Carrier-mediated Exchange Interaction Research Articles

Finding simplicity: unsupervised discovery of features, patterns, and order parameters via shift-invariant variational autoencoders * *Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide

Recent advances in scanning tunneling and transmission electron microscopies (STM and STEM) have allowed routine generation of large volumes of imaging data containing information on the structure and functionality of materials. The experimental data sets contain signatures of long-range phenomena such as physical order parameter fields, polarization, and strain gradients in STEM, or standing electronic waves and carrier-mediated exchange interactions in STM, all superimposed onto scanning system distortions and gradual changes of contrast due to drift and/or mis-tilt effects. Correspondingly, while the human eye can readily identify certain patterns in the images such as lattice periodicities, repeating structural elements, or microstructures, their automatic extraction and classification are highly non-trivial and universal pathways to accomplish such analyses are absent. We pose that the most distinctive elements of the patterns observed in STM and (S)TEM images are similarity and (almost-) periodicity, behaviors stemming directly from the parsimony of elementary atomic structures, superimposed on the gradual changes reflective of order parameter distributions. However, the discovery of these elements via global Fourier methods is non-trivial due to variability and lack of ideal discrete translation symmetry. To address this problem, we explore the shift-invariant variational autoencoders (shift-VAEs) that allow disentangling characteristic repeating features in the images, their variations, and shifts that inevitably occur when randomly sampling the image space. Shift-VAEs balance the uncertainty in the position of the object of interest with the uncertainty in shape reconstruction. This approach is illustrated for model 1D data, and further extended to synthetic and experimental STM and STEM 2D data. We further introduce an approach for training shift-VAEs that allows finding the latent variables that comport to known physical behavior. In this specific case, the condition is that the latent variable maps should be smooth on the length scale of the atomic lattice (as expected for physical order parameters), but other conditions can be imposed. The opportunities and limitations of the shift VAE analysis for pattern discovery are elucidated.

Correlation between crystal defects and room temperature ferromagnetism of hydrothermally grown Eu substituted ZnO nanorods

The hydrothermal method has been used to grow zinc oxide nanorods on a glass substrate with different Europium (Eu) concentrations. The Eu doping effect on the structural, optical, and magnetic properties of the samples was systematically investigated. The undoped sample exhibited diamagnetic behaviour, while soft room temperature ferromagnetism was observed in all the Eu substituted samples. With regard to the hysteresis loops, the highest coercivity, saturated magnetic moment, and retentive magnetic moment values were observed to be 41.64 Oe, 18.86 × 10−6 emu, and 2.17 × 10−6 emu, respectively, for ZnO:Eu 7%. According to the relationships between the magnetic properties, Eu doping levels, and defects concentrations, it can be concluded that the room temperature ferromagnetism observed in the samples is caused by crystal point defects including oxygen vacancies and zinc interstitials, rather than carrier-mediated exchange interactions, which is also consistent with the bound magnetic polarons theory.

Role of H-bond along with oxygen and zinc vacancies in the enhancement of ferromagnetic behavior of ZnO films: An experimental and first principle-based study

Magnetic properties of H2-ambience annealed pure ZnO films have been reported. ZnO films prepared by using DC magnetron sputtering show room temperature ferromagnetism (RT-FM) after annealing in hydrogen atmosphere at two different temperatures (350 °C and 400 °C). Enhancement in O-H concentration has been observed by analyzing core-level spectra recorded using x-ray photoelectron spectroscopy (XPS). The valence band spectra suggest the enhancement in carrier concentration after annealing in H2-ambience. Enhancement in the vacancy type defects have been observed by positron annihilation spectroscopy and tailoring of defects have been observed by photoluminescence spectra. Electrical transport results also indicate rapid decrease in activation energies corresponding to thermally activated band and nearest neighbor hopping (NNH) conduction, i.e. increase in carrier concentration after annealing. Increase in saturation magnetization and Zn vacancy (evident from M-H and PL data) along with the O-H concentration, after annealing in H2-ambience, suggest that Zn vacancies along with O-H concentration play a crucial role behind the ferromagnetic nature of the films. Surface morphology as examined by atomic force microscopy (AFM) shows that there is no significant variation of grain size and surface roughness of the films after annealing. The experimental results are further verified theoretically by performing the density functional theory calculations. Based on both experimental results and theoretical model, it has been explained that the enhancement of magnetic moment of H2-ambience annealed ZnO is because of the increased carrier mediated exchange interaction among the localized magnetic moment at VZn sites.

Effect of rare earth orthoferrite GdFeO3 on the band gap modulation and magnetic switching in Bi0.5Na0.5TiO3 ceramics

ABSTRACT This manuscript illustrates the structural, dielectric, optical and magnetic behaviour of [((Bi0.5Na0.5)1-xGdx)(Ti1-xFex)O3] O3 [BNTGF], x = 0.0,0.1, 0.2, 0.3, 0.4 and 0.5 prepared by mixed-oxide technique. The structural and phase identification is confirmed by X-ray diffraction, Scanning electron microscopy, Fourier transform infrared Spectroscopy and Raman analysis. The dielectric constants of the GdFeO3 modified ceramics are less than the reported values of the parent materials. Inverse variation of ferroelectric properties such as remnant polarization (Pr ) and coercive field (Ec) with GdFeO3 doping is clear from room temperature hysteresis loops. Appearance of weak ferromagnetism in the samples is established from the room temperature magnetic hysteresis loops due to the carrier-mediated exchange interactions with smaller values of magnetic parameters. The energy gap of the materials obtained from UV-vis spectra shows a drastic decrease compared to that of pure Bi0.5Na0.5TiO3.

Geçiş Metali Katkılı ZnO’in Ferromanyetizma ve Büyüme Korelasyonunun İncelenmesi

In this study, a series of 3d transition metal (TM) (Co, Ni and Fe) doped ZnO particles synthesized by simple co-precipitation method. Structural, morphological and magnetic properties were investigated to determine correlation between growth process and ferromagnetism of metal doped ZnO samples. All samples had ZnO hexzagonal würtzite structure and slightly shift of ZnO indexed peaks was observed by TM incorporation in ZnO. Although ZnO samples had granular forms, agglomerative forms had emerged in TM:ZnO samples due to the fast growth process. The pure ZnO sample transformed paramagnetic to ferromagnetic by TM incorporation due to carrier mediated exchange interaction between TM ions. The results demonstated that doping with Fe3+ ions in ZnO structure had enhanced ferromagnetism although fast growth and high particle agglomeration.

Oxygen vacancy enhanced room temperature ferromagnetism in Ar+ ion irradiated WO3 films

Room-temperature ferromagnetism in WO3 films was enhanced by 130 keV Ar+ ion irradiation. The X-ray diffraction (XRD) and Raman measurements not only confirmed the monoclinic phase of the irradiated WO3 films, but also showed that oxygen vacancy (VO) defects were formed. The analysis of photoluminescence spectra strongly reconfirmed the presence of oxygen vacancy. X-ray photoelectron spectroscopy (XPS) measurements revealed that the contents of VO and induced W5+ ions increase with increasing irradiation fluence and rich W5+-VO defect complexes in the irradiated WO3 films were formed. Further, the magnetic measurements exhibited a 2-fold enhancement in the saturation magnetization at the largest fluence of 3 × 1016 ions/cm2. At lower irradiation fluence, a bound magnetic polaron model was proposed to reveal the ferromagnetic exchange coupling resulting from overlapping of VO+ and VO++ defect states, and 5d1 states of W5+. At high irradiation fluence, the carrier concentration reaches 1.02 × 1020/cm3 and carrier-mediated exchange interactions result in the film's ferromagnetism.

Correlation between structure, dielectric and multiferroic properties of lead free Ni modified BaTiO3 solid solution

Present study highlights the influence of Ni doping on the structural, optical, dielectric, ferroelectric and magnetic properties of nanocrystalline BaTi1-xNixO3 (0 ≤ x ≤ 0.06) ceramics synthesized by sol-gel auto combustion process. Phase identification and crystal structure are examined through Rietveld refinement analysis that ensures mono-phase nature with tetragonal crystal structure in P4mm space group. The observed variation in the structural parameters viz. lattice constants, unit cell volume, bond lengths (Ti-O) and bond angles (Ti-O-Ti) are the direct evidence of distortion produce in the unit cell under the effect of Ni doping. The FTIR studies confirm perovskite structure and presence of stretching/bending vibrations of the various bands present in the samples. The optical properties divulge a minor alteration in optical bandgap under the influence of Ni content. Dielectric studies reveal higher value of the dielectric constant for pristine sample in the low frequency region, but its value decreases on Ni doping. The dielectric response, analyzed through UDR model, exhibits deviation from linear behavior at higher frequencies. Ferroelectric measurements demonstrate that the pristine sample has higher values of remnant polarization (Pr) and maximum polarization (Pm) which decrease linearly with the increase in Ni doping. Magnetic hysteresis loops at room temperature establish a weak ferromagnetic nature of the samples that arises due to the carrier-mediated exchange interactions with smaller values of magnetic parameters. These investigations ensure that the ferromagnetism can be induced in BaTiO3 by appropriate doping of Ni ions, which may find their potential use in the field of multiferroics.

Temperature dependence of magnetic anisotropy in heavily Fe-doped ferromagnetic semiconductor (Ga,Fe)Sb

We study the temperature dependence of magnetic anisotropy (MA) of ferromagnetic semiconductor (Ga0.7,Fe0.3)Sb thin films with thicknesses of 20 nm and 40 nm using ferromagnetic resonance. With decreasing temperature from 300 to 10 K, the easy magnetization axis of the 40-nm-thick film changes from in-plane to perpendicular at 250 K, while that of the 20-nm-thick film lies in the film plane at all measurement temperatures (10–300 K). In the 40-nm-thick film, the uniaxial perpendicular anisotropy (K2⊥) significantly increases with decreasing temperature and surpasses the thin-film shape anisotropy, leading to perpendicular magnetic anisotropy (PMA). We present a possible scenario that the increase in K2⊥ originates from the formation of the Fe-rich nanocolumns with the perpendicular shape anisotropy, which induces the PMA in the whole film at low temperatures due to carrier-mediated exchange interactions between Fe spin in the Fe-rich nanocolumns and Fe spin in the (Ga,Fe)Sb matrix. Our findings on the temperature dependence of MA of heavily Fe-doped (Ga,Fe)Sb provide insights into the mechanism of high-Curie-temperature ferromagnetism.

Variations of energy band gap and magnetic properties upon quantum confinement effects on the Cr doped ZnO nanoparticles

In this paper, Cr doped ZnO nanoparticles with different at % ratio (0, 2, 4, 6, 8, 10) (NPs) were synthesized with a precipitation method in ethanol. The synthesized nanoparticles have been analyzed using thermogravimetric analysis (TGA), x-ray diffraction (XRD), Energy dispersive x-ray spectroscopy (EDXS), transmission electron microscope (TEM), UV–vis spectrometer and Vibrating sample magnetometer (VSM) measurements. The XRD studies showed that the phase singularity of ZnO wurtizte hexagonal type structure. The surface morphology investigations by TEM confirmed the reduction of the particle size with increasing Cr content estimated from Williamson-Hall analysis of the diffraction peaks. The analysis of the UV–vis spectra of Cr doped NPs reveals that the wavelength of the absorption maxima λmax blue shifted to lower wavelength with increases the Cr content indicating the decrease in the particle size and recommends the increasing of energy gap with Cr content into ZnO semiconducting NPs. The observed increase of the energy band gap of Cr-doped ZnO nanoparticles caused by the decrease in the particle size with Cr concentration is attributed to the quantum confinement effect. The magnetization measurements showed room temperature ferromagnetism for all synthesized nanoparticles which can be attributed to the carrier-mediated exchange interaction improved by intrinsic defects.

Onset of multiferroicity in nickel and lithium co-substituted barium titanate ceramics

The structural, magnetic and ferroelectric properties of nickel and lithium co-substituted barium titanate were investigated in this work. Ba(1−x)LixNix/2TiO3 (x = 0, 0.02, 0.04 and 0.08) ceramics were synthesized via solid-state reaction with the assistance of microwave heating of the starting materials. The tetragonal structure has been observed in all samples, and it is confirmed by the Rietveld refinement study. The morphological study has been carried out by FE-SEM. Electron spin resonance (ESR) has been used to study the electron interaction and to verify the magnetism behavior of present samples. No resonance signal was observed in pure BaTiO3 samples. However, the resonance signal has appeared in the co-substituted samples. The result shows that the electron interactions are strongly affected by Ni2+ and Li+ concentrations. M-H loop was traced using VSM at room temperature. The results confirm that the sample with x = 0 shows an anti-ferromagnetic response. However, a ferromagnetic hysteresis loop arises with co-substitution. The emergence of M-H loops confirms the appearance of magnetic properties in Ni2+ and Li+ co-substituted BaTiO3 ceramics. The origin of magnetic behavior could be due to the carrier-mediated exchange interactions. Room temperature P-E hysteresis loop has been investigated at an applied electric field of 35 kV/cm and 33 Hz frequency. Measurements of room temperature ferroelectric and magnetic hysteresis loops indicate that the Ni2+ and Li+ co-substituted BaTiO3 ceramics show ferroelectricity and ferromagnetism simultaneously.

Gate-Tunable Spin Exchange Interactions and Inversion of Magnetoresistance in Single Ferromagnetic ZnO Nanowires.

Electrical control of ferromagnetism in semiconductor nanostructures offers the promise of nonvolatile functionality in future semiconductor spintronics. Here, we demonstrate a dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). Ferromagnetism in our ZnO NWs arose from oxygen vacancies, which constitute deep levels hosting unpaired electron spins. The magnetic transition temperature of the studied ZnO NWs was estimated to be well above room temperature. The in situ UV confocal photoluminescence (PL) study confirmed oxygen vacancy mediated ferromagnetism in the studied ZnO NW FET devices. Both the estimated carrier concentration and temperature-dependent conductivity reveal the studied ZnO NWs are at the crossover of the metal-insulator transition. In particular, gate-induced modulation of the carrier concentration in the ZnO NW FET significantly alters carrier-mediated exchange interactions, which causes even inversion of magnetoresistance (MR) from negative to positive values. Upon sweeping the gate bias from -40 to +50 V, the MRs estimated at 2 K and 2 T were changed from -11.3% to +4.1%. Detailed analysis on the gate-dependent MR behavior clearly showed enhanced spin splitting energy with increasing carrier concentration. Gate-voltage-dependent PL spectra of an individual NW device confirmed the localization of oxygen vacancy-induced spins, indicating that gate-tunable indirect exchange coupling between localized magnetic moments played an important role in the remarkable change of the MR.

Gate-tunable indirect exchange interaction in spin-orbit–coupled mesoscopic rings

We study the carrier-mediated exchange interaction, the so-called Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, between two magnetic impurity moments embedded in a semiconductor mesoscopic ring. We treat the ring in the presence of an Aharonov-Bohm–type magnetic flux and the Rashba and Dresselhaus spin-orbit interactions (RSOI and DSOI). Energy eigenvalues of the system are obtained within a tight-binding framework and the strength of the indirect exchange interaction vs. RSOI strengths are plotted for different values of DSOI strength. The results show that the type of the impurity magnetic order, ferromagnetic (F) or antiferromagnetic (AF), depends on the RSOI and DSOI strengths. This leads to a full electrical control on the magnetic alignment of the system through, e.g., an external gate voltage.

Ferromagnetism in Ti-doped ZnO thin films

We report our study on the origin of ferromagnetism in Ti-doped ZnO. A series of Ti doped ZnO films with increasing concentration of Ti dopant were grown and characterized in terms of structural, electrical, and magnetic properties. We found that Ti has a low solubility in the ZnO wurtzite structure. This favors stabilization of a large number of Zn vacancies, and theoretical calculations have shown that they can carry substantial magnetic moment. A carrier mediated exchange interaction between Zn vacancies is at the origin of the surprisingly high magnetic moment we measure in this compound.

Magnetic and ferroelectric properties of Ni doped BaTiO3

Our paper reports structural, magnetic and ferroelectric properties of Ni doped barium titanate. Ceramics samples of BaTi1−xNixO3 (here, x=0, 0.05, 0.1, and 0.15) were prepared through solid state sintering technique. It is revealed from structural studies that the phase transformation from tetragonal to hexagonal phase is strongly influenced by Ni doping concentration. Magnetism is induced in doped composition and the value of magnetism is dependent upon Ni ion concentration. This enhanced magnetic effect is likely to originate from carrier-mediated exchange interactions. Ferroelectricity was observed in all samples at room temperature, but it was decreased by Ni doping.

Effect of Ni-doping concentration on structural, optical and magnetic properties of CdSe nanorods

Cd1−xNixSe (x=0.00, 0.05, 0.10 and 0.15) nanorods have been synthesized using solvothermal route. X-ray diffraction (XRD) studies show that the nanorods possess wurtzite phase having hexagonal structure. Transmission electron microscopy (TEM) reveals the formation of nanorods having lengths 150–200nm and diameters 10–15nm. This has been further confirmed by high-resolution transmission electron microscopy (HRTEM) wherein lattice fringes corresponding to CdSe have been well observed. Reflectance measurements illustrate the red-shift in band gap with increase of Ni-doping concentration. Raman spectra display the shifting of longitudinal optical (LO) phonon modes, and modes activation due to doping in CdSe nanorods. Magnetic hysteresis (M–H) loops at room temperature reveal the ferromagnetism in nanorods. The saturation magnetization values have been observed to increase upto 10% due to carrier mediated exchange interactions, whereas a decrease has been observed for 15% Ni-doping concentration due to super-exchange interactions.

Enhancement of room temperature ferromagnetism in Cd1−xNixSe nanoparticles

Cd1−xNixSe (x = 0.0, 0.02, 0.05 and 0.1) nanoparticles have been synthesized by chemical route. X-ray diffraction analysis shows crystalline nature of synthesized nanoparticles possessing wurtzite phase having hexagonal structure. Transmission electron microscopy depicts spherical morphology and uniform particle size distribution of pure and Ni-doped CdSe nanoparticles. The blue-shift in band gap has been observed with Ni-doping concentration. Photoluminescence study shows the presence of intrinsic defects (VCd–VSe) in the synthesized nanoparticles. Electron spin resonance (ESR) analysis reveals the long range ferromagnetic ordering in pure and doped nanoparticles. ESR study also indicates that Ni ions exist in +2 oxidation state in host nanoparticles. The magnetic hysteresis (M-H) loops display ferromagnetism at room temperature in pure and Ni-doped CdSe nanoparticles. The increase of ferromagnetic behavior has been observed with Ni-doping concentration. M-H analyses indicate that defects and carrier mediated exchange interactions are responsible for ferromagnetic ordering, in the present study.

ChemInform Abstract: Electron Carrier‐mediated Room Temperature Ferromagnetism in Anatase (Ti,Co)O2

AbstractReview: 64 refs.

Growth and characterization of nonpolar, heavily Mn-substituted ZnO films

Eight percent of Mn was successfully diluted into nonpolar ZnO films deposited by pulsed laser deposition on single crystal (100) SrTiO3 substrates. X-ray diffraction patterns and energy-dispersive X-ray spectroscopy confirmed high crystallinity of the films and excluded unintentional magnetic doping. A unique surface domain structure was observed by scanning electron microscope and atomic force microscope, which might play a vital role to strain release induced by lattice mismatch between nonpolar (11–20) ZnO film and (100) SrTiO3 substrate. In addition, the films showed strong ferromagnetism with a large coercivity HC ∼ 180 Oe at room temperature. The large magnetic moment is ascribed to carrier-mediated exchange interaction between the Mn ions, where the majority of the carriers are oxygen vacancies.

Microbial Treatment of Lateritic Ni-ore for Iron Beneficiation and Their Characterization

Abstrac t Th is study aims at studying effect of anaerobic dissimilatory iron (III) reducing bacterial consortium on different phases of iron present in lateritic n ickel ore. Such conversion in lateritic nickel ore are helpful in better recovery of sorbed metal values like Ni and Co by subsequent bioleaching or acid leaching. Here properties of thermally and microbially reduced lateritic nickel ore are co mpared vis-a-v is orig inal ore. An anaerobic d issimilatory iron (III) reducing bacterial consortium capable of using glucose as carbon source and laterit ic n ickel ore as terminal electron acceptor was used for microbial reduction of ore under anaerobic condition. M icrobial reduction changes the initial light bro wn colour of the lateritic n ickel ore to dark bro wn. The change in colour is due to the conversion of goethite to magnetite, wh ich is confirmed fro m the XRD pattern. The FTIR spectra and the UV-Visib le spectra support the presence both goethite and hematite. The study shows that changes in phases brought about by microb ial treat ment are d ifferent than those by thermal treat ment. The carrier mediated exchange interaction between Fe 2+ and Fe 3+ ions in lateritic n ickel o re samp le treated with IRB consortium is responsible for higher ferro magnetic ordering. The thermal reduction of the same sample showed lowering of ferro magnetic ordering due to the decreasing percentage of Fe 2+ and Fe 3+ ions.

ELECTRON CARRIER-MEDIATED ROOM TEMPERATURE FERROMAGNETISM IN ANATASE (Ti,Co)O2

Since the discovery of room temperature ferromagnetism in ( Ti,Co ) O2 , the mechanism has been under discussion for a decade. Particularly, the central concern has been whether or not the ferromagnetic exchange interaction is mediated by charge carriers like ( Ga,Mn ) As . Recent two studies on the control of ferromagnetism in anatase ( Ti,Co ) O2 at room temperature via electric field effect [Y. Yamada et al., Science332, 1065 (2011)] and chemical doping [Y. Yamada et al., Appl. Phys. Lett.99, 242502 (2011)] indicate a principal role of electrons in the carrier-mediated exchange interaction. In this article, the authors review fundamental properties of anatase ( Ti,Co ) O2 and discuss the carrier mediated ferromagnetism.