Carrier-controlled ferromagnetism in transparent oxide semiconductors

Structural, Magnetic and Gas Sensing Activity of Pure and Cr Doped In2O3 Thin Films Grown by Pulsed Laser Deposition

Pure In2O3 and 6% Cr-doped In2O3 thin films were prepared on a silicon (Si) substrate by pulsed laser deposition technique. The obtained In2O3/In2O3:Cr thin films structural, morphological, optical, magnetic and gas sensing properties were briefly investigated. The X-ray diffraction results confirmed that the grown thin films are in single-phase cubic bixbyte structure with space group Ia-3. The SEM analysis showed the formation of agglomerated spherical shape morphology with the decreased average grain size for Cr doped In2O3 thin film compared to pure In2O3 film. It is observed that the Cr doped In2O3 thin film shows the lower band gap energy and that the corresponding transmittance is around 80%. The X-ray photoelectron spectroscopy measurements revealed that the presence of oxygen vacancy in the doped In2O3 film. These oxygen defects could play a significant role to enhance the sensing performance towards chemical species. In the magnetic hysteresis loop, it is clear that the prepared films confirm the ferromagnetic behaviour and the maximum saturation value of 39 emu/cc for Cr doped In2O3 film. NH3 gas sensing studies was also carried out at room temperature for both pure and Cr doped In2O3 films, and the obtained higher sensitivity is 182% for Cr doped In2O3, which is about nine times higher than for the pure In2O3 film due to the presence of defects on the doped film surface.

Study on the effect of Sn concentration on the structural, optical, and electrical properties of (Al0.55In0.45)2O3:Sn films

The MOVPE method is applied to deposit the (Al_0.55In_0.45)₂O₃:Sn films with high mobility as well as low resistivity.

Amorphous nonstoichiometric oxides with tunable room-temperature ferromagnetism and electrical transport

Material functionalities strongly depend on the stoichiometry, crystal structure, and homogeneity. Here we demonstrate an approach of amorphous nonstoichiometric inhomogeneous oxides to realize tunable ferromagnetism and electrical transport at room temperature. In order to verify the origin of the ferromagnetism, we employed a series of structural, chemical, and electronic state characterizations. Combined with electron microscopy and transport measurements, synchrotron-based grazing incident wide angle X-ray scattering, soft X-ray absorption and circular dichroism clearly reveal that the room-temperature ferromagnetism originates from the In_0.23Co_0.77O_1-v amorphous phase with a large tunable range of oxygen vacancies. The room-temperature ferromagnetism is tunable from a high saturation magnetization of 500 emu cm^-3 to below 25 emu cm^-3, with the evolving electrical resistivity from 5 × 10³ μΩ cm to above 2.5 × 10⁵ μΩ cm. Inhomogeneous nano-crystallization emerges with decreasing oxygen vacancies, driving the system towards non-ferromagnetism and insulating regime. Our work unfolds the novel functionalities of amorphous nonstoichiometric inhomogeneous oxides, which opens up new opportunities for developing spintronic materials with superior magnetic and transport properties.

Defect-Rich Dopant-Free ZrO2 Nanoclusters and Their Size-Dependent Ferromagnetism

As an intermediate form of matter between a single atom or molecule and the bulk, nanoclusters (NCs) provide novel properties because of their high surface area-to-volume ratios and distinct physical and electronic structures. These ultrasmall NCs offer a new approach to advance charge-spin manipulation for novel devices, including spintronics and magnetic tunneling junctions. Here, we deposit monosized ZrO₂ NCs over a large area by using gas-phase aggregation followed by in situ size selection by a quadrupole mass filter. These size-specific NCs exhibit sub-oxide photoemission features at binding energies that are dependent on the cluster size (from 3 to 9 nm), which are attributed to different oxygen vacancy defect states. These dopant-free ZrO₂ NCs also show strongly size-dependent ferromagnetism, which provides distinct advantages in solubility and homogeneity of magnetism when compared to traditional dilute magnetic semiconductors. A defect-band hybridization-induced magnetic polaron model is proposed to explain the origin of this size-dependent ferromagnetism. This work demonstrates a new protocol of magnetization manipulation by size control and promises potential applications based on these defect-rich size-selected NCs.

Tuning the CrIV/CrIII Valence States in Purple Cr-Doped SnO2 Nanopowders: The Key Role of CrIV Centers and Defects

A low content of chromium (≤5 mol %) has been incorporated into a SnO₂ cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O₂ flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr⁴⁺ and Cr³⁺ ions as well as ferromagnetic Cr⁴⁺-Cr³⁺ and antiferromagnetic Cr³⁺-Cr³⁺ interactions. The strong purple hue is related to the isolated Cr⁴⁺ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10Dq value around 2.4 eV, whereas the 10Dq value is around 2 eV for isotropic Cr³⁺ ions, partially substituted for Sn⁴⁺ ions in cassiterite. Just after the coprecipitation process, only Cr³⁺ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr⁴⁺ content remains high after annealing at 800 °C for 2 days especially for the highest Cr rate (2 and 5 mol %), leading to a darker purple color, but unfortunately the Cr³⁺ content also increases for a higher Cr concentration. A lighter purple hue can be reached after calcination at a higher temperature (T = 1000 °C) for a shorter time (4 h) but with a lower Cr content to avoid Cr clusters. This is due to stabilizing a high content of isolated Cr⁴⁺ species and limiting the Cr⁴⁺-Cr³⁺ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr⁴⁺ rate and the Cr⁴⁺-Cr³⁺ clusters create local defects whose concentration strongly varies with a total Cr content, which have then been demonstrated to strongly influence the optical and magnetic properties.

d0-Ferromagnetism in SHS Titanium Nitride Treated by Ball Milling

In this work, the influence of mechanical treatment (mechanical milling) of the TiN titanium nitride powder produced by self-propagating high-temperature synthesis on the magnetic properties of the milled powders is investigated. The effect of d0-magnetization was observed. The TiN powders were characterized by scanning electron microscopy, X-ray diffraction, vibrating-sample magnetometry, specific surface area measurement, and chemical analysis. The results show that the mechanical treatment of the TiN titanium nitride powder influences the magnetization in a nonmonotonic manner. The conditions of mechanical treatment corresponding to the best value of specific magnetization of milled powders were established. The specific magnetization depended on three measured parameters: specific surface area, coherent scattering region, and average particle size. It was shown that unit cell parameters of milled TiN titanium nitride powders have not been changed with the increasing of duration milling time. The calculated values of CSR of mechanically treated powders decreased with the increasing of duration of milling time. The values of macrostrains were negative. Mechanical treatment of the TiN titanium nitride powders has led to a change in the nitrogen content from 21.4 to 20.0 wt.%. Stoichiometry of the TiN titanium nitride varied from TiN0.903-TiN0.886; therefore, the observed d0-magnetization effect is associated with a defective surface structure of mechanically treated powders.

Nonvolatile Control of the Electronic Properties of In2-xCrxO3 Semiconductor Films by Ferroelectric Polarization Charge

A series of Cr-doped In_2-xCr_xO₃ (ICO) semiconductor thin films were epitaxially grown on (111)-oriented 0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃ (PMN-0.29PT) single-crystal substrates by the pulsed laser deposition. Upon the application of an electric field to the PMN-0.29PT substrate along the thickness direction, we realized in situ, reversible, and nonvolatile control of the electronic properties and Fermi level of the films, which are manifested by abundant physical phenomena such as the n-type to p-type transformation, metal-semiconductor transition, metal-insulator transition, crossover of the magnetoresistance (MR) from negative to positive, and a large nonvolatile on-and-off ratio of 5.5 × 10⁴% at room temperature. We also strictly disclose that both the sign and the magnitude of MR are determined by the electron carrier density of ICO films, which could modify the s-d exchange interaction and weak localization effect. Our results demonstrate that the ferroelectric gating approach using PMN-PT can be utilized to gain deeper insight into the carrier-density-related electronic properties of In₂O₃-based semiconductors and provide a simple and energy efficient way to construct multifunctional devices which can utilize the unique properties of composite materials.

Unusual Oxygen Partial Pressure Dependence of Electrical Transport of Single-Crystalline Metal Oxide Nanowires Grown by the Vapor-Liquid-Solid Process

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO₂ nanowires grown by a vapor-liquid-solid (VLS) process. The electrical conductivity of a single SnO₂ nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10^-3 to 10^-4 Pa. This contradicts the conventional trend of n-type SnO₂ semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is attributed to the intrinsic differences between the two crystal growth interfaces (LS and VS interfaces) in the critical nucleation of the crystal growth and impurity incorporation probability as a function of the oxygen partial pressure. The impurity incorporation probability at the LS interface is always lower than that at the VS interface, even under reduced oxygen partial pressures.

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 [Formula: see text]. Both the experimental and theoretical investigation show that the exchange interaction between Cu²⁺-Cu²⁺ ions mediated by [Formula: see text] is responsible for RTFM in Cu-doped ZnO.

Magnetic field control of charge excitations in CoFe2O4

We combine magnetic circular dichroism and photoconductivity with prior optical absorption and first principles calculations to unravel spin-charge interactions in the high Curie temperature magnet CoFe2O4. In addition to revising the bandgap hierarchy, we reveal a broad set of charge transfer excitations in the spin down channel which are sensitive to the metamagnetic transition involving the spin state on Co centers. We also show photoconductivity that depends on an applied magnetic field. These findings open the door for the creation and control of spin-polarized electronic excitations from the minority channel charge transfer in spinel ferrites and other earth-abundant materials.

Demonstration of Ru as the 4th ferromagnetic element at room temperature

Development of novel magnetic materials is of interest for fundamental studies and applications such as spintronics, permanent magnetics, and sensors. We report on the first experimental realization of single element ferromagnetism, since Fe, Co, and Ni, in metastable tetragonal Ru, which has been predicted. Body-centered tetragonal Ru phase is realized by use of strain via seed layer engineering. X-ray diffraction and electron microscopy confirm the epitaxial mechanism to obtain tetragonal phase Ru. We observed a saturation magnetization of 148 and 160 emu cm^-3 at room temperature and 10 K, respectively. Control samples ensure the ferromagnetism we report on is from tetragonal Ru and not from magnetic contamination. The effect of thickness on the magnetic properties is also studied, and it is observed that increasing thickness results in strain relaxation, and thus diluting the magnetization. Anomalous Hall measurements are used to confirm its ferromagnetic behavior.

Intrinsic Ferromagnetism in the Diluted Magnetic Semiconductor Co:TiO_{2}

Here we present a study of magnetism in Co_{0.05}Ti_{0.95}O_{2-δ} anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transmission electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while a very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure (≈10^{-6}  torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO_{2} is an intrinsic diluted magnetic semiconductor.

β-Ag3 RuO4, a Ruthenate(V) Featuring Spin Tetramers on a Two-Dimensional Trigonal Lattice

Open-shell solids exhibit a plethora of intriguing physical phenomena that arise from a complex interplay of charge, spin, orbital, and spin-state degrees of freedom. Comprehending these phenomena is an indispensable prerequisite for developing improved functional materials. This type of understanding can be achieved, on the one hand, by experimental and theoretical investigations into known systems, or by synthesizing new solids displaying unprecedented structural and/or electronic features. β-Ag3 RuO4 may serve as such a model system because it possesses a remarkable anionic structure, consisting of tetrameric polyoxoanions (Ru4 O16 )(12-) , and is an embedded fragment of a 2D trigonal MO2 lattice. The notorious frustration of antiferromagnetic (AF) exchange couplings on such lattices is thus lifted, and instead strong AF occurs within the oligomeric anion, where only one exchange path remains frustrated among the relevant six. The strong magnetic anisotropy of the [Ru4 O16 ](12-) ion, and the effectively orbital nature of its net magnetic moment, implies that this anion may reveal the properties of a single-molecule magnet if well-diluted in a diamagnetic matrix.

Probing high temperature ferromagnetism and its paramagnetic phase change due to Eu3+ incorporation in ZnO nanophosphors

Ferromagnetic oxide semiconductors exhibiting efficient luminescent properties together with robust ferromagnetism above room temperature form an exclusive class of spintronic materials endowed with both charge and spin degrees of freedom.

Reduction of Mn3+ to Mn2+ and near infrared plasmonics from Mn–Sn codoped In2O3 nanocrystals

Isovalent doping in In₂O₃ using Mn³⁺ precursor leads to aliovalent Mn²⁺ doped In₂O₃ NCs which tune SPR band from near to mid-infrared region.

Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy

This investigation studies the various magnetic behaviors of graphene oxide (GO) and reduced graphene oxides (rGOs) and elucidates the relationship between the chemical states that involve defects therein and their magnetic behaviors in GO sheets. Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature. Scanning transmission X-ray microscopy and corresponding X-ray absorption near-edge structure spectroscopy were utilized to investigate thoroughly the variation of the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups, as well as the C 2p(σ*)-derived states in flat and wrinkle regions to clarify the relationship between the spatially-resolved chemical states and the magnetism of GO, M-rGO and H-rGO. The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO. Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

High-Curie-Temperature Ferromagnetism in (Sc,Fe)F3 Fluorides and its Dependence on Chemical Valence

A magnetic metal-fluoride system is shown for the first time to have a high Curie temperature (≈545 K). The magnetism correlates intimately with the Fe(2+)/Fe(3+) ratio. As the ratio increases, the weak magnetism displayed by unordered magnetic moments intensifies, and these magnetic moments align in parallel. Simultaneously, a magneto-volume effect is also shown to increase the lattice volume.

Unexpected observation of spatially separated Kondo scattering and ferromagnetism in Ta alloyed anatase TiO2 thin films

We report the observation of spatially separated Kondo scattering and ferromagnetism in anatase Ta_0.06Ti_0.94O₂ thin films as a function of thickness (10–200 nm). The Kondo behavior observed in thicker films is suppressed on decreasing thickness and vanishes below ~25 nm. In 200 nm film, transport data could be fitted to a renormalization group theory for Kondo scattering though the carrier density in this system is lower by two orders of magnitude, the magnetic entity concentration is larger by a similar magnitude and there is strong electronic correlation compared to a conventional system such as Cu with magnetic impurities. However, ferromagnetism is observed at all thicknesses with magnetic moment per unit thickness decreasing beyond 10 nm film thickness. The simultaneous presence of Kondo and ferromagnetism is explained by the spatial variation of defects from the interface to surface which results in a dominantly ferromagnetic region closer to substrate-film interface while the Kondo scattering is dominant near the surface and decreasing towards the interface. This material system enables us to study the effect of neighboring presence of two competing magnetic phenomena and the possibility for tuning them.

Alteration of architecture of MoO₃ nanostructures on arbitrary substrates: growth kinetics, spectroscopic and gas sensing properties

MoO3 nanostructures have been grown in thin film form on five different substrates by RF magnetron sputtering and subsequent annealing; non-aligned nanorods, aligned nanorods, bundled nanowires, vertical nanorods and nanoslabs are formed respectively on the glass, quartz, wafer, alumina and sapphire substrates. The nanostructures formed on these substrates are characterized by AFM, SEM, GIXRD, XPS, micro-Raman, diffuse reflectance and photoluminescence spectroscopy. A detailed growth model for morphology alteration with respect to substrates has been discussed by considering various aspects such as surface roughness, lattice parameters and the thermal expansion coefficient, of both substrates and MoO3. The present study developed a strategy for the choice of substrates to materialize different types MoO3 nanostructures for future thin film applications. The gas sensing tests point towards using these MoO3 nanostructures as principal detection elements in gas sensors.

Observation of the origin of d0 magnetism in ZnO nanostructures using X-ray-based microscopic and spectroscopic techniques

Efforts have been made to elucidate the origin of d(0) magnetism in ZnO nanocactuses (NCs) and nanowires (NWs) using X-ray-based microscopic and spectroscopic techniques. The photoluminescence and O K-edge and Zn L3,2-edge X-ray-excited optical luminescence spectra showed that ZnO NCs contain more defects than NWs do and that in ZnO NCs, more defects are present at the O sites than at the Zn sites. Specifically, the results of O K-edge scanning transmission X-ray microscopy (STXM) and the corresponding X-ray-absorption near-edge structure (XANES) spectroscopy demonstrated that the impurity (non-stoichiometric) region in ZnO NCs contains a greater defect population than the thick region. The intensity of O K-edge STXM-XANES in the impurity region is more predominant in ZnO NCs than in NWs. The increase in the unoccupied (occupied) density of states at/above (at/below) the conduction-band minimum (valence-band maximum) or the Fermi level is related to the population of defects at the O sites, as revealed by comparing the ZnO NCs to the NWs. The results of O K-edge and Zn L3,2-edge X-ray magnetic circular dichroism demonstrated that the origin of magnetization is attributable to the O 2p orbitals rather than the Zn d orbitals. Further, the local density approximation (LDA) + U verified that vacancies in the form of dangling or unpaired 2p states (due to Zn vacancies) induced a significant local spin moment in the nearest-neighboring O atoms to the defect center, which was determined from the uneven local spin density by analyzing the partial density of states of O 2p in ZnO.

Investigating the development of spurious magnetism in single crystalline BaTi₀.₉₅Fe₀.₀₅O₃-δ with high δ by local structural probes

BaTi₀.₉₅Fe₀.₀₅O₃-δ has been suggested to be an intrinsic dilute magnetic oxide with a clear dependence of magnetism on oxygen vacancy concentration. However, it has also been shown that the dopant Fe ions distribute themselves rather inhomogeneously within the lattice, though without disrupting the crystal phase of the parent BaTiO₃. With the help of x-ray absorption spectroscopy (XAS), here we find that the incorporation of a larger amount of anion vacancy pushes this inhomogeneity to the extreme, leading to the precipitation of Fe metal clusters. It is also observed that the residual solid, without the Fe-metal cluster, undergoes massive structural and compositional reorganization.

X-Ray Magnetic Dichroism in the Cobalt-Doped Indium Tin Oxide from First Principle Calculations

The electronic structure of the Co-doped indium tin oxide (ITO) diluted magnetic semiconductors (DMSs) were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The X-ray absorption spectra (XAS) and X-ray magnetic circular dichroism (XMCD) spectra at the Co L3, In M2, Sn M2, and O K edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The calculated results are compared with available experimental data.

Microscopic distribution of metal dopants and anion vacancies in Fe-doped BaTiO(3-δ) single crystals

A detailed microscopic structural study on two single crystalline dilute magnetic oxides, BaTi0.95Fe0.05O(3-δ) with and without perceptible δ, has been carried out. Although it has been reported earlier that varying δ significantly affects high temperature ferromagnetism, the real distribution/redistribution of vacancies and dopant Fe ions inside the 6H hexagonal structure was never probed. This study reveals that oxygen vacancies reduce the dopant Fe(3+) ions to Fe(2+) and mostly accumulate around these Fe(2+) ions. Another distinct trend is the tendency of the dopant Fe ions to get closer instead of being distributed randomly, thereby creating Fe2(2+)O(9-δ') like dimers within the 6H hexagonal unit cell. This experimental observation definitively confirms previous hypotheses based on theoretical models.

Oxygen-vacancy-induced antiferromagnetism to ferromagnetism transformation in Eu₀.₅Ba₀.₅TiO₃₋δ multiferroic thin films

Oxygen vacancies (V(O)) effects on magnetic ordering in Eu₀.₅Ba₀.₅TiO₃₋δ (EBTO₃₋δ) thin films have been investigated using a combination of experimental measurements and first-principles density-functional calculations. Two kinds of EBTO₃₋δ thin films with different oxygen deficiency have been fabricated. A nuclear resonance backscattering spectrometry technique has been used to quantitatively measure contents of the V(O). Eu₀.₅Ba₀.₅TiO₃ ceramics have been known to exhibit ferroelectric (FE) and G-type antiferromagnetic (AFM) properties. While, a ferromagnetic (FM) behavior with a Curie temperature of 1.85 K has been found in the EBTO₃₋δ thin films. Spin-polarized Ti(3+) ions, which originated from the V(O), has been proven to mediate a FM coupling between the local Eu 4f spins and were believed to be responsible for the great change of the magnetic ordering. Considering the easy formation of V(O), our work opens up a new avenue for achieving co-existence of FM and FE orders in oxide materials.

Defect-driven magnetism in luminescent n/p-type pristine and Gd-substituted SnO2 nanocrystalline thin films

The effects of rare-earth-element Gd doping on the intrinsic magnetic ordering, photoluminescence, and electrical-conducting properties of the pristine SnO(2) nanocrystalline thin films fabricated by radio-frequency (RF) sputtering are investigated. The pristine SnO(2) thin film exhibits significant ferromagnetism while Gd doping results in an absence of intrinsic ferromagnetism. The presence of large amounts of singly ionized oxygen vacancies (V(O)(+)) is traced by photoluminescence spectroscopic analysis and they are found to be responsible for the observed ferromagnetism in pristine SnO(2) thin films. A significant reduction of oxygen vacancies is observed after Gd doping, and that might be insufficient to mediate long-range ferromagnetic ordering between V(O)(+) defects in a Gd-doped SnO(2) system. Although the associated magnetic moment is increased by 1 order of magnitude, because of the insertion of Gd(3+) ions, which have localized f-shell paramagnetic moment, there is no intrinsic FM ordering. Hall measurement reveals that the pure SnO(2) exhibits n-type behavior whereas Gd-doped SnO(2) films show the p-type conductivity with higher resistivity. The studies demonstrate that only structural defects such as V(O)(+) defects, not magnetic ions such as Gd(3+), are responsible for inducing ferromagnetism in SnO(2) thin films.

Spin-Controlled Vertical-Cavity Surface-Emitting Lasers

We discuss the concept of spin-controlled vertical-cavity surface-emitting lasers (VCSELs) and analyze it with respect to potential room-temperature applications in spin-optoelectronic devices. Spin-optoelectronics is based on the optical selection rules as they provide a direct connection between the spin polarization of the recombining carriers and the circular polarization of the emitted photons. By means of optical excitation and numerical simulations we show that spin-controlled VCSELs promise to have superior properties to conventional devices such as threshold reduction, spin control of the emission, or even much faster dynamics. Possible concepts for room-temperature electrical spin injection without large external magnetic fields are summarized, and the progress on the field of purely electrically pumped spin-VCSELs is reviewed.

Oxide nanowires for spintronics: materials and devices

Spintronics, or spin-based data storage and manipulation technology, is emerging as a very active research area because of both new science and potential technological applications. As the characteristic lengths of spin-related phenomena naturally fall into the nanometre regime, researchers start applying the techniques of bottom-up nanomaterial synthesis and assembly to spintronics. It is envisaged that novel physics regarding spin manipulation and domain dynamics can be realized in quantum confined nanowire-based devices. Here we review the recent breakthroughs related to the applications of oxide nanowires in spintronics from the perspectives of both material candidates and device fabrication. Oxide nanowires generally show excellent crystalline quality and tunable physical properties, but more efforts are imperative as we strive to develop novel spintronic nanowires and devices.

Quantum phase transition from superparamagnetic to quantum superparamagnetic state in ultrasmall Cd(1-x)Cr(II)(x)Se quantum dots?

Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II-VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p-d exchange interaction between the spin-up Cr(II) t(2g) level and the Se 4p. In this study, ultrasmall (~3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD.

Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (T_c>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn_0.05Ge_0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn_0.05Ge_0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn_0.05Ge_0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism.

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ∼1 and ∼10  nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)↔Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.

Metal-functionalized single-walled graphitic carbon nitride nanotubes: a first-principles study on magnetic property

The magnetic properties of metal-functionalized graphitic carbon nitride nanotubes were investigated based on first-principles calculations. The graphitic carbon nitride nanotube can be either ferromagnetic or antiferromagnetic by functionalizing with different metal atoms. The W- and Ti-functionalized nanotubes are ferromagnetic, which are attributed to carrier-mediated interactions because of the coupling between the spin-polarized d and p electrons and the formation of the impurity bands close to the band edges. However, Cr-, Mn-, Co-, and Ni-functionalized nanotubes are antiferromagnetic because of the anti-alignment of the magnetic moments between neighboring metal atoms. The functionalized nanotubes may be used in spintronics and hydrogen storage.

Room-temperature electric-field controlled ferromagnetism in Mn0.05Ge0.95 quantum dots

Room-temperature control of ferromagnetism by electric fields in magnetic semiconductors has been actively pursued as one of important approaches to realize practical spintronic and nonvolatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (T(c) > 300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Recently, Mn(0.05)Ge(0.95) quantum dots (QDs) were demonstrated to have a T(c) above 300 K. However, the field control of ferromagnetism based on hole-mediated effect remained at low temperatures and thus prohibited spintronic devices operable at ambient environment. Here, we report a successful demonstration of electric-field control of ferromagnetism in the Mn(0.05)Ge(0.95) quantum dots up to 300 K. We show that, by using quantum structure, high-quality material can be obtained and effective hole mediation due to quantum confinement effect can be achieved. Upon the application of gate bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer, that is, the Mn(0.05)Ge(0.95) quantum dots, was manipulated through the change of hole concentration. Our results are fundamentally and technologically important toward the realization of room-temperature spin field-effect transistors and nonvolatile spin logic devices.