Optical and magnetic properties of laser-deposited Co-doped ZnO thin films

We report the optical and magnetic properties of laser-deposited Zn_1−xCo_xO (x=0.06-0.3) thin films with no intentional electrical carrier doping. The analysis of the high-temperature magnetization data provides an unambiguous evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Co ions in Zn_1−xCo_xO alloy, yielding the value of the effective exchange integral J₁/k_B to be about −27 K. The low-temperature magnetization data reveals a spin glass transition in Zn_1−xCo_xO alloy for the Co content x>0.15, giving the value of the spin freezing temperature T_f to be ∼8 and ∼12 K for x=0.2 and 0.25, respectively. Optical spectra analysis shows a linear increase of the band gap E_g with the increase of the Co content following E_g=3.231+1.144x eV.     

Microstructure, magnetic and optical properties of sputtered polycrystalline ZnO films with Fe addition

Microstructure, magnetic and optical properties of polycrystalline Fe-doped ZnO films fabricated by cosputtering with different Fe atomic fractions (x_Fe) have been examined systematically. Fe addition could affect the growth of ZnO grains and surface morphology of the films. As x_Fe is larger than 7.0%, ZnFe₂O₄ grains appear in the films. All the films are ferromagnetic. The ferromagnetism comes from the ferromagnetic interaction activated by defects between the Fe ions that replace Zn ions. The average moment per Fe ion reaches a maximum value of 1.61 μ_B at x_Fe = 4.8%. With further increase in x_Fe, the average moment per Fe ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Fe ions. The optical band gap value decreases from 3.245 to 3.010 eV as x_Fe increases from 0% to 10%. Photoluminescence spectra analyses indicate that many defects that affect the optical and magnetic properties exist in the films.     

Ferromagnetism in ZnO doped with Co by ion implantation

The importance of doping ZnO with magnetic ions is associated with the fact that this oxide is a good candidate for the formation of a magnetic-diluted semiconductor. Most of the studies reported in Co-doped ZnO were carried out in thin films, but the understanding of the modification of the magnetic behaviour due to doping demands the study of single-crystalline samples. In this work, ZnO single crystals were doped at room temperature with Co by ion implantation with fluences ranging between 2×10¹⁶ and 1×10¹⁷ ions cm⁻² and implantation energy of 100 keV. As implanted samples show a superparamagnetic behaviour attributed to the formation of Co clusters, room temperature ferromagnetism is attained after annealing at 800 °C, but no magnetoresistance was detected in the temperature range from 10 to 300 K.     

Ordinary and extraordinary Coulomb blockade magnetoresistance in a (Ga, Mn)As single electron transistor

In the conventional Ohmic regime, magnetoresistance effects comprise the ordinary responses to the external magnetic field and extraordinary responses to the internal magnetization. Here we study magnetoresistance effects in the Coulomb blockade regime using a ferromagnetic (Ga, Mn)As single electron transistor. We report measurements of the magneto-Coulomb blockade effect due to the direct coupling of high external magnetic fields and the Coulomb blockade anisotropic magnetoresistance associated with magnetization rotations in the ferromagnet. The latter, extraordinary magnetoresistance effect is characterized by low-field hysteretic magnetoresistance which can exceed three orders of magnitude. The sign and size of this magnetoresistance signal is controlled by the gate voltage, and the data are interpreted in terms of anisotropic electrochemical shifts induced by magnetization reorientations. Non-volatile transistor-like applications of the Coulomb blockade anisotropic magnetoresistance are briefly discussed.     

Influence of size effect and sputtering conditions on the crystallinity and optical properties of ZnO thin films

The effects of the thickness variation, substrate type and annealing on the crystallinity parameters, luminescent and optical properties of the zinc oxide (ZnO) thin films were reported. The thin films were deposited on the glass and the amorphous quartz substrates by the standard RF-magnetron sputtering method using ZnO targets in the argon atmosphere. It has been found that the films deposited on the glass substrate manifest a clear size effect. Both the structural and the optical parameters show clearly minima on their thickness dependences. It has been shown that annealing of the comparatively thick ZnO films leads to increase of the crystallite sizes that are followed by a considerable rise of the cathodoluminescence intensity. The corresponding model of the crystallite growth is proposed.     

Carrier spin polarization in ferromagnet-semiconductor (Ga,Mn)As/GaAs structures

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet-semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.     

Dielectric properties of ferroelectric/DMS heterointerface using YMnO3 and Ce doped Si

Bottom gate type Al/Si:8.2 at%Ce/YMnO₃/Pt capacitor was fabricated. Although it was polycrystalline, we successfully obtained Si:8.2 at%Ce film on ferroelectric YMnO₃. The dielectric properties of the capacitor were carefully investigated. Although the capacitance shows frequency dispersion, the capacitor exhibits a ferroelectric type C-V hysteresis loop. From the PUND and P-V measurements, ferroelectric polarization was distinguished from the another polarization, Based on these dielectric measurements, effect of polarization induced by the ferroelectric YMnO₃ on the carrier modulation in the diluted magnetic semiconductor, Ce doped Si film was discussed.     

The influence of Mn content on luminescence properties in Mn-doped ZnO films deposited by ultrasonic spray assisted chemical vapor deposition

Mn-doped ZnO (Zn_1−xMn_xO, 0 ≤ x ≤ 0.1) films are prepared by an ultrasonic spray assisted chemical vapor deposition method. X-ray diffraction and Raman scattering show that all the Zn_1−xMn_xO films are good wurtzite structures without any impurity phases. Cathodoluminescence spectra show that ultraviolet emission and green luminescence can be observed. The intensity of ultraviolet emission decreases with the increment of x, while the intensity of green luminescence increases with the increment of x when x ≤ 0.02. However, when x (x > 0.02) is further increased, the intensity of green luminescence decreases gradually, and the green luminescence disappears when x is above 0.075. We consider that the change of the luminescence is related to the competition between the radiative recombination and the non-radiative recombination.     

Observation of higher magnetization on the substitution of Al for Co in La2MnCoO6

Upon substitution of non-magnetic Al³⁺ for diamagnetic, low-spin, Co³⁺ in ferromagnetic La₂MnCoO₆, the ferromagnetic moment, measured at 82 K and 15 kOe, is found to increase initially with Al content and then decreases, though the magnetic transition temperature decreases continuously on increasing x in La₂MnCo_1−xAl_xO₆.     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.     

Preparation of large Co nanosheets with enhanced coercivity by a magnetic-field-assisted solvothermal approach free of surfactants, complexants or templates

We presented a magnetic-field-assisted solvothermal route to prepare hexagonal close-packed (hcp) large Co nanosheets free of any surfactants, complexants or templates. Scanning electron microscope (SEM) images showed that the typical size and thickness of the Co nanosheets were several microns and ca. 50 nm, respectively. Magnetic measurement indicated the coercivity of the sample reached 317.3 Oe, which was much higher than those of bulk Co metal and previously reported Co nanoplatelets. It was found that the solvents played a crucial role in determining the morphology and the crystal structures of the Co products, and the introduction of ethanol conduced to the formation of pure hcp phase Co microcrystal. The external magnetic field was also found to be indispensable for the formation of sheet-shaped morphology.     

Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films

Co-doped TiO₂ films were fabricated under different conditions using reactive facing-target magnetron sputtering. Co doping improves the transformation of TiO₂ from anatase phase to rutile phase. The chemical valence of doped Co in the films is +2. All the films are ferromagnetic with a Curie temperature above 340 K. The average room-temperature moment per Co of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases from 0.74 μ_B at x=0.03 to 0.02 μ_B at x=0.312, and decreases from 0.54 to 0.04 μ_B as x increases from 0.026 to 0.169 for the Co-doped TiO₂ films fabricated at 0.27 Pa. The ferromagnetism originates from the oxygen vacancies created by Co²⁺ dopants at Ti⁴⁺ cations. The optical band gaps value (E_g) of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases linearly from 3.35 to 2.62 eV with the increasing x from 0 to 0.312. For the Co-doped TiO₂ films fabricated at 1.86 Pa, the E_g decreases linearly from 3.26 to 2.53 eV with increasing x from 0 to 0.350.     

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.     

Heterogeneous magnetic and electronic states in Ca1−xLaxMnO3−δ (x⩽0.12) single crystals with electron doping

The magnetic, transport, and optical properties of electron-doped Ca_1−xLa_xMnO_3−δ single crystals with x  ?0.12 were studied. The magnetic measurements show that in single crystals with x=0$x=0$ and 0.05 the G-type AFM phase with weak FM component is realized and in crystals with x=0.10$x=0.10$ and 0.12 the G- and C-type AFM phases coexist. The C-type magnetic structure arises at less concentration of La than in polycrystalline samples as a result of oxygen vacancies being additional source of electrons. Under magnetic transitions in the G- and C-type phases, resistivity and magnetoresistance of the doped single crystals have anomalies. Optical absorption in IR range indicates formation of a charge gap in crystals with x=0.10$x=0.10$ and 0.12 at appearance of the C-AFM and monoclinic phase with orbital/charge ordering. By comparing optical and transport properties, heterogeneous electronic state and its relation with heterogeneous magnetic state are shown.     

Characteristics of ZnO:Al thin films co-doped with hydrogen and fluorine

Fluorine and hydrogen co-doped ZnO:Al (AZO) films were prepared by radio frequency (rf) magnetron sputtering of ZnO targets containing 1 wt.% Al₂O₃ on Corning glass at substrate temperature of 150 °C with Ar/CF₄/H₂ gas mixtures, and the structural, electrical and optical properties of the as-deposited and the vacuum-annealed films were investigated. In as-deposited state, films with fairly low resistivity of 3.9-4 × 10⁻⁴ Ω cm and very low absorption coefficient below 900 cm⁻¹ when averaged in 400-800 nm could be fabricated. After vacuum-heating at 300 °C, the minimum resistivity of 2.9 × 10⁻⁴ Ω cm combined with low absorption loss in visible region, which enabled the figure of merit to uplift as high as 4 Ω⁻¹, could be obtained for vacuum-annealed film. It was shown that, unlike hydrogenated ZnO films which resulted in degradation upon heating in vacuum at moderately high temperature, films with fluorine addition could yield improved electrical properties mostly due to enhanced Hall mobility while preserving carrier concentration level. Furthermore, stability in oxidizing environment could be improved by fluorine addition, which was ascribed to the filling effect of dangling bonds at the grain boundaries. These results showed that co-doping of hydrogen and fluorine into AZO films with low Al concentration could be remarkably compatible with thin film solar cell applications.     

The origin and control of the sources of AMR in (Ga,Mn)As devices

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analysed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5 nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [A.W. Rushforth, et al., Phys. Rev. Lett. 99 (2007) 147207] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of the spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential.     

Effects of substrate temperature and Zn addition on the properties of Al-doped ZnO films prepared by magnetron sputtering

Al-doped ZnO (AZO) films prepared at different substrate temperature and AZO films with intentional Zn addition (ZAZO) during deposition at elevated substrate temperature were fabricated by radio frequency magnetron sputtering on glass substrate, and the resulting structural, electrical, optical properties together with the etching characteristics and annealing behavior were comparatively examined. AZO films deposited at 150 °C showed the optimum electrical properties and the largest grain size. XPS analysis revealed that AZO films deposited at elevated temperature of 450 °C contained large amount of Al content due to Zn deficiency, and that intentional Zn addition during deposition could compensate the deficiency of Zn to some extent. It was shown that the electrical, optical and structural properties of ZAZO films were almost comparable to those of AZO film deposited at 150 °C, and that ZAZO films had much smaller etching rate together with better stability in severe annealing conditions than AZO films due possibly to formation of dense structure.     

Comparison of the effects of cationic,anionic and nonionic surfactants on the properties of Sr-hexaferrite nanopowder synthesized by a sol–gel auto-combustion method

In this research, Sr-hexaferrite nanopowders were synthesized by a sol–gel auto-combustion route using three different surfactant types: cationic, anionic and nonionic. The obtained powders were characterized by FTIR, XRD and SEM techniques and their physical properties were compared. The results showed a decrease in crystallite size of the resultant Sr-hexaferrite powder in presence of cationic and anionic surfactants, while there are no significant changes in presence of nonionic one. However, all three types of surfactants resulted in a decrease in the formation temperature of Sr-hexaferrite.     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Pre-edge features in X-ray absorption structure of Mn in GaMnN,GaMnAs and GeMn

_Ga1-xMnxN${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{N}$ samples with a wide range of concentrations, from x=0.003$x=0.003$ to 0.057, were grown by molecular beam epitaxy. X-ray diffraction and the simulation of the extended X-ray absorption fine structure (EXAFS) confirmed the wurtzite structure of the samples, without any secondary phase, and the location of Mn in the gallium sublattice of GaN. The valence state of Mn was studied using the X-ray absorption near-edge structure (XANES) at the K-edge of Mn. The shape of the measured XANES spectra does not depend on the Mn concentration: that implies the same valence state and local atomic structure around the Mn atom in all samples. A calculation of the electronic band structure of GaMnN, GaMnAs, GeMn and the XANES spectra of Mn in GaMnN was performed using the linearized augmented plane wave (LAPW) method. The calculated spectra fit well our experimental data. In particular, the comparison demonstrates that a peak in the pre-edge structure is directly related to the 3+$3+$ valence state of Mn. This was confirmed experimentally by measurements of the Mn K-edge in GaMn²⁺As${\mathrm{GaMn}}^{2+}\mathrm{As}$ and ZnMn²⁺Te${\mathrm{ZnMn}}^{2+}\mathrm{Te}$. An application of proposed interpretation for GeMn is discussed.