superconducting quantum interference device （SQUID）. The XRD patterns does not show any signal of precipitates that are different from wurtzite type ZnO when Co content is lower than x = 0.10. An EXAFS technique for the Co K-edge has been employed to probe the local structures around Co atoms doped in ZnO powders by fluorescence mode. The simulation results for the first shell EXAFS signals indicate that Zn sites can be substituted by Co atoms when Co content is lower than x = 0.05. The SQUID results show that the samples （x 〈 0.05） exhibit clear hysteresis loops at 300K, and magnetization versus temperature from 5 K to 350K at H = 100 Oe for the sample x = 0.02 shows that the samples have ferromagnetism above room temperature. A double-exchange mechanism is proposed to explain the ferromagnetic properties of the samples.     

The electronic and magnetic properties of （Mn,C）-codoped ZnO diluted magnetic semiconductor

The electronic and magnetic properties of (Mn,C)-codoped ZnO are studied in the Perdew-Burke-Ernzerhof form of generalized gradient approximation of the density functional theory. By investigating five geometrical configurations, we find that Mn doped ZnO exhibits anti-ferromagnetic or spin-glass behaviour, and there are no carriers to mediate the long range ferromagnetic (FM) interaction without acceptor co-doping. We observe that the FM interaction for (Mn,C)-codoped ZnO is due to the hybridization between C 2p and Mn 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. Meanwhile, we demonstrate that ZnO co-doped with Mn and C has a stable FM ground state and show that the (Mn,C)-codoped ZnO is FM semiconductor with super-high Curie temperature (T C = 5475 K). These results are conducive to the design of dilute magnetic semiconductors with codopants for spintronics applications.     

First-principles study of electronic structure and ferromagnetism in Ti-doped ZnO

We perform first-principles spin polarized calculations of the electronic structure of Ti-doped in ZnO. Ferromagnetism in Ti-doped ZnO is identified, which is in agreement with recent experimental and calculated results. A net magnetic moment of 0.715μ_B is found per Ti. At a Ti concentration of 12.5%, total energy calculations show that the ferromagnetic state is 68 meV lower than the antiferromagnetic state. The electronic states near Fermi energy are dominated by strong hybridization between O 2p and Ti 3d, which is just the origin of impurity band in Ti-doped ZnO and also implies that the Ti-O bond is quite covalent instead of purely ionic. Since there is no magnetic element in this compound, Ti-doped ZnO appears to be an unambiguous dilute magnetic semiconductor.     

Tunable interlayer exchange coupling energy by modification of Schottky barrier potentials

We theoretically demonstrate that the interlayer exchange coupling (IEC) energy can be manipulated by means of an external bias voltage in a F₁/NM/F₂/S (F₁: ferromagnetic, NM: nonmagnetic metallic, F₂: ferromagnetic, S: semiconductor layers) four-layer system. It is well known that the IEC energy between two ferromagnetic layers separated by nanometer thick nonmagnetic layer depends on the spin-dependent electron reflectivities at the interface in F₁/NM/F₂ trilayer system. We apply such dependence to the F₁/NM/F₂/S four-layer system, where the reflectivity of NM/F₂ interface also depends on F₂/S interface due to the multiple reflection of an electron like optics. Finally, the IEC energy depends on the spin-dependent electron reflectivity not only at the interfaces of F₁/NM/F₂, but also at the interface of F₂/S. Naturally the Schottky barrier is formed at the interface between metallic ferromagnetic layer and semiconductor, the Schottky barrier height and thickness can be tailored by an external bias voltage, which causes the change of the spin-dependent reflectivity at F₂/S interface. We show that the IEC energy between two ferromagnetic layers can be controlled by an external bias voltage due to the electron-optics nature using a simple free-electron-like one-dimensional model.     

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc. These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, X-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution X-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.     

H<Subscript>2</Subscript>O<Subscript>2</Subscript>-molecular beam epitaxy of high quality ZnO

We have studied the growth and characterization of ZnO epilayers on (0001)-sapphire by H₂O₂-molecular beam epitaxy (MBE). A high temperature (HT) MgO buffer followed by a low-temperature ZnO buffer was introduced in order to accommodate the lattice mismatch between ZnO and sapphire. The surface morphology of the samples was studied using atomic force microscopy (AFM), and scanning electron microscopy (SEM). The crystalline quality of the layers was investigated by employing high resolution X-ray diffractometry (HRXRD) and high resolution transmission electron microscopy (HRTEM). The electrical properties of the grown ZnO layers were studied by Hall-effect measurements in a standard van der Pauw configuration. The measured surface roughness for the best layers is as low as 0.26 nm rms. HRXRD measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO heteroepitaxially grown on (0001)-sapphirewith a HT MgO buffer layers. The influence of the growth conditions on the crystalline quality is discussed. The FWHM of the HRXRD (0002) rocking curves measured for the 2-inch ZnO-on-sapphire is as low as 27 arcsec with a very high lateral homogeneity across the whole 2-inch ZnO epilayers. The results indicate that H₂O₂-MBE is a suitable technique to fabricate ZnO epilayers of very high quality. PACS 61.10.Nz; 68.37.Lp; 81.05.Dz; 81.15.Hi     

Synthesis and magnetic properties of Zn1−xMnxO films prepared by the sol-gel method

We report on the ferromagnetic characteristics of Zn_1−xMn_xO films (x=0.1-0.3) prepared by the sol-gel method on silicon substrates using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Magnetic measurement show that the Curie temperature (T_C) and the coercive field (H_C) were ∼39 K and ∼2100 Oe for the film of x=0.2, respectively. EDS and TEM measurements indicate that Mn content at the interface is significantly higher than that at the center of the Zn_0.8Mn_0.2O film showing the ratio, Zn:Mn:O≅1:12:15. This experimental evidence suggests that ferromagnetic precipitates containing manganese oxide may be responsible for the observed ferromagnetic behavior of the film.     

Dependences of the surface and the optical properties on the O2/O2 + Ar flow-rate ratios for ZnO thin films grown on ZnO buffer layers

ZnO active layers on ZnO buffer layers were grown at various O₂/O₂ + Ar flow-rate ratios by using radio-frequency magnetron sputtering. Atomic force microscopy images showed that the surface roughnesses of the ZnO active layers grown on ZnO buffer layers decreased with decreasing O₂ atmosphere, indicative of an improvement in the ZnO surfaces. The type of the ZnO active layer was n-type, and the resistivity of the layer increased with increasing O₂ atmosphere. Photoluminescence spectra from the ZnO active layers grown on the ZnO buffer layers showed dominant peaks corresponding to local levels in the ZnO energy gap resulting from oxygen vacancies or interstitial zinc vacancies, and the peak positions changed significantly with the O₂/O₂ + Ar flow rate. These results can help improve understanding of the dependences of the surface and the optical properties on the O₂/O₂ + Ar ratio for ZnO thin films grown on ZnO buffer layers.