Existence Of Room Temperature Ferromagnetism Research Articles

SYNTHESIS, ELECTRONIC STRUCTURE, AND MAGNETIC PROPERTIES OF NANOCRYSTALLINE OXYGEN-DEFICIENT TiO2–δ(B)

Nanocrystalline TiO2 polymorphs with nonstoichiometry, i.e. having intrinsic and/or impurity structural defects, attract considerable interest due to enhanced optical and catalytic activity, improved electronic properties, and existence of room-temperature ferromagnetism. Herein, due to combination of hydrothermal technology, ion exchange process, and high-temperature vacuum annealing, a gray-colored bronze titanium dioxide with a belt-like nanostructure has been successfully shynthesized. The electron paramagnetic resonance data indicate that coloring of bronze polymorph during vacuum annealing is caused by the appearance of F-centers within a crystal lattice (electron trapped in the anionic vacancies). X-ray diffraction, electron-microscopic studies, and adsorption measurements has no reveal obvious changes in phase composition, microstructure, and texture during vacuum heat treatment (in comparison with bronze TiO2 treteated under air). Spectrophotometry experiments illustrate a "narrowing" of the TiO2(B) band gap from 3.23 to 3.04 eV and an evolution of the optical absorption in the visible and near infrared ranges due to appearance of oxygen vacancies. The study of magnetic characteristics showed that TiO2–δ(B) has a ferromagnetic ordering at room temperature, and the nature of ferromagnetism in it is clearly defective (magnetic properties are determined by the exchange interaction between electrons located in the vacancy zone). At low temperatures (4 K) the magnetic properties of oxygen-deficient bronze titanium dioxide become similar to those for quasi-stoichiometric TiO2(B) prepared through annealing under oxidizing atmosphere (air).

Synthesis of Sn1−x NixO2 nanoparticles: Observation of room temperature structural, optical and magnetic behavior

Ni-doped SnO2 (Sn1−xNixO, x = 0.03, 0.05, 0.10, 0.12, and 0.15) powder samples were synthesized in a chemical auto combustion method and followed by calcination at 850 ºC for 5 h. Initially, their structural characteristics were investigated through X-ray diffraction (XRD) which reveals that the prepared samples are nanocrystalline rutile tetragonal structure similar to host SnO2. The surface morphological image analyses were done by scanning electron microscopy (SEM) which revealed the spherical nature of grains along with the presence of voids. From Raman spectroscopy study, the presence of fundamental vibration modes of SnO2 like A1g modes at 634 cm−1, B2g modes at 773 cm−1 and Eg modes at 473 cm−1 along with the formation of defects i.e. oxygen vacancies in the sample have been confirmed. Similarly, Fourier transforms infrared spectroscopy (FTIR) shows the prominent peaks of Sn-O and Sn-O-Sn vibrations around 560 cm−1and 615 cm−1 respectively, and confirms the incorporation of Ni ions into pure SnO2 lattice by the small shifting in the positions of the peaks. Furthermore, from UV–visible spectroscopy, the decrease of bandgap with increasing Ni concentration has been noticed. The existence of room temperature ferromagnetism in Ni-doped SnO2 nanoparticles has been confirmed from the hysteresis loop of the vibrating sample magnetometer measurement (VSM) measurement. From all the above characterizations, it has been confirmed that up to x = 0.12 of Ni has been incorporated into host material SnO2.

Fabrication of vertically aligned ferromagnetic ZnO nanopillar arrays on sapphire substrates by polymer-assisted deposition

Vertically aligned ferromagnetic ZnO nanopillar arrays have been fabricated on single-crystal sapphire substrates through polymer-assisted deposition. X-ray diffraction and transmission electron microscopy analysis show that the arrays are ZnO hexagonal crystal structure along the c-axial direction. The existence of room temperature ferromagnetism of the arrays was confirmed by the superconducting quantum interference device measurements. The enhancement of ferromagnetism in ZnO nanopillar arrays is found to be correlated with the increase in Zn1− vacancies and the decrease in oxygen interstitials, which was shown by photoluminescence measurements of the samples. Our results provide a viable way to fabricate ferromagnetic vertical arrays of ZnO nanopillars.

Investigations on structural, optical and ferromagnetic properties of Ni doped ZnO nanotwins

Ni doped ZnO nanotwin rods (nanotwins) have been fabricated by hydrothermal technique. Incorporation of Ni to the ZnO host lattice induces the morphological amendment which are being optimized and are investigated through scanning electron microscope. The diverse nature of interplanar spacing and orientation at the interface of ZnO nanotwins have been investigated through transmission electron microscope. Further, the local structure of Ni in the ZnO matrix has been studied through X-ray diffraction and X-ray photoelectron spectral analysis. Here, texture coefficient of (200) and photoluminescence spectra have been considered for the determination of defect states and the grain boundary mismatch. The existence of room temperature ferromagnetic property of the ZnO nanotwins has been manifested using magnetization hysteresis behavior. The optimized Ni ion concentration of 5% in ZnO nanorods yields nanotwins forming stable defects at Zn interstitial, oxygen vacancy and oxygen antisite helps to promote the ferromagnetic interaction to accomplish higher magnetic moment. Also, the presence of uncompensated surface spins in terms of charges on the surface of the nanoparticles have been analyzed through chronocoulometric analysis. The measured magnetic moment is 132 memu/g which is about 12 times greater than that of the pristine ZnO. In addition, the existence of room temperature ferromagnetism in ZnO nanotwins has been explained through bound magnetic polaron mechanism.

Effect of doping and annealing on the electronic structure and magnetic properties of nanoscale Co and Zn co-doped SnO2: An experimental study and first-principles modeling

The influence of Co and Zn co-doping and annealing on the structure and magnetic properties of SnO2 nanoparticles were investigated in relation to electronic structure. The powder X-ray diffraction patterns revealed single phase tetragonal rutile structure for both as-synthesized and annealed Co and Zn co-doped SnO2 samples. The results of XPS and first-principles modeling emphasized that both the co-doped metals incorporated to SnO2 host lattice with an oxidation state of 2 + inducing the formation of oxygen vacancies under cation substitution of tetravalent Sn. Moreover, XPS spectra showed the O/(Sn + Co + Zn) ratio which has been strongly reduced under annealing for creating additional oxygen vacancies. Field dependent magnetization studies demonstrated the existence of room temperature ferromagnetism (RTFM) for the as-synthesized co-doped (Co, Zn: SnO2) and all the annealed samples. The origin of observed RTFM was correlated to the oxygen vacancies. The results of first-principles modeling demonstrated that the increase in the concentration of Co/Zn dopant offers not just an increase of total magnetic moment of the system but also control the magnetic interactions between magnetic moments on metal ions and oxygen atoms. The combination of experimental and theoretical techniques unveiled a correlation between observed RTFM and dopant atom interactions within a host lattice. Our findings provide new insight into the interaction of transition metals (Co/Zn) within co-doped systems and new evidence for the oxygen vacancy-mediated room temperature ferromagnetism.

Effect of phase transition on room temperature ferromagnetism in cerium doped ZnS nanorods

The effects of Ce doping on the structure, optical, oxidation, thermal and magnetic properties of ZnS:Ce nanorods synthesized by a chemical co-precipitation method were reported. The crystalline phase transformation from cubic to hexagonal structure was observed upon doping ZnS with Ce. Magnetic measurements showed the existence of room temperature ferromagnetism in Ce-doped ZnS nanorods. X-ray photoelectron spectroscopic (XPS) measurements provided evidence for Zn—S bonds and oxidation state of Ce in the near-surface region. Raman spectrum provided evidence for the presence of defects as well as hexagonal structure of 5 wt.% Ce doped ZnS nanorods. Ce substitution induced shape evolution was studied by using TEM. DRS spectra further validated the incorporation of Ce3+ ions. The present study reveals that Ce doped ZnS nanorods may find applications in spintronic devices.

Microwave assisted synthesis of Sn(1−x)CoxO2 nanoparticles: effect of impurity phase formation on structural, optical and electrochemical properties

Nanocrystalline Sn(1−x)CoxO2 (x = 0, 0.01, 0.03 and 0.05) nanoparticles have been synthesized under microwave irradiation using oxalic acid (H2C2O4) as reducing agent. X-ray diffraction studies show that the prepared nanoparticles are crystalline with tetragonal crystal system and with few additional phases, which is further supported by Raman spectroscopy. The variation in molar concentration of Co2+ ions had a significant effect on size and morphological aspects which is evident from high resolution microscopic images. The emission owing to band edge position, recombination with V o ++ centers and recombination with shallow level defects were observed from the Photoluminescence spectrum. The existence of room temperature ferromagnetism has been clearly observed using magnetization hysteresis measurements and the magnetic moments are found to decrease with the increased cobalt ions concentration. The decrease in charge carriers that arises due to the formed impurity phases and its significant effect on electron transfer capability have been discussed with the assistance of cyclic voltammetry. This study possibly will bring better insight for understanding the effects of doping concentration, formation of impurity phases and explicate the consequences of them on various properties of nanostructured materials.

Electron spin splitting in polarization-doped group-III nitrides

The Rashba spin-orbit splitting parameter has been calculated in wurtzite GaN/AlGaN heterostructures. Despite the fact that wide-bandgap semiconductors are expected to have a smaller spin-orbit coupling parameter than that in InGaAs-based III-V materials, the electron spin-split energy in GaN/AlGaN heterostructure is predicted to have the same order of magnitude, due to the strong polarization field at the interface and polarization-induced doping. This, taken together with the existence of room-temperature ferromagnetism in GaN(Mn), could make the GaN-based material system competitive in spintronic applications.

Magnetism of Co-doped titania thin films prepared by spray pyrolysis

Co-doped TiO2 (anatase) films prepared by spray pyrolysis at 500 °C are investigated for the possible existence of room temperature ferromagnetism (RTF). These films were characterized by x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy at room temperature, followed by studies of the temperature and magnetic field dependence of the magnetization by squid magnetometry. For Co doping of upto 10 at. %, no phase other than anatase could be detected, and there is no RTF in these films. Instead, the magnetic susceptibility χ fits the modified Curie–Weiss law χ=χ0+C/(T+θ), with θ≃5 K and magnitude of C consistent with Co2+ paramagnetism in Ti1−xCoxO2−x. Only for T⩽5 K, a hysteresis loop with coercivity Hc=200 Oe is observed. These observations suggest Co2+ paramagnetism, with exchange interaction becoming important for T⩽5 K in the case of 10% Co/TiO2 films.