The important role of Mn in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga_1−xMn_xN) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga_1−xMn_xN films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn³⁺ due to the thermal treatment. Mn³⁺, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn³⁺ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn³⁺ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn_xGa_y formation.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

Magnetic properties of p-doped GaMnN diluted magnetic semiconductors containing clusters

Magnetic properties of p-doped GaMnN diluted magnetic semiconductors, having both randomly distributed Mn ions and Mn_xN_y clusters, are presented under the theory based on the hole-mediated ferromagnetism. The critical temperature of the second order phase transition between ferromagnetic and paramagnetic phases and the magnetization as a function of temperature are obtained from the free energy calculation. The Curie temperature of the p-doped GaMnN containing clusters depends not on the type of clusters but on the composition rate of clusters. The behavior of the spontaneous magnetization as a function of temperature is strongly affected by carrier concentration. The p-doped GaMnN diluted magnetic semiconductors containing clusters have room temperature ferromagnetism regardless of the magnetic type of clusters, as long as hole-mediated spin-spin interactions occur in them.     

Anomalous electrical transport properties of Ag/Al bilayers grown on Si by molecular beam epitaxy

We have investigated the temperature dependent electrical resistivity, ρ(T), of Ag(100 nm)/Al(10 nm) bilayers grown on Si(111) and quartz substrates using molecular beam epitaxy (MBE). Bilayers grown on Si exhibited an anomalous negative temperature coefficient of resistivity (TCR) in the temperature range of 140-165 K of the ρ(T) plot. However, at temperatures below and above this negative TCR region, ρ(T) exhibited a characteristic positive TCR of metallic alloys. No such resistive anomaly was observed for the bilayers grown on quartz substrates. The observed resistive anomaly could be qualitatively explained by assuming two parallel conduction channels, that is, one at the interface having high Si content and obeying the polaronic behavior at <165 K and another far away from the interface having almost no Si impurity and thus exhibiting pure metallic behavior down to 4 K. In addition, bilayers exhibited a sharp resistive transition at ∼6.5 K, indicating a possibility of a new Ag-Al alloy being a superconducting material.     

Magnetic properties of doped ZnO prepared by different synthetic routes

We have studied the magnetic properties of Zn_0.96M_0.04O (M=Mn, Fe, Co) compounds prepared using several routes. The low temperature ceramic synthesis gave multiphasic samples and show ferromagnetic behavior. Single phases can be obtained by heating at higher temperatures (∼900–1100 °C). The use of very low oxygen pressure also favours the preparation of single-phases. We were also successful in preparing single-phase samples at very low temperature (∼400 °C) by using a sol-gel method. All of the samples without noticeable secondary phases in the X-ray patterns behave as conventional paramagnets. This is true even for the samples with very low grain size. Samples exhibiting secondary phases reveal spontaneous magnetization even at room temperature in some cases. Our results strongly support that ferromagnetism at room temperature is always due to the presence of secondary phases and not to the doping of ZnO.     

Strain-symmetrized Si/SiGe multi-quantum well structures grown by molecular beam epitaxy for intersubband engineering

Three strain-symmetrized Si/SiGe multi-quantum well structures, designed for probing the carrier lifetime of intrawell intersubband transitions between heavy hole 1 (HH1) and light hole 1 (LH1) states with transition energies below the optical phonon energy, were grown by molecular beam epitaxy at low temperature on fully relaxed SiGe virtual substrates. The grown structures were characterized by using various experimental techniques, showing a high crystalline quality and very precise growth control. The lifetime of the LH1 excited state was determined directly with pump-probe spectroscopy. The measurements indicated an increase of the lifetime by a factor of ∼2 due to the increasingly unconfined LH1 state, which agreed very well with the design. It also showed a very long lifetime of several hundred picoseconds for the holes excited out of the well to transit back to the well through a diagonal process.     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.     

Observation of ferromagnetism at room temperature for Cr ions implanted ZnO thin films

Single crystalline ZnO films were grown on c-plane GaN/sapphire (0 0 0 1) substrates by molecular beam epitaxy. Cr⁺ ions were implanted into the ZnO films with three different doses, i.e., 1 × 10¹⁴, 5 × 10¹⁵, and 3 × 10¹⁶ cm⁻². The implantation energy was 150 keV. Thermal treatment was carried out at 800 °C for 30 s in a rapid thermal annealing oven in flowing nitrogen. X-ray diffraction (XRD), atomic force microscopy, Raman measurements, transmission electron microscopy and superconducting quantum interference device were used to characterize the ZnO films. The results showed that thermal annealing relaxed the stress in the Cr⁺ ions implanted samples and the implantation-induced damage was partly recovered by means of the proper annealing treatment. Transmission electron microscopy measurements indicated that the first five monolayers of ZnO rotated an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. The magnetic-field dependence of magnetization of annealed ZnO:Cr showed ferromagnetic behavior at room temperature.     

Raman Spectroscopy and Magnetic Properties of Mn-Doped ZnO Bulk Single Crystal

Mn doped ZnO bulk single crystals are grown by the modified Bridgman method. The as-grown crystals are red in eolour. The additional Raman mode observed at 524cm^-1 is attributed to the Mn ions incorporating into ZnO crystal. The crystal exhibited paramagnetie under lower applied field below 2280 Oe. Then diamagnetism is observed in the crystal when the magnetic field rises up and becomes dominant under applied field above 5270 Oe. The magnetic susceptibility dependence on the temperature follows a Curie law indicating a typical paramagnetie characteristic under an applied field of 2kOe. No ferromagnetic ordering is observed in the as-grown Mn-doped ZnO crystal.     

Electrical and optical properties of ZnO films grown by molecular beam epitaxy

Zinc oxide (ZnO) films have been grown on sapphire by molecular beam epitaxy (MBE), and it is found that the grain size of the ZnO films increased with increasing the growth temperature. Photoluminescence (PL) study shows that the intensity ratio of near-band-edge emission to deep-level-related emission (NBE/DL) of the ZnO is significantly enhanced with increasing the growth temperature, and the dependence of the carrier mobility on the growth temperature shows very similar trend, which implies that there is a community factor that determines the optical and electrical properties of ZnO, and this factor is suggested to be the grain boundary. The results obtained in this paper reveal that by reducing the grain boundaries, ZnO films with high optical and electrical properties may be acquired.     

ZnO-Fe3O4 composite prepared by sol-gel method with H2 deoxidation

Fe doped ZnO powder samples (Fe/Zn=0.05 and 0.1) were prepared by sol-gel method with H₂ deoxidation at 450 ^°C for several hours or just heated in air at the same temperature. It was showed by vibrating sample magnetometer (VSM) that samples heat treated in H₂ could show strong ferromagnetism at room temperature while samples treated in air only show very weak magnetism. XRD using Co kα X-ray revealed that the samples heated in H₂ were not pure phase but like a granular system and the magnetism mainly results from Fe₃O₄ in samples while samples heated in air showed pure ZnO phase. Our work indicated that H₂ deoxidation treatment may be an effective technique to fabricate such magnetic semiconductor-like materials with Curie temperature higher than room temperature.     

Zn-interstitial-enhanced ferromagnetism in Cu-doped ZnO films

Copper-doped ZnO (ZnO:Cu) films exhibiting room-temperature (RT) ferromagnetism were prepared by filtered cathodic vacuum arc (FCVA) technique. The ZnO:Cu films deposited at RT showed the strongest magnetic moment of 0.40 μ_B/Cu as compared with the samples prepared at elevated temperatures. The observed strong ferromagnetism in the RT-deposited ZnO:Cu films could be partly associated with Zn-interstitial defects. The degradation of magnetic moment in the ZnO:Cu prepared at high temperatures and annealed at elevated temperatures might be attributed to the out-diffusion of Zn interstitials to the ZnO lattice.     

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.     

Growth and characterization of rocksalt MnS/GaAs epilayers by hot-wall epitaxy

Epitaxial growth characteristics of α-MnS on GaAs(1 0 0) substrates have been investigated by X-ray diffraction and double crystal rocking curve measurements. Growth of stoichiometric α-MnS films has been performed by hot-wall epitaxy using Mn and ZnS as a source of sulfur. The films on GaAs(1 0 0) at low substrate temperature exhibit multiphase crystal structures of zincblende and rocksalt, and the main structure is changed to rocksalt with increasing substrate temperature. Photoluminescence spectrum of the α-MnS epilayer at 5 K exhibits broad emission bands, which are attributed to Mn²⁺ ions. The band gap energy of the α-MnS epilayer at room temperature was also estimated to be about 3.3 eV by reflection.     

X-ray absorption spectroscopic and magnetic characterization of cobalt-doped zinc oxide nanocrystals prepared by the molten-salt method

The local atomic arrangement and electronic structure of the Co-doped Zn_1−xCo_xO nanocrystal have been quantitatively examined along with its magnetic properties. According to our analysis using powder X-ray diffraction, electron microscopy, and Zn K-edge X-ray absorption spectroscopy (XAS), phase-pure wurzite-structured Zn_1−xCo_xO nanocrystals have been successfully synthesized via the molten-salt method. The Co K-edge XAS analysis clearly demonstrates that all the Co²⁺ ions are substituted for the tetrahedral Zn sites of the Wurzite structure with a coordination number of 3.9 and a bond distance of 1.97 Å, ruling out the presence of magnetic impurity phase and Co-metal cluster. Magnetization measurements reveal that the present Zn_1−xCo_xO sample does not show any ferromagnetic transition down to 2 K. In this regard, we can conclude that Co-doped zinc oxide is not ferromagnetic but the previously reported ferromagnetism in this phase would be an extrinsic property.     

Ferromagnetic properties of Fe-substituted ZnO-based magnetic semiconductor

The diluted magnetic semiconductor Zn_1−x⁵⁷Fe_xO (x=0.01, 0.02, 0.03) compounds were prepared by the solid-state reaction method. The crystal structure of _Zn0.97⁵⁷_Fe0.03O${\mathrm{Zn}}_{0.97}{}^{57}{\mathrm{Fe}}_{0.03}\mathrm{O}$ at room temperature is determined to be a hexagonal structure of P6₃mc with lattice constants a₀=3.252 Å and c₀=5.205 Å by Rietveld refinement. The Bragg factors R_B and R_F were determined as 3.23% and 2.81%. From the inverse susceptibility versus T curve, the paramagnetic Curie temperature is found to be 2.7 K and effective moment is found to be 4.01 μ_B, thereby suggesting that the exchange interactions between Fe ions are ferromagnetic. Mössbauer spectra of _Zn0.97⁵⁷_Fe0.03O${\mathrm{Zn}}_{0.97}{}^{57}{\mathrm{Fe}}_{0.03}\mathrm{O}$ have been taken at various temperatures ranging from 4.2 to 295 K. Mössbauer spectrum for _Zn0.97⁵⁷_Fe0.03O${\mathrm{Zn}}_{0.97}{}^{57}{\mathrm{Fe}}_{0.03}\mathrm{O}$ at 4.2 K has shown ferromagnetic phase (sextet), and the spectra were fitted based on a random distribution model of Fe ions.     

Synthesis of GaN phase by ion implantation

GaN phase is synthesized using systemic implantation of nitrogen ions of multiple energies (290, 130 and 50 keV) into Zn-doped GaAs (1 0 0) at room temperature and subsequent annealing at 850 °C for 30 min in Ar + H₂ atmosphere. The implanted doses of nitrogen ions are 5 × 10¹⁶ and 1 × 10¹⁷ ions-cm⁻². Glancing angle X-ray diffraction studies show that hexagonal phase of GaN were formed. The photoluminescence studies show the emission from the band edge as well as from point defects.     

Improving magnetic properties of barium hexaferrites by La or Pr substitution

La or Pr substituted barium hexaferrites, Ba_1−x(La or Pr)_xFe₁₂O₁₉, x=0.00-0.20, were successfully prepared by a citrate combustion process. The sintered bodies were structurally and magnetically studied by powder X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). All XRD patterns show the single phase of the magnetoplumbite barium ferrite without other intermediate phases. Magnetization (M_S) and coercive field (H_C) could be improved by substitutions of La or Pr ions on Ba ion basis sites. The M_S reveal magnetic behavior with respect to La or Pr ions content, showing an increase at first and then a decrease. The H_C increases remarkably with increasing La or Pr ions content.     

Optical and magneto-optical properties of room-temperature ferromagnetic In(Ga)MnAs layers, deposited by pulse laser ablation

Spectral dependences of refractive and absorption indices n(hν), k(hν) (hν=1.2-4.4 eV) and the transversal Kerr effect δ(hν) (hν=0.5-4.4 eV) in In(Ga)MnAs layers fabricated by laser deposition have been investigated. Spectra of the diagonal and off-diagonal components of the dielectric permittivity tensor of these layers have been calculated. Comparison of the spectral dependences δ(hν), ε′(hν) and ε′₂×(hν)² of the In(Ga)MnAs layers with similar spectra for MnAs has been carried out. Particular features in the spectra of the In(Ga)MnAs layers have been explained by a competition of contributions of the In_1−x(Ga_1−x)Mn_xAs host and MnAs inclusions.     

Crystallographic and magnetic properties of (Nd,Dy)3Fe27.5(Ti,Mo)1.5 compounds

A systematic study of the formation, structure and magnetic properties of (Nd,Dy)₃Fe_27.5(Ti,Mo)_1.5 compounds has been performed. Rietveld analyses of the X-ray patterns of the samples indicate that the concentrations of Ti and Mo affect the formation and structural properties slightly, whereas different rare-earth (Nd and Dy) contents influence them significantly. It is found that high Dy contents make it difficult to form the 3:29-type structures. The Curie temperatures of Nd_2.1Dy_0.9Fe_27.5Ti_1.5−xMo_x decrease monotonically as more Ti was replaced by Mo but their saturation magnetizations remain almost unchanged; in contrast, for Nd_3−yDy_yFe_27.5TiMo_0.5, their saturation magnetizations decrease monotonically with increasing Dy contents while their Curie temperatures are constant.