Ab Initio Study of Magnetism in III-V- and II-VI-Based Diluted Magnetic Semiconductors

Electronic structure and magnetic properties of Ga_1–x Mn _x As, Ga_1–x Mn _x N, Zn_1–x M _x O, and Zn_1–x M _x Te (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64-atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMSs. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMSs strongly correlates with the occupation and energy position of 3d-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM vs. AFM competition is discussed.     

Preparation and Characterization of IV–VI Diluted Magnetic Semiconductor Ge1?xCrxTe

IV–VI diluted magnetic semiconductor Ge_1–x Cr _x Te films with different Cr compositions up to x = 0.33 were successfully prepared by a sputtering method. Curie–Weiss behavior, giving that the paramagnetic Curie temperature is 29 K and the effective number of the Bohr magneton is 4.67, is clearly observed for a Ge_1–x Cr _x Te (x = 0.07) film. All the films show ferromagnetic order at low temperatures. The easy direction of the magnetization orients perpendicular to the plane. As the Cr composition increases, the ferromagnetic Curie temperature determined from the temperature dependence of the residual magnetization tends to increase up to 25 K for x = 0.33.     

Observation of Combined Ferromagnetic/Paramagnetic Phase in Ga1?xMnxAs by Magnetic Circular Dichroism

Magnetic properties of a series of Ga_1–xMn_xAs layers with different Mn and hole concentrations has been studied by measuring magneto-absorption and magnetic circular dichroism (MCD). We first focus on comparing the MCD spectra of samples with unambiguous ferromagnetic or paramagnetic magnetization. We then investigate MCD in a sample with parameters between these two extremes, and interpret the observed behavior in terms of the coexistence of ferromagnetic domains and paramagnetic regions that may result, for example, from inhomogeneities in the sample caused, e.g., by compositional or doping fluctuations.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

Magnetic Behaviors of Ferromagnetic Semiconductor Zn1?xCrxTe Grown by MBE

The magnetic and structural properties of MBE-grown films of Zn_1–xCr_xTe were investigated. The magnetization versus magnetic field (M–H) measurement of Zn_1–xCr_xTe (x = 0.01–0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T_C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M–T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.     

Mn- and Cr-Doped InN: A Promising Diluted Magnetic Semiconductor Material

Cr- and Mn-doped InN films were successfully grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. Low temperature GaN buffer layers grown by metal-organic vapor-phase epitaxy were used to accommodate the large lattice mismatch between InN and sapphire. A high n-type carrier concentration of 1.5×10²⁰ cm^–3 was measured in InN films with 3% Cr-doping. Films of this type exhibit a well-defined in-plane magnetic hysteresis loop and remanence for temperatures varying from 5 to 300K. The Mn-doped films, however, turned out to exhibit less clear magnetic properties. Thus, ferromagnetism in Cr-doped InN can be concluded from our measurements.     

Laser-Controlled Magnetization in a Single Magnetic Semiconductor Quantum Dot

The photoluminescene signal of individual semimagnetic CdSe–Zn_0.75Mn_0.25Se quantum dots is used to study the magnetization of the Mn²⁺ spin system in the exchange field of a single exciton. We demonstrate that by increasing the laser excitation power a significant blue shift of the photoluminescence signal occurs. This is attributed to a laser-induced demagnetization, i.e. the laser-generated carriers heat the Mn²⁺ spin system via spin–flip exchange scattering.     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1–x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^–3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.     

Theory of Cyclotron Resonance and Magneto-Optics in n- and p-Type InMnAs in Ultra-high Magnetic Fields

We present a theory for the electronic and optical properties of n- and p-type In_1–x Mn _x As in ultra-high magnetic fields. An eight-band effective mass model based on the Pidgeon–Brown model and including the wavevector dependence of the electronic states as well as the s–d and p–d exchange interactions with Mn d-electrons is used to determine the electronic states. The optical properties such as cyclotron resonance are computed using Fermi's golden rule. Comparison of the theory with ultra-high magnetic field (>50 T) cyclotron resonance experiments shows that the electron cyclotron resonance peak shifts with Mn doping and that the shift allows one to extract the Mn-electron/hole exchange parameters, and . The hole cyclotron resonance shows multiple resonance peaks, which we attribute a heavy to heavy and light to light hole transitions.     

Low Temperature Annealing Studies of Ga1?xMnxAs

High and low field magnetotransport measurements, as well as SQUID measurements of magnetization, were carried out on Ga_1–x Mn _x As epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280^°C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga_1–x Mn _x As annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga_1–x Mn _x As.     

Cyclotron Resonance of Electrons and Holes in Paramagnetic and Ferromagnetic InMnAs-Based Films and Heterostructures

We have studied the cyclotron resonance of electrons and holes in various types of InMnAs-based structures at ultrahigh magnetic fields. Our observations, in conjunction with an eight-band effective mass model including the s–d and p–d exchange interactions with Mn d-electrons, unambiguously suggest the existence of s-like and p-like delocalized carriers in all samples studied. The samples studied include Paramagnetic n-type In_1–x Mn _x As films (x 0.12) grown on GaAs, ferromagnetic p-type In_1–x Mn _x As films (x 0.025) grown on GaAs with Curie temperatures (T _C) > 5 K, paramagnetic n-type In_1–x Mn _x As/InAs superlattices, ferromagnetic p-type In_1–x Mn _x As/GaSb heterostructures (x 0.09) with T _C = 30-60 K, and ferromagnetic (In_0.53Ga_0.47)_1–x Mn _x As/In_0.53Ga_0.47As heterostructures (x 0.05) grown on InP with T _C up to 120 K.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Magnetic Order in GaMnAs Layers

MBE grown Ga_1–x Mn _x As layers were investigated by means of magnetic resonance techniques. Two phases can be distinguished: an almost isotropic ferromagnetic phase in insulating layers and an anisotropic ferromagnetic phase in the metallic Ga_1–x Mn _x As. Under a strong magnetic field the field-induced insulator-to-metal transition is accompanied by the change from the ferromagnetic to the ferrimagnetic phase.     

Effective removal and fixation of Cr(VI) from aqueous solution with Friedel's salt

Friedel's salt (3CaO·Al₂O₃·CaCl₂·10H₂O or Ca₄Al₂(OH)₁₂Cl₂(H₂O)₄) is a calcium aluminate hydrate formed by hydrating cement or concrete in seawater at a low cost. In the current study, we carefully examined the adsorption behaviors of Friedel's salt for Cr(VI) from aqueous solution at different concentrations and various initial pHs. The adsorption kinetic data are well fitted with the pseudo-first-order Lageren equation at the initial Cr(VI) concentration from 0.10 to 8.00 mM. Both the experimental and modeled data indicate that Friedel's salt can adsorb a large amount of Cr(VI) (up to 1.4 mmol Cr(VI)/g) very quickly (t_1/2 = 2–3 min) with a very high efficiency (>99% Cr(VI) removal at [Cr] < 4.00 mM with 4.00 g/L of adsorbent) in the pH range of 4–10. In particular, the competitive adsorption tests show that the Cr(VI) removal efficiency is only slightly affected by the co-existence of Cl⁻ and HCO₃⁻. The Cr(VI)-fixation stability tests show that only less than 0.2% adsorbed Cr(VI) is leaching out in water at pH 4–10 for 24 h because the adsorption/exchange of Cr(VI) with Friedel's salt leads to the formation of a new stable phase (3CaO·Al₂O₃·CaCrO₄·10H₂O). This research thus suggests that Friedel's salt is a potential cost-effective adsorbent for Cr(VI) removal in wastewater treatment.     

Anomalous Conductivity in Random Magnetic (Ga,Cr) As

The electrical conductivity of molecular beam epitaxy grown zinc blende Ga_1–x Cr _x As, x = 0.1, exhibits anomalous behavior below room-temperature. The room-temperature resistivity is small, = 0.1 cm, and comparable to (Ga,Mn)As. Near room temperature, the conductivity is activated, following = ₀ exp(–E _A/kT ), with an activation energy of E _A = 63 meV. In this activated region, Hall measurements also show activated behavior in the hole concentration where p 10²⁰ cm^–3, indicating that Cr can also act as an acceptor similar to Mn. For decreasing temperature, the resistivity increases rapidly because of hopping or tunneling conduction, and becomes strongly insulating at low temperatures. From T = 20 to 200 K the conductivity follows exp[–(T ₁/T)^1/2] over eight orders of magnitude range of conductivity, possible evidence of tunneling between metallic-like polarons.     

Growth and Optical Properties of Quantized CdS Crystallites in TEOS Silica Xerogels

Quantized CdS crystallite-doped tetraethylorthosilicate (TEOS) silica xerogels are prepared by the sol-gel method. In this method, cadmium acetate [Cd(CH₃COO)₂2H₂O]-doped TEOS alcogel is formed by the hydrolysis and polycondensation of ethanolic TEOS in the presence of hydrochloric acid (HCl) and ammonium hydroxide (NH₄OH) catalysts and Cd(CH₃COO)₂.2H₂O. The CdS crystallites are formed in the alcogel by the reaction of Cd(CH₃COO)₂.2H₂O present in the gel and methanolic sodium sulfide (Na₂S), which is added over the alcogel. The effect of CdS/TEOS, EtOH/TEOS, S/Cd molar ratios, and temperature on the optical properties and CdS crystallite sizes in the xerogels are studied. A blue shift is observed in optical absorption spectra by decreasing the CdS/TEOS molar ratio from 2 × 10^–2 to 1 × 10^–4. It is observed that the crystallite size is increased from 1.6 to 3.4 nm by increasing the EtOH/TEOS molar ratio from 2 to 20, respectively, for a constant CdS/TEOS molar ratio of 5 × 10^–4. Emission spectra of xerogels are measured and found that the emission peak maxima shifted toward lower energies (higher wavelengths) by increasing the CdS/TEOS molar ratio in the xerogels. It is known from the X-ray diffraction (XRD) measurements of CdS-doped xerogels that the CdS crystallite structure in the xerogels is hexagonal wurtzite. The crystallite sizes were calculated from the XRD patterns and tight bonding calculations. There is a significant change in the color and size of CdS crystallite in the xerogels with a variation in temperature from 200 to 400°C.     

Electrochemical behaviour of iron—chrome alloys in relation to pitting corrosion

The polarization behaviour and pitting corrosion of Fe-Cr alloys of 7, 13, 18, 24 wt% Cr were studied. Potentiodynamic and galvanostatic measurements were performed in the absence and presence of Cl^–. As the Cr content increases the active dissolution current densities decrease while the passive range and transpassive current densities increase. Polarization parameters gave for the passive transition of the alloys a Cr concentration of -13%. An increase of Cl^– concentration causes the progressive destruction of passivity. It interfered with O₂ evolution, and then destroyed the transpassive region. Still higher Cl^– concentrations initiated pitting corrosion as shown by the oscillations in potential of the galvanostatic polarization curves supported by visual observation. Results are discussed on the basis of competitive adsorption between the aggressive and inhibitor anions for the active sites on the alloys' surface.     

Magnetization of EuS–PbS Multilayers with Antiferromagnetic Interlayer Coupling

SQUID magnetometry is applied to study the temperature and magnetic field dependence of magnetization M(T, H) of semiconductor EuS–PbS ferromagnetic multilayers grown on insulating KCl(100) and conducting n-PbS(100) monocrystalline substrates. For low external magnetic fields (of the order of 10 Oe) and PbS spacer layers thinner than about 2 nm, we observe in EuS–PbS–EuS trilayers the strongly nonmonotonic temperature dependence of magnetization with almost zero total magnetic moment below the Curie temperature. The application of the magnetic field of the order of 100 Oe restores the regular monotonic increase of magnetization with decreasing temperature. To explain this M(T,H) dependence we present a model that considers the competition of three (temperature dependent) contributions to the total magnetic energy of the trilayer: the antiferromagnetic interlayer interaction energy, the Zeeman energy, and the energy of in-plane magnetocrystalline anisotropy.     

Charging Effects on Nuclear Spin Relaxation in Quantum Dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with 2D gas. We show that the nuclear spin relaxation rate T ₁ ^–1 is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin–orbit (SO) coupling the relaxation rate is maximal in the CB valleys whereas for the weak SO coupling the maximum of 1/T ₁ is near the CB peaks. The physical mechanism of nuclear spin relaxation rate at strong SO coupling is identified as Debye–Mandelstam–Leontovich–Pollak–Geballe relaxational mechanism.