Effect of annealing on the electric and magnetic properties of GaMnAs and Be-codoped GaMnAs

GaMnAs and Be-codoped GaMnAs films grown via molecular beam epitaxy (MBE) were heat treated and the stability of Mn in the matrix was investigated. MnAs had a stable phase at the low growth temperature, but MnGa was stable at the annealing temperature. Be-codoping did not prevent the precipitation processes, but Be itself was stable during the annealing process to maintain the GaAs matrix at the high conductivity.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

Angular dependence of tunneling magnetoresistance in magnetic semiconductor heterostructures

Theoretical studies on spin-dependent transport in magnetic tunnel heterostructures consisting of two diluted magnetic semiconductors (DMS) separated by a nonmagnetic semiconductor (NMS) barrier, are carried in the limit of coherent regime by including the effect of angular dependence of the magnetizations in DMS. Based on parabolic valence band effective mass approximation and spontaneous magnetization of DMS electrodes, we obtain an analytical expression of angular dependence of transmission for DMS/NMS/DMS junctions. We also examine the dependence of spin polarization and tunneling magnetoresistance (TMR) on barrier thickness, temperature, applied voltage and the relative angle between the magnetizations of two DMS layers in GaMnAs/GaAs/GaMnAs heterostructures. We discuss the theoretical interpretation of this variation. Our results show that TMR of more than 65% are obtained at zero temperature, when one GaAs monolayer is used as a tunnel barrier. It is also shown that the TMR decreases rapidly with increasing barrier width and applied voltage; however at high voltages and low thicknesses, the TMR first increases and then decreases. Our calculations explain the main features of the recent experimental observations and the application of the predicted results may prove useful in designing nano spin-valve devices.     

Magnetic coupling in ferromagnetic semiconductor GaMnAs/AlGaMnAs bilayer devices

We carefully investigated the ferromagnetic coupling in the as-grown and annealed ferromagnetic semiconductor GaMnAs/AlGaMnAs bilayer devices. We observed that the magnetic interaction between the two layers strongly affects the magnetoresistance of the GaMnAs layer with applying the out of plane magnetic field. After low temperature annealing, the magnetic easy axis of the AlGaMnAs layer switches from out of plane into in-plane and the interlayer coupling efficiency is reduced from up to 0.6 to less than 0.4. However, the magnetic coupling penetration depth for the annealed device is twice that of the as-grown bilayer device.     

Comparison of annealing effects on Zn-doped GaMnAs and undoped GaMnAs epilayers

To compare the annealing effects on GaMnAs-doped with Zn (GaMnAs:Zn) and undoped GaMnAs (u-GaMnAs) epilayers, we grew GaMnAs thin films at 200 °C by molecular beam epitaxy (MBE) on GaAs substrates, and they were annealed at temperatures ranging from 220 °C to 380 °C for 100 min in air. These epilayers were characterized by high-resolution X-ray diffraction (XRD), electrical, and magnetic measurements. A maximum resistivity at temperatures T_m close to the Curie temperatures T_c was observed from the measurement of the temperature-dependent resistivity ρ(T) for both the GaMnAs:Zn and the u-GaMnAs samples. We found, however, that the maximum temperature T_m observed for GaMnAs:Zn epilayers increased with increasing annealing temperature, which was different from the result with the u-GaMnAs epilayers. The formation of GaAs:Zn and MnAs or Mn-Zn-As complexes with increasing annealing temperature is most likely responsible for the differences in appearance.     

Emitting heterostructures with a bilayer InGaAs/GaAsSb/GaAs quantum well and a GaMnAs ferromagnetic layer

The radiative and magnetic properties of novel heterostructures with a bilayer InGaAs/GaAsSb/GaAs quantum well and a GaMnAs ferromagnetic layer are studied. The circular polarization of electroluminescent radiation is observed at temperatures from 10 to 160 K. The magnetic field dependences of the degree of circular polarization are nonlinear with a hysteresis loop at temperatures from 10 to 50 K, and they become linear at higher temperatures. The magnitude of polarization at the saturation magnetization of GaMnAs in the 2000 Oe field remains at the level of ~0.2%.     

The tunneling magnetoresistance in GaMnAs/GaAs/GaMnAs junctions

The tunneling magnetoresistance (TMR) in GaMnAs/GaAs/GaMnAs magnetic tunnel junctions is studied under an extended coherent tunneling approach where both the contributions of the light holes and the heavy holes and their mutual competitions are investigated. It is shown that the TMR ratio can increase with decreasing the barrier strength, which is different from the results in the conventional magnetic tunnel junctions but a good news for the applications. It is also shown that the presence of the pinholes in the thin barrier layer gives a possible explanation of the peak in the barrier thickness dependence of the TMR ratio.     

Ultrafast optical study of magnons in the ferromagnetic semiconductor GaMnAs

Coherent oscillations of the magnetization were observed in magneto-optical Kerr measurements in thin films of the ferromagnetic semiconductor GaMnAs. For magnetic fields oriented in the film plane, two precession modes were observed. Their frequencies increase with the field when it is along the [100] axis, whereas they behave non-monotonically when the field is oriented along the in-plane hard axis [110]. Spectra are also presented for fields applied normal to the film plane. From the measured field-dependence of the magnon frequency, the spin stiffness and magnetic anisotropy constants were obtained.     

Effect of thermal annealing on the magnetic anisotropy of GaMnAs ferromagnetic semiconductor

We have systematically investigated the influence of annealing on the magnetic anisotropy properties of GaMnAs film using an epilayer with a Mn concentration of 6.2%. The GaMnAs epilayer was grown by molecular beam epitaxy and the planar Hall effect measurement was used to monitor the magnetic anisotropy of the film. We found significant annealing-induced changes in the magnetic anisotropy properties of the GaMnAs film that depended on the annealing conditions. For example, the cubic anisotropy that gave a four-fold symmetry of magnetic easy axes decreased while the uniaxial anisotropy that gave a two-fold symmetry of magnetic easy axes increases in the samples annealed temperature below 300 °C. In particular, the uniaxial anisotropy along the [010] direction in as-grown GaMnAs film changed to the [100] direction by rotating by 90° after the sample was annealed at 300 °C for 3 h. This investigation thus indicates that the magnitude and the direction of the magnetic anisotropy in the GaMnAs film can be effectively controlled by choosing an appropriate annealing time and temperature.     

Ferromagnetism in laser-deposited GaMnAs layers

The properties of the GaMnAs layers grown by laser deposition on semi-insulating GaAs(100) substrates at temperatures from 300 to 650°C have been investigated. A strong anisotropy of the hysteretic curve of the angle of rotation of the plane of polarization with a change in the magnetic field direction in the sample plane was found during investigation of the magneto-optical Kerr effect (300 K). The domain structure in GaMnAs layers has been observed for the first time at room temperature by magnetic-force microscopy.     

Nickel nanofibers and nanowires: Elaboration by reduction in polyol medium assisted by external magnetic field

Nickel nanowires, with diameter 250 nm and a length of several microns, were prepared by the polyol process (chemical reduction) while an external magnetic field of 1.4 T has been applied during preparation. This combination has allowed the elaboration of Ni nanowires with a yield of over 90%. X-ray diffraction (XRD) showed that these nanowires crystallize with the face-centered-cubic structure. Magnetic static measurements showed the effect on the nanoparticles’ morphology of the external magnetic field applied during the synthesis. They also allowed studying the effect of the external magnetic field on the magnetic properties of nanowires as a function of their orientation. When nanowires are aligned parallel with magnetic field, the hysteresis loop obtained is very open with a coercivity field (Hc) value of 385 Oe and a high remanence to saturation ratio Mr/Ms of 0.85.     

Spatially resolved inhomogeneous ferromagnetism in (Ga,Mn)as diluted magnetic semiconductors: a microscopic study by muon spin relaxation

Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-microSR) as well as by transport and magnetization measurement techniques. LE-microSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.     

Electrical Properties of Nanostructured Magnetic Colloid and Influence of Magnetic Field

We investigate the electrical properties of the nanostructured magnetic colloid without and with magnetic field. The competition between the directional motion of the charged magnetic nanoparticles and other minor nonmagnetic impurities （also small amount of ions） under applied voltage and their random orientation due to thermal activation is implemented to elaborate the electrically conduction mechanism under zero magnetic field. Two equivalent electric circuits are employed for explaining the charging and discharging processes. The tunnelling conduction mechanism upon application of externally magnetic field may exist in the nanostructured magnetic colloid. The alternation of the two conduction mechanisms accounts for the current spikes when the magnetic field is switched on or off. This work presents the peculiar electrical phenomena of the magnetically colloidal system.     

Magnetostriction of a single crystal of nickel observed with an X-ray four-circle goniometer

We proposed a technique to observe magnetostrictive coefficients of a single crystal specimen with X-ray diffraction. An angle between a direction of crystallographic orientation and a direction of magnetic field could be estimated with two kinds of diffraction peaks which were found with an X-ray four-circle goniometer. The magnetostriction was measured by a shift of Bragg angle. This technique was suitable for a case to observe the magnetostrictive coefficient which varied as a function of the magnetic field direction. We applied the technique to a single crystal specimen of nickel and showed dependences of the magnetostriction on the magnetic field strength and its direction around an axis of easy magnetization at room temperature.     

Influence of phase segregation process on transport properties of dilute magnetic semiconductors

Effect of the phase segregation process on the transport properties of GaMnAs has been investigated. It was shown that the change of the transport properties in precipitated GaMnAs is not only the result of the decreasing Mn concentration in the semiconductor matrix, but the carriers’ localization effect in the vicinity of the cluster–matrix interfaces as well.     

Ultrafast photo-induced turning of magnetization and its relaxation dynamics in GaMnAs

We report that,by linearly polarized pumping of different wavelengths,Kerr transients appear at zero magnetic field only in the case when GaMnAs samples are initialized at 3 K by first applying a 0.8 Tesla field and then returning to zero field.We find that,instead of magnetization precession,the near-band gap excitation induces a coherent out-of-plane turning of magnetization,which shows very long relaxation dynamics with no precession.When photon energy increases,the peak value of the Kerr transient incre...     

Absence of metal-insulator transition and coherent interlayer transport in oriented graphite in parallel magnetic fields

Measurements of the magnetoresistivity of graphite with a high degree of control of the angle between the sample and magnetic field indicate that the metal-insulator transition, shown to be induced by a magnetic field applied perpendicular to the layers, does not appear in parallel field orientation. Furthermore, we show that interlayer transport is coherent in less ordered samples and high magnetic fields, whereas appears to be incoherent in less disordered samples. Our results demonstrate the two-dimensionality of the electron system in ideal graphite samples.     

杂质与缺陷对GaMnAs光学特性影响的研究

提高GaMnAs材料中Mn的含量可以提高其居里温度, 但随之而来也会引入很多缺陷。为了研究高含量Mn引入的缺陷对稀磁半导体材料的影响, 本文对低温分子束外延技术（LT-MBE）生长的GaMnAs外延层进行了光电导以及红外等光谱的分析。通过对样品的光谱分析, 发现样品中存在大量的As反位缺陷（AsGa）、Mn的间隙位缺陷（MnI）、以及在生长和退火过程中产生的Mn以及MnAs团簇等缺陷, 这些缺陷都会影响外延层的光谱特性, 同时也会影响器件的电学性能。     

Epitaxial ferromagnetic thin films and heterostructures of Mn-based metallic and semiconducting compounds on GaAs

We present two approaches to integrate magnetic materials with III–V semiconductors. One is epitaxial ferromagnetic metallic films and heterostructures on GaAs (0 0 1) substrates. Although crystal structure, lattice constant, chemical bonding and other properties are dissimilar, ferromagnetic hexagonal MnAs thin films and MnAs/NiAs ferromagnet/nonmagnet heterostructures (HSs) are grown on GaAs by molecular beam epitaxy (MBE). Multi-stepped magnetic hysteresis are controllably realized in MnAs/NiAs HSs, making this material promising for the application to multi-level nonvolatile recording on semiconductors. The other approach is to prepare a new class of GaAs based magnetic semiconductor, GaMnAs, by low-temperature molecular beam epitaxy (LT-MBE) on GaAs (0 0 1). New III–V based superlattices consisting of ferromagnetic semiconductor GaMnAs and nonmagnetic semiconductor AlAs are also successfully grown. Structural and magnetic properties of these new heterostructures are presented.     

Features of the magnetoplastic effect in a beryllium condensate

The effect of a weak dc magnetic field on inelastic characteristics of a magnesium-thermal beryllium condensate (99.95 wt % Be) has been studied. It has been shown that the magnetic field differently affects the defect subsystem of beryllium and, depending on sample treatment, leads to its hardening or softening. Based on an analysis of amplitude dependences of the internal friction and elastic moduli, the dislocation velocity after magnetic field exposure has been estimated. It has been shown that the magnetic aftereffect in Be is significantly stronger than the effect in situ.