Study on Mn-induced Jahn–Teller distortion in BiFeO3 thin films

Present work reports Raman spectroscopy study of single-phase Mn-doped BiFeO₃ [BiFe_1?x Mn _x O₃ (0 ≤ x ≤ 0.20)] polycrystalline thin films carried out in backscattering geometry. De-convolution of Raman spectra showed a gradual transition in the crystal symmetry from rhombohedral (?R) to multiphase [rhombohedral (?R) + tetragonal (?T)] structure with increasing Mn doping concentration in BiFe_1?x Mn _x O₃ (BFMO) thin films. X-ray diffraction (XRD) along with Le-Bail extraction refinement confirms that the structural symmetry lowering in BFMO thin films occurs at about 10 % Mn doping concentration. A blue shift is observed in the direct energy band gap of BFMO thin films from 2.53 to 2.87 eV (at T = 295 K) and is attributed to the local symmetry lowering and local induced strain in Fe³⁺ environment resulted from Jahn–Teller distortion in (MnFe)³⁺O₆ octahedral unit. Second-derivative analysis of FTIR spectra in the spectral regions (420–470) cm^?1 and (480–680) cm^?1 further indicates the favourable structure distortion leading to the simultaneous exhibition of enhanced ferromagnetic and ferroelectric properties owing to Mn substitution in host BiFeO₃ lattice.     

Optical Study of Plasmon–LO Phonon Modes in Ga1−x Mn x As

Ga_1?xMn _x As layers with Mn fractions 0 ≤ x ≤ 2.8 % grown on GaAs(001) substrates by low-temperature molecular beam epitaxy were investigated using micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of coupled modes of longitudinal optical phonons and hole plasmons. A full line-shape analysis of the spectra was performed within a Lindhard–Mermin model for the dielectric function, including intraband and interband hole transitions. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the number of Mn atoms on Ga lattice sites is deduced from high-resolution X-ray diffraction. After annealing at 450°C the Raman lines of elemental As are observed, which are due to the precipitation of As on the sample surface.     

Distinct magnetic response of nanograined Zn1−xMnxO (x = 0, 0.02, 0.1) powders and thin films: Focus on the effect of the working atmosphere

Nanocrystalline Zn_1?xMn_xO (where x = 0, 0.02, 0.1) powders and thin films were prepared by a polymeric precursor method and pulsed laser deposition, respectively. The wet chemistry method was chosen to synthesize the powders in order to improve key parameters as purity and grain-size distribution. Optimization procedures, encompassing substrate temperature, laser fluence and background gas conditions were performed and are intended to improve the crystalline quality of the films. X-ray diffraction studies reveal no clear evidence (within the detection limit of the technique) of affectation by impurities generated during the synthesis. Films prepared under optimized conditions feature a mosaic spread <0.3° and nearly bulk-like c-axis lattice parameter (5.198 Å). The substitution of Mn²⁺ cations in the tetrahedral sites of the wurtzite structure of pristine ZnO is confirmed both X-ray diffraction and Raman spectroscopy. Magnetic measurements, in turn, show that the powders are paramagnetic for temperatures above 3 K, while the thin films, grown in vacuum, are ferromagnetic at room temperature. These results indicate that variations in the preparation parameters have a marked influence on the magnetic responses of the Zn_1?xMn_xO system.     

Composition-dependent structural, optical and electrical properties of In x Ga1−x N (0.5 ≤ x ≤ 0.93) thin films grown by modified activated reactive evaporation

In this report, we have studied the compositional dependence of structural, optical and electrical properties of polycrystalline In _x Ga_1?x N thin films grown by modified activated reactive evaporation. The growth was monitored by optical emission spectroscopy. The thickness of the films was in the range ~600–800 nm. The phase, crystallinity and composition of the films were determined by X-ray diffraction, Raman spectroscopy and energy dispersive X-ray analysis. The surface morphology was studied by atomic force microscopy. The band gaps of these films obtained from transmittance and photoluminescence measurements were found to vary from 1.88 to 3.22 eV. All the films show n-type conductivity. The carrier concentration was found to be decreasing with increase in gallium incorporation which is in good agreement with the free carrier absorption observed in transmittance spectra.     

Effect of Mn doping concentration on structural,vibrational and magnetic properties of NiO nanoparticles

The Ni_1?xMn_xO (x?=?0.00, 0.02, 0.04 and 0.06) nanoparticles were synthesized by chemical precipitation route followed by calcination at 500?°C for 4?h. The prepared samples were characterized by energy dispersive analysis of X-rays (EDAX), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and vibrating sample magnetometer (VSM). Rietveld refinement of XRD data confirms the structural phase purity and XRD patterns are well indexed to NaCl like rock salt fcc crystal structure with Fm-3m space group. The particle size of Mn doped samples is found to be less than that of pure NiO sample. However, the particle size increases slightly on increasing the Mn concentration due to surface/grain boundary diffusion. The vibrational properties of the synthesized nanoparticles were investigated by Raman and FT-IR spectroscopy. The results of room temperature magnetization (M-H) and temperature dependent magnetization (M-T) measurements are explained with a core-shell model. The synthesized nanoparticles show weak ferromagnetic and super-paramagnetic like behavior at room temperature.     

Structural, optical and photoluminescence studies of heavily Mn-doped ZnO nanoparticles annealed under Ar atmosphere

Undoped and heavily Mn-doped with ZnO nanoparticles (Zn_1?xMn_xO, x?=?0.0, 0.05, 0.1 and 0.2) annealed under Ar atmosphere have been synthesized by a sol–gel method. The structural properties and optical absorption of the prepared samples have been examined by powder X-ray diffraction, energy dispersive X-ray analysis, Fourier transform infrared (FTIR) spectroscopy and UV–visible spectrophotometer. Hexagonal wurtzite structure of the samples is confirmed by the XRD spectra. The average crystalline size of the Zn_1?xMn_xO nanoparticles has been calculated from X-ray line broadening and is decreased from 35.73 to 18.24?nm with increase in Mn concentrations from 0.0 to 0.2. The increase in lattice parameters indicates the substitution of Mn in ZnO lattice. SEM and TEM photographs indicated that the grain size of undoped ZnO is bigger than the Mn-doped ZnO which is due to the limitations of grain growth upon Mn doping. The presence of functional groups and the chemical bonding due to Mn doping is confirmed by FTIR spectra. PL spectra of the Zn_1?xMn_xO system showed that the shift in near band edge emission at 390?nm and a blue band emission at 450–490?nm which confirms the substitution of Mn.     

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single phase samples of Ni(Cr_1?xMn _x )₂O₄ (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr_1?xMn _x )₂O₄. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I4₁/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (T_C) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr₂O₄ system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.     

Hydrothermal synthesis of Zn-doped Ni–Mn–Al–O thin films toward high-performance negative temperature coefficient thermistor

In this study, Zn-doped Ni–Mn–Al–O negative temperature coefficient thermistor (NTC) film with high electrical performance has been demonstrated. XRD, XPS, SEM and electrical measurements were carried out to explore the impact of Zn-doping. The XRD analysis of NiMn_1.8?xAl_0.2Zn_xO₄ films confirmed the cubic spinel crystal phase regardless of x. The SEM image illustrated that the morphology of NiMn_1.8?xAl_0.2Zn_xO₄ is closely related to the Zn doping amount. The XPS showed that the relative molar content of Mn³⁺ and Mn⁴⁺ increased with the increase of Zn doping concentration. Remarkably, the Zn-doped Ni–Mn–Al–O film presented the typical NTC characteristics, and the room temperature resistance (R₂₅) increased with the improvement in Zn doping concentration. Moreover, the thermistor constant B_25/50 remained reasonably high from 4088 to 4272 K. Meanwhile, the aging test showed that the Zn-doped films were more stable, even aging at 150 °C for 500 h.     

Electronic and optical properties of (Al x Ga1−x )1−y Mn y As single crystal: a new candidate for integrated optical isolators and spintronics

We have explored the electronic and optical properties of cubic (Al _x Ga_1?x )_1?y Mn _y As system using the FP-LAPW method. The unit cell has 64 atoms, so that one manganese (Mn) atom is placed in the position of gallium site, which corresponds to 3.125 % doping concentration with x = 12.5 %. Our calculations, using local density approximation + U (Hubbard parameter) scheme, predict that the ferromagnetic state for AlGaMnAs, with a magnetic moment of about 4.014 μ_B per Mn dopant is more favorable. Despite its electronic properties being strongly affected by inducing small amounts of Mn substitutional atoms in the cationic sublattice of AlGaAs, (Al _x Ga_1?x )_1?y Mn _y As possesses optical properties strictly less than those of Al _x Ga_1?x As, especially its optical conductivity at the peak 1.256 eV. The results indicate that AlGaMnAs may be a good candidate for optoelectronics when exploited in optical fiber networks, and it can still be of great interest because of its promising potential when used for spintronics.     

High temperature growth, charge distribution and magnetism in Co and Mn co-doped ZnO

Zn_1?2xCo_xMn_xO (x = 0.03 and 0.05) bulk samples were synthesised by a standard high temperature solid state reaction technique at 1,400 °C. Characterization techniques X-ray diffraction, Vibrating Sample Magnetometry and Scanning Electron Microscopy with Energy Dispersive Analysis of X-rays were used to investigate effect of transition metal element Co and Mn incorporated in the system using high temperature technique. The magnetic, structural, electron density distribution and bonding properties have been investigated for the samples prepared in this work. X-ray diffraction analysis of the synthesized samples showed a single phase ZnO wurtzite structure for x = 0.03 and very slight segregation of a spinel phase for x = 0.05. Magnetization measurements indicate no evidence of ferromagnetism in co-doped (Co and Mn) ZnO. The structural and electron density analysis presented in this work is based on the actual concentrations of Co and Mn ions introduced in the matrix which nearly coincide with the nominal concentrations.     

Structure and magnetic properties of (Co,Mn) co-doped ZnO diluted magnetic semiconductor nanoparticles

The ZnO, Zn_0.96Mn_0.04O, Zn_0.95Mn_0.04Co_0.01O, Zn_0.94Mn_0.04Co_0.02O and Zn_0.92Mn_0.04Co_0.04O nanoparticles were synthesized by simple chemical precipitation technique. The effects of co-doping on the structure and magnetic properties of these nanoparticles were studied. The X-rays diffraction (XRD) scans were performed in the 2θ range of 20°–80°. The XRD patterns, at 300 K, of all the pure and co-doped ZnO samples confirmed the formation of wurtzite-type structure. X-ray diffraction and transmission scanning electron microscope analysis indicated that the high spin Co²⁺ and Mn²⁺ ions were substituted for the Zn²⁺ ions at tetrahedral sites. The average size of the nanoparticles were increased from 17 to 24 nm with the increase of dopants concentration. Moreover, Energy Dispersive X-ray spectroscopy (EDX) confirmed the synthesis results. All Zn_0.96?xMn_0.04Co _x O (x?=?0.0, 0.1, 0.2 and 0.4) nanoparticles samples were observed to be paramagnetic below 300 K. However, a large increase in the magnetization was observed below 40 K. This behavior, along with the negative value of the Curie–Weiss constant obtained from the linear fit to the susceptibility data below room temperature, indicated the ferromagnetic nature of the samples. The origin of ferromagnetism is likely to be the intrinsic characteristics of the Co and Mn doped samples. The high magnetization was noted for the 1 wt% Co co-doped Mn–ZnO annealed samples as compared to other samples with Co concentration above and below this threshold concentration.     

Investigation of structural and some physical properties of Cr substituted polycrystalline Eu<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Mn<Subscript>1−x</Subscript>Cr<Subscript>x</Subscript>O<Subscript>3</Subscript> (0 ≤ x ≤ 0.1) manganites

The polycrystalline samples with nominal composition Eu_0.5Sr_0.5Mn_1?x Cr _x O₃ (0 ≤ x ≤ 0.1) were prepared by the conventional solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy and electrical resistivity behavior without and with magnetic field. The structural parameters obtained by using Rietveld refinement of X-ray diffraction data showed that all samples crystallize with orthorhombic perovskite type symmetry with Pbnm space group. The scanning electron micrograph images reveal that the increase in Cr substitution hinders grain growth and grain connectivity. The temperature dependence of electrical resistivity show the semiconducting nature of these compounds and support the small polaron hoping model and variable range hopping conduction model. The calculated hopping distance and activation energy decreased as rate of Cr content increased whereas density of states at Fermi level increased. A large negative magnetoresistance is also present in the sample at the lowest temperature of measurements.     

Structural and Magnetic Properties of Pr0.6Sr0.4Mn1−x Fe x O3 (0≤x≤0.3) Manganites Oxide Prepared by the Ball Milling Method

We present the structural and magnetic properties of Pr_0.6Sr_0.4Mn_1?x Fe _x O₃ (x=0, 0.1, 0.2, and 0.3) compounds. Samples have been prepared by the ball milling method. Rietveld refinements of the X-ray powder diffraction data show that all our synthesized samples are single phase and crystallize in the orthorhombic symmetry with the Pnma space group. The unit cell volume increases with increasing the Fe content. The infrared spectrum shows two active bands, which can be ascribed to the internal stretching and bending modes. The magnetization measurements versus temperature showed that Fe doping leads to a weakening of the ferromagnetic ordering at low temperature, the Curie temperature T _C decreases from 300 K for x=0.0 to 88 K for x=0.2. The magnetization versus applied magnetic measurements at low temperature lead to conclude that the substitution of Mn³⁺ ions by Fe³⁺ ions triggers antiferromagnetic interactions between the Fe³⁺ and Mn⁴⁺ spins, and also the magnetization versus applied magnetic measurements at room temperature shows a small hysteresis loop and a low coercive field, which indicates that the samples are superparamagnetic.     

Structural,Magnetic, Optical and Electrical Properties of Ba Substituted BiFeO3

Ba²⁺ substituted bismuth ferrite (BFO) (Bi_1?x Ba _x FeO₃, x=0, 0.1, 0.2, and 0.3) was prepared by chemical route using nitrates as a precursor, and sintered at 830 °C for 1 hr. The structural, optical, magnetic, and electrical properties of the prepared samples were investigated to understand the effect of Ba²⁺ substitution in BFO. The samples were characterized by X-ray diffraction (XRD), and shows an impurity phase which reduces drastically on substitution. Magnetic measurements were carried out at room temperature up to a field of 20 kOe. Magnetic hysteresis loops revealed a significant change in magnetization on Ba²⁺ substitution in BFO with unsaturated nature. Further, the Fourier transformed Infrared (FTIR) and Ultra Violet Visible (U-V Vis) spectra of the samples at room temperature shows as a change in spectral behavior on substitution. The AC electrical conductivity analyzed through Impedance analyser at different temperatures and frequencies. The ac conductivity and its activation energies for the samples are found to be frequency and temperature dependent and also vary with Ba²⁺ substitution in BFO.     

Oxygen vacancy-related dielectric relaxation and electrical conductivity in La-doped Ba(Zr0.9Ti0.1)O3 ceramics

Ba_1?xLa_x (Zr_0.9Ti_0.1)_1?x/4O₃ (BLZT) ceramics with x = 0.02 (BLZT-1), 0.04 (BLZT-2), 0.06 (BLZT-3) and 0.08 (BLZT-4) were prepared by a solid-state reaction route. Crystal structure of the BLZT ceramics was determined using X-ray diffraction and Raman spectroscopy. While the ceramics for x ≤ 0.04 are pure phase with cubic perovskite structure, pyrochlore La₂Zr₂O₇ appears in the samples with x = 0.06 and 0.08. Dielectric properties as function of temperature and frequency showed more than one dielectric anomaly were found at high temperatures during heating but they weakened or disappeared during cooling. Both dielectric relaxation and electrical conductivity were taken into account in point defect mechanism. The double-ionized and short-range hopping of oxygen vacancy should be mainly responsible for the dielectric anomalies and conduction behavior. Activation energy of conductivity E _con is lower than half of the band gap E _g obtained by UV–Vis spectroscopy, which results from emergency of oxygen vacancies. In visible light region, the ceramics show a strong absorption with band gap of about 3.57 eV.     

Structure and Magnetic Properties of Mn-doped CoFe 2 O 4 Nanoparticles Prepared by Solvothermal Route

Spinel Co_1?xMn_xFe₂ O ₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) nanoparticles were synthesized by a solvothermal method using polyethylene glycol (PEG) as a surfactant. X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy and vibrating sample magnetometer (VSM) techniques were used to characterize phase, morphology, valence states, and magnetic properties of the samples. XRD analysis showed single phase of spinel structure, and the decrease of lattice constant on the effective Mn substitution was investigated. The effect of the PEG on the spherical aggregates of Co_1?xMn_xFe₂ O ₄ nanoparticles was observed. The result of XANES spectra showed Mn²⁺/Mn³⁺, Fe³⁺, and Co²⁺ exist in the samples. The samples showed ferromagnetism at room temperature with a maximum saturated magnetization of 72 emu/g and the smallest coercivity of 45 Oe for x = 1.0. The origin of ferromagnetic behavior is believed to be due to the occupation of Mn²⁺/Mn³⁺ ions.     

Effect of dopant concentration on the structural, electrical and optical properties of Mn-doped ZnO films

The Mn-doped ZnO (Zn_1 − xMn_xO) thin films with manganese compositions in the range of 0-8 at.% were deposited by radio-frequency (RF) magnetron sputtering on quartz glass substrates at room temperature (RT). The influence of Mn concentration on the structural, electrical and optical properties of Zn_1 − xMn_xO films has been investigated. X-ray diffraction (XRD) measurements reveal that all the films are single phase and have wurtzite structure with (002) c-axis orientation. The chemical states of Mn have been identified as the divalent state of Mn²⁺ ions in ZnO lattice. As the content of Mn increases, the c-lattice constant and the optical band gap of the films increase while the crystalline quality deteriorates gradually. Hall-effect measurements reveal that all the films are n-type and the conductivity of the films has a severe degradation with Mn content. It is also found that the intensity of RT photoluminescence spectra (PL) is suppressed and saturates with Mn doping.     

Impact of Mn Substitution at Ru Site in?RuSr2(Eu1.4Ce0.6)Cu2O10?�� Magneto-Superconductor

We report the effect of Mn ion substitution on the structural, superconducting and magnetic properties of polycrystalline Ru_1?x Mn _x Sr₂(Eu_1.4Ce_0.6)Cu₂O_10??? system with x=0.0 to 0.50. All the samples crystallizes with tetragonal structure in I4/mmm space group. RuSr₂(Eu_1.4Ce_0.6)Cu₂O_10??? (EuRu-1222) is a reported magneto-superconductor with magnetic ordering at 100 K and superconductivity occurs at ?40 K. The exact nature of Ru spin magnetic ordering is still being debated and no conclusion has been reached yet. Here, we found the superconducting transition temperature T _c=15 K from the dc magnetization measurements for undoped sample. It is observed that the superconducting transition temperature decreases with the Mn doping at Ru site. DC magnetic susceptibility measurements exhibited ferromagnetic like transition for all the synthesized samples. It was also observed that the net magnetic moment decreases gradually with Mn doping, though not monotonically. It seems that doping of Mn in EuRu-1222 at Ru site enhance the AFM ordering of Ru spins and suppress the FM component. Our results point out possible coupling between superconductivity and magnetism.     

Spin-phonon coupling and improved multiferroic properties of Zr substituted BiFeO3 nanoparticles

Zr substituted BiFeO₃ (BiFe_1?xZr_xO₃ with x = 0.03, 0.07, 0.10 and 0.15) nanoparticles were synthesized by sol–gel method. Powder X-ray diffraction studies showed rhombohedral crystal structure for x = 0.03–0.10 samples. The substitution induced structural transformation from rhombohedral to triclinic phase has been observed for x = 0.15. Raman analysis confirmed this structural transformation as also the distortion induced spin phonon coupling. Enhanced magnetic behaviour with saturation magnetization of 7.62 emu/g has been observed in x = 0.07 sample. The dielectric measurements indicated the strong magneto-electric coupling in the range of Neel temperature. The impedance study over a wider frequency and temperature range suggests decrease in conductivity in the samples with increasing Zr concentration. UV–Vis diffuse reflectance spectra shows the strong absorption of visible light suggesting the band gap values from 2.22 to 2.15 eV corresponding to x = 0.03–0.15 compositions.     

MgAl2O4–LaCr0.5Mn0.5O3 composite ceramics for high temperature NTC thermistors

New negative temperature coefficient (NTC) ceramics based on xMgAl₂O₄–(1 ? x)LaCr_0.5Mn_0.5O₃ (x = 0, 0.4, 0.6, 0.8) composition were investigated. The composite ceramics (x = 0.4, 0.6, 0.8) consisted of two phases: a cubic spinel MgAl₂O₄ phase and an orthorhombic perovskite LaCr_0.5Mn_0.5O₃ phase isomorphic to LaCrO₃. The brighter regions were the LaCr_0.5Mn_0.5O₃ and the darker was the MgAl₂O_4. There was ion diffusion between these two phases. X-ray photoelectron spectroscopy analysis corroborated the presence of Cr³⁺, Cr⁴⁺, Mn³⁺ and Mn⁴⁺ ions on lattice sites, which may result in the hopping conduction. The ρ ₃₀₀ and B _400/800 constant increased with increasing MgAl₂O₄ content. The values of ρ ₃₀₀, B _400/800 and activation energy of NTC thermistors were in the range 1.76–1.22 × 10⁸ Ωcm, 2,646–8,711 K, 0.228–0.746 eV, respectively. This means the electrical properties can be adjusted to desired values, depending on the MgAl₂O₄ content.