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.     

Magnetic and transport properties of SmCoAsO1−xFx

A series of SmCoAsO_1−xF_x (with x=0, 0.05, 0.1, and 0.2) samples have been prepared by solid state reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has been doped into the lattice. The magnetic and transport properties of the samples have been investigated. Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism, ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Néel temperatures were almost independent of the F content and metamagnetic transitions were observed below antiferromagnetic Néel temperatures. With increasing F content, high temperature (below 142 K) ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction in the region of 2-300 K and Fermi liquid behavior at low temperatures were shown in all samples. Transport properties at applied magnetic fields showed anomalies at low temperatures.     

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.     

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.     

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.     

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 LaFe11.5−xCoxSi1.5C0.2 compounds for magnetic refrigeration near room temperature

The influence of cobalt on the microstructural, magnetic and magnetocaloric properties of LaFe_11.5−xCo_xSi_1.5C_0.2 (x=0.50–0.85) compounds was investigated. The ingots were prepared by using a vacuum induction melting furnace. Before annealing, a large amount of 1:13 phase was distinctly observed. Nearly single 1:13 phase was obtained after annealing at 1353 K for only 3 days. The easy formation of 1:13 phase in the annealing process could be attributed to carbon doping. The Curie temperature (T_C) increases linearly with increasing the cobalt content. Although the maximum magnetic entropy changes of the compounds decrease rapidly when T_C rises from 275 to 298 K, and it decreases mildly when T_C continues to rise. Two composite refrigerants based on the compounds are proposed. Their entropy changes remains approximately constant over the temperature range from 266 to 292 K and 289 to 309 K.     

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.     

Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Superconducting properties of SmFeAsO1−x prepared under high-pressure condition

Synthetic conditions such as stoichiometries, temperature and pressure are optimized to achieve a high quality oxygen deficient SmFeAsO_0.6 superconductor. Both electric and magnetic measurements show a sharp superconducting transition at about 55 K. Several important physical parameters are deduced. The apparent superconducting gap observed in heat capacity with 2Δ_o/k_BT_c of 4.57 larger than that of previous fluorine replaced samples indicate that this superconductivity will not strongly conflict with the phonon-mediated BCS mechanism. The mean free length ?=18.8 nm and the coherent length ξ=2.3-3.3 nm show that the superconductivity is in the clean limit.     

Magnetic phase diagram of the Ca1−xMnxO systems

The magnetic properties of the Ca_1−xMn_xO systems in the range 0?x?1 have been studied by mean field theory and high-temperature series expansions (HTSEs). By using the first theory, we have evaluated the nearest neighbour and the next-neighbour super-exchange interaction J₁(x) and J₂(x) respectively, in the range 0.45?x?1. The corresponding classical exchange energy for magnetic structure is obtained for the Ca_1−xMn_xO systems. The HTSEs combined with the Padé approximants (PA) method is applied to the Ca_1−xMn_xO systems; we have obtained the magnetic phase diagrams (TN or TSG versus dilution x) in the range 0?x?1. The obtained theoretical results are in agreement with experimental ones obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) are deduced in the range 0?x?1.     

Structural, optical and electrical properties of Zn1−xCdxO thin films prepared by PLD

Ternary polycrystalline Zn_1−xCd_xO semiconductor films with cadmium content x ranging from 0 to 0.23 were obtained on quartz substrate by pulse laser deposited (PLD) technique. X-ray diffraction measurement revealed that all the films were single phase of wurtzite structure grown on c-axis orientation with its c-axis lattice constant increasing as the Cd content x increasing. Atomic force microscopy observation revealed that the grain size of Zn_1−xCd_xO films decreases continuously as the Cd content x increases. Both photoluminescence and optical measurements showed that the band gap decreases from 3.27 to 2.78 eV with increasing the Cd content x. The increase in Cd content x also leads to the broadening of the emission peak. The resistivity of Zn_1−xCd_xO films decreases evidently for higher values of Cd content x. The shift of PL emission to visible light as well as the decrease of resistivity makes the Zn_1−xCd_xO films potential candidate for optoelectronic device.     

Pressure dependent elastic properties of diluted magnetic semiconductors: Hg1−xMnxS (x=0.02 and 0.07)

A theoretical study of the elastic properties in diluted magnetic semiconductors Hg_1−xMn_xS (x=0.02 and 0.07) using an effective interionic interaction potential (EIoIP) in which long-range Coulomb interactions, charge transfer mechanism (three body interaction) and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is considered. Particular attention is devoted to evaluate Poisson's ratio ν, the ratio R_S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and thermodynamic property as Debye temperature is calculated. By analyzing Poisson's ratio ν and the ratio R_S/B we conclude that Hg_1−xMn_xS is brittle in zinc blende (B3). To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_1−xMn_xS compounds and still awaits experimental confirmations.     

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.     

BiFeO3/Zn1−xMnxO bilayered thin films

BiFeO₃/Zn_1−xMn_xO (x = 0-0.08) bilayered thin films were deposited on the SrRuO₃/Pt/TiO₂/SiO₂/Si(1 0 0) substrates by radio frequency sputtering. A highly (1 1 0) orientation was induced for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO thin films demonstrate diode-like and resistive hysteresis behavior. A remanent polarization in the range of 2P_r ∼ 121.0-130.6 μC/cm² was measured for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO (x = 0.04) bilayer exhibits a highest M_s value of ∼15.2 emu/cm³, owing to the presence of the magnetic Zn_0.96Mn_0.04O layer with an enhanced M_s value.     

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.     

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.     

Structural and photoluminescence properties of thermally evaporated Cd1−xMnxS nano-crystalline films

Thin films of Cd_1−xMn_xS (0≤x≤0.5) were formed on glass substrates by resistive vacuum thermal evaporation. All the films were deposited at 300 K and the films were annealed at 373, 473 and 573 K for 1 h in a vacuum of 10⁻⁶ mbar. Atomic force microscopy (AFM) studies showed that all the films investigated were in nano-crystalline form with a grain size in the range 36-82 nm. All the films exhibited a wurtzite structure of the host material. The lattice parameters varied linearly with composition following Vegard’s law in the entire composition range. Photoluminescence studies showed that two distinct emission bands were observed for each Cd_1−xMn_xS compound. One corresponds to internal transition and the other one is due to the transition of Mn²⁺ ions in interstitial sites or in small ‘Mn’ chalcogenic clusters.     

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.