Bound Magnetic Polaron Research Articles

Origin of ferromagnetism in Cu-doped ZnO

It is widely reported during last decade on the observation of room temperature ferromagnetism (RTFM) in doped ZnO and other transition metal oxides. However, the origin of RTFM is not understood and highly debated. While investigating the origin of RTFM, magnetic ion doped oxides should be excluded because it is not yet settled whether RTFM is intrinsic or due to the magnetic ion cluster in ZnO. Hence, it is desirable to investigate the origin of RTFM in non-magnetic ion doped ZnO and Cu-doped ZnO will be most suitable for this purpose. The important features of ferromagnetism observed in doped ZnO are (i) observation of RTFM at a doping concentration much below than the percolation threshold of wurtzite ZnO, (ii) temperature independence of magnetization and (iii) almost anhysteretic magnetization curve. We show that all these features of ferromagnetism in ZnO are due to overlapping of bound magnetic polarons (BMPs) which are created by exchange interaction between the spin of Cu2+ ion and spin of the localized hole due to zinc vacancy ({V}_{Zn}). Both the experimental and theoretical investigation show that the exchange interaction between Cu2+-Cu2+ ions mediated by {V}_{Zn} is responsible for RTFM in Cu-doped ZnO.

Bandgap Engineering and Signature of Ferromagnetism in Ti1−xMnxO2­ Diluted Magnetic Semiconductor Nanoparticles: A Valence Band Study

Diluted magnetic semiconductor Ti1−xMnxO2­ (0.0 ≤ x ≤ 0.06) nanoparticles have been synthesized by sol–gel technique. Phase purity, structural, micro‐structural, and vibrational properties of the samples have been studied by X‐ray diffraction, transmission electron microscopy (TEM), high‐resolution TEM, and Raman spectroscopy. UV–Vis and photoluminescence spectroscopy clearly indicate the tuning of bandgap and appearance of different defect states (oxygen vacancies) with Mn‐doping, respectively. Chemical states and surface stoichiometry of the samples have been probed by X‐ray photoemission spectroscopy (XPS). Shifting of binding energy of Ti2p toward lower value and appearance of Mn2+, Mn3+, and Mn4+confirm Mn doping into TiO2 and also indicate that Mn‐doping reduces the number of oxygen vacancies in the system. Valence band studies have been done by XPS and ultraviolet photoemission spectroscopy (UPS) valence band spectra. Combined result of valence band spectra and optical data reveals shortening of HOMO–LUMO gap with increasing Mn‐concentration. Room temperature ferromagnetism, originating from oxygen vacancies, has been explained on the basis of the bound magnetic polaron (BMP) model. Resistivity measurements have been conducted to examine the semiconducting behavior and to study the electrical conduction mechanism. It is revealed that the thermally activated conduction (Arrhenius) mechanism is valid in the high temperature region whereas Mott's variable‐range hopping (VRH) mechanism is applicable in low temperature region.

Hydrogen influenced the structural and optical properties of Ni-doped BiOCl nanocomposite: creation of FM properties

Bismuth oxychloride (BiOCl) nanocomposite powders doped with nickel ions were synthesized by sol–gel route using bismuth carbonate. The study focused on the determination the essential conditions required to create stable room-temperature ferromagnetic (RT-FM) properties in host BiOCl crystalline medium by means of doping with Ni2+ ions (BiOCl:Ni). This work was based on the following ideas; the presence of Ni2+ dopant impurity ions in BiOCl crystalline lattice could support the dissociation of H2 molecules during the hydrogenation process that can develop the crystalline–electronic medium (CEM) of BiOCl for carrying out the spin–spin (S.S) long-range interaction between dopant Ni2+ ions and thus create FM properties. Structural, optical, and magnetic properties of pure and Ni-doped BiOCl nanocomposite samples were systematically investigated. The structural properties were studied by X-ray diffraction (XRD) method. The optical properties were investigated by diffuse reflection spectroscopy (DRS). The study disclosed the conditions necessary to create RT-FM properties in hydrogenated BiOCl:Ni nanopowders. The physical explanations and discussions were given in the framework of bound magnetic polaron (BMP) theory. Thus, BiOCl nanocrystalline powder could be used as a potential candidate for optical applications with tailored magnetic properties.

Doping effect and oxygen defects boost room temperature ferromagnetism of Co-doped ZnO nanoparticles: experimental and theoretical studies.

Co-doped ZnO nanoparticles with different dosage concentrations were fabricated by a thermal decomposition method. The nanoparticles show a pure wurtzite structure without the formation of a secondary phase or Co clusters, in which Co ions present as Co2+ and occupy Zn2+ tetrahedral sites within the ZnO matrix. All the samples show ferromagnetic properties at room temperature with nonzero coercivity and remanence magnetization. Besides, the magnetic data is also fitted by the model of bound magnetic polarons (BMP). By increasing the Co2+ doping concentration, the saturation magnetization values of Co-doped ZnO nanoparticles increase first and then decreases, which is related to the variation tendency of oxygen defects on the surface and the number of BMPs. This phenomenon can be ascribed to the formation of defect-induced BMPs, in which ferromagnetic coupling occurs at lower Co2+ concentration and Co2+–O2−–Co2+ antiferromagnetic coupling arises at higher Co2+ concentration. Air annealing experiments further demonstrate this result, in which the saturation magnetization of Co-doped ZnO nanoparticles is reduced after annealing in Air. The doping effect and oxygen defects on the magnetic ordering of Co-doped ZnO were calculated using density functional theory. The calculation results reveal that stable long-range magnetic ordering in Co-doped ZnO nanoparticles is mainly attributed to the localized spin moments from 3d electrons of Co2+ ions. Both the experimental and theoretical studies demonstrate that the ferromagnetism in Co-doped ZnO nanoparticles is originated from the combined effects of Co doping and oxygen vacancies. These results provide an experimental and theoretical view to understand the magnetic origination and tune the magnetic properties of diluted magnetic semiconductors, which is of great significance for spintronics.

Study of the hydrogenation and re-heating of Co-doped ZnO and In2O3 Nano composites

Nanocomposite samples of zinc oxide (ZnO) and indium oxide (In2O3) doped with Co (at 1.5% molar concentration) are synthesized through conventional solid-state reaction technique. These samples are taken in a quartz tube and placed in reduction furnace for post-annealing in the presence of hydrogen for ~ 10 h at around 550 °C. The X-ray diffraction (XRD) study confirms the formation of hexagonal wurtzite structure for ZnO, Zn0.985CO0.015O and Zn0.985CO0.015O:H samples whereas cubic bixbyite structure for In2O3, (In0.985CO0.015)2O3 and (In0.985CO0.015)2O3:H samples. The magnetization data investigations depict that both undoped ZnO and In2O3 have small negative susceptibilities and exhibit diamagnetic behavior at room temperature. The doping of Co ions induces paramagnetic property in both the samples. Interestingly, the hydrogenated samples Zn0.985CO0.015O:H and (In0.985CO0.015)2O3:H exhibited a perceptible ferromagnetic behavior at 300 K whereas it overturns significantly after long air sintering. An effort has been made to fit the experiential M-H data of hydrogenated samples to the Bound Magnetic Polaron (BMP) model. MATLAB-based simulations have been performed to validate the homogeneity in particle distribution of the samples evinced through SEM micrographs. A multivariate assessment viz. hierarchical cluster analysis is also executed to corroborate and strengthen the experimental findings of magnetic properties.

The aggregation of Mn2+, its d-d transition in CdS:Mn(II) nanobelts and bound magnetic polaron formation at room temperature

Tuning the photoluminescence (PL) and magnetic properties of 1D semiconductor nanostructures is extremely important in processing light, improving the speed and storage capacity for optoelectronic and spintronic applications. Here, we have reported the 1D Cd1−xMnxS (x = 0–0.102) nanobelts (NBs) and investigated their optical and magnetic properties. These NBs were synthesized by chemical vapor deposition method. The successful incorporation of Mn ions into an individual CdS NB has been confirmed through several characterization tools: SEM-EDX analysis, significant higher angle and phonon mode shifts were observed in the XRD and Raman spectra. Room-temperature PL showed two emission peaks at the near band edge. The first peak is related to exciton magnetic polaron (EMP) and the second one appeared on the low-energy side of the band edge emission and showed very large red-shift (∼33 nm) compared to EMP, which is attributed to bound magnetic polaron (BMP). BMP emission was detected for the first time in CdS low-dimensional nanostructures. Our study showed that Mn ions tuned CdS emission more than 400 nm (from 512 to 929 nm) covering the whole visible spectral region up to the near infrared region for the first time, and significantly boosted the room-temperature ferromagnetism, which shows promise for optoelectronic and spintronic applications.

Diluted magnetic semiconducting properties of nanocrystalline Zn0.98X0.02O (X = Fe, Ga, Ni) thin films deposited by PLD technique for spintronic applications

Structural, optical and diluted magnetic semiconducting properties of pristine ZnO, Fe-doped (Zn0.98Fe0.02O), Ga-doped (Zn0.98Ga0.02O) and Ni-doped (Zn0.98Ni0.02O) ZnO thin films deposited by pulsed laser deposition technique have been studied. X-ray diffraction (XRD) studies reveal the nanocrystalline nature in single wurtzite phase of all the films. Optical bandgap of the films found to depend upon the transition/post-transition metal doping and has a maximum value (∼3.65 eV) for Zn0.98Ga0.02O film and lowest value (∼3.23 eV) for Zn0.98Ni0.02O film. SQUID-vibrating sample magnetometer studies confirm the ferromagnetic nature of all the films with well defined field dependent magnetization (M−H) curves and temperature dependent zero field cooled (ZFC) and field cooled (FC) curves. The origin of ferromagnetism in the present films has been analyzed under bound magnetic polarons (BMPs) and grain boundary specific area models. The present films are found to follow the grain boundary specific area model for the origin of ferromagnetism.

Effects of Y doping with point defects on the ferromagnetic properties of ZnO(0001)-Zn polar surface

The ferromagnetic properties of Y-doped ZnO(0001)-Zn polar surface with and without point defects, including oxygen vacancy (VO), hydrogen interstitial (Hi), and zinc vacancy (VZn), are calculated using the first-principles method. Results show that Y-doped ZnO with or without VO is nonmagnetic. Hi in Y-doped ZnO with VO can combine adjacent O atoms to form a bound magnetic polaron. Adjacent H–O bound magnetic polaron (BMPs) create an interconnected BMP cluster, which induces the ferromagnetism of ZnO. Unpaired O-2p electrons induced by VZn causes ZnO to have a long-range ferromagnetic order through the double-exchange mechanism. Ferromagnetic quenching occurs in the ZnO lattice when the ratio of Y to VZn is 2:1. Both ferromagnetic mechanism can be achieved at room temperature.

Characterization of europium oxychloride nanopowders codoped with Cu/Ni ions synthesized by solvothermal route: significant effect of post-annealing in hydrogen

Europium oxychloride nanopowders codoped with copper/nickel ions were synthesized by solvothermal route followed by a subsequent heat treatment. The present study focuses on determination the conditions required to create stable ferromagnetic (RT-FM) properties in EuOCl crystalline powder. For that purpose, nickel (Ni2+) ions were used as dopant. It was found that such doping could not create FM properties in EuOCl unless followed by hydrogenation. The presence of transition metal (TM) Cu2+ as impurity dopant ions assists to dissociate H2 molecules into H atoms. Moreover, dopant Cu2+ ions donate polarons (electrons) that develop the electronic medium (EMI) of the spin-spin (S.S) long-range interaction in the EuOCl crystalline lattice. Systematic structural, optical, and magnetic properties of pure and Cu/Ni-codoped EuOCl samples were investigated. Their variations with post-annealing treatment in different conditions (temperature and atmosphere) were also studied. The optical properties were studied by diffuse reflection spectroscopy (DRS) method. The magnetic measurements reveal that hydrogenated Cu/Ni-codoped EuOCl nanopowders have RT-FM properties with saturation magnetisation ~ 1.7 emu/g. The physical explanations and discussions were given in the framework of bound magnetic polarons (BMP) theory. Thus, EuOCl nanocrystalline powder could be used as a potential candidate for optical applications with tailored magnetic properties.

Hall effect driven by non-collinear magnetic polarons in diluted magnetic semiconductors

In this letter, we develop the theory of Hall effect driven by non-collinear magnetic textures (topological Hall effect—THE) in diluted magnetic semiconductors (DMSs). We show that a carrier spin-orbit interaction induces a chiral magnetic ordering inside a bound magnetic polaron (BMP). The inner structure of non-collinear BMP is controlled by the type of spin-orbit coupling, allowing us to create skyrmion- (Rashba) or antiskyrmion-like (Dresselhaus) configurations. The asymmetric scattering of itinerant carriers on polarons leads to the Hall response which exists in weak external magnetic fields and at low temperatures. We point out that DMS-based systems allow one to investigate experimentally the dependence of THE both on a carrier spin polarization and on a non-collinear magnetic texture shape.

Investigations on the physical properties of Mn-modified ZnO samples prepared by sol–gel route

In this report, we have synthesized successfully sol–gel derived Zn1−xMnxO (x = 0.02, 0.04 and 0.06) nanoparticles to see the effect of Mn doping on structural, optical and magnetic properties of ZnO. The phase purity and structural analysis of all the samples have been made by the X-ray diffraction (XRD) technique with Rietveld refinement using the FullProf software. This study clearly revealed that Mn-doped ZnO nanoparticles exhibit hexagonal wurtzite structure with P63mc symmetry. Lattice parameters found to be increased with Mn doping, this shows that Mn2+ is successfully substituted on Zn2+ sites. The morphology of the nanoparticles was examined by FE-SEM. UV–Vis, FTIR, PL and VSM techniques have been used to see the optical and magnetic response of all the samples. UV–Vis spectra clearly indicate the sharp increment in the band gap energy with Mn doping up to 3.22 eV might be due to the Burstein–Moss effect. FT-IR studies have been utilized to find out the different phonon modes present in the prepared samples. Photoluminescence study revealed a blue shift of the near band emission (NBE) and an increase in the intrinsic defects (viz. VO and OZn) density with increasing Mn concentration up to a certain extent of doping (6%). Magnetic measurement of the Mn doped ZnO samples shows bound magnetic polaron (BMP) induced room temperature ferromagnetism (RTFM) behavior, however, there is suppression of ferromagnetic behavior due to the existence of antiferromagnetic ordering also present in the samples, supported by the Curie–Weiss Law.

Room temperature magnetic ordering and analysis by bound magnetic polaron model of Yb3+ doped nanocrystalline zinc oxide (Zn0.98Yb0.02O)

Nanocrystalline ZnO and Yb-doped ZnO (Zn0.98Yb0.02O) are prepared by simple co-precipitation method. The pure crystallographic phases of ZnO and Zn0.98Yb0.02O are confirmed by Rietveld analyses of XRD patterns. Raman and photoluminescence spectra recorded at room temperature (RT) indicates that lattice defects and oxygen vacancy in Zn0.98Yb0.02O is more than that of ZnO. Magnetic measurements of ZnO and Zn0.98Yb0.02O are carried out from 300 to 5 K. Ferromagnetic ordering is observed in Zn0.98Yb0.02O at RT and the strength of the ordering increases with the lowering of temperature. Ferromagnetism of Zn0.98Yb0.02O is attributed to the oxygen vacancy introduced by annealing in vacuum. Magnetization vs. temperature curve is well fitted by the combination of 3D spin wave and Curie-Weiss law, which suggest simultaneous presence of para- and ferro-magnetic phases in Zn0.98Yb0.02O. Ferromagnetic ordering is successfully explained by vacancy mediated bound-magnetic polaron model.

Structural, dielectric and magnetic properties of particulate composites of relaxor (BaTi0.85Sn0.15O3) and ferrite (NiFe2O4) synthesized by gel-combustion method

Particulate composites of (1-x) BaTi0.85Sn0.15O3 – x NiFe2O4 (with x = 5, 10,15 and 20 wt%) were synthesized using the solid-state reaction method by sintering at 1350 °C for 4 h. Formation of the diphase composites was confirmed by X-ray diffraction (XRD) and Fourier Transform Infra-red (FTIR) techniques. Temperature (RT-200 °C) and frequency (20 Hz- 1 MHz) dependent of AC conductivity, dielectric constant and dissipation have been studied. The dielectric constant exhibits strong frequency dispersion in the range 20 Hz-1 kHz which is attributed to Maxwell-Wagner interfacial polarization occurring at grain-grain-boundaries interface/interface of grains of BTS-NF. The M-H curve of all the composites exhibited a hysteresis loops typical charcateistic of a ferromagnetic material. The ferromagnetic ordering in the composites on account of NiFe2O4 as a constituent is explained using bound magnetic polarons (BMPs) model. The experimental magnetic data have been fitted to BMP model. Value of Ms is smaller, whereas of Hc and Mr are higher of the composites compared to value for NiFe2O4. The temperature at which divergence in the M vs. T plot in ZFC and FC starts is higher for the composites than for NiFe2O4.

Synergistic effect of La and Co co-doping on room-temperature ferromagnetism enhancement of TiO2 nanoparticles

The synergistic influence of lanthanum and cobalt co-doping on room temperature ferromagnetism (RTFM) of TiO2 system is investigated. A series of Ti0.97−xCo0.03LaxO2 nanoparticles were prepared and their structures and properties were systematically studied with X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy, UV–vis spectrophotometer, Raman spectra and magnetic measurement techniques, respectively. Detailed experimental characterizations indicate that the as-prepared La and Co co-doped samples exhibit single anatase phase, and all the samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which were dramatically enhanced with La and Co co-doping, and the maximum saturation magnetization (Ms) reaches 1.38emu/g at the La content of 6mol%. It is speculated that oxygen vacancies modulated by ionic La play an important role in the enhanced RTFM, which can be attributed to the bound magnetic polarons (BMPs) formed via ferromagnetic coupling between two neighboring Co2+ ions mediated by oxygen vacancy (F+ center). Our results present an alternative method to obtain high performance RTFM.

Spin-induced magnetic anisotropy in novel Co-doped GaN nanoneedles and their related photoluminescence

We demonstrate the formation and excitation of exciton magnetic polarons (EMPs) and bound magnetic polarons (BMPs) in novel Co-doped GaN.

The Electronic Structures and Ferromagnetism of Cu-Doped ZnO: the First-Principle Calculation Study

We have carried out first-principle spin-polarized calculations to investigate the electronic structures and the magnetic properties of Cu-doped wurtzite ZnO. It was found that the CuZn substitution can introduce 1 μ B local magnetic moment into the doped system, whereas the ferromagnetic coupling between CuZns is so weak that the system cannot keep ferromagnetism at higher temperature. In Zn vacancy (VZn) and CuZn coexisted system, Cu is in the Cu 2+/Cu 3+ mixed valence under the influence of acceptor defect VZn. Cu-3d electrons couple with O(around VZn)-sp electrons and then form Cu2+ −VZn −Cu2+ or Cu2+ −VZn−−Cu3+ bound magnetic polaron (BMP) in the range of 1.27 ∼ 1.90 nm. The overlapping between BMPs is the main ferromagnetism origin of ZnO/Cu. According to our analysis, we thought that ZnO/Cu system will display stronger ferromagnetism when the Cu concentration attains 3% in atomic percentage. The calculated results and analysis will shed light on the origin of ferromagnetism in ZnO/Cu.

Defect mediated ferromagnetism in cluster free Zn1−xNixO nanopowders prepared by combustion method

Structural, optical, vibrational and magnetic properties of sol–gel auto-combustion assisted Zn1−xNixO (x=0–3mol%) nanoparticles have been characterized. The X-ray diffraction (XRD) studies show hexagonal wurtzite phase formation upto 1mol% doping, confirming the substitution of Ni2+ ions. Photoluminescence and Raman spectroscopy confirm existence of zinc interstitials (Zni) and oxygen vacancies (Vo). Both extrinsic Ni doping and intrinsic oxygen vacancy contribute to the observed ferromagnetism. Magnetization values derived from bound magnetic polaron (BMP) model match the observed changes in magnetization. Temperature dependent magnetization characteristics of 1mol% Ni doped ZnO exhibits distinct zero field cooled (ZFC) and field cooled (FC) curves.

Optical and magnetic properties of Co-doped ZnO synthesized by magnetic assisted hydrothermal method

Cobalt(Co) doped ZnO nanoparticles were synthesized by a hydrothermal method with a magnetic field pretreatment(in the hydrolysis reaction). The pretreatment time(t) was set as 0h(no pretreatment), 1h, 2h and 4h. The crystallite structure, the lattice constant have been estimated by X-ray diffraction (XRD) with Rietveld refinement, which shows slight lattice expansion due to Co doping. Scanning electron microscopy (SEM) reveals flower-like microstructure and transmission electron microscopy (TEM) indicates the single crystalline in nature. The defect, the optical and magnetic properties of the four obtained samples were systematically investigated by Raman spectroscopy, photoluminescence (PL) spectra and vibrating sample magnetometer (VSM) respectively. The results show that the pretreatment time of magnetic field has a significant influence on optical and magnetic properties of Co-doped ZnO, and the effective time of magnetic field in the hydrolysis reaction on the magnetic property is between 1h and 2h. When 1h ≤ t ≤ 2h, more Co atoms are incorporated into ZnO and the saturation magnetization (MS) increases by nearly three times, comparing with the sample without pretreatment. Samples of different pretreatment time show changes in the emission band of PL spectra and defect related bands of Raman spectra. The characterization of Raman scattering combined with the analysis of PL spectra suggests that the original of room temperature ferromagnetism(RTFM) may arise from the Co2+–VO+–Co2+ bound magnetic polaron(BMP).

Oxygen vacancy induced electrical conduction and room temperature ferromagnetism in system BaSn1−xNixO3 (0 ⩽ x ⩽ 0.20)

Powder and ceramics of few compositions of BaSn1−xNixO3 (with x = 0, 0.05, 0.10 and 0.20) have been prepared bysolid-state reaction route. Synthesized samples have been characterized using XRD, FTIR, SEM and XPS techniques. XPS studies have shown that all samples contain a significant number of oxygen vacancies (VO), Ba is present in the Ba2+ state, Sn is present in both Sn2+ and Sn4+ states and Ni exists mainly in the Ni2+ state. The AC conductivity spectra of all compositions followed universal Johnscher’s power law and the time-temperature superposition principle (TTSP). Above 300 °C conduction in these samples is governed by long-range diffusion of doubly ionized oxygen vacancies (). M-H loop at room temperature shows that undoped BaSnO3 is ferromagnetic with saturation magnetization (Ms) ~ 5.46 × 10−3 emu g−1 and coercive field (Hc) ~ 540 Oe, due to spins of electrons trapped in oxygen vacancies (F-center) and direct interaction between F-centers. Ni-doped samples exhibit both ferromagnetic and paramagnetic behavior and paramagnetic contribution increases with doping concentration. The room temperature magnetic parameters have been extracted by fitting the experimental data to the bound magnetic polaron (BMP) model. The saturation magnetization increases whereas coercive field decrease with increasing concentration of dopant (Ni).

On exceeding the solubility limit of Cr+3 dopants in SnO2 nanoparticles based dilute magnetic semiconductors

The paper investigates the magnetic behavior of chromium doped SnO2 Dilute Magnetic Semiconductor (DMS) nanoparticles, through structural, spectroscopic, and magnetic studies. A non-equilibrium solution combustion method is adopted to synthesize 0–5 at. % Cr doped SnO2 nanoparticles. The detailed spectroscopic studies on the system using micro-Raman spectroscopy, x-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy along with the structural analysis confirm the presence of Cr in 3+ oxidation state, which substitutes at Sn4+ site in SnO6 octahedra of the rutile structure. This doping is found to enhance the defects in the system, i.e., oxygen vacancies. All the synthesized SnO2 nanoparticles (with or without dopants) are found to exhibit Room Temperature Ferromagnetism (RTFM). This occurrence of RTFM is attributed to the magnetic exchange interaction through F-centers of oxygen vacancies as well as dopant magnetic impurities and explained through the Bound Magnetic Polaron (BMP) model of DMS systems. Nonetheless, as the doping of Cr is further increased beyond 2%, the solubility limit is achieved. This antiferromagnetic exchange interaction from interstitial Cr dopants dominates over the BMP mechanism and, hence, leads to the decrease in the net magnetic moment drastically.