Bound Magnetic Polaron Research Articles

Cobalt-doped ZnO nanoparticles and PLD-deposited thin film forms: structure, optical properties and nature of magnetic anisotropy.

Cobalt-doped zinc oxide nanoparticles (NPs) were synthesized using a modified sol-gel method. Thereafter, the obtained powder was deposited on a Suprasil glass substrate by employing a pulsed laser deposition (PLD) technique. X-ray diffraction analysis with Rietveld refinement confirmed a hexagonal wurtzite ZnO phase belonging to the P63 mc space group for both samples in the NP and thin film forms. In particular, the thin film exhibited an intensive (002) XRD peak, indicating that it had a preferred c-axis orientation owing to the self-texturing mechanism. No segregated secondary phases were detected. The crystallite structure, morphology, and size were investigated using high-resolution transmission electron microscopy (HRTEM). To study the crystalline quality, structural disorder, and defects in the host lattice, we employed Raman spectroscopy. UV-vis-NIR spectroscopy was performed to confirm the nature of the Co-doped ZnO NP powder and the film. The chemical states of oxygen and zinc in the thin film sample were also investigated via X-ray photoelectron spectroscopy (XPS). The M-T curve could be successfully fitted using both the three-dimensional (3D) spin-wave model and Curie-Weiss law, confirming the mixed state existence of weak ferromagnetic (FM) and paramagnetic (PM) phases. Magnetic interaction was quantitatively studied and explained by polaronic percolation of bound magnetic polarons (BMPs). Analysis of magnetic symmetry of the topological antiferromagnetic as-deposited thin film using torque measurements was performed. Based on a phenomenological model, it was revealed that the structure gives rise to uniaxial magneto-crystalline anisotropy (UMA) with the magnetic easy axis parallel to the c-axis.

XPS study of single phase formation of Eu2O3 nanoparticles for enhanced ferromagnetic ordering investigated with BMP and FCE models

The structural, optical, luminescence, electronic structure, and magnetic properties of the Eu2O3 nanoparticles synthesised using chemical co-precipitation routewere investigated while being annealed at two distinct temperatures500 °C and 800 °C. For Eu2O3 nanoparticles, the average particle size estimated using the Scherer equation, Williamson Hall plot, and size-strain plot is in the range of 23–44 nm while sustaining the body centre cubic structure (BCC). With an annealing temperature of 800 °C, the impurity phases of Eu(OH)3 and EuO are no longer visible in the sample that was with whenannealed at 500 °C. The luminescence properties of such cubic structures were characterized by an intense red emission centred at 613 nm. With the help of X-ray photoemission spectroscopy, the electronic structure of Eu2O3 nanoparticles is investigated. This revealed the formation of oxygen vacancies as well as the presence of the Eu cation in the two valence states + 2 and + 3. The Bound Magnetic Polaron Model and F Centre Exchange interaction, which establish long range magnetic ordering, are used to explain SQUID-VSM investigations that found clearly discernible ferromagnetic ordering in our samples at ambient temperature. The ferromagnetic ordering in these samples is explained by the creation of various complexes. It was observed that the synthesized Eu2O3 nanoparticles exhibit an efficient red-luminescence hence a potential candidate for application in lightings. The detailed outcomes of present study suggested the application of Eu2O3 nanoparticles as photocatalyst, optoelectronics and spintronics devices.

Ab initio calculations on magnetic and optical characteristics of pure ZnO and Mn(II)-doped ZnO monolayers with and without neutral charged vacancies defects

At high Curie temperatures, diluted magnetic semiconductors exhibit induced ferromagnetism and spin-dependent interactions, making them useful in magneto-electronic devices. In spite of the fact that ferromagnetism and optical emission dynamics are characterized by intricate microstructural and compositional features, the origins of these phenomena are not yet completely understood. Here, we used first principles calculations to study how vacancy defects affect the electrical, optical, and magnetic properties of pure ZnO and Mn@ZnO monolayers. In the pristine monolayer, the Zn vacancy produces magnetism with a total magnetic moment of 2 μB, but the oxygen vacancy does not. The magnetic semiconducting characteristic of the Mn substitution at the Zn site is shown by a total magnetization of 5 μB. The magnetic ground state of the ZnO monolayer doped with Mn is significantly influenced by the presence of native defects. More specifically, Zn vacancies are essential for stabilizing the FM states due to the bound magnetic polarons (BMPs) formation, but O vacancies have no influence on the magnetic ground state of the Mn(II)@ZnO monolayer. Based on the optical study, we discovered that Zn and O vacancies in the ZnO ML produce optical absorption peaks at 1.92 eV and 2.90 eV, respectively. The substitution of Mn(II) at Zn site not only increases the band gap of the ZnO monolayer but also produces d-d transition bands at lower energy side of fundamental energy gap peak. The Zn vacancies in the Mn(II)@ZnO ML produce an infrared absorption band around 1.13 eV, which could be due to the formation of the BMP, and enhance the optical absorption efficiency. The ZnO ML’s energy gap and Mn ion’s d-d transition bands exhibit a blueshift in AFM systems and a redshift in the FM systems. In far configuration, the Mn(II)@ZnO monolayer show no shift in the fundamental bandgap and intra-band transition of the Mn(II) spins due to the Mn(II) ion’s paramagnetic nature.

Effect of manganese (Mn) doping on the structural, dielectric, ferroelectric, and ferromagnetic properties of ceria (CeO2)

The Ce1-xMnxO2 ceramics were prepared using a solid-state route, with Mn doping concentrations of (x = 0.00, 0.03, 0.08, 0.13, and 0.18). Examination of X-ray diffraction patterns confirms the effective insertion of Mn into the CeO2 lattice, resulting in discernible changes in oxygen vacancy concentration and lattice expansion. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of Ce3+, Ce4+, Mn3+, and Mn2+ oxidation states. The Photoluminescence (PL) spectra reveal the existence of oxygen vacancies, as manifested by the appearance of peaks within the energy range spanning from 3.1 eV to 2.71 eV. Raman spectra reveal a symmetric vibration around 466.81 cm-1 in all samples. The surface morphology of the samples exhibited subtle changes in grain size upon the introduction of Mn into CeO2. The dielectric behavior, investigated across a range of frequencies with varying Mn doping concentrations, exhibits an increased dielectric constant in Mn-doped CeO2, particularly with a smaller dielectric loss at lower frequencies. The ferroelectric response revealed a maximum recoverable energy density of 0.56 mJ/cm2 with an efficiency of 47.86% for CeO2. The room temperature magnetic response of the samples improved as Mn concentration increased, which is well explained by the Bound Magnetic Polarons (BMPs) and exchange interactions model. Overall, the studies show the effect of structural, electrical, and magnetic properties after doping with Mn, opening avenues for diverse applications of these ceramics.

Magnetic field-enhanced oxygen reduction reaction for electrochemical hydrogen peroxide production with different cerium oxide nanostructures

We investigated cerium oxide nanoparticles of various morphologies (nanosheets, nanocubes, and nanoparticles) supported on carbon Vulcan XC-72 for the two-electron oxygen reduction reaction (ORR). A continuous magnetic field (2000 Oe) was used for the first time in the literature. The best results were for 5 % (w/w) CeO2 for all three different morphologies, more than doubling the ring current, enhancing the hydrogen peroxide selectivity from 51 % (Vulcan XC-72) to 84–89 %, and modifying the onset potential to lesser negative values. The presence of the magnetic field led to even higher ring currents with 5 % (w/w) CeO2, H2O2 selectivity from 54 % (Vulcan XC-72) to 88–96 % and changing even more the onset potential. Those results were correlated with the Zeeman effect, the Lorentz force, generating magnetohydrodynamic effects, the Kelvin force, and the formation of Bound Magnetic Polarons. This pioneering research introduces an innovative approach, highlighting the potential of an external continuous magnetic field.

Room-temperature ferromagnetic semiconductor Fe-doped β-Ga2O3 thin films with high saturation magnetization and low coercivity.

Emergent ferromagnetism in β-Ga2O3 with an ultra-wide bandwidth and high electrical breakdown strength offers exciting opportunities for fabricating robust spintronic devices. One pertinent obstacle in the material has been the low saturation magnetization, which precludes its practical application in magnetic devices. In this work, large-scale Fe-doped β-Ga2O3 diluted magnetic semiconductor (DMS) films are synthesized using a polymer-assisted deposition method, and the effect of Fe doping on their structural and magnetic properties is investigated. Remarkably, the optimal sample exhibits a high saturation magnetization (70 emu cm-3 at 300 K), much larger than those in previously reported stable oxide DMS films, as well as a low coercivity (12 Oe at 300 K). Further analysis shows that our samples manifest a typical bound magnetic polaron (BMP) model and the high saturation magnetization originates from the strong ferromagnetic coupling between the BMPs which is enhanced by Ga vacancies. The Fe-doped β-Ga2O3 thin films with high saturation magnetization and low coercivity may provide a promising platform for related semiconductor spintronics.

The Role of Concentration, Site, and O Vacancy on Magnetic and Optical Properties of Cu‐Doped Anatase TiO2

Cu‐doped is a dilute magnetic semiconductor with excellent electrical, magnetic, and optical properties. Herein, first‐principles methods are employed to investigate its electronic structure, magnetic properties, and optical behavior. The results demonstrate that Cu‐doped exhibits intrinsic ferromagnetism. The presence of O vacancies facilitates the ferromagnetic exchange between Cu ions by forming bound magnetic polarons (BMPs). This finding validates the BMPs model and explains for the decrease in magnetic properties during annealing under O2 conditions. As the concentration of Cu increases, the system undergoes a transition from a semiconductor to a metal. Cu ions exhibit a preference for a compact configuration and display either paramagnetism or antiferromagnetism. The spin polarization can be effectively controlled from 0% to 100% by adjusting the concentration and site of Cu. Additionally, Cu doping leads to a reduction in the bandgap and an extension of the absorption range into the infrared region. The absorption intensity is positively correlated with the concentration. The presence of a spin‐polarized intermediate band indicates a correlation between the spin of the excited electron and the energy of the absorbed photon. Overall, Cu‐doped shows significant potential for applications in spintronics and spin‐related optics, including photospintronics and spin photocatalysis.

Magnetic and electric behavior of spike- and petal-like nanoscaled Cu:ZnO superstructures diluted magnetic semiconductors

Room temperature ferromagnetism (RTFM) is the main condition for searching for materials to the application in spintronics. Beyond the candidates, the diluted magnetic semiconductors (DMS) are potential materials for such application. The introduction of a foreign transition metal ion in the semiconductor lattice may lead to the RTFM, where the intrinsic ferromagnetism developed by a nonmagnetic ion is preferable for spintronics application. In this work, single-phase Cu-substituted zinc oxide was synthesized by a fast polymeric precursor method, providing nanocrystallites from 4 to 11 nm. Copper hindered grain growth, even for high Cu concentrations. The crystallites assembled to superstructures with petal and spike-like morphologies, following an unconventional crystallization. The texturization observed during the crystallization led to the development of unexpected weak RTFM. The number of oxygen vacancies is too small to promote the RTFM since few atoms of copper find attached to the vacancies. Probably the RTFM arises from overlapped spin-split bound magnetic polarons (BMP) due to the interaction of the hybridized O 2p and localized unoccupied twofold t2 subband of Cu 3d states. Although the sintering of pure and Cu-doped ZnO particles was equivalent, the electrical properties diverged. The copper insertion enhanced the resistivity and the non-linearity coefficient reached values higher than the unity. This is maybe due to the presence of copper at the grain boundaries forming a Schottky barrier. Neither the non-ohmic electrical behavior nor the RTFM from nanostructured superstructures was ever reported before.

Oxygen vacancy driven luminescence, ferromagnetic and electronic structure properties of Eu doped CeO2 nanoparticles

Chemical co-precipitation route prepared nanoparticles of CeO2 and rare-earth cation Eu- doped Ceria Ce1-xEuxO2 (x = 0.03, 0.05, and 0.07) were investigated for various properties. Strained and phase maintained doped nanoparticles with crystallite sizes between 13 and 26 nm were confirmed by XRD, Rietveld refinement and UV–Vis–NIR measurements. Strained nanoparticles by replacement of host Ce with Eu in +2 and + 3 states were confirmed by X-ray Photoelectron Spectroscopy analyses which also, relate the crop of oxygen vacancies with the concentration of Eu doping by analysing Ce3+/Ce4+ ratio. Formation of various complexes involving host and doped cations and mediated by oxygen vacancies indicate the formation of F0, F+ and F2+ centres. Photoluminescence studies and their CIE chromaticity diagram underline the possible usage of these nanoparticles in Light Emitting Diodes due to production of light under 5000K. All the characterization techniques were on the same page in indicating our findings. F-centre exchange (FCE) and Bound Magnetic Polarons (BMP) mechanisms were underlined for the observed weak ferromagnetic behaviour along with M-T studies. A systematic study on the concentration of Eu-doping in CeO2 nano-particle to understand the role of oxygen vacancies and their effect on tailoring the luminescence, electronic structure and magnetic properties for variety of applications.

Impact of Cr doping on the structure, optical and magnetic properties of nanocrystalline ZnO particles

The doping of Cr ions on the structure, optical and magnetic properties of ZnO have been investigated. Zn1−xCrxO (0 ≤ x ≤ 0.05) samples were prepared by the solution combustion method. Rietveld refined x-ray diffraction results reveal that all the samples are in a hexagonal structure. Transmission electron microscopy (TEM) revealed that the particle size was 48.2 ±2.1 nm, 29.4 ± 2.2 nm and 25.3 ± 2.1 nm for the Zn1−xCrxO with x = 0, 0.03 and 0.05, respectively. The band-gap decreases slightly from 3.308 ± 0.003 eV to 3.291 ± 0.003 eV with increase of Cr content from x = 0 to 0.05, respectively. The field-dependent magnetization M(μοH) measurements of ZnO and Cr-doped ZnO exhibit room temperature ferromagnetism (RTFM). Photoluminescence and electron paramagnetic resonance exhibits negatively charged zinc vacancies (VZn−) and singly ionized oxygen vacancies (Vo+) in ZnO and these defects are responsible for RTFM. And the VZn− accountable for RTFM in Cr-doped ZnO. EPR results confirmed that Cr3+ ions replaces the Zn2+ sites in the hexagonal ZnO. The observed long-range ferromagnetism ordering is explained based on the bound magnetic polaron (BMP) mechanism. This work elucidates the origin of RTFM in synthesized ZnO and Cr-doped ZnO.

Enhancement of weak magnetism of 3C–SiC by 3d-transition metal doping

Defects seem to play a very decisive role in setting up long-range magnetic order in 3C–SiC. The feeble ferromagnetic response exhibited by SiC at room temperature, known as d0 magnetism, has been attributed to defects in the system. We present the strengthening of d0 magnetism through hole doping by 3d-transition metal (TM) substitution at the Si site. The valence state of the Mn, Co, and Ni has been found to be 2+ while Cr takes a 3+ valence state. The Mn-doped system exhibits the highest Curie temperature, saturation magnetization, and spontaneous magnetization value as compared to samples with other dopants. Detailed quantitative analysis of magnetization data and electron spin resonance spectra suggests the enhancement of the d0 magnetism due to the generation of holes caused by the TM doping at the Si site. The room temperature ferromagnetic response observed in the doped samples could be explained using modified Bound Magnetic Polaron (BMP) model.

O-vacancies induced in co-doped ZnO:(Cu,Fe) synthesized via hydrothermal method by pulsed magnetic field

Parent ZnO nanoparticles dually doped (Cu, Fe) were synthesized by the hydrothermal method in the presence of variant pulse magnetic field (PMF) at (0, 4T). X-Ray diffraction (XRD) was used to find crystal structure, Scanning Electron Microscope (SEM) revealed that the transformation of elongated spheres into spheres with increasing magnetic pulse (4T), Raman scattering peaks appeared at 437.70 and 436.63 cm−1 belonged to ZFCO-0T and ZFCO-4T indicated defects and inclusion of dopant elements in parent sample, XPS analysis showed that the decline in compressive strain enhances the contents of vacancies of oxygen (VO) for ZFCO-4T sample, and Bound Magnetic Polarons (BMPs) explained the RTFM order i.e.the sample ZFCO-4T has a higher number of polarons and they overlap excellently.

Effect of Ar:O2 ratio on magnetic properties of ZnO:Y thin films

4 at.% Y-doped ZnO (4YZO) thin film was fabricated by RF magnetron sputtering, on Si(111) substrate upon varying Ar:O2 ratios (75:25, 50:50, and 40:60). The effect of Ar:O2 ratio on morphological, structural, magnetic, and optical properties of these films have been investigated in detail. The films orientated along the (002) diffraction plane were confirmed by X-ray diffraction pattern. The defect centres in the prepared films were analysed using visible band emission from Photoluminescence spectra which showed the presence of oxygen vacancy (VO+) and zinc interstitial (Zni+). The number of donor defects (VO+ and Zni+) in the films were reduced with an increase in O2 content. The vibrating sample magnetometer results indicated that 4 at.% Y-doped ZnO thin film deposited at an Ar:O2 ratio of 75:25 exhibited ferromagnetic property with a saturation magnetization value of 2.01 emu/cm3. The observed magnetic coercivity value of 500 Oe, makes it suitable for non-volatile magnetic memory storage applications. The films prepared using an Ar:O2 ratio of 50:50 and 40:60 showed a blend of paramagnetic and weak ferromagnetic behaviour. The donor defects present in the films were responsible for the ferromagnetism in 4YZO thin films. The magnetic properties of 4YZO thin films have been explained using the bound magnetic polaron (BMP) model.

Ferromagnetism and dielectric properties in Zn0.95−xNdxTM0.05O (TM=Co, Fe) nanocrystals: Collective role of grain boundaries and oxygen vacancies

Herein, Zn0.95−xNdxTM0.05O (TM = Co, Fe) (x = 2%, 6%) nanocrystals with spherical shape, wurtzite-like hexagonal structure, and average crystallite size of 35 nm were prepared by sol-gel method, those exhibit ferromagnetic (FM) behavior at room temperature. X-ray photoelectron spectroscopy (XPS) results confirmed the successful incorporation of dopants ions such as Nd3+ and Co2+/Fe2+ into the host ZnO lattice. Size-strain analysis based on the Williamson-Hall plot was performed to evaluate the crystallite size and lattice strain. High-resolution transmission electron microscope characterization shows that the fine nanocrystals are separated by grain boundaries (GBs). The calculated specific grain boundary area (SGB) of 60 × 106m2m-3 in the co-doped samples is above the threshold value of 5.3 × 107 m2m-3. The UV–Visible analysis shows that the energy band gap decreases with increasing Nd3+ concentration. The photoluminescence spectra (PL) show a broad emission band in the visible region corresponding to the various structural defects and oxygen vacancies in the prepared nanocrystals responsible for the activation of bound magnetic polarons (BMPs). Fitting the experimental magnetization data with the BMP model based on the Langevin function shows that the density of magnetic polarons (1020cm−3) in the doped samples is equal to the typical percolation threshold. The dielectric constant and dielectric loss was observed to increase at lower frequency due to the presence of GBs and oxygen vacancies, and this behavior was explained based on Koop's phenomenological theory. Based on these results, it was demonstrated that the FM response and the dielectric properties in studied samples are strongly associated with GBs and oxygen vacancies.

A comparative study on the structural, optical, and magnetic behavior of transition metal (Co and Mn) ions substituted Cr2O3 nanoparticles

A comparative study on structural, optical, and magnetic properties of TMxCr2−xO3 (TM = Co and Mn) nanoparticles was synthesized using a simple co-precipitation method with subsequent heat treatment. The structural characterizations of as-prepared nanoparticles were performed using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. XRD pattern revealed the formation of rhombohedral Cr2O3 phase with space group R3¯c. The FTIR spectra illustrated the shifting of absorption bands of Cr2O3 nanoparticles with Co and Mn substitution due to strain induced within the crystal lattice. X-ray photoelectron spectroscopy (XPS) confirmed the presence of a mixed oxidation state of Cr and oxygen defects in the prepared nanoparticles. Furthermore, XPS spectra also demonstrated the presence of Co and Mn having +2 oxidation state. The reduction in optical band gap of host material was observed with Co and Mn substitution. The magnetic properties of as-synthesized nanoparticles were recorded using a SQUID magnetometer and temperature-dependent magnetic measurements illustrated the decrease of Neel temperature with Co and Mn substitution. It was also demonstrated that magnetization increases below about ∼ 120 K due to spin canting and spin frustration for both TM ions substituted Cr2O3 nanoparticles and is fitted using the 3D spin-wave model with Curie-Weiss law. The magnetic studies revealed the mixed paramagnetic (PM) and antiferromagnetic (AFM) order at room temperature and AFM mix with ferromagnetic (FM) order at low temperature explained using the bound magnetic polarons (BMP) model.

Enhanced room-temperature ferromagnetism in Co and Er co-implanted MgO film

The present study reports a high ferromagnetism acquired by co-implanting Co and Er ions in MgO films prepared by filtered cathode vacuum arc on Al2O3 substrates. The resulting MgO films were analyzed with respect to their structures, surface morphologies, and magnetic properties. Magnetic measurements show that substantial intrinsic ferromagnetism has been found for all the doped MgO. The ferromagnetism for the Er-doped MgO is larger than that of the Co-doped MgO and it is further strengthened by Er-Co co-doping. The X-ray photoelectron spectroscopy analysis substantiates that a large number of oxygen vacancies (Vo) are generated after the doping of Er and Co ions. The origin of the magnetism can be described by the Vo mediated bound magnetic polaron (BMP) model. The increased number of BMP with higher moments and the coupling interaction of Er and Co accounts for the strong ferromagnetism in the co-doped films. Our research provides a new idea for manufacturing high-quality diluted magnetic oxides by co-doping rare earth and transition metal ions in MgO samples.

Effect of annealing time on structural, optical and magnetic properties of TiO2 nanoparticles

The structural, optical and magnetic behavior of undoped TiO2 nanoparticles synthesized by sol-gel technique annealed at 400 °C in air for different time periods were studied. Tetragonal anatase phase was formed in all the samples as studied from X-ray diffraction, and Raman spectroscopy. HRTEM showed the particles in the nanometer range (13–16 nm) Photoluminescence and XPS show the formation of oxygen vacancies and Ti3+ions. Clear hysteresis is observed at low field, but there is a lack of saturation magnetization, suggesting the presence of paramagnetic/diamagnetic contributions. The bound magnetic polaron (BMP) is proposed to be the mechanism of ferromagnetism (FM) which is formed by overlapping of orbital of trapped electrons in F+ center with the unpaired (3d1) electron of Ti3+ions. The increase of magnetic moment with the increase in annealing time can be due to a greater number of BMPs participating in FM exchange and the same have formed due to the formation of large number of oxygen vacancies and Ti3+ ions in TiO2 nanocrystals.

Structural, optical, and magnetic properties of V-doped ZnO nanoparticles and the onset of ferromagnetic order

Pure and vanadium (V) doped ZnO nanoparticles (NPs) were successfully synthesized via a modified sol-gel method. The wurtzite crystal structure of ZnO remained intact with the vanadium doping concentrations under the present synthesis environment as confirmed by an array of advanced analytical techniques; such as Rietveld refined X-ray diffraction (XRD) pattern, as well as bond valence sum (BVS) analysis. SEM and TEM techniques were used to examine the morphology of the samples. UV-Vis-NIR spectra showed an absorption band in the UV range with reduction in the band gap from 3.23 eV to 3.20 eV when V content increases. Photoluminescence (PL) shows that doping with V causes the creation of some defects in the band gap of ZnO. The energy position of the obtained PL emission introduces a blue shift when the measurement temperature increases. The M-T curve is successfully fitted using both three dimensional (3D) spin wave model and Curie-Weiss law which attests to the mixed state existence of ferromagnetic (FM) and paramagnetic (PM) phases. The electron paramagnetic resonance (EPR) analysis supports the XRD results and provides evidence of oxygen vacancies (VO). Magnetic interaction is quantitatively studied and explained by polaronic percolation of bound magnetic polarons (BMP) produced by VO defects.

Computational insights into electronic, magnetic and optical properties of Mn(II)-doped ZnTe with and without vacancy defects

First principles calculations have been performed to investigate the optoelectronic and magnetic properties of Mn(II); ZnTe DMS with and without native defects using first principle calculations. Our results find that anti-ferromagnetic coupling dominates in the pure Mn(II)-doped ZnTe system due to super-exchange (SE) mechanism. The impact of p-type and n-type defects on magnetic coupling in Mn(II)-doped ZnTe were discussed and we found that p-type defect such as Zn vacancy defects play important role in stabilization of FM state due to the formation of bound magnetic polaron (BMP) while n-type defect such as Te vacancy defect does not affect the ground state of Mn(II); ZnTe. The magnetic coupling of Mn(II) spins in the absence and presence of Zn vacancy were explained on the basis of phenomenological band structure model. The optical absorption spectra of pure ZnTe and Mn(II); ZnTe with and without vacancies have been investigated and we found that single Mn(II)-doping in the lattice of ZnTe generates an absorption band at energy around 2.02 eV, which is assigned to intra-band d-d transitions of Mn(II) dopant. The band at energy around 0.44 eV in the absorption spectrum of Mn(II)-doped ZnTe with Zn-vacancy may be due to the acceptor states produced by Zn vacancy at Fermi level and band around 1.6 eV in the spectrum of Te vacancy defect system are related to the donor states produced by Te vacancy defect. Moreover, the correlation of magnetic coupling with intra-band d-d transition bands and fundamental bandgaps of ZnTe were also discussed and it was found that intra-band transition peak of Mn(II) ions and optical band gap of ZnTe are blue-shift in AFM coupled Mn(II) ions and are red-shift in FM coupled ions system. Our present findings highlight the worthwhile half-metallic properties of Mn(II)-doped ZnTe, which can be obtained via Zn vacancy defect engineering; this can find broad applications for the fabrication of optoelectronic and spintronic devices.

Ortho-atomic projector assisted DFT+U study of room temperature Ferro- and antiferromagnetic Mn-doped TiO2 diluted magnetic semiconductor

Using ortho-atomic Hubbard-corrected density functional theory, we present magnetic properties, ferromagnetic transition temperature T c , Neel temperature T N , electronic structure, structural formation energy, and crystal structure of anatase (Ti16−x Mn x O32 for x = 1, 2, 3, 4 and 6). According to the calculated formation energy, an oxygen-rich condition is more structurally stable than a Ti-rich situation. The geometric and lattice parameter optimization result indicates that Mn2+ exists in the system. In addition, when Mn is substituted in the system, some of the bonds that connect oxygen with the Mn ion are broken, and the bond lengths show variation; this results in defects, which cause the formation of oxygen vacancies that enhance both ferromagnetic and antiferromagnetic properties of the system. According to the structure of electronic bands and PDOS analysis, 2p and 3d hybridization lead to a decrease in the edge of the CB and an increase in the edge of the VB, narrowing the bandgap from 3.23 eV to 2.21 eV(for 4.17% of Mn), 2.13 eV (for 8.33% of Mn) and 1.69 eV (for 12.5% of Mn). Results also showed that both ferromagnetic and antiferromagnetic properties are energetically favored. Furthermore, the material exhibits antiferromagnetic with predicted T N to be 804.13 K for a small concentration of Mn dopant (for 4.17% of Mn), which is higher than room temperature. In contrast, the calculated ferromagnetic temperature (T c ) was increased from 255.42 K (for 8.33% of Mn) to 385.63 K (for 12.5% of Mn), most likely as a cause of bound magnetic polarons (BMPs) production. Result also reveals oxygen vacancy for the relaxed pristine (Ti16O31) a non-zero magnetic moment and a total magnetization of 2.00 μ B which is observed as a result of an ortho-atomic projector. The present findings suggest that TiO2 could be tailored through doping and oxygen vacancy for versatile spintronic applications, inconsistent with previous experimental results.