Observation Of Room Temperature Ferromagnetism Research Articles

Surface magnetism in ZnO/Co3O4 mixtures

We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides, respectively. Here, we present a detailed study on the electronic structure of this material in order to account for the unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction Co+3→Co2+ at the particle surface takes place as evidenced by x-ray absorption spectroscopy measurements and optical spectroscopy. This reduction allows explaining the observed ferromagnetic signal within the well established theories of magnetism in oxides.

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In 1−xFe x) 2O 3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In 2O 3 samples (In 1− x Fe x ) 2O 3 (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In 2O 3 lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In 2O 3 sample. The highest remanence magnetization moment (6.624 × 10 −4 emu/g) is reached in the sample with x = 0.03.

Observation of room temperature ferromagnetism in ZnTe:Cr films grown onto glass substrate by thermal evaporation method

ZnTe and ZnTe:Cr films were prepared onto glass substrates using thermal evaporation method. Structural properties of the prepared samples were analyzed using X-ray diffractometer, and the presence of ZnCrTe phase was identified along with poor crystallinity. Composition analysis was done using XPS and the Cr content in the film was found to be 0.05 atomic percent. Transmittance spectra were recorded using UV-Vis spectrophotometer. The valence state of Cr in ZnTe:Cr film is determined to be +2 using electron spin resonance (ESR) spectroscopy. Magnetic moment data as a function of magnetic field were recorded using Superconducting Quantum Interference Device (SQUID) magnetometer at temperatures 5, 77 and 300 K. The results showed minority ferromagnetic behavior even at room temperature. Magnetic domains were observed using Magnetic Force Microscopy and the average value of domain size is 3.7 nm.

Ferromagnetism in transparent Mn(II)-doped indium tin oxide films prepared by sol–gel coating

We observe remarkably strong room temperature ferromagnetism [∼1.5 μ B/Mn(II)] in optically transparent Mn(II)-doped indium tin oxide (ITO) films. The nanocrystalline films with average grain size 10–22 nm and thickness 150–350 nm are prepared by sol–gel coating technique on soda lime silica glass substrate. The ferromagnetic property is, of course, weak for films deposited on pure silica glass substrate. The structural parameters of the films appear to be governing the magnetic property strongly which vary appreciably depending on the substrate. The observation of room temperature ferromagnetism in transparent conducting ITO films may find a plethora of applications in the area of magneto-optics.

Observation of room temperature ferromagnetism in Mn–Fe doped ZnO

A room temperature ferromagnetic Fe doped ZnO bulk sample has been synthesized by co-doping 2 at% Mn with 2 at% Fe in ZnO. The final firing temperature for the preparation of the Zn0.96Mn0.02Fe0.02O sample is the same as the final firing temperature for the preparation of the Zn0.98Mn0.02O sample and in this case it is found to be 490 °C. It has been found that room temperature ferromagnetism in Zn0.96Mn0.02Fe0.02O is a factor of 1.6 times more than that observed in Zn0.98Mn0.02O samples. The size of the saturation magnetic field for the Zn0.96Mn0.02Fe0.02O sample is, however, only a factor of 7 times more than that observed for nanocrystalline undoped ZnO.

Size‐Controlled Ex‐nihilo Ferromagnetism in Capped CdSe Quantum Dots

(Ga,Mn)As, [4,5] (Zn,M)O [6–9] , and (Ti,M)O2 [10–12] with M ¼ V, Cr, Mn, Co, Ni, and Cu. Despite the numerous reports on the successful observation of room temperature ferromagnetism (RTFM) in a number of these systems in apparent agreement with computations based on density functional theory, the issue remains unsettled and contentious for a number of reasons including lingering doubt of the possible role of undetected ferromagnetic impurities such as Fe, Co, and Ni. In some systems such as (Zn,Ni)O [13] and (Zn,Cr)Te [14] the observed RTFM has been linked to clustering of Ni and Cr nanocrystals, respectively. In this paper, we report the observation of RTFM in CdSe quantum dots (QD) capped with TOPO (tri-n-octylphosphine oxide). This RTFM is labeled as ex-nihilo since the RTFM is due to the marriage of two diamagnetic materials viz. CdSe and TOPO, possibly resulting from charge transfer from Cd d-band to the oxygen atoms of TOPO. We further show that the RTFM varies inversely with the size of the QD, in agreement with our calculations. CdSe is a II-VI semiconductor with direct band gap Eg ¼ 1.74 eV at 300 K. Semiconductors nanocrystals (NCs) such as CdSe are often called quantum dots (QD) when the size of the first Bohr radius exceeds the crystallite size D, leading to quantum confinement effects such as increase in the band-gap Eg with decrease in D. For the CdSe NCs, Eg can be tuned to cover the whole visible range by change in D, thus making this system potentially useful for solar energy applications. [15,16]

High temperature ferromagnetism in single crystalline dilute Fe-dopedBaTiO3

We report the observation of room temperature ferromagnetism in high quality, single crystalline dilute Fe-doped $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$. The large equilibrium solubility of Fe ions in the matrix refutes uncertainties about secondary phase magnetism, which has often eclipsed this interesting field of research. While room temperature ferromagnetism is observed at and above 5% Fe concentrations, one finds a highly concave temperature dependence of the susceptibility. Using detailed ab initio calculation, this has been related to intrinsic magnetic inhomogeneities arising from positional disorder. Apart from providing a mechanism for the observed high temperature ferromagnetism, our results point out that intrinsic disorder is a generic and essential component of dilute magnetism.

Ferromagnetism in Cu-doped ZnO films: Role of charge carriers

We report the observation of room temperature ferromagnetism in Cu-doped (5%) ZnO films grown on c-plane sapphire substrates. Films were prepared by pulsed laser deposition technique and were thoroughly characterized using several state-of-the-art characterization techniques. Hall measurements showed that the films are of n-type with a carrier concentration of 3×1017cm−3. Magnetization measurements showed that the films exhibit room temperature ferromagnetism with a saturation magnetization of ∼1.45μB∕Cu atom. When additional carriers were introduced in the films, ferromagnetism was completely vanished. Our results show that the p-type nature of the film is not essential for realizing ferromagnetic characteristics; however, the concentration of n-type carriers should not exceed a critical value.

Study of ferromagnetism in Co-doped rutile powders and float-zone grown single crystals

Co-doped rutile samples in the form of both powders and bulk single crystals have been studied with particular emphasis on the dependence of their magnetic, compositional and structural properties upon the type of atmosphere used during their preparation. Both powders and single crystals were characterized using X-ray diffractometry and vibrating sample magnetometry, while the crystals were also studied using the X-ray Laue technique, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that an oxygen deficient environment during the preparation of Co-doped TiO 2 powders is crucial for the observation of room temperature ferromagnetism, while preparation in oxygen rich conditions destroys the ferromagnetism due to the formation of the paramagnetic second phase CoTiO 3. Floating zone growth of crystals under oxygen also led to the formation of material containing second phase CoTiO 3 that was paramagnetic at room temperature, while crystals grown under argon were ferromagnetic and contained Co-rich inclusions.

Synthesis and characterization of room-temperature ferromagnetism in Fe- and Ni-co-doped In 2O 3

Observation of room-temperature ferromagnetism in Fe- and Ni-co-doped In 2O 3 samples (In 0.9Fe 0.1− x Ni x ) 2O 3 (0⩽ x⩽0.1) prepared by citric acid sol–gel auto-igniting method is reported. All of the samples with intermediate x values are ferromagnetic at room-temperature. The highest saturation magnetization (0.453 μ B/Fe+Ni ions) moment is reached in the sample with x=0.04. The highest solubility of Fe and Ni ions in the In 2O 3 lattice is around 10 and 4 at%, respectively. The 10 at% Fe-doped sample is found to be weakly ferromagnetic, while the 10 at% Ni-doped sample is paramagnetic. Extensive structure including Extended X-ray absorption fine structure (EXAFS), magnetic and magneto-transport including Hall effects studies on the samples indicate the observed ferromagnetism is intrinsic rather than from the secondary impurity phases.

Room temperature ferromagnetism in Cr-doped In2O3 on high vacuum annealing of thin films and bulk samples

We report on the observation of room temperature ferromagnetism in Cr-doped In2O3 bulk samples and spin-coated thin films. The samples showed a clear ferromagnetism above 300K with magnetic moments of 0.008 and 0.22μB∕Cr at 300K for the bulk and thin film, respectively, only after high vacuum (HV) annealing at 600°C. The vacuum annealed Cr-doped In2O3 thin films showed a typical semiconducting behavior with a room temperature resistivity of 0.73Ωcm, while bulk samples were more conducting (23mΩcm). We present systematic investigations on the influence of HV annealing on the carrier concentrations, resistivity, and magnetic properties of the samples.

Effects of Annealing on Structural and Magnetic Properties of Cobalt Implanted TiO2 Thin Films

AbstractSince the observation of room-temperature ferromagnetism (RTFM) in Co-doped anatase TiO2 [1], there have been many reports on the study of the magnetic properties of Co-doped TiO2 prepared by various methods with diversified results. The origin of the RTFM in these systems is still a topic of controversy today. In this work, TiO2 thin films were prepared by RF sputtering onto thermally grown oxide layers on Si substrates. Cobalt implantation was performed using a metal vapor vacuum arc (MEVVA) ion source to various doses ranging from 3×1015 cm-2 to 4×1016 cm-2. Post-implantation annealing was performed in a vacuum chamber at various temperatures ranging from 400°C to 700°C for 2 hours and 4 hours. Characterization of these films as-implanted and after thermal annealing under various conditions was performed using Rutherford backscattering spectrometry, energy filtered and high-resolution transmission electron microscopy, x-ray diffractometry, x-ray photoelectron spectroscopy, and vibrating sample magnetometry. The dependence of the magnetic properties on the implantation and annealing conditions were studied in detail. Clear RTFM properties were observed. The saturation magnetic moment per implanted Co atom (MS) seems to increase with increasing dose within the implantation dose range in this study. At a fixed dose, the MS value also shows a generally increasing trend with increasing annealing temperature and annealing time. Quite a number of samples showed MS values exceeding the bulk Co value of 1.69 ìB/Co significantly and the maximum MS value observed is about 3.16 µB/Co. Such high MS values indicate that the RTFM must not come from Co clusters alone. Possible origins of the RTFM properties will be discussed in conjunction with the structural properties.

Ferromagnetism in epitaxial Zn0.95Co0.05O films grown on ZnO and Al2O3

AbstractIn this article, the possible mechanisms resulting in strong ferromagnetic coupling in transition metal(TM)‐doped ZnO and other diluted magnetic semiconductors (DMS) are reviewed and the prerequisites for the observation of room temperature ferromagnetism in TM‐doped ZnO are defined. In order to study the ferromagnetic behavior we have grown epitaxial Zn0.95Co0.05O films simultaneously on (0001) ZnO and Al2O3 substrates by laser molecular beam epitaxy at different deposition temperatures. A systematic study of the structural and magnetic properties has been performed to reveal their interdependence. Room temperature ferromagnetism has been found in Zn0.95Co0.05O films grown on ZnO, whereas for films deposited on sapphire only weak ferromagnetic signals have been detected which could not unambiguously be separated from those of the substrate. The different behavior is explained by different structural properties and defect densities in both films. Our experimental findings are in good agreement with a spin split impurity band model, where strong ferromagnetic exchange in ZnO:Co2+ is obtained by a strong hybridization between the magnetic Co2+ ion states and the donor states due to a large density of native defects. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room-temperature ferromagnetism inGe1−xMnxnanowires

We report an observation of room temperature ferromagnetism in Ge nanowires doped with Mn. High-density arrays of ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\phantom{\rule{0.2em}{0ex}}(x=1%--5%)$ nanowires with the smallest diameter of 35 nm have been synthesized within the pores of anodized aluminium oxide membranes using a supercritical fluid inclusion-phase technique. Structural analysis of the nanowires proved the existence of a highly crystalline germanium host lattice containing discrete manganese atoms. All of the nanowires studied displayed ferromagnetic properties at room temperature. Ferromagnetic ordering reaches a maximum at intermediate Mn concentrations. The magnetic properties of the nanowires can be understood by considering the influence of co-dopant nonmagnetic impurities and nanowire/membrane interfaces.

Magnetic and spectroscopic characteristics of ZnMnO system

We report on the observation of room-temperature ferromagnetism in epitaxial (Zn,Mn)O films grown by a pulsed-laser deposition technique using high-density targets. The X-ray, microscopic, spectroscopic and magnetic properties of target material containing 6 at.% of Mn and films were compared. The target shows the presence of large clusters exhibiting paramagnetic behavior. However, ferromagnetic properties were observed in (Zn,Mn)O films grown at a substrate temperature of 500 °C and with an oxygen partial pressure of 1 mTorr. Although, crystalline quality of the film improves with increasing substrate temperature, the ferromagnetism becomes weaker.

New Possibilities for Ferromagnetic Semiconductors

AbstractNew candidate ferromagnetic semiconductors have recently been grown by doping semiconducting transition-metal oxides with magnetic impurities. Some of these exhibit ferromagnetism at and above room temperature. The critical question is why? In many cases, complex solid-state chemistry is involved in the synthesis and drives the resulting properties. The observation of room-temperature ferromagnetism in these materials must be accompanied by a careful identification of the phases and structures present in order to accurately identify the origin of the magnetism.