Semimagnetic Materials Research Articles

Influence of slot wedge material on permanent magnet losses in a traction motor with tooth coil windings

Permanent magnet (PM) machines with tooth coil windings are designed and analyzed. We study how the slot wedge material affects the behavior of PM losses when using tooth coil windings, which generate a significant air gap flux density harmonic content. The losses and cogging torques of a surface permanent magnet rotor and an interior permanent magnet rotor are compared in no-load and load situations. A finite element analysis applying Cedrat's Flux2D is made. Motors of this size can have high permanent magnet eddy current losses, and therefore, segmented magnets are often preferred even though interior magnets and semi-magnetic wedge material are used. The study shows that tooth coil windings may benefit from using semi magnetic material as a wedge material as it decreases the stator iron losses and, especially, losses in the permanent magnets.

MAGNETIC PROPERTIES Cu DOPED ZnO:Fe SEMICONDUCTORS

Magnetization measurements were performed on a series of Zn 0.9-x Fe 0.1 Cu x O samples (0 ≤ x ≤ 0.10). Small hysteresis losses are seen at low temperatures. The ferromagnetic exchange interaction has been evaluated from the inverse DC magnetic susceptibility versus T and the molecular field theory. We found that Cu -doping greatly enhances the FM exchange interaction, increases the saturation magnetization, hysteresis losses and the magnetic susceptibility. Arrott-Kouvel plot has been used to study the spontaneous magnetic moment at low temperatures. We show that hysteresis losses cannot be taken as the only evidence for the appearance of the ferromagnetic order in these semiconducting semi-magnetic materials.

Transport properties of a Ga1-xMn xAs/Ga1-yAl yAs double-barrier

We study the transport properties of a spin filter consisting of a double-barrier resonant tunneling device in which the well is made of a semimagnetic material. Even if the device could be made of several materials, we discuss here the case of a Ga1-xMnxAs/Ga1-yAlyAs system because it can be integrated into the well known AlAs/GaAs technology. We solve the Hamiltonian H = HK+HP+HE+HM+Hh-i+Hh-h. Its terms represent the kinetic energy, the double-barrier profile, the applied bias, the magnetic interaction, the hole-impurity attraction and the hole-hole repulsion, respectively. A very simple one-dimensional Green function is introduced to solve self-consistently the Poisson equation for the profile due to the charge distribution. A real space renormalization formalism is used to calculate exactly the currents. In a previous work we have shown that the Rashba effect is weak. Therefore the results show very well defined spin-polarized currents. Our results confirm that this system is a good device for spintronics.

A selfconsistent calculation of the transport properties of a double barrier spin filter

A double barrier resonant tunneling device in which the well is made of a semi-magnetic material can work as an efficient spin filter. Today it is possible to make semiconductors that are ferromagnetic at room temperature. Therefore the device studied here has a great potential to be used as a polarizer, an analyzer and other spintronic applications. We discuss here the case of a Ga1-xMn xAs/Ga1-yAl yAs system because it can be integrated into the well known AlGaAs/GaAs technology. Our tight-binding Hamiltonian includes the kinetic energy, the double-barrier profile, the electric field, the magnetic term, the hole-impurity and the hole-hole interactions. The profile and the charge distribution are calculated self-consistently. In previous works we studied this system by solving the Hamiltonian in the reciprocal space, in order to simplify the treatment of the Poisson equation for the charge distribution. Here we introduce a simple one dimensional Green function that permits to solve all terms in the real space. Besides, a real space renormalization formalism is used to calculate exactly the electronic currents for each spin polarization. The results confirm that the proposed system is a good device for spintronics.

Progress in III-nitrides: Process issue and purity perspective

The growth of good quality layers of gallium nitride (GaN) as suitable for epitaxial growth is of great technological importance. Chloride vapour phase epitaxy (Cl-VPE) has been employed to grow good quality layers of GaN. The grown layers have been extensively characterized for their structural and optical properties. MOVPE grown GaN layers have been used to address process issues on device structuring and fabrication. GaN samples with different transition metal dopants have been synthesized and their usefulness as semi-magnetic materials, which are also identified as dilute magnetic semiconductors (DMS), have been evaluated. Better results have been obtained on the magnetic characteristics of GaN with ruthenium as the dopant. Nano dimensional structures of GaN have been obtained with excellent control of the growth parameters.

Electrophysical properties of semimagnetic solid solutions Hg1−xMnxTe

A comprehensive study of the electrophysical properties of the semimagnetic ternary solid solution Hg1−xMnxTe, an alternative material to Hg1−xCdxTe, is reported. The charge-carrier scattering, optical, photoelectric, and magnetic properties of the material are investigated. Values are obtained for the effective masses of electrons and holes, the ionization energy of the acceptor level, and the g factor of the charge carriers as functions of the manganese telluride concentration and the densities of electrons and holes at temperatures of 300 and 77 K. It is shown that the methods of radio spectroscopy can be used for diagnostics of the semimagnetic material. Photodiode structures having characteristics close to the values for working in the background-limited regime are obtained by the use of Schottky barriers, diffusion in mercury vapor, and implantation of B+ ions into p-Hg1−xMnxTe samples.

Exciton condensation in semimagnetic CdTe/CdMnTe multiple quantum wells

The exciton condensation in a magnetic field, parallel to the interfaces, tunable type-I/type-II multiple quantum well structure made by semimagnetic materials of CdTe/CdMnTe has been investigated theoretically. In the type-II regime, the magnetic field results in suppression of exciton condensation, which in turn is attributed to the field-induced polarization of exciton, the increases of magnetic confinements and interacting strengths of electrons and holes. Detailed calculations show an ∼10meV insulating gap in the case of B=20T and n≈16×1010cm−2, and exhibit a transition from Bose–Einstein condensation to BCS condensed phase with increased excitonic density.

High-magnetic-field EPR of Cr-based diluted magnetic semiconductors

We have studied magnetic interactions in chromium-based diluted magnetic semiconductors by measuring in detail the electron paramagnetic resonance spectrum of ${\mathrm{Cr}}^{2+}$ over the frequency range between 30 and 210 GHz at high magnetic fields up to 20 T oriented along different crystal axes. At low temperatures, crystals of chromium-alloyed zinc chalcogenides ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}$ (S, Se, Te) demonstrate complex properties that are neither of Brillouin nor of Van Vleck type. The behavior of these ${\mathrm{Cr}}^{2+}$ impurities in semimagnetic materials results from the interaction of the ground state with the low-lying electronic states of the $3d{\mathrm{Cr}}^{2+}$ ion and from the Jahn-Teller-induced distortion of the ${\mathrm{Cr}}^{2+}$ position in the host lattice. We measured optical transitions between these states and we could determine the magnetic-field dependence and the anisotropy of the electronic levels. We explain our results with the crystal-field model proposed by Vallin et al.,1 including a cubic crystal field and Jahn-Teller distortion, and we evaluate the dependence of the model parameters on the semiconductor host lattice.

Anisotropic Zeeman splitting in semimagnetic quantum-well structures

The Zeeman splitting pattern in semimagnetic superlattices and asymmetric double quantum wells composed of different sequences of semimagnetic and nonmagnetic well and barrier materials have been investigated experimentally and theoretically, in particular, in dependence on the orientation of an external magnetic field and the lattice mismatch induced biaxial strain. The magneto-optical anisotropy, which appears by varying the tilting angle of the magnetic field, can be explained by coupling of heavy- and light-hole states of the ${\ensuremath{\Gamma}}_{8}$ valence band. Theoretical results have been obtained by transfer matrix method and are compared with experimental photoluminescence and photoluminescence excitation spectra obtained on superlattices and asymmetric double quantum wells.

Electron Transport in Submicron Wires of Semiconductors

We review the methods of fabrication and transport properties of submicron II-VI, IV-VI and IlI-V semiconductor wires. Devices were prepared by electron-beam lithograply and used for detailed magnetotransport studies, carried out at low (down to 30 mK) temperatures. We discuss a number of novel features obtained in ballistic, diffusive and localized transport regimes. In particular, we describe the universal conductance fluctuations for semimagnetic materials (CdMnTe) and discuss the edge channel transport for PbTe, PbSe and GaAs/GaA1As systems.

Experimental determination of the effect of alloy composition on the band alignment of the CdTe-Cd1-xMnxTe heterojunction.

A technique based on photoluminescence excitation measurements is described for the direct determination of the band alignment in a quantum well formed from nonmagnetic and semimagnetic materials. Results are presented for a series of CdTe-${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te quantum well samples. These show that the valence band offset, when expressed as a fraction of the total band discontinuity, is a function of the alloy concentration x in the barrier.

Magneto-Optical Anisotropy in Semimagnetic Materials: The Interplay of Strain and Magnetic Field

Magneto-Optical Anisotropy in Semimagnetic Materials: The Interplay of Strain and Magnetic Field

Phonon-assisted interband magneto-optical transitions in HgCdMnTe

Photovoltaic oscillations due to interband magneto-optical transitions have been observed on the quaternary open-gap semimagnetic alloys Hg1-x-yCdxMnyTe. In addition to the conventional transitions, phonon-assisted interband excitations have been observed for the first time on a semiconductor. It is argued theoretically that the resonant phonon-assisted interband transitions must originate from the camel-back shaped Landau subbands. Consequently, the authors observations confirm the existence of such subbands in the Gamma 8 valence bands of semimagnetic materials. The determined optic phonon energies correspond to the longitudinal HgTe-related mode.

Magneto-optics in II–VI compound type III superlattices

Magneto-optical experiments on n-type Hg 1- x Zn x Te-CdTe superlattices yield the determination of the conduction band dispersion. Comparison with theoretical calculations demonstrates the semimetallic character of the superlattices and support a rather large value of the valence band offset. Similar experiments performed on a Hg 1- x Mn x Te-CdTe superlattice at various temperatures show evidence of the exchange interaction in this new semimagnetic material.

Magneto-optical studies of CdTe/CdMnTe semimagnetic semiconductor superlattices under high pressure

Abstract The photoluminescence (PL) of a CdTe/CdMnTe superlattice has been studied at pressures up to 4.1GPa, where the phase transition occurs. PL is observed up to this pressure, and it moves to higher energy with pressure at 66meV/GPa. This result is consistent with theory. Magnetic fields decrease the band-gap of the semimagnetic CdMnTe barriers and this reduces the PL energy. The pressure dependence of this effect is expected to provide a stringent test of the theory of semimagnetic materials and of superlattices.

K

We present a k\ensuremath{\cdot}p theory of semiconductor superlattices in an applied magnetic field. We consider superlattices with a [001] growth axis and the magnetic field along the growth axis. A single-basis set for the constituent materials is provided by a zone-center pseudopotential calculation with a reference Hamiltonian. The ${\ensuremath{\Gamma}}_{15}$ valence and ${\ensuremath{\Gamma}}_{1}$ conduction states are coupled with a spinor and treated explicitly. Nearby energy states are treated in L\"owdin perturbation theory with the k\ensuremath{\cdot}p operator and the difference between the material pseudopotential and the reference pseudopotential as the perturbation. The calculation is carried out consistently to first order in wave functions and second order in energies. Magnetic, exchange (in semimagnetic materials), spin-orbit, and strain (in strained-layer superlattices) interactions are included between the explicitly included states. When inversion-asymmetry and warping terms are dropped in the Hamiltonian, a Landau index becomes a good quantum number. Bloch and evanescent states are computed for a fixed Landau index in each material. Interface matching of the constituent-material bulk eigenfunctions is accomplished with use of results derived for the normal component of the current density operator. The Landau indices are not mixed by the interface matching. Superlattice translational symmetry is used to derive an eigenvalue equation for the superlattice wave vectors and eigenfunctions. The numerical implementation of the formal results is described and used to investigate a nonmagnetic superlattice ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0}$.${48\mathrm{I}\mathrm{n}}_{0.52}$As and a semimagnetic superlattice ${\mathrm{Hg}}_{0.95}$${\mathrm{Mn}}_{0.05}$Te/${\mathrm{Cd}}_{0.78}$${\mathrm{Mn}}_{0.22}$Te.

II-VI Compounds with Fe - New Family of Semimagnetic Semiconductors

AbstractSeveral experimental methods: absorption, photoemission and transport measurements were used to determine the energy position of substitutional Fe2+ (3d6) donor state in the band structure of the semimagnetic semiconductor Hg1-v-xCdvFexSe for 0≤v≤0.7 and v+x=l, and x≤0.15. For v≤0.40, Fe2+(3d6) level is a resonant donor located in the conduction band. For v=O (HgSe) we obtain 230 meV for the position of Fe2+(3d6) level with respect to the bottom of the conduction band which coincides with the position of the Fermi level for electron concentration N ≅5x1018 cm-3. Surprisingly, the mobility of free electrons (T∼4.2K) is abnormally high and the Dingle temperature measured in quantum magnetoresistivity oscillations (SdH effect) and magnetooptical measurements is abnormally low. Because of the Coulomb interaction between the ionized donors, at low T, there will appear some correlation of their positions. This may lead to a kind of “liquefying” of the system of ions and to its “crystallisation” (i.e. formation of a superlattice or hyperlattice of ionized donors) at even lower T. The space-ordering of ionized donors influences dramatically the free-carrier scattering and correspondingly explains the high mobility and low Dingle temperature. Finally, we shall also present some magnetic properties of these new semimagnetic materials.