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.     

Engineering of Ternary Ferromagnetic Semiconductors in Heterojunctions

Compatibility of room temperature ferromagnetic chalcopyrite compounds with Si, Ge, and GaAs wafers was considered. Physical–chemical aspects of heterophase epitaxial growth were analyzed. Ternary ferromagnetic compounds II–IV–V₂Mn already successfully grown on CdGeP₂ and ZnGeP₂ semiconductor single crystals are suggested to serve as good heteropartners as well for spin injecting contacts on Si- and GaAs-based devices.     

Growth and characterization of CuInSe2 thin films

Films of CulnSe₂ have been grown using a technique similar to close-spaced vapour transport. The effect of substrate temperature and the distance of the substrate from the source have been optimised to grow well-oriented chalcopyrite phase of CuInSe₂. D.c. conductivity and Hall coefficient studies have been made in the temperature range 77–300 K. The films grown at a substrate temperature of 350 °C have an electron mobility of 3.4×10³ cm²V^–1 s^–1 at 77 K.     

Pressure Effects on the Structural and Elastic Properties of AgGaS2–AgGaSe2Crystals: An X-ray Diffraction Study

The structural and elastic properties of AgGa(S_{1 – x} Se _x )₂crystals with the tetragonal chalcopyrite structure were studied by x-ray powder diffraction at pressures of up to 4 GPa. All of the crystals were found to have anisotropic elastic properties, with thec-axis compressibility notably exceeding the a-axis compressibility. The compression along the longer bond Ag–S(Se) is smaller than that along the Ga–S(Se). The compressibility anisotropy in the solid solutions and AgGaSe₂is stronger than that in AgGaS₂. The composition dependences of the elastic constants are nonlinear.     

Magnetic properties of proximity-induced superconducting matrix in multifilamentary wires

Critical current densities J_cp of proximity-induced superconducting matrices in NbTi multifilamentary wires are estimated from measured twist-pitch dependence of magnetization. The values of J_cp are 2–4 orders of magnitude smaller than those of the superconducting filaments and decrease rapidly by raising temperature T and magnetic field B_e. The interfilamentary spacing d_N of submicrons results in zero screening length at measured temperatures ranging 4.5 to 8 K and the induced superconductivity is suggested to be type-II. The upper critical field B_c2p is obtained by applying the scaling law to J_cp data. B_c2p's at 4.5 K are 1.2–3.0 T for d_N = 0.20–0.59 m. The Cooper pair penetration length K _N ^–1 estimated from the temperature dependence of B_c2p shows the clean limit characteristic as T^–1 and 0.32 m for d_N = 0.59 m wire at 4.5 K. K _N ^–1 estimated from the temperature dependence of J_cp agrees consistently with those from B_c2p(T).     

Magnetic Properties and Spin-Glass State of CdCr2 – xInxSe4 and Cd1 – yInyCr2Se4 Solid Solutions

CdCr_{2 – x} In _x Se₄ and Cd_{1 – y} In _y Cr₂Se₄ solid solutions were prepared and their stoichiometry ranges were determined. Increasing the In content was found to increase the lattice parameter and decrease the Curie temperature. In both solid-solution systems, paramagnetic–ferromagnetic–spin-glass phase transitions were revealed.     

Dielectric and Magnetic Properties of SrTiO3–BiMnO3 Solid Solutions

SrTiO₃–BiMnO₃ solid solutions with a cubic perovskite structure are synthesized in the range 30–70 mol % BiMnO₃. The solid solutions containing 60 and 70 mol % BiMnO₃ exhibit high dielectric losses in the microwave range and are, therefore, potential microwave absorbing materials. The real part of rf magnetic susceptibility of the sample containing 60 mol % BiMnO₃ lies in the range 2–3.     

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.     

Magnetic order in superconducting Dy x Ce1−x Ru2

Magnetization and the ac susceptibility have been measured for x = 0–0.4 in Dy _x Ce_1–x Ru₂. The magnetization curves are different from that expected for a well-established spin-glass. The magnetic order in superconducting Dy _x Ce_1–x Ru₂ is not of the spin-glass type, rather is that of the nearly ferromagnetic type. This type of magnetic order and superconductivity coexist in the form of domains.     

Physicochemical Properties of (CuAlSe2)x(2ZnSe)1 – xSolid Solutions

(CuAlSe₂) _x (2ZnSe)_{1 – x} solid solutions were prepared by a single-zone method. According to x-ray diffraction characterization, the solid solutions had the chalcopyrite structure for x> 0.7 and sphalerite structure for x< 0.7. CuAlSe₂, ZnSe, and (CuAlSe₂) _x (2ZnSe)_{1 – x} crystals were grown by chemical vapor transport and were used for microhardness tests and transmission measurements near the fundamental edge. The results demonstrate that the microhardness and band gap of the solid solutions pass through a maximum and minimum, respectively.     

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.     

Surface characterization of sprayed CuGa x In1−x Se2 layers

Small-area polycrystalline CuGa _x In_1–x Se₂-based solar cells produced in the laboratory have been reported with efficiencies up to 18.8%. This success at achieving high-efficiency devices has stimulated a parallel research effort to produce large-area modules based on the use of CuGa _x In_1–x Se₂ with reduced cost of production. One method that has potential to achieve these objectives is chemical spray pyrolysis. Preliminary studies have resulted in devices with efficiencies of 4–5%. Further improvements toward implementing higher-efficiency devices are expected to result from a better understanding of the physical and chemical properties of the chalcopyrite layers produced. In this work we have investigated, for the first time, the properties of these layers using X-ray photoelectron spectroscopy (XPS) to identify the secondary phases and residual impurities present. The layers studied were for In/Ga mole ratios of x=0, 0.2, 0.4, 0.6, 0.8, and 1.0 in CuGa _x In_1–x Se₂. The layers were also profiled by sputter etching away the surface of the layers and repeating the XPS measurements. The effects of annealing the layers in air, vacuum, and selenium vapor were also investigated.     

Magnetic ordering in Cu-Zn ferrite

The variation of magnetization with temperature of the Zn _x Cu_1–x Fe₂O₄ system has been obtained between 300 K and the Néel temperature at a constant magnetic field of 5.57×10⁵ A m^–1 for x=0 to 0.8. The observations indicate the existence of a Yafet-Kittel (Y-K) type of magnetic ordering in the mixed ferrites. A molecular field analysis of the Y-K spin-ordering using a three-sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Néel temperatures of Cu-Zn ferrites were also determined from Mössbauer studies.     

Curie Temperatures of III–V Diluted Magnetic Semiconductors Calculated from First-Principles in Mean Field Approximation

Electronic structure and ferromagnetism in III–V compound-based diluted magnetic semiconductors (DMS) are investigated based on first-principles calculations by using the Korringa-Kohn-Rostoker method combined with the coherent-potential-approximation. The stability of the ferromagnetic phase in GaN-, GaAs-, GaP-, GaSb-based DMS is investigated systematically. The calculations show that 3d-impurities from the first-half of the transition metal series favor the ferromagnetic state, while impurities from the latter-half of the series exhibit spin-glass behavior. This chemical trend in the magnetism is explained by the double exchange mechanism taking the local symmetry at the impurity gap states into account. Curie temperatures of GaAs- and GaN-based DMS are estimated by using the Heisenberg model in a mean field approximation with the parameters calculated from first-principles. It is suggested that room-temperature ferromagnetism can be realized in these systems.     

Magnetic properties of Nd–Fe–B/FeMn multilayer films

[Nd–Fe–B(x nm)/FeMn(d nm)]_n thin films were deposited by magnetron sputtering on Si (100) substrates heated at 650 °C. The influence of the composition and thickness of FeMn layer on the structure and magnetic properties of Nd–Fe–B films are investigated. The Nd–Fe–B/FeMn multilayer films present an enhanced coercivity and a reduced saturation magnetization, in comparison with those of a Nd–Fe–B single layer. The coercivity of [Nd–Fe–B(x nm)/FeMn(5 nm)]_n films increases with increasing the period number of FeMn layer for the same thickness of magnetic layer, while the coercivity in [Nd–Fe–B(50 nm)/FeMn(5 nm)]_n films increases with decreasing the period number of Nd–Fe–B/FeMn bilayers. The coercivity H_c of about 17.2 kOe is achieved in the Nd–Fe–B(50 nm)/FeMn(5 nm) film.     

Magnetic and Electrotransport Properties of the Anion-Deficient Manganites with Perovskite Structure

The chemical phase content and microstructure as well as magnetic and magnetotransport properties for the doped anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 orthomanganites have been experimentally studied. The high-quality anion-deficient ceramic with controlled chemical phase content and microstructure has been obtained from the parent stoichiometric one at the 800 C in vacuum. The average grain size is ~5 m. It is established that the samples in the region of 0 x 0.125 are O-orthorhombic perovskites whereas in the 0.175 x 0.30—rhombohedric. As doping level increases along with the oxygen vacancies number the samples in ground state undergo a transition from the antiferromagnet A-type (x=0) through the inhomogeneous magnetic state (0 < x 0.175 to the cluster spin glass one (0.175 < x 0.30. The temperature of magnetic moment freezing is ~ 45 K. All the samples are semiconductors and show considerable magnetoresistance over a wide temperature range with peak for x = 0.175 only. Fitting of the electrical resistivity to T^–1 and T^–1/4 realized. The concentration dependences of the activation energy, spontaneous magnetization, coercivity as well as magnetic transition temperatures for the anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 ortomanganites have been established. Obtained experimental results are interpreted in terms of the oxygen vacancies, isotropic superexchange Mn³⁺-O–Mn³⁺ interactions and phase separation models.     

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.     

Magnetic field dependence of the thermopower of dilute aluminum alloys

Measurements are presented of the thermopowerS of pure Al and dilute Al alloys from 2 to 6 K in magnetic fields up to 20 kG. For all samples the data are consistent with the equationS(H)=A(H)T+B(H)T ³ , allowing a separation of the electron diffusion thermopower componentS _e (H)=A(H)T from the phonon-drag componentS _g (H)=B(H)T ³ . Zero-field characteristic values ofS _e were obtained for the impurities Cu, Cd, Tl, and Sn:S _e (Cu) =–0.6T×10^–8 V/K;S _e (Cd)=–4T×10^–8 V/K;S _e (Tl)=–3T×10^–8 V/K;S _e (Sn) =–2T×10^–8 V/K. The phonon-drag coefficientB was found to vary from impurity to impurity, a variation attributed to anisotropic electron-impurity scattering. Upon application of a transverse magnetic field,A(H) was found first to become more positive, and then to saturate in value at high fields. The quantity A=A(H)–A(H=0) was found to be very nearly the same for all impurities, varying in value only from 2.1–2.6×10^–8 V/K. Both the general behavior ofA(H) and this magnitude for A are shown to be determined primarily by the form of the Fermi surface of pure Al.B(H) was also observed to vary with magnetic field. This variation is tentatively attributed to a combination of a changing importance with magnetic field of electrons in the second and third Brillouin zones of Al and anisotropic electron-impurity scattering, but no detailed explanation is yet available. Finally, the Wiedemann-Franz ratio of an impure Al-Cu sample is found to have the value 2.43 ⁵ ±0.02×10^–8 W·/K ² at 4.5 K, independent ofH up to 20 kG, the highest field used. This value is in good agreement with the expected value :L ₀ =2.45×10^–8 W·/K ² .This work was supported in part by the U.S. Atomic Energy Commission under Contract No. AT(11-1)-1247 and in part by the N.S.F. under Grant No. GH-34124. Based primarily upon a thesis submitted by one of us (R.S.A.) in partial fulfillment of the requirements for the Ph.D. degree at Michigan State University.     

Magnetic properties of hydrothermally synthesized strontium hexaferrite as a function of synthesis conditions

Fine particles of strontium hexaferrite, SrFe₁₂O₁₉, with a narrow size distribution have been synthesized hydrothermally from mixed aqueous solutions of iron and strontium nitrates under different synthesis conditions. The relationship between the synthesis variables (temperature, time and alkali molar ratio) and the magnetic properties has been investigated. The results have shown that, as the synthesis temperature increases, the saturation magnetization of the particles increases up to a plateau and the coercivity decreases. As the alkali molar ratio R(=OH^–/NO ₃ ^– ) increases, the coercivity decreases and goes through a local minimum, while the saturation magnetization increases and goes through a local maximum. Increasing the synthesis time from 2 h to 5 h has no significant effect on the saturation magnetization, but decreases the coercivity. An anisotropic sintered magnet with a high saturation magnetization value of 67.26 e.m.u g^–1 (4320 G) has been fabricated from the hydrothermally synthesized powders.Relationship between the c.g.s and S.I.units which are used in this paper are as follows: 1 erg = 10^–7 J, 1 e.m.u. cm^–3 = 12.57×10^–7 Wom^–2 (tesla), 1 oersted (Oe) = 79.6 A m^–1, 1 G = 10^–4 tesla (T).     

Exciton Spin Manipulation in ZnMnSe-Based Structures

Strong effect of structural design on spin functionality is observed in quantum structures based on II–VI semiconductors. Spin switching is realized when using a thin layer of Zn^0.95Mn^0.05Se diluted magnetic semiconductor (DMS) as a spin manipulator. This is evident from the polarization of photoluminescence related to a spin detector (an adjacent nonmagnetic quantum well (QW)) measured under the resonant excitation of the spin-up and spin-down states of the DMS, which is identical in value but opposite in sign. The achieved spin switching is suggested to reflect fast carrier diffusion from the DMS due to the absence of an energy barrier between the upper spin state of the DMS layer and the QW. On the other hand, the spin alignment is accomplished in the tunneling structures where the presence of the energy barrier inserted between a spin manipulator (i.e., a ZnMnSe/CdSe DMS superlattice) and a spin detector ensures a slow escape rate from the DMS layer.