Solvothermal synthesis and magneto-optical properties of Zn1−xNixO hierarchical microspheres

Zn_1−xNi_xO (x=0-0.25) hierarchical microspheres were synthesized via a solvothermal process in ethylene glycol. The magnetic microspheres were characterized by X-ray powder diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectra, X-ray photoelectron spectroscopy, room-temperature photoluminescence spectra, and vibrating sample magnetometer. The as-prepared samples take on a well-defined spherical architecture following the processes of spontaneous aggregation and localized Ostwald ripening. Dependence of the magnetization and morphology on Ni²⁺ content was observed. Magnetic hysteresis loops reveal that the Ni-doped ZnO microspheres exhibit ferromagnetic loops at room temperature.     

Room-temperature ferromagnetism in Zn1−xMnxO

In view of recent controversies on above room-temperature ferromagnetism (RTFM) in transition-metal-doped ZnO, the present paper aims to shed some light on the origin of ferromagnetism by investigating annealing effects on structure and magnetism for polycrystalline Zn_1−xMn_xO powder samples prepared by solid-state reaction method and annealed in air at different temperatures. Magnetic measurements indicate that the samples are ferromagnetic at room temperature (RTFM). Room temperature ferromagnetism has been observed in the sample annealed at a low temperature of 500 °C with a saturated magnetization (M_s) of 0.159 emu/g and a coercive force of 89 Oe. A reduction in RTFM is clearly observed in the sample annealed at 600 °C. Furthermore, the saturation magnetic moment decreases with an increase in grain size, suggesting that ferromagnetism is due to defects and/or oxygen vacancy confined to the surface of the grains. The experimental results indicate that the ferromagnetism observed in Zn_1−xMn_xO samples is intrinsic rather than associated with secondary phases.     

X-ray photoelectron study of Sn1−xMnxTe semimagnetic semiconductors

X-ray photoelectron (XPS) studies of core-levels in Sn_1−xMn_xTe (x < 0.1) semimagnetic semiconductors have been performed. The spectra were acquired under UHV conditions from the clean (as-cleaved or in-situ scraped) crystal surface. The single-phase NaCl structure of the alloys studied was verified by X-ray diffraction (XRD). The structure of Sn 3d and Te 3d core-levels in SnMnTe was found fully consistent with that of SnTe. Remarkable qualitative similarity of the Mn 2p spectrum of Sn_1−xMn_xTe (x = 0.09) with the case of zinc-blende MnTe [R.J. Iwanowski, M.H. Heinonen, E. Janik, Chem. Phys. Lett. 387 (2004) 110] has been shown: (1) the same binding energies (BEs) of the main contributions to the Mn 2p_3/2 line, related to Mn²⁺ state of the bulk MnTe bond; (2) occurrence of low BE component in the Mn 2p spectrum, indicative of clean-surface species containing reduced-valence Mn ions (i.e. Mn^q+, where 0 < q < 2); (3) strong satellites of the 2p_3/2 (Mn²⁺ related) parent lines. In SnMnTe, the highest intensity ratio of the satellite to main peak (ever reported for Mn 2p photoelectron spectrum) was revealed; this was interpreted in terms of the so-called charge-transfer model.     

Structural,optical and morphological properties of Ga1−xMnxAs thin films deposited by magnetron sputtering for spintronic device applications

In this work, GaMnAs alloy materials were deposited on 7059 Corning glass and GaAs (1 0 0) substrates via RF magnetron sputtering technique. A concentration of Mn about 0.28 was obtained by Energy Dispersive X-ray spectroscopy. The substrate temperature was changed from 440 to 520 °C and layer thicknesses between 172 and 514 nm were obtained. Characterization by atomic force microscopy and X-ray diffraction were performed to determinate surface morphology and crystal structure, respectively. From transmittance spectral measurements we were able to determine optical constants: band gap energy (E_g), absorption coefficient (α), and refraction index (n). A correlation between morphological properties and substrate type was also studied. Diluted magnetic semiconductors like GaMnAs are considered among promising materials for the development of new spin-electronic devices.     

On the paramagnetic behavior of heavily doped Zn1−xMnxO films fabricated by Pechini’s method

Heavily doped Zn_1−xMn_xO (x = 0.3) films were prepared by polymeric precursor method onto glass substrates and their structural, morphological, optical and magnetic properties carefully studied. Undoped ZnO films were also prepared for the purpose of comparison. The polymeric precursor method consists in preparing a coating solution from the Pechini process followed by a three-step thermal treatment of the as deposited films at temperatures up to 550 °C for 30 min. X-ray diffraction (XRD) analysis reveals the typical hexagonal wurtzite structure of the undoped ZnO film. The addition of Mn ions leads to a dramatic reduction of the crystalline quality of film although no evidence of affectation by secondary phases is found. The affectation of the ZnO structure may be due to the formation of Mn clusters and generation of defects such as vacancies and interstitials. Here, the solubility limit of the Mn ions in ZnO should play an important role and it is discussed in the framework of ionic radius and valence states. The scanning electron microscopy (SEM) analysis shows that the surface of the doped sample was affected by the presence of cracks due, probably, to the expansion of the lattice constant of Zn_0.7Mn_0.3O caused by the Mn incorporation in the ZnO lattice. The existence of cluster-type structures on the surface is corroborated by atomic force microscopy (AFM). The EDX analysis, carried out on some areas in the film, yielded Mn/Zn ratios of about 0.3, which points out to an effective Mn incorporation in the film. On the other hand, the absorption edge of the doped films is red shifted to 2.9 eV (3.24 eV for undoped ZnO film) and the absorption edge is less sharp due, probably, to amorphous states appearing in the band gap. No evidence of dilute magnetic semiconductor mean-field ferromagnetic behavior is observed. The temperature dependence of the magnetization follows a Curie law suggesting pure paramagnetic behavior. The very small s-shape behavior of M versus H (without hysteresis) observed at room temperature on selected areas would stem from Mn clusters which are easily formed in transition metal doped ZnO.     

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.     

Magnetism in Zn1−xMnxO crystal prepared by hydrothermal method

Crystal Zn_1−xMn_xO magnetic semiconductors have been obtained by using a hydrothermal method for the first time at temperature of 703 K with substituent fraction ranging from x=0 to 0.04. X-ray diffraction and optical absorption measurements provide evidence for the locating at Zn site of Mn ion in ZnO crystals. The non-monotonic variation of band gap indicates the short-ranged interactions of sp-d electrons. However, no evidence of ferromagnetism is found in these systems down to T=2 K. The magnetization is found to be contributed from both free spins and spins associated with antiferromagnetic clusters. The antiferromagnetism is confirmed by fitting a Curie-Weiss function.     

Spin-polarized structural, electronic and magnetic properties of diluted magnetic semiconductors Cd1−xMnxTe in zinc blende phase

We have investigated the structural, electronic and magnetic properties of the diluted magnetic semiconductor (DMS) Cd_1−xMn_xTe (for x=0.75 and 1.0) in the zinc blende (B3) phase by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the frame work of spin-polarized density functional theory (SP-DFT). The electronic exchange-correlation energy is described by generalized gradient approximation (GGA). We have calculated the lattice parameters, bulk modulii and the first pressure derivatives of the bulk modulii, spin-polarized band structures, and total and local densities of states. We estimated the spin-exchange splitting energies Δ_x(d) and Δ_x(pd) produced by the Mn3d states, and we found that the effective potential for the minority spin is more attractive than that of the majority spin. We determine the s-d exchange constant N₀α (conduction band) and p-d exchange constant N₀β (valence band) and these somewhat agree with a typical magneto-optical experiment. The value of calculated magnetic moment per Mn impurity atom is found to be 4.08 μ_B for Cd_0.25Mn_0.75Te and 4.09 μ_B for Cd_0.0Mn_1.0Te. Moreover, we found that p-d hybridization reduces the local magnetic moment of Mn from its free space charge value of 5.0 μ_B and produces small local magnetic moments on the nonmagnetic Cd and Te sites.     

Study of Zn1−xCoxO (0.02≤x≤0.08) dilute magnetic semiconductor prepared by mechanosynthesis route

Single-phase Zn_1−xCo_xO (0.02≤x≤0.08) dilute magnetic semiconductor is prepared by mechanical milling process. The shift of XRD peaks towards the higher angle and a redshift in the band gap compared to the undoped ZnO ensure the incorporation of Co²⁺ ions in the semiconductor host lattice. Pure Zn_xCo_1−xO phases show the paramagnetic behavior in the temperature range 80 K≤T≤300 K. The room temperature volume magnetic susceptibility (χ_v) estimated in case of Zn_0.96Co_0.04O is ∼10⁻⁵ emu/Oe cm³. The temperature dependence of susceptibility χ_v can be fitted well with Curie law confirming the paramagnetic interaction. The observed crystal-field splitting of 3d levels of Co²⁺ ions inside Zn_1−xCo_xO has been successfully interpreted using Curie law.     

Nanoscratch study of Zn1−xMnxO heteroepitaxial layers

We investigated the nanotribological properties of Zn_1−xMn_xO epilayers (0 ≤ x ≤ 0.16) grown by molecular beam epitaxy (MBE) on sapphire substrates. The surface roughness and friction coefficient (μ) were analyzed by means of atomic force microscopy (AFM) and hysitron triboscope nanoindenter techniques.The nanoscratch system gave the μ value of the films ranging from 0.17 to 0.07 and the penetration depth value ranging 294-200 nm when the Mn content was increased from x = 0 to 0.16. The results strongly indicate that the scratch wear depth under constant load shows that higher Mn content leads to Zn_1−xMn_xO epilayers with higher shear resistance, which enhances the Mn-O bond. These findings reveal that the role of Mn content on the growth of Zn_1−xMn_xO epilayers can be identified by their nanotribological behavior.     

Structure and magnetic properties of Sn1−xMnxO2

The microstructure and magnetic properties have been investigated systematically for Sn_1−xMn_xO₂ polycrystalline powder samples with x=0.02-0.08 synthesized by a solid-state reaction method. X-ray diffraction revealed that all samples are pure rutile-type tetragonal phase and the cell parameters a and c decrease monotonously with the increase in Mn content, which indicated that Mn ions substitute into the lattice of SnO₂. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism. Furthermore, magnetic investigations demonstrate that magnetic properties strongly depend on doping content, x. The average magnetic moment per Mn atom decreases with increase in the Mn content, because antiferromagnetic super-exchange interaction takes place within the neighbor Mn³⁺ ions through O²⁻ ions for the samples with higher Mn doping. Our results indicate that the ferromagnetic property is intrinsic to the SnO₂ system and is not a result of any secondary magnetic phase or cluster formation.     

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO₂ powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5 at%. Room temperature M-H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn_0.987Mn_0.013O₂ showed a coercivity H_c=545 Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54 μ_B of the Sn_0.9921Mn_0.0079O₂ sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn_1−xMn_xO₂ are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.     

Structural and photoluminescence analysis of Zn1 − xMnxO nanocrystalline powders

This Letter reports on structural and photoluminescence properties of Zn_1 − xMn_xO nanocrystalline powders, which were synthesized by using oxalate precursor decomposition method. From the XRD features, we have noticed that all samples exhibit wurtzite crystal structure. The origin of photoluminescence properties of Mn doped and undoped ZnO have been discussed.     

Theoretical study of magnetic ordering and electronic properties of AgxAl1−x N compounds

Ferromagnetic ordering of silver impurities in the AlN semiconductor is predicted by plane-wave ultrasoft pseudopotential and spin-polarized calculations based on density functional theory (DFT). It was found that an Ag impurity atom led to a ferromagnetic ground state in Ag_0.0625Al_0.9375N, with a net magnetic moment of 1.95 μ_B per supercell. The nitrogen neighbors at the basal plane in the AgN₄ tetrahedron are found to be the main contributors to the magnetization. This magnetic behavior is different from the ones previously reported on transition metal (TM) based dilute magnetic semiconductor (DMS), where the magnetic moment of the TM atom impurity is higher than those of the anions bonded to it. The calculated electronic structure band reveals that the Ag-doped AlN is p-type ferromagnetic semiconductor with a spin-polarized impurity band in the AlN band gap. In addition, the calculated density of states reveals that the ferromagnetic ground state originates from the strong hybridization between 4d-Ag and 2p-N states. This study shows that 4d transition metals such as silver may also be considered as candidates for ferromagnetic dopants in semiconductors.     

Thermoluminescence studies of bismuth doped BaxCa1−xS nanostructures

Bismuth doped Ba_1−xCa_xS:Bi (x=0-1) nanocrystallities have been prepared by the solid state reaction method and characterized by XRD and TEM. X-ray diffraction analysis shows the formation of the compounds in cubic structure at room temperature. Only partial replacement of Ba is possible and we found that Ba_0.5Ca_0.5S:Bi could not be prepared due to the difference between ionic radii of barium and calcium. Thermoluminescence studies of these samples after exposure to UV radiation have been carried out. The TL glow curve of Ba_xCa_1−xS:Bi has been found to be a simple structure with a single peak at 405, 428 and 503 K for x=1, 0.8 and 0, respectively. The kinetic parameters at various heating rates namely activation energy (E), order of kinetics (b) and frequency factor (s) of the Ba_1−xCa_xS:Bi (x=0.2) (0.4 mol%) sample have been determined using Chen’s method. The deconvolution of curve was done using the GCD function suggested by Kitis. The effect of different heating rates and different amount of dose has also been discussed.     

Synthesis, structural and magnetic study of polycrystalline LaNi1−xMnxO3 films

Polycrystalline double perovskite LaNi_1−xMn_xO₃ (x=0.3, 0.4, 0.5 and 0.7, which is defined as Mn03, Mn04, Mn05 and Mn07, respectively) thin films are successfully deposited on Si (1 0 0) substrates via chemical solution deposition method. Their structural and magnetic properties are measured. All the thin films are of single phase. Raman spectra indicate that relative intensity of Mn05 is stronger than that of others that can be attributed to the higher degree of B-site ordering. The low temperature magnetic moment of Mn05 is about 500 emu/cm³, which is obviously larger than that of Mn03 and Mn07 because of the long-range ordering of Mn and Ni ions in Mn05.     

First-principles study of structural, electronic and magnetic properties in Cd1−xFexS diluted magnetic semiconductors

In this paper, we report theoretical investigations of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Cd_1−xFe_xS with x=0.25, 0.5 and 0.75 in zinc blende (B3) phase using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the density functional theory and the generalized gradient approximation. The analysis of band structures, density of states, total energy, exchange interactions and magnetic moments reveals that both the alloys may exhibit a half-metallic ferromagnetism character. The value of calculated magnetic moment per Fe impurity atom is found to be 4 μ_B. Moreover, we found that p-d hybridization reduces the local magnetic moment of Fe from its free space charge value of 4 μ_B and produces small local magnetic moments on Cd and S sites.     

Study of the local micro-structure and magnetic and transport properties of CrxGe1−x thin films

The local micro-structure as well as the magnetic and transport properties of Cr_xGe_1−x films prepared by means of magnetron sputtering have been investigated. Structural analysis shows that Cr atoms are situated in substitutional sites in the Ge lattice. Electrical transport properties indicate that Cr introduces a shallow acceptor level at 0.016 eV from the valence band implying Cr substituting for Ge. The low temperature ferromagnetism observed in the films is mediated mainly by ferromagnetic superexchange interactions between diluted Cr ions.     

MnxGe1−x thin layers studied by TEM, X-ray absorption spectroscopy and SQUID magnetometry

Mn_0.06Ge_0.94 samples have been grown by molecular-beam epitaxy on Ge(0 0 1)2 × 1. High-resolution transmission electron microscopy shows the coexistence of an ordered diluted Mn_0.06Ge_0.94 film and of nanoscopic crystallites, which were identified as Mn₅Ge₃ by electron diffraction. The magnetic properties of the Mn_0.06Ge_0.94 samples show a superposition of a paramagnetic behavior, due to the interaction of Mn atoms diluted in the Ge host, and a ferromagnetic behavior attributed to the Mn₅Ge₃ crystallites dispersed into the films. The Mn L_2,3 X-ray absorption spectra of the Mn_0.06Ge_0.94 films exhibit a lineshape typical of metallic Mn, with considerably reduced multiplet structure.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.