Ferromagnetic Properties of Mn-Doped III–V Semiconductor Quantum Wells

We investigate magnetic properties of In_1–xMn_xP and Ga_1–xMn_xN quantum wells in the mean-field approximation and show the difference between them. In the case of the In_1–xMn_xP, the dependence of the Curie temperature (T_c) on the hole density exhibits a step-like behavior, reflecting the effect of a two-dimensional Fermi (hole) gas, when the hole–hole exchange interaction is ignored. When we take into account the hole–hole exchange interaction, however, this behavior is broken by the appearance of peaks at the specific two-dimensional carrier densities, and T_c is substantially enhanced in this region. In the case of Ga_1–xMn_xN, the step-like behavior is obscure, and it appears that T_c increases rather continuously with the increasing two-dimensional (2D) carrier density. This shows very weak step-like behavior compared to other III-Mn-V DMS quantum wells, because the hole effective mass of Ga_1–xMn_xN is very large, and the large hole effective mass reduces the energy splitting due to the confinement effect. In a multi-heavy-valence-subband model, the Curie temperature of the In_1–xMn_xP quantum well is about 68 K with 6.5×10¹² holes per cm² and the Mn mole fraction x = 0.05 and the exchange constant J_pd = 0.15 eV nm³. The Curie temperature of the p-type Ga_1–xMn_xN quantum well can be above room temperature, unless the spin-exchange interaction integral is abnormally small.     

Correlation of Mn Lattice Location, Free Hole Concentration, and Curie Temperature in Ferromagnetic GaMnAs

We provide experimental evidence that the electrical and magnetic characteristics of Ga_1–x Mn _x As for a given x depend primarily on the distribution of Mn atoms over their different possible locations in the crystal lattice. Using combined channeling Rutherford backscattering and particle-induced X-ray emission, we show that optimal postgrowth annealing—which leads to an increase of the Curie temperature T _C and is accompanied by an increase of free hole concentration and saturation magnetization—is caused by the reduction in the number of Mn atoms occupying interstitial positions. On the other hand, when Ga_1–x Mn _x Asis additionally doped with Be, we observe that—while the hole concentration remains nearly constant—there occurs a strong decrease of T _C together with a dramatic increase in the concentration of Mn interstitials. These results indicate that there is a thermodynamic limit imposed on the maximum Curie temperature in Ga_1–x Mn _x As.     

Cyclotron Resonance of Electrons and Holes in Paramagnetic and Ferromagnetic InMnAs-Based Films and Heterostructures

We have studied the cyclotron resonance of electrons and holes in various types of InMnAs-based structures at ultrahigh magnetic fields. Our observations, in conjunction with an eight-band effective mass model including the s–d and p–d exchange interactions with Mn d-electrons, unambiguously suggest the existence of s-like and p-like delocalized carriers in all samples studied. The samples studied include Paramagnetic n-type In_1–x Mn _x As films (x 0.12) grown on GaAs, ferromagnetic p-type In_1–x Mn _x As films (x 0.025) grown on GaAs with Curie temperatures (T _C) > 5 K, paramagnetic n-type In_1–x Mn _x As/InAs superlattices, ferromagnetic p-type In_1–x Mn _x As/GaSb heterostructures (x 0.09) with T _C = 30-60 K, and ferromagnetic (In_0.53Ga_0.47)_1–x Mn _x As/In_0.53Ga_0.47As heterostructures (x 0.05) grown on InP with T _C up to 120 K.     

Alloying (In,Mn)As and (Ga,Mn)As: Ferromagnetic (In,Ga,Mn)As Lattice-Matched to InP

An effect of alloying two ferromagnetic semiconductors (In,Mn)As and (Ga,Mn)As on the ferromagnetic properties of resultant (In,Ga,Mn)As alloys is reported. For conditions close to lattice-matching to InP substrates, y = 0.53 in (In _y Ga_1–y )_1–x Mn _x As, ferromagnetism up to Curie temperatures T _C = 100–110 K could be achieved for a Mn composition x = 0.13. Trends in the Curie temperature in (In,Ga,Mn)As are compared with (Ga,Mn)As and (In,Mn)As as a function of Mn content. Hole concentrations determined from magnetotransport, taking into account the anomalous Hall contribution to Hall resistance, gives p/Mn = 0.03 ratio to Mn composition in metallic case for x = 0.13. We mention the possible role of chemical ordering (short range) of Mn impurity atoms on hole concentration and, consequently, for the ferromagnetic properties.     

Low Temperature Annealing Studies of Ga1?xMnxAs

High and low field magnetotransport measurements, as well as SQUID measurements of magnetization, were carried out on Ga_1–x Mn _x As epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280^°C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga_1–x Mn _x As annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga_1–x Mn _x As.     

Quantum-confinement effect on recombination dynamics and carrier localization in cubic InN and InxGa1 − xN quantum boxes

In this study, the quantum confinement effect on recombination dynamics and carrier localization in cubic InN (c-InN) and cubic In_xGa_1 − xN (c-In_xGa_1 − xN) low dimensional structures are theoretically examined. The small InN and In-rich In_xGa_1 − xN low dimensional structures show a strong quantum confinement effect, which results in ground states away from the band edge and discrete eigen-states. Depending on composition, temperature, and size of the InN and In_xGa_1 − xN low dimensional structures, quantum confinement effect can affect the exciton dimensions (D). In InN quantum cubes, the strong quantum confinement effect leads to temperature-dependent radiative lifetimes showing a large size variation. The nearly-temperature-independent and shorter radiative lifetimes in small InN and In-rich In_xGa_1 − xN low dimensional structures suggest that the strong quantum confinement leads to 0 D carrier confinement, stronger carrier localization, and high recombination efficiency. Reported radiative lifetimes were found to match very well with our simulation results of In-rich quantum cubes, which indicates that spontaneous emissions come from the radiative recombination of localized excitons in In-rich In_xGa_1 − xN clusters. The simulation results could provide important information for the designs and interpretations of c-InN and c-In_xGa_1 − xN devices.     

Magnetic and transport properties of Mn3+xGa1?xN compounds

Mn_3+x Ga_1−x N compounds with x = 0.0 and 0.1 were prepared by re-sintering Mn₂N_0.86, Ga bulk and Mn powders. These compounds are deduced to be the N-deficiency ones. In Mn₃GaN, a step-like magnetic transition, from frustrated antiferromagnetism to paramagnetism with increasing temperature, occurs at 370 K, while the same magnetic transition of Mn_3.1Ga_0.9N is far above 380 K. The enhanced magnetization of Mn₃GaN at low temperatures is ascribed to the fast lowering of antiferromagnetism. The electrical resistivity of Mn₃GaN exhibits a typically metallic conducting behavior with a positive magnetoresistance of 4–7%.     

Magnetic and Electrical Properties of Random and Digital Alloys of GaSb:Mn

We have investigated the effect of Sb/Ga flux ratio on the magnetic and electronic properties of Mn-incorporated GaSb random and digital alloys grown by low-temperature molecular beam epitaxy. The magnetic and magnetotransport properties of Ga_1–xMn_xSb random alloys are strongly dependent on Sb/Ga flux ratio. Clear square-like hysteresis loops were observed for Sb/Ga flux ratios between 4.60 and 5.25. The coercive field and negative magnetoresistance increase with decreasing Sb/Ga flux ratio, while the Curie temperature remains constant at approximately 23 K, with no systematic dependence on the hole density. In contrast, the Curie temperatures for the GaSb:Mn digital alloys with different Mn surface coverages depend significantly on the Sb/Ga flux ratio, and it is also directly correlated with the hole density.     

Properties of Quaternary Alloy Magnetic Semiconductor (InGaMn)As Grown on InP

We have studied properties of quaternary alloy magnetic semiconductor (InGaMn)As grown on InP substrates by low-temperature molecular-beam epitaxy (LT-MBE). A large MCD peak whose intensity is larger than 500 mdeg for (InGaMn)As was observed. This peak intensity was about three times larger than that of typical (GaMn)As films. Relatively high Curie temperature of 83 K of [(In_0.53Ga_0.47)_0.88Mn_0.12]As was observed by Hall measurements. The carrier concentration of [(In_0.53Ga_0.47)_0.88Mn_0.12]As was estimated to be more than 1.0 × 10²¹ cm^–3 by using the Curie–Weiss fitting of the Hall coefficient R _H, indicating that more than 40% of Mn atoms are activated. This means that (InGaMn)As has a higher activation ratio of Mn as acceptors than (GaMn)As.     

The crystal structure,magnetic and magnetocaloric properties of Mn8−xCrxGa5

Mn₈Ga₅ alloy has certain significance in the study of materials for magnetic refrigeration applications due to its low coercivity. In this paper, the structure, magnetic properties and magnetocaloric effects (MCE) of Mn_8?xCr_xGa₅ (x?=?1.3, 1.7) polycrystalline alloys were first studied by using X-ray diffraction (XRD) technique, scanning electron microscope (SEM), energy-dispersion (EDS) analysis and magnetization measurement. The two samples crystallized in a cubic (Zn₈Cu₅)-type structure with space group of I-43 m (No. 217). The lattice constants and crystallite sizes of Mn_8?xCr_xGa₅ increase with the increase of Cr concentration. As the Cr content increases, the Curie temperature (T_c) of Mn_8?xCr_xGa₅ (x?=?1.3, 1.7) is increased from 145 to 187 K. The saturation magnetic moments of Mn_8?xCr_xGa₅ are 33.8 (x?=?1.3) and 36.6 (x?=?1.7) emu/g under the magnetic field of 16 kOe at 80 K, respectively. A second-order magnetic phase transition occurred in Mn_8?xCr_xGa₅ alloy samples. The maximum magnetic entropy changes (|?S_M|) of Mn_8?xCr_xGa₅ are 0.9 (x?=?1.3) and 0.8 J·kg^?1·K^?1 (x?=?1.7), and the relative cooling power (RCP) values are 25 (x?=?1.3) and 17 J/kg (x?=?1.7) under 15 kOe magnetic field.