Room-temperature ferromagnetism in Sn1−xMnxO2 nanocrystalline thin films prepared by ultrasonic spray pyrolysis

Sn_1−xMn_xO₂ (x=0.01-0.05) thin films were synthesized on quartz substrate using an inexpensive ultrasonic spray pyrolysis technique. The influence of doping concentration and substrate temperature on structural and magnetic properties of Sn_1−xMn_xO₂ thin films was systematically investigated. X-ray diffraction (XRD) studies of these films reflect that the Mn³⁺ ions have substituted Sn⁴⁺ ions without changing the tetragonal rutile structure of pure SnO₂. A linear increase in c-axis lattice constant has been observed with corresponding increase in Mn concentration. No impurity phase was detected in XRD patterns even after doping 5 at% of Mn. A systematic change in magnetic behavior from ferromagnetic to paramagnetic was observed with increase in substrate temperature from 500 to 700 °C for Sn_1−xMn_xO₂ (x=0.01) films. Magnetic studies reveal room-temperature ferromagnetism (RTFM) with 3.61×10⁻⁴ emu saturation magnetization and 92 Oe coercivity in case of Sn_1−xMn_xO₂ (x=0.01) films deposited at 500 °C. However, paramagnetic behavior was observed for the films deposited at a higher substrate temperature of 700 °C. The presence of room-temperature ferromagnetism in these films was observed to have an intrinsic origin and could be obtained by controlling the substrate temperature and Mn doping concentration.     

The important role of Mn in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga_1−xMn_xN) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga_1−xMn_xN films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn³⁺ due to the thermal treatment. Mn³⁺, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn³⁺ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn³⁺ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn_xGa_y formation.     

Effects of Fe doping and Fe-N-codoping on magnetic properties of SnO2 prepared by chemical co-precipitation

The effects of Fe-doping and Fe-N-codoping on the magnetic properties of SnO₂, prepared by chemical co-precipitation technique, are investigated in details. We found that the paramagnetism is the dominant magnetic interaction in Fe doped SnO₂. A weak antiferromagnetic coupling between Fe²⁺ ions is also confirmed through Zero field-cooled (ZFC) and field-cooled (FC) magnetization studies. On the other hand, hystersis behavior is observed for Fe-N-codoped SnO₂ samples with coercivity H_c∼420 and 352 Oe for x=0.05 and 0.10, respectively. As no other secondary or impurity phase is detected by XRD study and the presence of N is confirmed by EDX analysis, this observed ferromagnetism is originated due to the substitution of N in Sn_1−xFe_xO₂. N doping at the oxygen site can be regarded as defect and introduces a hole in this system. As a result, a hole-induced ferromagnetism might be the origin of the observed ferromagnetism in Fe-N-codoped SnO₂ samples.     

Studies on the structural and electrical properties of F-doped SnO2 film prepared by APCVD

Fluorine-doped tin oxide films (SnO₂:F, FTO) were deposited by atmosphere pressure chemical vapor deposition (APCVD) on Na-Ca-Si glass coated with a diffusion barrier layer of SiO_xC_y. The effects of post-heating time at 700 °C on the structural and electrical properties of SnO₂:F films were investigated. The results showed that SnO₂:F films were polycrystalline with tetragonal SnO₂ structure, SnO phase was present in SnO₂ film, and abnormal grain growth was observed. The element distribution in the film depth was measured with X-ray photoelectron spectroscopy (XPS) and revealed that when the heating time increased from 202 s to 262 s, the oxygen content in the surface increased from 78.63% to 83.38%. The resistivity increased from 3.13 × 10⁻⁴ for as-deposited films to 4.73 × 10⁻⁴ Ω cm when post-heated for 262 s. Hall mobility is limited by the ionized impurity scattering rather than the grain boundary scattering.     

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.     

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.     

A study of room-temperature ferromagnetism in transition metal and fluorine-doped spray-pyrolyzed SnO2 thin films

Room-temperature ferromagnetism has been observed in Co- or Mn-doped SnO₂ and Co- and F-co-doped SnO₂ thin films. A maximum magnetic moment of 0.80μ_B/Co ion has been observed for Sn_0.90Co_0.10O_1.925−δF_0.075 thin films, whereas in the case of Sn_1−xMn_xO_2−δ it was 0.18μ_B/Mn ion for x=0.10. The magnetization of both Sn_1−xCo_xO_2−δ and Sn_1−xCo_xO_2−y−δF_y thin films depends on the free carrier concentration. An anomalous Hall effect has been observed in the case of Co-doped SnO₂ films. However, the same was not observed in the case of Mn-doped SnO₂ thin films. Carrier-mediated interaction is convincingly proved to be the cause of ferromagnetism in the case of Co:SnO₂. It is, however, proposed that no carrier-mediated interaction exists in the case of Mn:SnO₂. Present studies indicate that dopants and hence electronic cloud-lattice interaction plays an important role in inducing ferromagnetism.     

Recent developments in the formation and structure of tin-iron oxides by laser pyrolysis

Complex oxides demonstrate specific electric and magnetic properties which make them suitable for a wide variety of applications, including dilute magnetic semiconductors for spin electronics. A tin-iron oxide Sn_1−xFe_xO₂ nanoparticulate material has been successfully synthesized by using the laser pyrolysis of tetramethyl tin-iron pentacarbonyl-air mixtures. Fe doping of SnO₂ nanoparticles has been varied systematically in the 3-10 at% range. As determined by EDAX, the Fe/Sn ratio (in at%) in powders varied between 0.14 and 0.64. XRD studies of Sn_1−xFe_xO₂ nanoscale powders, revealed only structurally modified SnO₂ due to the incorporation of Fe into the lattice mainly by substitutional changes. The substitution of Fe³⁺ in the Sn⁴⁺ positions (Fe³⁺ has smaller ionic radius as compared to the ionic radius of 0.69 Å for Sn⁴⁺) with the formation of a mixed oxide Sn_1−xFe_xO₂ is suggested. A lattice contraction consistent with the determined Fe/Sn atomic ratios was observed. The nanoparticle size decreases with the Fe doping (about 7 nm for the highest Fe content). Temperature dependent ⁵⁷Fe Mössbauer spectroscopy data point to the additional presence of defected Fe³⁺-based oxide nanoclusters with blocking temperatures below 60 K. A new Fe phase presenting magnetic order at substantially higher temperatures was evidenced and assigned to a new type of magnetism relating to the dispersed Fe ions into the SnO₂ matrix.     

Yb effect on the spectroscopic properties of Er-Yb codoped SnO2 thin films

We have investigated the optical properties of sol-gel thin films of tin dioxide (SnO₂) codoped with Er³⁺-Yb³⁺ as a function of Yb³⁺ concentration. The Judd-Ofelt model has been applied to absorption intensities of Er³⁺ (4f¹¹) transitions to establish the so-called Judd-Ofelt intensity parameters: Ω₂, Ω₄, Ω₆. Various spectroscopic parameters were obtained to evaluate their dependence and the potential of the samples as a laser material in the eye-safe laser wavelength (1.53 μm) as a function of Yb³⁺ concentration. An amelioration of the quality factor Ω₄/Ω₆ was found with Yb content. Both the IR photoluminescence (PL) intensity and the up-conversion emission, from Er³⁺ ion in SnO₂, were found to increase with Yb concentration. We show that the Yb³⁺ ion acts as sensitizer for Er³⁺ ion and contributes largely to the improvement of the spectroscopic properties of SnO₂:Er. The mechanism of up-conversion emission is discussed and a model is proposed. The results showed that sol-gel SnO₂ is promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable up-conversion lasers.     

Effect of process parameters on surface morphology and characterization of PE-ALD SnO2 thin films for gas sensing

Tin dioxide (SnO₂) thin films were deposited by plasma enhanced-atomic layer deposition (PE-ALD) on Si(1 0 0) substrate using dibutyl tin diacetate (DBTA) ((CH₃CO₂)₂Sn[(CH₂)₃-CH₃]₂) as precursor. The process parameters were optimized as a function of substrate temperature, source temperature and purging time. It is observed that the surface phenomenon of the thin films was changed with film thickness. Atomic force microscopy (AFM) images and X-ray diffraction (XRD) pattern were used to observe the texture and crystallanity of the films. The films deposited for 100, 200 and 400 cycles were characterized by XPS to determine the chemical bonding properties. XPS results reveal that the surface dominant oxygen species for 100, 200 and 400 cycles deposited films are O₂⁻, O⁻ and O²⁻, respectively. The 200 cycles film has exhibited highest concentration of oxygen (O⁻) species before and after annealing. Conductivity studies revel that this film has best adsorption strength to the oxygen ions forming on the surface. The sensor with 200 cycles SnO₂ thin film has shown highest sensitivity to CO gas than other films. A correlation between the characteristics of Sn3d_5/2 and O1s XPS spectra before and after annealing and the electrical behavior of the SnO₂ thin films is established.     

Structural, optical and electrical properties of Cd-doped SnO2 thin films grown by RF reactive magnetron co-sputtering

Transparent conducting SnO₂:Cd thin films were prepared by RF reactive magnetron co-sputtering on glass slides at a substrate temperature of 500 °C using CdO as cadmium source. The films were deposited under a mixed argon/oxygen atmosphere. The structural, optical and electrical properties were analyzed as a function of the Cd amount in the target. The X-ray diffraction shows that polycrystalline films were grown with both the tetragonal and orthorhombic phases of SnO₂. The obtained films have high transmittance and conductivity. The figure of merit of SnO₂:Cd films are in the order of 10⁻³ Ω⁻¹, which suggests that these films can be considered as candidates for transparent electrodes.     

p-type conduction in nitrogen-doped SnO2 films grown by thermal processing of tin nitride films

p-type nitrogen-doped SnO₂ (SnO₂:N) films were grown by thermal processing of amorphous tin nitride films at temperatures between 350 and 500?^°C in flowing O₂?CAr gas mixture. From high-resolution X-ray photoelectron spectroscopy (XPS) and X-ray diffraction patterns, it is deduced that the N atoms replace the O atoms in the SnO₂ lattice. The N dopant is more tightly bound in SnO₂:N at higher thermal oxidation temperatures deduced from the XPS results. The hole concentration obtained at an oxidation temperature of 400?^°C is 1.87×10¹⁹?cm^?3, which is dramatically enhanced compared to previous reports. Our results indicate that the high-temperature thermal oxidation of tin nitride is a facile and effective route to alleviate the self-compensation effect, reduce the content of ??-N₂ double donors, and reinforce the stability of N dopant in the SnO₂:N films.     

Effect of Sr concentration on microstructure and magnetic properties of (Ba1−xSrx)(Ti0.3Fe0.7)O3 ceramics

Fe-doped (Ba_1−xSr_x)TiO₃ ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤x≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti sites mediated by the O²⁻ ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe³⁺ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba_0.25Sr_0.75)TiO₃, is an order of magnitude higher than that of the Fe-doped BaTiO₃ system with similar Fe concentration and preparation conditions, which may indicate (Ba_1−xSr_x)TiO₃ as a more suitable matrix material for multiferroic research.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

The synthesis and the magnetic properties of Gd-doped FexCo1−x/CoyFe3−yO4 micro-octahedrons composites

Gd³⁺-substituted micro-octahedron composites (Fe_xCo_1−x/Co_yGd_zFe_3−y−zO₄) in which the Fe-Co alloy has either a bcc or fcc structure and the oxide is a spinel phase were fabricated by the hydrothermal method. The X-ray diffraction (XRD) patterns indicate that the as-synthesized Gd³⁺-substituted micro-octahedron composites are well crystallized. Scanning electron microscopy (SEM) images show that the final product consists of larger numbers of micro-octahedrons with the size ranging from 1.3 to 5 μm, and the size of products are increased with increasing the concentration of KOH. The effect of the Co²⁺/Fe²⁺ ratio (0?Co²⁺/Fe²⁺?1) and substitution Fe³⁺ ions by Gd³⁺ ions on structure, magnetic properties of the micro-octahedrons composites were investigated, and a possible growth mechanism is suggested to explain the formation of micro-octahedrons composites. The magnetic properties of the structure show the maximal saturation magnetization (107 emu/g) and the maximal coercivity (1192 Oe) detected by a vibrating sample magnetometer.     

Temperature-dependent magnetization in (Mn, N)-codoped ZnO-based diluted magnetic semiconductors

The influences of Mn doping on the structural quality of the Zn_xMn_1−xO:N alloy films have been investigated by XRD. Chemical compositions of the samples (Zn and Mn content) and their valence states were determined by X-ray photoelectron spectrometry (XPS). Hall effect measurements versus temperature for Zn_xMn_1−xO:N samples have been designed and studied in detail. The ferromagnetic transitions happened at different T_C should explain that the magnetic transition in field-cooled magnetization of Zn_1−xMn_xO:N films at low temperature is caused by the strong p-d exchange interactions besides magnetic transition at 46 K resulting from Mn oxide, and that the room temperature ferromagnetic signatures are attributed to the uncompensated spins at the surface of anti-ferromagnetic nano-crystal of Mn-related Zn(Mn)O.     

Chemical synthesis of nanocrystalline SnO2 thin films for supercapacitor application

Nanocrystalline SnO₂ thin films were deposited by simple and inexpensive chemical route. The films were characterized for their structural, morphological, wettability and electrochemical properties using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy techniques (SEM), transmission electron microscopy (TEM), contact angle measurement, and cyclic voltammetry techniques. The XRD study revealed the deposited films were nanocrystalline with tetragonal rutile structure of SnO₂. The FT-IR studies confirmed the formation of SnO₂ with the characteristic vibrational mode of Sn-O. The SEM studies showed formation of loosely connected agglomerates with average size of 5-10 nm as observed from TEM studies. The surface wettability showed the hydrophilic nature of SnO₂ thin film (water contact angle 9°). The SnO₂ showed a maximum specific capacitance of 66 F g⁻¹ in 0.5 Na₂SO₄ electrolyte at 10 mV s⁻¹ scan rate.     

Investigation of structure and magnetic properties of the as-deposited and post-annealed iron nitride films by reactive facing-target sputtering

Structure and magnetic properties of the as-deposited and post-annealed iron nitride films have been investigated systematically. A series of phases containing α-Fe, ?-Fe₃N, ξ-Fe₂N and γ″-FeN were obtained as nitrogen flow rate (FN₂) increases from 0.5 to 30 sccm. An increase of the nitrogen concentration in the as-deposited films could be concluded from the phase transition with the increasing FN₂. After being annealed, some of the iron nitride phases are decomposed and γ′-Fe₄N appears in the films. The magnetic characteristics are dependent on FN₂, which can be ascribed to the facts that the nitrogen in the films turns the valence states of Fe into Fe⁺ or Fe^dipole with high magnetic momentum or ever H-like bond Fe^+/dipole with low magnetic momentum based on the bond-band-barrier correlation mechanism.