Room-temperature ferromagnetism in Dy films doped with Ni

Temperature, magnetic field and spectral dependences of magneto-optical effects (MOEs) in bi-layer films Dy_(1−x)Ni_x-Ni and Dy_(1−x)(NiFe)_x-NiFe were investigated, x changes from 0 to 0.06. Peculiar behavior of the MOEs was revealed at temperatures essentially exceeding the Curie temperature of bulk Dy which is explained by the magnetic ordering of the Dy layer containing Ni under the action of two factors: Ni impurities distributed homogeneously over the whole Dy layer and atomic contact of this layer with continues Ni layer. The mechanism of the magnetic ordering is suggested to be associated with the change of the density of states of the alloy Dy_(1−x)Ni_x owing to hybridization with narrow peaks near the Fermi level character for Ni.     

Theory of ferromagnetism in doped excitonic condensates

Nesting in a semimetal can lead to an excitonic-insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. We present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.     

Stabilization of ferromagnetism in Mn doped ZnO with C co-doping

We present a method for stabilizing ferromagnetism in Mn doped ZnO. We find that Mn doped ZnO show anti-ferromagnetic order in the absence of additional carriers. When Mn doped ZnO is co-doped with C atom at O sites ferromagnetic state gets stabilized. The C doping creates holes which leads to stabilization of ferromagnetic state via hole mediated double exchange mechanism.     

First-principles study on the origin of ferromagnetism in n-type Cu-doped ZnO

Based on the density functional calculations with the GGA+U correction, we elucidate the origin of the experimentally reported ferromagnetism in n-type Cu-doped ZnO. Pure Cu-doped ZnO shows the unoccupied 3d states in the gap introduced by Cu, resulting in the insulating ground state and weak magnetic exchange interactions, in contrast to the half-metallic ground state and high ferromagnetic stability predicted by the calculations without U correction. However, the electron traps induced by Cu in n-type Cu-doped ZnO may lead to the partial occupancy of the Cu gap states, which stabilize the ferromagnetic ordering between two Cu atoms.     

Origins of ferromagnetism in transition metal doped diamond

Using first-principles calculations, we investigated the electronic and magnetic properties of Mn-doped, Fe-doped, and Co-doped diamond. It is found that the Mn-, Fe-, and Co-doped diamond are stabilized in ferromagnetic configurations. The origins of the magnetic ordering are explained successfully by the phenomenological band coupling model based on the p–d and d–d level repulsions between the dopant ions and host elements. According to Heisenberg model, high Curie temperature may be expected for Mn-, Fe-, and Co-doped diamond if there are no native defects or other impurities.     

Evidence of the Zn vacancy acting as the dominant acceptor in n-type ZnO

We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated (E(el)=2 MeV, fluence 6 x 10(17) cm(-2)) ZnO samples. The Zn vacancies are identified at concentrations of [V(Zn)] approximately 2 x 10(15) cm(-3) in the as-grown material and [V(Zn)] approximately 2 x 10(16) cm(-3) in the irradiated ZnO. These concentrations are in very good agreement with the total acceptor density determined by temperature dependent Hall experiments. Thus, the Zn vacancies are dominant acceptors in both as-grown and irradiated ZnO.     

Weak ferromagnetism in BiFeO3 doped with titanium

The Bi_1−xA_xFe_1−xTi_xO₃ (A—Ca, Sr, Pb, Ba) and BiFe_1−xTi_xO_3+δ systems have been studied using X-ray, neutron powder diffraction and magnetization measurements in a magnetic field up to 14 T. It was found that all Bi_1−xA_xFe_1−xTi_xO₃ solid solutions are rhombohedral up to x=0.3. In the case of BiFe_1−xTi_xO_3+δ the rhombohedral distortion preserved up to x=0.11. A homogeneous weakly ferromagnetic state was found for Bi_1−xCa_xFe_1−xTi_xO₃ (0.15≤x≤0.25) and BiFe_1−xTi_xO_3+δ (0.06≤x≤0.11), probably due to magnetoelectric interactions, whereas Bi_1−xA_xFe_1−xTi_xO₃ (A—Sr, Pb, Ba) compounds above doping level x>0.1 seem to be collinear antiferromagnets.     

Unusual ferromagnetism in nanoparticles of doped oxides and manganites

The observation of unusually large ferromagnetism in the nanoparticles of doped oxides and enhanced ferromagnetic tendencies in manganite nanoparticles have been in focus recently. For the transition metal doped oxide nanoparticles a phenomenological ‘charge transfer ferromagnetism’ model has been recently proposed by Coey et al. From a microscopic calculation with charge transfer between the defect band and mixed valent dopants, acting as reservoir, we show how the unusually high ferromagnetic response develops. The puzzle of nanosize-induced ferromagnetic tendencies in manganites is also addressed within the same framework where lattice imperfections and uncompensated charges at the surface of the nanoparticle are shown to reorganize the surface electronic structures with enhanced double exchange.     

Electron irradiation-induced defects in lithium doped n-type silicon with different oxygen concentrations

Electron irradiation-induced defects and their annealing behaviour in lithium (Li) doped n-type silicon containing different oxygen concentrations have been studied with deep level transient spectroscopy (DLTS) in conjunction with the capacitance-voltage (C-V) method. Two Li-related defects E(0.17) and E(0.50) situated at, respectively, 0.17 and 0.50 eV below the conduction band minimum have been observed on different conditions. It has been shown that oxygen in silicon can restrain the interaction between Li and radiation-induced defects. Only when the concentration of Li is not far less than that of oxygen in silicon can lithium interact effectively with radiation-induced defects.     

Ultrafast enhancement of ferromagnetism via photoexcited holes in GaMnAs

We report on the observation of ultrafast photoenhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100 ps time scale, after an initial subpicosecond demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature T(c) and dominates the demagnetization component when approaching T(c). We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20 K and a transient increase in T(c).     

The dopant concentration and annealing temperature dependence of ferromagnetism in Co-doped ZnO thin films

A series of Zn_1−xCo_xO thin films with the atomic fraction, x, in the range of 0.03–0.10 were deposited on glass substrates at room temperature by magnetron co-sputtering technique and subsequently coupled with the post-annealing treatment for half hour at different temperatures (350 °C and 500 °C) under vacuum. A systematic study was done on the structural, optical and magnetic properties of Zn_1−xCo_xO thin films as a function of Co concentration and annealing temperature. X-ray diffraction and UV–vis spectroscopy results indicated that there are not any secondary phases and Co²⁺ substituted for Zn²⁺ of ZnO host. Magnetic hysteresis loops were observed at room temperature, indicating that both the as-deposited samples and the annealed ones exhibit the room temperature ferromagnetism. It was also found that the magnetic saturation moment per Co atom decreases with increasing Co concentration, while the post-annealing treatment can enhance the magnetic moment of the films effectively.     

Resonant Raman spectroscopy of ethylene-acetylene copolymers doped with iodine

The relative contents of short and long conjugated chains in ethylene-acetylene copolymers (EAC) are determined by varying the lasing wavelength, as was done earlier for pure poly(acetylene). The Raman spectra of the copolymer samples doped with iodine contain the bands attributed to the iodine polyions I ₃ ^? and I ₅ ^? Unlike poly(acetylene), the ethylene-acetylene copolymer is characterized by the Raman spectra in which the intensities of the bands assigned to the I ₅ ^? polyions are higher than those of the I ₃ ^? polyions even at low degrees of doping. The structural features responsible for this difference are discussed.     

Phenomena in silicon photodiodes doped with Zn and S

The electrophysical and photoelectrical properties of Si〈S〉 were investigated and the noise and threshold characteristics of photoresistors made of this material were considered. Such photoresistors had detectivity D* (5.3; 570.1)= 5.15 × 10¹⁰ and 2.0 × 10⁻¹¹cm Hz W⁻¹, near to the background limited performance for a field of view 40° and 10° respectively, at a temperature 78 K and an electrical field of 300 V cm⁻¹. Paraphotosensitivity of a diode made of Si doped with Zn was investigated. Some peculiarities of the current-voltage characteristics of p⁺nn⁺ and p⁺np⁺ Si〈Zn〉 structures were observed. Generation-recombination noise spectral density calculations are given for the case of doping the semiconductors with double-charged deep centres using the “impedance field” method.     

The role of N dopant in inducing ferromagnetism in (ZnO)n clusters (n = 1-16)

The effect of nitrogen doping on the magnetic properties of (ZnO)(n) clusters (n = 1-16) has been investigated using spin polarized density functional theory. The total energy calculations suggest that N is more stable at the O site than at the Zn site in (ZnO)(n) clusters and induces a magnetic moment of 1 μ(B)/N atom. The N-Zn-N configuration is more stable than isolated N for 3D structures. The N dopants do not show any tendency for clustering. The binding energy is found to decrease with the increase in the number of N dopants. The magnetic moment increases gradually with the increase in the number of atoms with 1 μ(B)/N atom for n ≤ 4 and less than 1 μ(B)/N for n > 4. The local magnetic moment is mainly localized at the N site with a small magnetic moment induced at the O site. The presence of a Zn vacancy (V(Zn)) induced an additional magnetic moment of 2 μ(B) on the nearest O atoms. The N dopant prefers to form a N-V(Zn) pair. The combination of N and V(Zn) in 3D structures leads to a total magnetic moment of 3 μB. The Mulliken charge transfers from Zn to N and O in all N doped (ZnO)(n) clusters. The calculated results are consistent with existing experimental and theoretical results.     

Effect of annealing atmosphere on ferromagnetism in Mn doped ZnO films

This paper reports that Zn_0.97Mn_0.03O thin films have been prepared by radio-frequency sputtering technology followed by rapid thermal processing in nitrogen and oxygen ambient respectively. Magnetic property investigation indicates that the films are ferromagnetic and that the Curie temperature (T_c) is over room temperature. It is observed that the saturation magnetization of the films increases after annealing in nitrogen ambience but decreases after annealing in oxygen. Room temperature photoluminescence spectra indicate that the amount of defects in the films differs after annealing in the different ambiences. This suggests that the ferromagnetism in Zn_0.97Mn_0.03O films is strongly related to the defects in the films.     

Absence of intrinsic ferromagnetic interactions of isolated and paired Co dopant atoms in Zn1-xCoxO with high structural perfection

We report element specific structural and magnetic investigations on Zn(1-x)Co(x)O epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism having an unprecedented degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of 4.8 (mu B). However, the total magnetization is reduced by approximately 30%, demonstrating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetic interactions for isolated or paired Co dopant atoms in Co:ZnO films.     

High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high-T_c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T_g=(20-550) °C on single-crystal GaAs or Al₂O₃ substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5-500 K. The optimum growth temperature, T_g, for Si:Mn/GaAs layers with T_c≈400 K is shown to be about 400 °C. The Si:Mn/Al₂O₃ layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.     

Disorder-induced enhancement of conductance in doped nanowires

A new mechanism is proposed to explain the enhancement of conductance in doped nanowires.It is shown that the anomalous enhancement of conductance is due to surface doping.The conductance in doped nanowires increases with dopant concentration,which is qualitatively consistent with the existing experimental results.In addition,the I-V curves are linear and thus suggest that the metal electrodes make ohmic contacts to the shell-doped nanowires.The electric current increases with wire diameter (D) and decreases exponentially with wire length (L).Therefore,the doped nanowires have potential application in nanoscale electronic and optoelectronic devices.     

Enhancement of ferromagnetism and multiferroicity in Ho doped Fe rich BiFeO3

Crystalline solid solution of BiHo_XFe_1+XO₃ (X=0, 0.05, 0.1, and 0.15) ceramics have been successfully synthesized by slow step sintering schedule. At particular value of X (X=0.1) the sample showed good crystallinity with almost impurity free phases. It was observed that at particular concentration of X (X=0.1), the sample showed enhanced M–H loop by Ho doping in presence of excess Fe. Furthermore, the M–H loop gets reduced with the increase in value of X (X=0.15). Compared to the pure BFO, the dielectric properties for X=0.1 composition are improved due to the decreased oxygen vacancies by the stabilized oxygen octahedron. Raman spectra over the frequency range of 100–1500 cm^?1 have been systematically investigated with different concentrations of X. Besides the changes of the peak position and the line width of the all modes, the prominent frequency shift, the line broadening and variation of the intensity for the two-phonon mode between 1270 and 1280 cm^?1 were observed with increase in value of X. All these results indicate the existence of strong spin–phonon coupling in Ho doped Fe rich BFO.     

Formation of heavily doped n-type layers in GaAs by multiple ion implantation

Rare earth doping of CaF₂ produces material which gives strong thermoluminescence signals. In an attempt to separate the influence of impurity and intrinsic defects CaF₂ has been implanted with ions of Ce, Dy, Mn, Ca and F. Comparisons are made with chemically doped samples and the effect of thermal treatments have been made in all cases. The cerium and dysprosium ions influence both the shallow charge trapping levels, which determine the temperature of the glow peaks, and the recombination sites which control the photon spectra.

After implantation the strong TL signals show emission at wavelengths near 360 nm for Ce, 480 nm for Dy and for Mn.

Re-excitation of the trapping levels reveals selective emission for some defects, restructuring of less stable defects and major changes in defect concentrations with thermal treatment. The effects of the impurity and intrinsic defects on the spectra are discussed. One major observation is that addition of cerium to “pure” samples does not enhance the TL sensitivity, whereas Dy and Mn both show new glow peaks. In the case of Dy it is thought that the charge trap and the luminescent site are directly linked within one complex defect.