Room Temperature Ferromagnetism In Co Research Articles

Room temperature ferromagnetism in Co and Ni co-doped ZnO particles: validation through density functional theory

Room temperature ferromagnetism in Co and Ni co-doped ZnO particles: validation through density functional theory

Effect of cobalt doping on the structural, optical and magnetic properties of sol-gel derived ZnS nanocrystalline thin films and ab initio studies

The effect of Co doping on the structural, optical and magnetic properties of sol-gel derived ZnS films was investigated for their dilute magnetic semiconducting behaviour. X-ray diffraction pattern shows nanocrystalline nature of the films. Raman spectroscopy was used to study the vibrational properties of the films and also to identify any kind of secondary phases. The surface morphology and roughness of the films was analysed through scanning electron microscopy and atomic force microscopy. The chemical binding state of Co in ZnS lattice was confirmed from the X-ray photoelectron spectroscopy. The optical band gap is found to be decreasing with the incorporation of Co in ZnS. The red emission ~1.7 eV ensures tetrahedral occupation of Co atoms in ZnS lattice. The observed room temperature ferromagnetism in Co:ZnS films is attributed to carrier induced magnetization as well as to the intrinsic defects. The origin of magnetism in Co:ZnS system was studied in terms of the spin-polarized electronic density of states obtained through the density functional theory.

Effect of cobalt doping on ZnO thin films deposited by sol-gel method

This paper reports the effect of cobalt doping on ZnO thin films. Undoped and doped films were deposited on corning glass substrates by sol-gel method with different concentration of cobalt (0–10%). X-ray diffraction patterns show the polycrystalline nature of the films with the preferred orientation along c-axis. No other cobalt metal cluster and impurity phases have been observed with Co doping up to 5%. Raman spectra confirms the substitution of Co ion in place of Zn ion, however the films with Co concentration above 5% show peaks at 490cm−1, 525cm−1 and 715cm−1 which can be attributed to the formation of spinel ZnCo2O4 structure. Surface morphology and topography were studied using field emission scanning electron microscope and atomic force microscopy. The X-ray photoelectron spectroscopy results indicate that the Co ions are in +2 charge state in the films. The optical transmittance of Co doped ZnO thin films reduces up to 80% as compared to undoped ZnO thin film in the visible region. The bandgap was found to be increasing in the range of 3.26–3.31eV with Co doping whereas it decreases for higher doping of Co concentration. The M-H and M-T curve confirms the room temperature ferromagnetism in Co doped ZnO thin films.

Above Room Temperature Ferromagnetism in Co- and V-Doped TiO2 — Revealing the Different Contributions of Defects and Impurities

We report recent experimental results on the mag- netic, magnetotransport, and magneto-optical properties of Co- and V-doped TiO2−δ magnetic oxides at the doping level around 1 at. %. The samples were prepared using rf magnetron sputtering in identical conditions that allows to compare the mechanisms of above-room-temperature ferro- magnetism observed in both cases of doping. In spite of the comparable values of magnetic moment around 1 ÷ 2.5 µB per 3d impurity derived from macroscopic magnetic mea- surements for both systems, the magneto-optical response

Spectroscopic investigation of an intrinsic room temperature ferromagnetism in Co doped ZnO nanoparticles

Pure and Co substituted ZnO nano crystalline particles were prepared by solution combustion technique using l-Valine as a fuel. As synthesized powder samples were characterized by X-ray diffractometer and SQUID magnetometer to confirm the formation of single phase wurtzite structure and to study the bulk magnetic response of the sample, respectively. Magnetic studies show that Co doped ZnO nanoparticles exhibit ferromagnetism (FM) at room temperature (RT). Furthermore, the electronic structure and element specific magnetic properties were investigated by near-edge X-ray absorption fine structure (NEXAFS) and X-ray magnetic circular dichroism (XMCD) measurements, respectively. The effect of Co substitution on the spectral features of Co–ZnO at O K-edge, Co L3,2 edge, Zn L3,2 edge have been investigated. The spectral features of NEXAFS at Co L3,2 edge is entirely different from the spectral features of metallic clusters and other impurity phases, which rules out the presence of impurity phases. The valence state of ‘Co’ ion is found to be in +2 state. The FM nature of the sample was confirmed through XMCD spectra, which is due to the incorporation of divalent ‘Co’ ions. Hence the presented results confirm the substitution of ‘Co’ ions at ‘Zn’ site in the host lattice, which is responsible for the RTFM.

Effect of oxygen deficiency on room temperature ferromagnetism in Co doped ZnO

We investigated the correlation between magnetization and oxygen vacancies in Zn0.95Co0.05O nanoparticles. Enhanced magnetizations were found in SiO2 nanopowders and carbon nanotubes (CNTS) treated Zn0.95Co0.05O, which are attributed to minimizing nanoparticle size and increasing oxygen vacancy concentration. After oxygen annealing, the magnetization of both non-treated Zn0.95Co0.05O and CNTS treated Zn0.95Co0.05O decreased sharply with the filling of the oxygen vacancies, while the SiO2 treated Zn0.95Co0.05O was influenced little as the amorphous SiO2 shell prevents the diffusion of oxygen into magnetic particles. It demonstrated that the ferromagnetism comes from the interfacial oxygen deficiency and is tunable by changing the oxygen vacancies.

Effect of defects on room-temperature ferromagnetism in Co and Na co-doped ZnO

The role of defects in the room temperature ferromagnetism of the Co–ZnO based diluted magnetic semiconductor (DMS) was investigated by co-doping the DMS with Na. The structure characterizations indicate that both Na and Co ions enter into the ZnO lattice without the formation of secondary phase. The oxygen vacancy of ZnCoNaO increased while the carrier concentration decreased compared with that of ZnCoO, leading to the enhancement of the ferromagnetic property in the ZnCoNaO. The observed ferromagnetism introduced by Na ions is attributed to the exchange interaction via the electron trapped oxygen vacancies coupled with the magnetic Co ions.

Room temperature ferromagnetism in Co doped ZnO within an optimal doping level of 5%

We report on the structural, micro-structural and magnetic properties of Zn1−xCoxO (0≤x≤0.1) system. Electron probe micro-structural analysis on 5% Co doped ZnO indicates the presence of segregated cobalt oxide which is also confirmed from the Co 2p core level X-ray photoelectron spectrum. The presence of oxygen defects in lower percentage of Co doped ZnO (≤5%) enhances the carrier mediated exchange interaction and thereby enhancing the room-temperature ferromagnetic behaviour. Higher doping percentage of cobalt (>5%) creates weak link between the grains and suppresses the carrier mediated exchange interaction. This is the reason why room temperature ferromagnetism is not observed in 7% and 10% Co doped ZnO.

ROOM TEMPERATURE FERROMAGNETISM IN Co-DOPED ZnO NANOPARTICLES: MILLING TIME DEPENDENCE AND ANNEALING EFFECT

We report on the occurrence of room temperature ferromagnetism in Co -doped ZnO nanoparticles (NPs). Doping is performed by ball milling of 3 wt% of Co mixed with ZnO nanopowders (commercial) for durations of 2–8 h. X-ray diffraction data and high-resolution transmission electron microscopy (HRTEM) confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO . The magnetization (M–H curve) measured at room temperature exhibits the clear ferromagnetic characteristic with saturation magnetization (Ms) and coercive field (Hc) of the order of 3–4 emu/g and 225 Oe, respectively. Post-growth annealing at 250°C results in an increase of Ms by a small magnitude, while annealing at 500°C results in reduction of Ms. UV–visible absorption spectra show small redshift in the absorption peaks in the Co -doped ZnO NPs due to the incorporation of Co atoms in ZnO lattice. Room temperature photoluminescence studies show enhanced near-band-edge emission at 378 nm in the doped NPs as compared to the undoped NPs indicating low density of defects in the doped ZnO crystals. Contribution of intrinsic defects and magnetic impurities in the observed ferromagnetism is discussed.

Structural, electrical and magnetic properties of Co and Fe co-doped ZnO nanoparticles prepared by solution combustion method

Abstract Nano-sized Co and Fe co-doped ZnO (Zn 1− x Fe 0.05 Co x O where x = 0, 0.005, 0.01, 0.02) powders were prepared by simple solution combustion method. The powder samples were characterized by X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and Energy dispersive analysis of X-rays (EDAX). The XRD patterns showed the formation of single phase wurtzite structure except for x = 0. The high porosity in the material was confirmed by SEM images. The DC electrical conductivity measurements were carried out in the temperature range 300–650 K and it was found to increase with increase in temperature. At room temperature, DC electrical conductivity was evidenced to decrease with increase in Co content. The room temperature ferromagnetism in Co and Fe co-doped ZnO nanoparticles was found and confirmed by hysteresis in the M – H curve.

Donor defects enhanced ferromagnetism in Co:ZnO films

We experimentally report the role of donor defects on the origin of room temperature ferromagnetism in Zn 0.985Co 0.015O films. Undoped ZnO, Co:ZnO and (Co, N):ZnO thin films are fabricated by radio-frequency magnetron sputtering. The local structural measurements indicate that Co 2+ substitutes for Zn 2+ in the ZnO wurtzite lattice with the upper limit of ~ 10% Co metal of the total cobalt dopants. The Raman spectra reveal vibrational modes at 273, 470, 639, 691 and 854 cm − 1 in addition to the host phonons of ZnO, indicating the substitutional behavior of N 3− for O 2− and the increase in donor defects. Acceptor doping with nitrogen compensates the electron carriers and hence decreases the electron carrier concentration. Combined with the enhancement of ferromagnetism, it is found that the donor defects other than electron carriers are responsible for the room temperature ferromagnetism in Co:ZnO films.

Enhancement of ferromagnetic properties in In 1.99Co 0.01O 3 by additional Cu doping

This paper reports room-temperature ferromagnetism in Co- and Cu-doped In 2 O 3 samples synthesized by a solid-state reaction method. Structure and composition analyses revealed that Co and Cu were incorporated into the In 2 O 3 lattices. Photoluminescence measurement revealed an additional emission at 520 nm from these doped samples. The magnetic measurement showed that additional Cu doping greatly enhanced the ferromagnetism of In 1.99 Co 0.01 O 3 bulk samples. The implication of the effects of additional Cu doping is also discussed.

Enhanced room temperature ferromagnetism in Co- and Mn-ion-implanted silicon

The authors report on ferromagnetism at room temperature in cluster-free, cobalt- and manganese-ion-implanted crystalline silicon. Through magnetic and structural analysis it is shown that the ion-implanted Si consists of two layers of Co- and Mn-containing silicon: (1) an amorphous Si layer on the surface and (2) single crystalline Si beneath. The amorphous layer shows very little magnetism by itself but seems to be responsible for partially canceling out or masking the ferromagnetism in the crystalline Si. The authors also observe that etching of the amorphous Si layer dramatically enhances the measured magnetism by as much as 400%.

The role of oxygen vacancies in magnetism of Co xTi 1− xO 2−δ films on Si(001) prepared by sol–gel method

Co x Ti 1− x O 2−δ films have been prepared on Si(001) substrates by sol–gel method. When heat treated in air, Co x Ti 1− x O 2−δ films are non-ferromagnetic at room temperature. However, after further vacuum annealing or hydrogenation, Co x Ti 1− x O 2−δ films show room-temperature ferromagnetism (RTFM). When the vacuum annealed Co x Ti 1− x O 2−δ films are reheated in air, the magnetic moments of the films strongly reduce. After these films are vacuum annealed once again, the magnetic moments are greatly enhanced, confirming the role of vacuum annealing in ferromagnetism of Co x Ti 1− x O 2−δ films. The x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and measurements of magnetization ( M ) vs temperature ( T ) fail to detect Co clusters in the vacuum annealed and the hydrogenated Co x Ti 1− x O 2−δ films. Oxygen vacancies are formed in Co x Ti 1− x O 2−δ films after vacuum annealing and hydrogenation, determined by XRD and XPS measurements. These results indicate that oxygen vacancies created by vacuum annealing and hydrogenation play an important role in the generation of RTFM in Co x Ti 1− x O 2−δ films.

The influence of hydrogen annealing on magnetism of Co-doped TiO 2 films prepared by sol–gel method

Co-doped TiO 2 (Co x Ti 1− x O 2, 0.05⩽ x⩽0.2) films have been prepared on Si (0 0 1) substrates by sol–gel method. When heat treated in air, Co x Ti 1− x O 2 films are non-ferromagnetic at room temperature. However, after further annealed in a flowing hydrogen atmosphere, Co x Ti 1− x O 2 films show room-temperature ferromagnetism (RTFM). Measurements of magnetization ( M) vs. temperature ( T), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) fail to detect Co clusters in the hydrogenated Co 0.1Ti 0.9O 2 films, suggesting that RTFM in the hydrogenated Co 0.1Ti 0.9O 2 films may be intrinsic. But, metal Co appears in the hydrogenated Co 0.2Ti 0.8O 2 films, showing that RTFM in the hydrogenated Co 0.2Ti 0.8O 2 films is as least partly due to metal Co. These results indicate that hydrogen annealing can produce room-temperature ferromagnetism in Co x Ti 1− x O 2 films, but it should be carefully designed to avoid the formation of metal Co in the hydrogenated Co x Ti 1− x O 2 films.

Nature of magnetism in Co- and Mn-doped ZnO prepared by sol-gel technique

Magnetic properties of sol-gel-prepared bulk samples of Co0.05Zn0.95O and Mn0.05Zn0.95O are reported before and after annealing in 5%H2∕95%Ar at 573 K for 6 h. The as-prepared samples are paramagnetic with the magnetic susceptibility χ following the Curie-Weiss law: χ=χ0+C∕(T−θ). The magnitudes of C are consistent with the magnetic moments expected for the Co2+ and Mn2+ states. After hydrogenation, the magnetism of Mn∕ZnO is unchanged but Co∕ZnO acquires room-temperature ferromagnetism (RTFM) with a magnetic moment of 0.35μB∕Co site and hysteresis loop with coercivity Hc≃600Oe, remanence Mr≃0.45emu∕g, and saturation magnetization Ms≃1.2emu∕g. Electron magnetic-resonance spectroscopy at 9.28 GHz gives signals corresponding to the Co2+ and Mn2+ states for the paramagnetic states and a broad FM signal for the hydrogenated Co∕ZnO. This difference under hydrogenation between Co∕ZnO and Mn∕ZnO suggests that n-type doping leads to stabilizing of RTFM in Co∕ZnO but not in Mn∕ZnO, the latter perhaps requiring p-type doping.

Possible metamagnetic origin of ferromagnetism in transition-metal-doped SnO2

Room-temperature ferromagnetism has been observed recently in Co and Fe doped SnO2 powders. In Sn0.99Co0.01O2 powders prepared at 600 °C, ferromagnetism emerged following a sharp transition in the M vs H data at a critical field ∼25kOe. Broad susceptibility maxima were observed in ferromagnetic Sn1−xCoxO2 and Sn1−xFexO2 powders centered near 290 and 260 K, respectively, indicating spin fluctuation effects. High temperature magnetic measurements of Sn0.99Co0.01O2 and Sn0.99Fe0.01O2 obtained TC’s of 440 and 850 K respectively. However, unlike conventional ferromagnets, the magnetic transitions at the TC were associated with sharp changes in M. It is argued that these observed unusual magnetic features indicate a metamagnetic origin of the room-temperature ferromagnetism in Co and Fe doped SnO2.