(Ga,Mn)As稀磁半导体的杂质动力学模拟研究

结合第一性原理计算和动力学蒙特卡罗模拟研究了稀磁半导体(Ga,Mn)As中Mn杂质的沉积动力学规律。利用第一性原理计算和爬坡弹性带方法计算了Mn杂质的跃迁势垒和结合能,并把这些能量作为动力学蒙特卡罗模拟(Ga,Mn)As微观结构演化的输入数据。结果表明在外延生长退火下长时间的微观结构演化的背后机制是Ga空位调节Mn原子在Ga子晶格上进行扩散。这种扩散会导致Mn原子的聚集,进而降低了居里温度。此外,随着退火温度的升高Mn团簇聚集的速率也更快。在高温退火下容易导致相分离。     

Manifestation of Local Magnetic Domain Reversal by Spin-Polarized Carrier Injection in (Ga,Mn)As Thin Films

Recently, we have reported that the spin-polarized holes generated by the irradiation with circularly polarized light can change the magnetization orientation of III–V ferromagnetic semiconductor (Ga,Mn)As via p–d exchange interaction. In this paper, we report that a small portion of change does not return to the initial state after the light is turned off. This residual component, named as the memorization effect, exhibits ferromagnetic characteristics. This fact strongly suggests that small magnetic domains having the perpendicular magnetic axis are rotated by photogenerated carrier spins.     

Temperature Peculiarities of Magnetic Anisotropy in (Ga,Mn)As: The Role of the Hole Concentration

It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently small. In such films, the easy axis assume a standard in-plane orientation when the temperature is increased towards the Curie temperature. The findings are shown to corroborate the predictions of the mean-field Zener model for strained (III,Mn)V ferromagnetic semiconductors.     

Spin Carrier Exchange Interactions in (Ga,Mn)N and (Zn,Co)O Wide Band Gap Diluted Magnetic Semiconductor Epilayers

(Ga,Mn)N and (Zn,Co)O wide band gap diluted magnetic semiconductor epilayers have been investigated by magneto-optical spectroscopy. In both cases, absorption bands observed below the energy gap allow us to study the nature of the valence and spin state of the incorporated magnetic element. Exchange interactions between magnetic ions and carriers can be observed by analyzing the magnetic circular dichroism in transmission or the exciton Zeeman splitting in reflection for (Zn,Co)O. A first estimation of the exchange integrals can be given for both materials.     

Magnetic and Magnetotransport Properties of Diluted Magnetic Semiconductor (Ga,Mn)Sb Crystals

We report the magnetic and magnetotransport properties of Ga_1−x Mn _x Sb crystals grown with different Mn doping concentrations (x=0.01, 0.02, 0.03 and 0.04) by the horizontal Bridgman technique. A systematic reduction in lattice parameter with increase in Mn concentration is observed through X-ray diffraction studies. The crystals show negative magnetoresistance and anomalous Hall effect below 10 K. Anomalous Hall coefficient is negative and decreases with increasing Mn concentration. Temperature dependence on magnetization measurement shows a magnetic ordering below 10 K which could arise from GaMnSb alloy formation. Also, ferromagnetism is observed till room temperature due to the presence of MnSb clusters. Existence of MnSb clusters is verified through scanning electron microscopy. The carrier concentration increases with Mn doping and this results in decrease of resistivity. The observed magnetic and transport properties indicate the presence of ferromagnetic phase below 10 K in the studied system.      

Rotation of Ferromagnetically Coupled Mn Spins in (Ga,Mn)As by Hole Spins

We present experimental data obtained from ferromagnetic p-(Ga,Mn)As layers that indicate the collective rotation of ferromagnetically coupled Mn spins by optically generated spin-polarized holes through the p–d exchange interaction. The rotation occurs reversibly between the in-plane and perpendicular directions, causing a large change in perpendicular magnetization without the application of a magnetic field. Pump-and-probe experiments suggest that the rotation of Mn spins take place in the picosecond time domain.     

Photoinduced Magnetization Rotation and Precessional Motion of Magnetization in Ferromagnetic (Ga,Mn)As

Dynamics of photoinduced magnetization have been studied in ferromagnetic semiconductor (Ga,Mn)As thin films by time-resolved polar Kerr rotation. Ultrafast (150 fs) magnetization rotation has been realized using both chirality of excitation light and p–d exchange interaction. We also report on the occurrence of photoinduced magnetization precession in longer time domain. This is initiated by a change in hole concentration and resultant change in magnetic anisotropy. The overall behaviors of these photoinduced magnetization dynamics are discussed.     

Effect of GaAs Intermediary Layer Thickness on the Properties of (Ga,Mn)As Tri-Layer Structures

We have investigated the magnetic properties, current–voltage characteristics, and tunnel magnetoresistance of (Ga,Mn)As/GaAs/(Ga,Mn)As tri-layers having different thickness of the intermediary GaAs layer (1–7 nm). When the thickness of GaAs layer is less than 6 nm, the two (Ga,Mn)As layers couples ferromagnetically. For the tri-layers with thicker GaAs, the GaAs acts as an effective barrier for the carrier transport and decouples the two (Ga,Mn)As layers, which results in tunnel magnetoresistance ratio as high as 100% at 5 K.     

Magnetization Reversal by Electrical Spin Injection in Ferromagnetic (Ga,Mn)As-Based Magnetic Tunnel Junctions

We report current-induced magnetization reversal in a ferromagnetic semiconductor-based magnetic tunnel junction (Ga,Mn)As/AlAs/(Ga,Mn)As prepared by molecular beam epitaxy on a p-GaAs(001) substrate. A change in magneto-resistance that is asymmetric with respect to the current direction is found with the excitation current of 10⁶ A/cm². Contributions of both unpolarized and spin-polarized components are examined, and we conclude that the partial magnetization reversal occurs in the (Ga,Mn)As layer having smaller magnetization with the spin-polarized tunneling current of 10⁵ A/cm².     

Influence of Codoping on the Magnetoresistance of Paramagnetic (Ga,Mn)As

We studied the correlation between magnetoresistance (MR) effects and the type of conducti- vity in paramagnetic Ga_1–x Mn _x As with x < 0.5%. Series of samples of (Ga,Mn)As with different codoping levels of Te have been prepared by metal-organic vapor-phase epitaxy (MOVPE). With increasing Mn-content from doping levels to x = 0.5% in paramagnetic Ga_1–x Mn _x As, the MR at 1.6 K changes continuously from positive to strongly negative. This manifests the complex interplay between p–d exchange induced valence band splitting and the metal–insulator transition. Codoping with Te leads eventually to a dominant conduction band transport. It is characterized by a small negative contribution at low magnetic fields to the parabolic MR similar to that found in highly n-doped diamagnetic semiconductors. It shows that the Mn-induced conduction band splitting is much smaller than the valence band splitting, i.e., |N₀| |N₀|.     

Anomalous Conductivity in Random Magnetic (Ga,Cr) As

The electrical conductivity of molecular beam epitaxy grown zinc blende Ga_1–x Cr _x As, x = 0.1, exhibits anomalous behavior below room-temperature. The room-temperature resistivity is small, = 0.1 cm, and comparable to (Ga,Mn)As. Near room temperature, the conductivity is activated, following = ₀ exp(–E _A/kT ), with an activation energy of E _A = 63 meV. In this activated region, Hall measurements also show activated behavior in the hole concentration where p 10²⁰ cm^–3, indicating that Cr can also act as an acceptor similar to Mn. For decreasing temperature, the resistivity increases rapidly because of hopping or tunneling conduction, and becomes strongly insulating at low temperatures. From T = 20 to 200 K the conductivity follows exp[–(T ₁/T)^1/2] over eight orders of magnitude range of conductivity, possible evidence of tunneling between metallic-like polarons.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

Electric Field Effect on the Magnetic Properties of III–V Ferromagnetic Semiconductor (In,Mn)As and ((Al),Ga,Mn)As

We have investigated the magnetotransport properties of field-effect transistors (FET) having a III–V ferromagnetic semiconductor channel layer. One can control not only the ferromagnetic transition temperature T _C but also the magnetization and the coercive force of (In,Mn)As channel layers isothermally and reversibly by gate electric fields. A small change of the magnetization upon application of gate electric fields is also observed in FETs with a (Ga,Mn)As channel. Results on a (Al,Ga,Mn)As channel FET are also presented.     

Strain-Controlled Variation of Magnetoresistive and Magnetic Anisotropy in (Ga,Mn)As

The influence of strain on the anisotropic magnetoresistance (AMR) and magnetic anisotropy (MA) of (Ga,Mn)As is systematically investigated in layers grown on relaxed (In,Ga)As/GaAs templates. By choosing different In contents, the vertical strain ε _zz in the (Ga,Mn)As layers could be varied over a wide range from tensile to compressive, including the case of (nearly) zero strain. The AMR and the MA of the as-grown and annealed samples were probed at 4.2 K by means of angle-dependent magnetotransport measurements. The respective resistivity and anisotropy parameters, determined by an appropriate fitting procedure, were found to linearly vary with ε _zz . The MA was additionally manipulated by preparing narrow stripes from the (Ga,Mn)As layers, resulting in an in-plane uniaxial contribution to the free energy, which is not present in unstrained samples.