Ferromagnetism in InMnAs layers at room temperature

The InMnAs layers with ferromagnetic properties at room temperature are prepared by laser ablation. This is confirmed by the results of investigating the anomalous Hall and magneto-optical Kerr effects and by magnetic-force microscopy. According to x-ray diffraction data, the InMnAs layers have a fairly high crystal quality but contain inclusions of the MnAs hexagonal phase. An analysis of the electrical properties of the InMnAs layers suggests that the ferromagnetism revealed at room temperature cannot be accounted for by the presence of the MnAs phase but is associated with the charge carrier transfer in the InMnAs matrix.     

Ferromagnetism in Mn-doped GaAs layers: Effects of laser annealing

The properties of Mn-doped GaAs layers grown by laser deposition were investigated with measurements of Hall effect and magneto-optical Kerr effect (MOKE). The electrical and magnetic parameters of the layers were defined by growth temperature and quantity of sputtered Mn. It was shown that room-temperature ferromagnetism is revealed by MOKE and, after ruby laser 25 ns pulse annealing, by Hall effect measurements.     

Effect of compressive and tensile stresses in GaMnAs layers on their magnetic properties

The effect of elastic stresses (compressive, tensile) on the magnetic properties of epitaxial GaMnAs layers prepared by laser deposition of solid-state targets in a gas atmosphere on different buffer sublayers (In _x Ga_{1 − x} As and In _x Ga_{1 − x} P) and substrates (GaAs, InP) has been investigated. It has been established from the investigations of magnetic-field dependences of the Hall resistance that all layers exhibit ferromagnetic properties with the Curie temperature ∼50 K. It has been shown that, in the case of tensile stresses in GaMnAs layers (In _x Ga_{1 − x} As and In _x Ga_{1 − x} P buffers and InP substrate), the anomalous Hall effect shape demonstrates a predominant orientation of the easy-magnetization axis in the growth direction, unlike the GaMnAs layers prepared on a GaAs substrate (with compressive stresses), which demonstrate the predominance of the component of the magnetization vector in the layer plane.     

High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high-T_c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T_g=(20-550) °C on single-crystal GaAs or Al₂O₃ substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5-500 K. The optimum growth temperature, T_g, for Si:Mn/GaAs layers with T_c≈400 K is shown to be about 400 °C. The Si:Mn/Al₂O₃ layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.     

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.     

Ferromagnetism in epitaxial germanium and silicon layers supersaturated with managanese and iron impurities

The possibility of laser synthesis of diluted magnetic semiconductors based on germanium and silicon doped with manganese or iron up to 10–15 at % has been shown. According to data on the electronic levels of 3d atoms in semiconductors, Mn and Fe impurities are most preferable for realizing ferromagnetism in Ge and Si through the Ruderman-Kittel-Kasuya-Yosida mechanism. Epitaxial Ge and Si layers 50–110 nm in thickness were grown on gallium arsenide or sapphire single crystal substrates heated to 200–480°C. The content of a 3d impurity has been measured by x-ray spectroscopy. The ferromagnetism of layers and high magnetic and acceptor activities of Mn in Ge, as well as of Mn and Fe in Si, are manifested in the observation of the Kerr effect, anomalous Hall effect, high hole conductivity, and anisotropic ferromagnetic resonance at 77–500 K. According to the ferromagnetic resonance data, the Curie point of Ge:Mn and Si:Mn on a GaAs substrate and of Si:Fe on an Al₂O₃ substrate is no lower than 420, 500, and 77 K, respectively.     

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.     

Ferromagnetism in epitaxial layers of gallium and indium antimonides and indium arsenide supersaturated by manganese impurity

We report on laser synthesis of thin 30–200 nm epitaxial layers with mosaic structure of diluted magnetic semiconductors GaSb:Mn and InSb:Mn with the Curie temperature T_C above 500 K and of InAs:Mn with T_C no less than 77 K. The concentration of Mn was ranged from 0.02 to 0.15. In the case of InSb:Mn and InAs:Mn films, the additional pulse laser annealing was needed to achieve ferromagnetic behavior. We used Kerr and Hall effects methods as well as ferromagnetic resonance (FMR) spectroscopy to study magnetic properties of the samples. The anisotropy FMR was observed for both layers of GaSb:Mn and InSb:Mn up to 500 K but it takes place with different temperature dependencies of absorption spectra peaks. The resonance field value and amplitude of FMR signal on the temperature is monotonically decreased with the temperature increase for InSb:Mn. In the case of GaSb:Mn, this dependence is not monotonic.     

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.     

Antiferromagnetic s-d exchange coupling in GaMnAs

Measurements of coherent electron spin dynamics in Ga1-xMnxAs/Al0.4Ga0.6As quantum wells with 0.0006%相似文献   

Anisotropy of magnetoresistance in Be Co-doped GaMnAs

Magnetotransport properties of GaMnAs co-doped with Be have been studied as a function of applied magnetic field orientations and temperature. It was shown that magnetoresistivity phenomena in GaMnAs:Be depend not only on the relative orientation of the current and applied magnetic field, but also on the respective orientation of these two vectors to the crystalline axis.     

Ferromagnetism in γ-iron

F.c.c. γ-Fe(111)-films, prepared by epitaxial growth in UHV on Cu(111), are ferromagnetic. The spontaneous magnetization was determined as 0.58 ± 0.13 μ_B/atom. This is independent of film thickness (up to 80 Å) and applies both for island-films and for layer-grown films. The Curie-temperature for bulk material was estimated to T_c(∞) = 900 ± 300 K. The apparent contradiction to antiferromagnetism in γ-Fe-precipitates is explained by different lattice parameters.     

External control of a metal-insulator transition in GaMnAs wires

Quantum transport in disordered ferromagnetic (III,Mn)V semiconductors is studied theoretically. Mesoscopic wires exhibit an Anderson disorder-induced metal-insulator transition that can be controlled by a weak external magnetic field. This metal-insulator transition should also occur in other materials with large anisotropic magnetoresistance effects. The transition can be useful for studies of zero-temperature quantum critical phase transitions and fundamental material properties.     

Ultrafast enhancement of ferromagnetism via photoexcited holes in GaMnAs

We report on the observation of ultrafast photoenhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100 ps time scale, after an initial subpicosecond demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature T(c) and dominates the demagnetization component when approaching T(c). We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20 K and a transient increase in T(c).     

Ferromagnetism in Cu-doped silicon carbide

Cu doped silicon carbide is shown to be ferromagnetic based on experiment results and first-principles calculations. The magnetization value of the Cu doped silicon carbide decreased as the Cu concentration increased. When the films were annealed at 800 °C, the ferromagnetic signal was increased. Reduction of the C vacancy concentration will introduce a large total moment in the system. Theoretically, compared with the case of one Cu atom replacing one Si atom, increasing the Cu doping, changing the Cu atom location or including carbon vacancies in the calculations for the system all make the ferromagnetic moment decrease. One Cu atom replacing one Si atom with the addition of one C vacancy makes the energy band gap of the system disappear. Our investigations suggest that the ferromagnetism arises from the hybridization between Cu 3d orbital and C 2p orbital. Ferromagnetic moment is influenced by a symmetry-lowering distortion of the surrounding lattice by the Cu dopant.     

Ferromagnetism in the Hubbard model

Whether spin-independent Coulomb interaction can be the origin of a realistic ferromagnetism in an itinerant electron system has been an open problem for a long time. Here we study a class of Hubbard models on decorated lattices, which have a special property that the corresponding single-electron Schrödinger equation hasN _d-fold degenerate ground states. The degeneracyN _d is proportional to the total number of sites ||. We prove that the ground states of the models exhibit ferromagnetism when the electron filling factor is not more than and sufficiently close to=N _d/(2||), and paramagnetism when the filling factor is sufficiently small. An important feature of the present work is that it provides examples of three dimensional itinerant electron systems which are proved to exhibit ferromagnetism in a finite range of the electron filling factor.