High temperature ferromagnetism in GaAs-based heterostructures with Mn delta doping

We show that suitably designed magnetic semiconductor heterostructures consisting of Mn delta (delta)-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the two-dimensional hole gas wave function, realized remarkably high ferromagnetic transition temperatures (T(C)). A significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low-temperature annealing led to high T(C) up to 250 K. The heterostructure with high T(C) exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.     

Transport and ferromagnetism in InGaAs quantum well structures delta-doped with Mn

Samples containing InGaAs quantum wells with p-type conductivity delta-doped by Mn were synthesized and studied. Magnetic moment measurements on a SQUID magnetometer revealed the presence of ferromagnetism at temperatures T from 4.2 to 400 K. Anomalous Hall effect caused by additional sample magnetization was observed at temperatures of from about 30 to 80 K. The Shubnikov-de Haas effect was recorded at 4.2 K. Negative magnetoresistance changed sign for positive at T ≈ 30 K as the temperature increased.     

Unconventional ferromagnetism and transport properties of (In,Mn)Sb dilute magnetic semiconductor

Narrow-gap higher mobility semiconducting alloys In_1-xMn_xSb were synthesized in polycrystalline form and their magnetic and transport properties have been investigated. Ferromagnetic response in In_0.98Mn_0.02Sb was detected by the observation of clear hysteresis loops up to room temperature in direct magnetization measurements. An unconventional (reentrant) magnetization versus temperature behavior has been found. We explained the observed peculiarities within the frameworks of recent models which suggest that a strong temperature dependence of the carrier density is a crucial parameter determining carrier-mediated ferromagnetism of (III,Mn)V semiconductors. The correlation between magnetic states and transport properties of the sample has been discussed. The contact spectroscopy method is used to investigate a band structure of (InMn)Sb near the Fermi level. Measurements of the degree of charge current spin polarization have been carried out using the point contact Andreev reflection (AR) spectroscopy. The AR data are analyzed by introducing a quasiparticle spectrum broadening, which is likely to be related to magnetic scattering in the contact. The AR spectroscopy data argued that at low temperature the sample is decomposed on metallic ferromagnetic clusters with relatively high spin polarization of charge carriers (up to 65% at 4.2 K) within a cluster.     

Defects induced ferromagnetism in Mn doped ZnO

Single phase Mn doped (2 at%) ZnO samples have been synthesized by the solid-state reaction technique. Before the final sintering at 500 °C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 h). The grain sizes of the samples are very close to each other (～32±4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value for saturation magnetization (0.11 μ_B/Mn atom) was achieved for 96 h milled sample. Electrical resistivity has been found to increase with increase in milling time. The most resistive sample bears the largest saturation magnetization. Variation of average positron lifetime with milling time bears a close similarity with that of the saturation magnetization. This indicates the key role played by open volume vacancy defects, presumably zinc vacancies near grain surfaces, in inducing ferromagnetic order in Mn doped ZnO. To attain optimum defect configuration favorable for ferromagnetism in this kind of samples proper choice of milling period and annealing conditions is required.     

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.     

Defect-mediated ferromagnetism in ZnO:Mn nanorods

In this work, the structural, chemical and magnetic properties of ZnO:Mn nanorods were investigated. Firstly, well-aligned ZnO nanorods with their long axis parallel to the crystalline c-axis were successfully grown by the vapor phase transport technique on Si substrates coated with a ZnO buffer layer. Mn metal was then diffused into these nanorods at different temperatures in vacuum. From SEM results, ZnO:Mn nanorods were observed to have diameters of ～100 nm and lengths of 4 μm. XPS analysis showed that the Mn dopant substituted into the ZnO matrix with a valence state of +2. Magnetic measurements performed at room temperature revealed that undoped ZnO nanorods exhibit ferromagnetic behavior which may be related to oxygen vacancy defect-mediated d ⁰ ferromagnetism. ZnO:Mn samples were seen to show an excess room temperature ferromagnetism that is attributed to the presence of oxygen vacancy defects forming bound magnetic polarons involving Mn.     

Evolution of ferromagnetism with sputtering gas in Mn:ZnO films

The effect of introducing nitrogen and oxygen in the sputtering working gas on the magnetic properties of Mn:ZnO thin films has been investigated. A set of films has been characterized by X-ray diffraction, X-ray absorption near edge structure (XANES) and optical absorption spectroscopy to correlate its magnetic properties with Mn electronic characteristics. Mn²⁺ substituting Zn²⁺ in the wurtzite structure has been obtained for the films presenting considerably high saturation magnetization values. The change in the magnetic behaviour seems to be associated with the electronic carrier density in the films.     

Room-temperature ferromagnetism in pure and Mn doped SnO2 powders

We report here observation of ferromagnetism in pure and Mn doped SnO₂ powder with different Mn contents. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. The RTFM has been observed in the pure SnO₂ powder, which is believed to be defect induced, with a saturation magnetization of ～0.017 emu/g. The RTFM was enhanced considerably in the Mn doped samples and the magnetic properties strongly depend on doping content. A sample with 1% of Mn is ferromagnetic at room temperature with a saturation magnetization of ～0.98 emu/g, a remanent magnetization of ～27%, and a coercivity of ～270 Oe. The average magnetic moment per Mn atom decreases with increasing Mn content. Our results reveal that the large RTFM observed in Mn doped SnO₂ powder originates from a combination effect of oxygen vacancies and transition metal doping.     

Low-temperature electron transport in GaAs

For temperatures between 5 and 80 K, electron mobilities in n-type GaAs have been calculated using ionized impurity cross sections derived from the partial wave phase shift method. The mobilities obtained differ significantly from those calculated previously using Brooks-Herring theory. Although agreement with data from a number of experimental samples is good above 30 K, the theoretical mobilities tend to overestimate experiment at lower temperatures. In order to improve the agreement it may be necessary to incorporate the effects of multi-ion scattering and collision-broadening, and to generalize the screened Coulomb scattering potential.     

Low-temperature transport in Heisenberg chains

A technique to determine accurately transport properties of integrable and nonintegrable quantum-spin chains at finite temperatures by quantum Monte Carlo is presented. The reduction of the Drude weight by interactions in the integrable gapless regime is evaluated. Evidence for the absence of Drude weight in the gapless regime of a nonintegrable system with longer-ranged interactions is presented. We estimate the effect of the nonintegrability on the transport properties and compare with recent experiments on one-dimensional quantum-spin chains.     

Low-temperature thermal transport in nanowires

We propose a theory of low temperature thermal transport in nanowires in a regime in which competition between a phonon and flexural modes governs the relaxation processes. Starting with the standard kinetic equations for two different types of quasiparticles, we derive a general expression for the coefficient of thermal conductivity. The underlying physics of thermal conductance is completely determined by the corresponding relaxation times, which can be calculated directly for any dispersion of quasiparticles, depending on the size of a system. We show that, if the considered relaxation mechanism is dominant, then at small wire diameters the temperature dependence of thermal conductivity experiences a crossover from T^1/2 to T³-dependence. Quantitative analysis shows reasonable agreement with resent experimental results.     

Enhanced room temperature ferromagnetism and photoluminescence behavior of Cu-doped ZnO co-doped with Mn

Cu, Mn co-doped ZnO nanoparticles were successfully synthesized by the sol–gel technique. XRD pattern described that Mn-doping did not affect the hexagonal wurtzite structure of the samples and no secondary phases were found. The reduced crystallite size at Mn=2% is due to the suppression of grain surface growth by foreign impurity. The enhancement of crystal size after Mn=2% is due to the expansion of lattice volume produced by the distortion around the dopant ion. The better dielectric constant and conductivity noticed at Mn=2% are explained by charge carrier density and crystallite size. The suppression of broad UV band by Mn-doping is discussed based on the generation of non-radiative recombination centers. Hysteresis loop showed the clear room temperature ferromagnetism in all the samples and the magnetization increased with Mn-doping. Better electrical and magnetic behavior of Zn_0.94Cu_0.04Mn_0.02O sample is suggested for effective opto-magnetic devices.     

Possible origin of room-temperature ferromagnetism in undoped GaN epilayers implanted with Mn ions

Manganese ions were implanted into unintentionally doped GaN epilayers grown by metal organic chemical vapor deposition (MOCVD). The (Ga,Mn)N and Ga_xMn_y phases were formed after Mn-implanted undoped GaN epilayers annealed at 700 and 800 °C. The samples showed ferromagnetic behavior at room temperature with the highest magnetization obtained in the sample annealed at 800 °C. Ferromagnetic signal reduces as annealing temperature increased above 900 °C. It is believed that the room-temperature ferromagnetic property of Mn-implanted undoped GaN epilayers are mainly from (Ga_,Mn)N. The Ga_xMn_y phases play a critical role in providing holes and also contribute to increasing the ferromagnetic property.     

Microstructure and ferromagnetism of heavily Mn doped SiGe thin flims

Heavily Mn-doped SiGe thin films were grown by radio frequency magnetron sputtering and then treated by postgrowth thermal annealing. Structural characterizations reveal the coexistence of Mn-diluted SiGe crystals and Mn-rich nanoclusters in the annealed films. Magnetic measurements indicate the ferromagnetic ordering of the annealed samples above room temperature. The data suggest that the ferromagnetism is probably mainly contributed by the Ge-rich nanoclusters and partially contributed by the tensile-strained Mn-diluted Si Ge crystals. The results may be useful for room temperature spintronic applications based on group IV semiconductors.     

Giant strain-induced ferromagnetism in Fe59Mn17Al24

Giant strain-induced ferromagnetism (SIF) has been observed in Fe₅₉Mn₁₇Al₂₄ single crystals. The crystals in either the L2₁ or B2 ordered state are weakly magnetic, with saturation magnetization, M _s, of 9–10?emu/g. After a 60% thickness reduction, the M _s of the L2₁ crystal increased to 89?emu/g, whereas the M _s of the B2 crystal after a 53% reduction was 96?emu/g, an M _s approximately four times larger than that of the largest SIF observed in other ordered alloys. By comparison, mechanically alloyed powder of the same composition, which had a fully-disordered bcc structure, showed a similar M _s of ～90?emu/g. The increased M _s in strained Fe₅₉Mn₁₇Al₂₄ single crystals is attributed primarily to the generation of increased magnetic coupling due to chemical disordering. This large M _s is substantially reduced after annealing at 573?K.     

The energy of periodic surface structures in ferromagnetism

The periodic domain structures on unfavourably oriented surface layers of ferromagnetic materials were studied both experimentally and theoretically. The connection between the surface structure and the crystallographic orientation of the crystals was confirmed. All the terms contributing to the total energy of the surface layer were calculated. A general solution of the potential problem is given for arbitrary periodic distribution of the charges.

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Electric-field control of ferromagnetism in GaSb/Mn digital alloys

We have investigated the dependence of ferromagnetism in GaSb/Mn digital alloys on applied electric bias perpendicular to the digital layers. The remanent Hall resistance determined from hysteresis loops in R_Hall vs. magnetic field was used as a measure of the ferromagnetism present in two samples at temperatures between 5 and 60 K. With applied gate bias, the remanent Hall resistance measured in gated Hall bars was found to change systematically with applied bias. The changes agree qualitatively with changes in carrier concentration determined from the sheet resistance vs. bias. Ferromagnetism in these samples can be turned on and off with applied bias near the Curie temperature (as high as 60 K in one sample).     

Noncollinear ferromagnetism in (III,Mn)V semiconductors

We investigate the stability of the collinear ferromagnetic state in kinetic exchange models for (III,Mn)V semiconductors with randomly distributed Mn ions. Our results suggest that noncollinear ferromagnetism is common to these semiconductor systems. The instability of the collinear state is due to long-range fluctuations involving a large fraction of the localized magnetic moments. We address conditions that favor the occurrence of noncollinear ground states and discuss unusual behavior that we predict for the temperature and field dependence of its saturation magnetization.     

Point defects, ferromagnetism, and transport in calcium hexaboride

The formation energy and local magnetic moment of a series of point defects in CaB6 are computed using a supercell approach within the generalized gradient approximation to density functional theory. It is found that the substitution of Ca by La does not lead to the formation of a local moment, while a neutral B6 vacancy carries a moment of 2.4 Bohr magnetons. A plausible mechanism for the ferromagnetic ordering of these moments is suggested. Since the same broken B-B bonds appear on the preferred (100) cleavage planes of the CaB6 structure, it is argued that internal surfaces in polycrystals as well as external surfaces in general will make a large contribution to the observed magnetization.