Ferromagnetism in One Layer of Self-organized InMnAs Quantum Dots

One layer of self-assembled InMnAs quantum dots with InGaAs barrier was grown on high-resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of ferromagnetic structure was confirmed in the InMnAs dilute magnetic quantum dots. The one layer of self-assembled InMnAs quantum dots was found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T _C =80 K. It is likely that the ferromagnetic exchange coupling of sample with T _C =80 K is hole-mediated resulting in Mn substituting Ge. PL emission spectra of InMnAs samples grown at temperature of 210°C and 285°C show that the interband transition peak centered at 1.31 eV comes from the InMnAs quantum dot.     

Pressure-enhanced ferromagnetism and metallicity in La1.24Sr1.76Mn2O7 bilayered manganite system

We report the low temperature magnetization, electrical resistance, and magnetoresistance of La_1.24Sr_1.76Mn₂O₇ bilayer manganite system under hydrostatic pressure. At ambient pressure, the compound shows a sharp ferromagnetic transition (T _C) accompanied by a metal–insulator transition (T _MI) at 130 K. We observe that the T _C and T _MI increase with hydrostatic pressure at a rate of dT _C/dP = 2.08 K/kbar and dT _MI/dP = 2 K/kbar, respectively. Also, we observe an appreciable increase of magnetic moments at low temperatures with increasing pressure. The high temperature regime of temperature dependence of resistivity curves was fitted with the Emin–Holstein’s polaron hopping model and the calculated activation energy values suggest that the applied pressure weakens the formation of Jahn–Teller polarons. The magnetoresistance ratio (MRR) was measured at T _C and at 4.2 K upon an external magnetic field of 5 T. The observed MRR at T _C is about 210 % and the applied pressure increases the MRR significantly. These results can be interpreted by the pressure-enhanced overlap between the orbitals of Mn–O–Mn, which facilitates the charge transfer and hence enhances the ferromagnetism and metallicity.     

Distribution of the Impurity Resonance Energy in Bi2Sr2CaCu2O8+δ Observed by Scanning Tunneling Spectroscopy

Using scanning tunneling microscopy and spectroscopy (STM/STS) at 4.2 K, we observed Zn impurity resonance states in single crystals of Zn substituted Bi₂Sr₂Ca(Cu_1?x Zn _x )₂O_8+δ . Zn impurity resonances were characterized by the peak structure in the local density of states (LDOS). The peaks were appeared near the E _F on each of Zn impurity sites. The peak energy showed a wide distribution from ?7 to 5 meV. The average peak energy was ?1.5 meV and the variance was 1.8 meV. We investigated the correlation between the peak energy at a Zn site and the energy gap value in the region where the Zn site is embedded. Although we observed about hundred Zn sites, no correlation between the local energy gap and the peak energy was observed.     

Influence of GaNAs strain compensation layers upon annealing of GaIn(N)As/GaAs quantum wells

GaIn(N)As/GaAs and GaIn(N)As/GaNAs/GaAs quantum well samples, with and without GaNAs strain-compensating layers (SCL), were grown on GaAs (001) substrates by molecular beam epitaxy. Photoluminescence (PL) was used to study the effects of the GaNAs SCL on the properties of the Ga(In)NAs QWs upon annealing. It is found that an extra blue-shift (ΔE = 5.2 meV) in the PL emission from the GaInNAs QW sample at the initial annealing stage (t_ann = 40 s) was induced by the GaNAs SCLs. However, for the GaInAs QW sample, the blue-shift induced by GaNAs SCL is only 1.1 meV. As the annealing time was increased, the blue-shift of both GaInNAs and GaInAs QWs showed saturations at 16 meV and 8 meV, respectively. The PL blue-shifts were much enhanced by inserting GaNAs SCLs showing a non-saturable behavior. X-ray diffractions from the strain compensated GaIn(N)As QWs before and after annealing show no N atom diffusion but Ga/In atom interdiffusion across the QW interfaces. The Ga/In atom interdiffusion caused by annealing was also confirmed by high-resolution transmission electron microscopy from the GaInNAs/GaAs QW sample.     

Low Temperature Heat Capacity of Layered Superconductors SrNi2Ge2 and SrPd2Ge2

Low-temperature heat capacity C(T) of the weakly electron-correlated SrNi₂Ge₂ 122-layer compound undergoes a superconducting transition with onset at 1.4 K and a bulk T _c =0.75 K, where heat-capacity jump ratio ΔC(T _c )/γT _c =0.88–1.05. A small average superconducting energy gap E _g (ave)=2.21 kT _c =0.14 meV is derived for this multi-gap superconductor. Similar results for isostructural SrPd₂Ge₂ include T _c (onset)=3.5 K, bulk T _c of 2.92 K, ΔC(T _c )/γT _c =0.70 and E _g (ave)=2.54 kT _c =0.64 meV. The higher T _c onset could be associated with stoichiometric 1:2:2 grains in the polycrystalline samples. In addition, deviations of E _g /kT _c from the BCS ratio of 3.5 suggest that, just like their iron-based counterpart, these 122-layer germanides may also exhibit an unconventional, fully-opened multi-gap s-wave superconductivity.     

Study on Mn-induced Jahn–Teller distortion in BiFeO3 thin films

Present work reports Raman spectroscopy study of single-phase Mn-doped BiFeO₃ [BiFe_1?x Mn _x O₃ (0 ≤ x ≤ 0.20)] polycrystalline thin films carried out in backscattering geometry. De-convolution of Raman spectra showed a gradual transition in the crystal symmetry from rhombohedral (?R) to multiphase [rhombohedral (?R) + tetragonal (?T)] structure with increasing Mn doping concentration in BiFe_1?x Mn _x O₃ (BFMO) thin films. X-ray diffraction (XRD) along with Le-Bail extraction refinement confirms that the structural symmetry lowering in BFMO thin films occurs at about 10 % Mn doping concentration. A blue shift is observed in the direct energy band gap of BFMO thin films from 2.53 to 2.87 eV (at T = 295 K) and is attributed to the local symmetry lowering and local induced strain in Fe³⁺ environment resulted from Jahn–Teller distortion in (MnFe)³⁺O₆ octahedral unit. Second-derivative analysis of FTIR spectra in the spectral regions (420–470) cm^?1 and (480–680) cm^?1 further indicates the favourable structure distortion leading to the simultaneous exhibition of enhanced ferromagnetic and ferroelectric properties owing to Mn substitution in host BiFeO₃ lattice.     

Ferromagnetism in Mn-implanted Ge/Si Nanostructure Material

Multistacked Ge quantum dots (QDs) with Si spacers of different thicknesses have been grown on (100) Si substrates by rapid thermal chemical vapor deposition followed by Mn ion implantation and post-annealing. The presence of ferromagnetic structure was confirmed in the insulating (Si_0.45Ge_0.55)Mn_0.03 diluted magnetic quantum dots (DMQD) and semiconducting (Si_0.45Ge_0.55)Mn_0.05 DMQD. The DMQD materials were found to be homogeneous and to exhibit p-type conductivity and ferromagnetic ordering with a Curie temperature of T _C=350 and 160 K. The x-ray diffraction (XRD) data show that there is a phase separation of Mn₅Ge₃ from the MnGe nanostructure. The temperature-dependent electrical resistivity in semiconducting DMQD material indicates that manganese introduces two acceptor levels in germanium, at 0.14 eV from the valence band and 0.41 eV from the conduction band implying Mn substituting Ge. Therefore, it is likely that the ferromagnetic exchange coupling of DMQD material with T _C=160 K is hole mediated due to formation of bound magnetic polarons and the ferromagnetism in a sample with T _C>300 K is due to Mn₅Ge₃ phase.     

Investigation of Anomalous Magnetic Transition in Pr0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxides Pr_0.5Sr_0.5CoO₃ were prepared by the sol-gel method. We mainly study the anomalous magnetic transition of Pr_0.5Sr_0.5CoO₃. We report the investigations of polycrystalline samples of the metallic ferromagnetic material Pr_0.5Sr_0.5CoO₃ through measurements of X-ray diffraction, the magnetization, and the resistivity. We found an unusual anomaly around T _A=120 K, much below the ferromagnetic transition (T _C=228 K). Further using the variable temperature X-ray diffraction and electron spin resonance (ESR) measurements, we found that as the temperature goes down, the crystal structure changes obviously at T _A=120 K. We show that this actually results in anomalous magnetic transition.     

Epitaxial layers of CuInSe2 on GaAs

CuInSe₂ single-crystal films with (112) orientation and with thicknesses in the range 800–1200 Å were deposited onto semi-insulating (111)A-oriented GaAs substrates by flash evaporation under controlled growth conditions. Epitaxial growth began at a substrate temperature T_sub = 720 K and twins in the 〈221〉 direction were detected in reflection high energy electron diffraction investigations. The twin concentration decreased with increasing growth temperature. At T_sub = 870 K a change of the structure from chalcopyrite to sphalerite was observed. Films produced at T_sub ? 720 K showed n-type conductivity whereas at higher growth temperatures the films were always p type and showed a rapid increase in hole concentration with increasing substrate temperature. Two different acceptor levels with ionization energies of 92 meV and about meV were found.     

Tuning optical properties of high In content InGaAs/GaAs capped InAs quantum dots by post growth rapid thermal annealing

The effect of rapid thermal annealing on InAs quantum dots (QDs) capped with In_0.4Ga_0.6As/GaAs layer has been investigated by photoluminescence (PL). An unusual red shift of the PL emission peak has been observed for an annealing temperature (T_a) of 650 °C together with a pronounced improvement of the PL from the quantum well like heterocapping layer (QW). This behavior is attributed to the strain induced phase separation of the hetero-capping alloy. However, for T_a = 750 °C, a blue shift of the QDs PL peak has been observed with respect to that of the as-grown sample. For this annealing temperature the PL intensity of the QW exceeds that of the QDs indicating a relatively prominent In/Ga interdiffusion. When annealed at 850 °C, only the PL arising from the QW can be detected in addition to a broadened low energy side band indicating the dissolution of the QDs at that temperature.     

La0.7Sr0.3MnO3 Nanoparticles Synthesized via the (Pechini) Polymeric Precursor Method

In this paper, we report the preparation of La_0.7Sr_0.3MnO₃ colossal magnetoresistance nanoparticles by means of the polymeric precursor method, at a temperature of 650 °C. Rietveld refinement of the X-ray powder diffraction spectra shows that the chemically-synthesized manganite is single-phase with the space group R3C. By using the peak broadening technique and Scherrer’s formula, a grain size of ～15 nm was estimated. The ferromagnetic to paramagnetic transition is sharp with a Curie temperature of T _C ～360 K. In spite of the low annealing temperature, the insulator-metal-like transition temperature (T _p ) is close to T _C . Transport measurements on the prepared samples show a magnetoresistance change of ～10 % at room temperature in a field of 2 T. This high-field MR is probably due to grain-boundary effects.     

Temperature-dependent integral exciton absorption in semiconducting GaAs crystals

This Letter is devoted to an experimental investigation of the temperature dependence of the fundamental absorption edge of thin GaAs layers grown by molecular beam epitaxy. Critical temperature T _cr = 135 K, above which the integral absorption becomes constant, corresponding values of critical damping parameter ?? _c = 0.248 meV, and longitudinal-transverse splitting kh??_LT = 0.082 meV are determined experimentally. The temperature dependence of the true dissipative damping is separated.     

Optical Properties of Mn-Implanted GaN Nanorods

We have investigated the optical properties of vertical GaN nanorods with diameters of 150 nm grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy followed by Mn ion implantation and annealing. The GaN nanorods are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. For GaMnN nanorods, Arrhenius plots of the intensities of the Mn acceptor give a thermal activation energy of Δ=350 meV, indicating that the thermal quenching of the Mn-related PL peak is due to the dissociation of an acceptor-bound hole from the temperature-dependent PL spectra. This suggests that the Mn-bound holes in GaN nanorods exhibit the impurity states predicted by the hydrogen model.     

Effect of Zn, Ni and Fe Impurities on Bi2Sr1.6La0.4CuO6+��

We measured the change of T _c with the doping of the magnetic and nonmagnetic impurity in Bi₂Sr_1.6La_0.4CuO_6+?? (Bi2201). The reduction of T _c is about 20% in samples with Zn (1.1%), Ni (1.0%), and Fe (1.2%), and a slight difference was observed among these samples. We also observed the resonance peak at the Zn-impurity site by scanning tunneling spectroscopy at 4.2 K. The observed resonance peak showed the four-fold symmetry, and appeared at approximately ?1.5 meV.     

Fe doping in ZnS for realizing nanocrystalline-diluted magnetic semiconductor phase

Zn_1?x Fe _x S (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.12, and 0.15) samples have been synthesized by the chemical co-precipitation method, using ZnCl₂ and Na₂S as starting materials, FeCl₃ as a dopant and Ethylenediaminetetraacetic acid (EDTA) as capping agent. Investigations of the structural, optical, and magnetic properties of the prepared samples have been carried out. The results of X-ray diffraction (XRD), selected area electron diffraction of TEM images, Fourier transform infrared spectroscopy have shown that Fe ions are incorporated into the lattice of ZnS crystallites as substitutional impurity at Zn sites in the structure without disturbing the original ZnS wurtzite structure. The average crystallite size from the XRD data and transmission electron microscopy has been found to be in the range of 4–10 nm. Magnetization behavior (i.e., M–H Characteristics) shows the evolution of ferromagnetic behavior with Fe incorporation. It has been found that there is an optimum iron ion concentration for observing the maximum magnetization. The optimum iron ion concentration has been found to be, x = 0.08 in the synthesized samples of Zn_1?x Fe _x S. The observed magnetization behavior has been understood as ferromagnetic behavior of Fe ions. The decrease of ferromagnetism for the higher Fe ion concentration has been discussed as a result of Fe–Fe interaction in close proximity to result antiferromagnetism for decreasing the magnetization.     

Microstructural and compositional modification of In0·53Ga0·47As/In0·52Al0·48As multiquantum wells using rapid thermal annealing process

Abstract

Quantum well intermixing of In_0·53Ga_0·47As/In_0·52Al_0·48As multiquantum wells (MQWs) in an impurity free vacancy diffusion (IFVD) mechanism was investigated to observe the intermixing aspect and the effect of defects in MQWs with regard to the microstructural aspect using transmission electron microscopy. The MQWs were grown on a GaAs (100) substrate using compositionally graded buffer layers via molecular beam epitaxy, and the MQWs were annealed at 750 and 900°C for 30 s via rapid thermal annealing for quantum well intermixing using IFVD method. In the fabricated MQWs, defects, such as stacking faults, twins and dislocations, were not generated at 750°C. The diffusion of Ga in the well layer for the quantum well intermixing started from the top well layer, because the SiO₂ layer that supplied vacancies for the quantum well intermixing was at the top of the sample. Additionally, in the same well layer, the intermixing did not show equality, because these vacancies were not supplied homogeneously. Especially, in the 900°C annealed case, many dislocations were generated from the cladding layer. These dislocations contributed to new vacancy generation sites, thus the quantum well intermixing was accelerated.     

Interdiffusion effect on GaAsSbN/GaAs quantum well structure studied by 10-band k•p model

The effect of annealing on the photoluminescence (PL) in GaAsSbN/GaAs quantum well (QW) grown by solid-source molecular-beam epitaxy (SS-MBE) has been investigated. Low-temperature (4 K) PL peaks shift to higher energy sides with the increase of annealing temperature. An As-Sb atomic interdiffusion at the heterointerface is proposed to model this effect. The compositional profile of the QW after interdiffusion is modeled by an error function distribution and calculated with the 10-band k•p method. When the diffusion length equals to 1.4 nm, a corresponding transition energy blueshift of 36 meV is derived. This agrees with the experimental result under the optimum condition (750 °C at 5 min).     

Activation Energy of the Photo Induced Q2 Oxygen Ordered Phase in the La2CuO4.08 Superconductor

The interstitial oxygen ordering in a La₂CuO_{4+ y} (y = 0.08) single crystal with a T _c = 38 K has been investigated. We have studied the growth dynamics of the three dimensional (3D) ordered phase, called Q2, at different temperatures under X-ray illumination. The critical temperature T ₀～334 K of the order-disorder transition for the Q2 phase has been determined. We have found that the illumination of the sample with a continuous polarized synchrotron radiation X-ray flux stimulates the oxygen ordering kinetics. The photoinduced ordering process shows a threshold characteristic of cooperative phenomena and its kinetics shows a temperature dependence that is controlled by the activation energy E _A = 100 meV.     

The Superconducting Ferromagnet UCoGe

The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T _C=3 K and a superconductor with a transition temperature T _s=0.6 K. We review its basic thermal, magnetic—on the macro and microscopic scale—and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T _s=0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.     

Magnetic and transport polaron percolation in diluted GeMn films

Magnetic and electronic transport properties of Mn-doped Ge films have been studied as a function of Mn content. The films exhibit a long-range ferromagnetic behavior and a Mn dilution-dependent Curie temperature T_C. Resistivity shows an insulator-like character with two distinct activation energies below about 80 K, while the Hall coefficient evidences a strong contribution from the anomalous Hall effect, in a p-type material. At a characteristic temperature T_R, resistivity experiences a sudden reduction and the Hall coefficient reverses its sign from positive to negative. Moreover, around T_R, any residual remanence and coercivity disappear.The transport and magnetic results are deeply related and can be qualitatively explained by a percolation model based on bound magnetic polarons due to localized holes in the GeMn alloy lattice.