Photoinduced Barkhausen effect in the ferromagnetic semiconductor (Ga,Mn)As

Magnetization of ferromagnetic materials commonly occurs via random jumps of domain walls between pinning sites, a phenomenon known as the Barkhausen effect. Using strongly focused light pulses of appropriate power and duration we demonstrate the ability to selectively activate single jumps in the domain wall propagation in (Ga,Mn)As, manifesting itself as a discrete photoinduced domain wall creep as a function of illumination time. The propagation velocity can be increased over 7 orders of magnitude varying the illumination power density and the magnetic field.     

Enhancement of the Curie temperature of ferromagnetic semiconductor(Ga,Mn)As

In this review article,we review the progress made in the past several years mainly regarding the efforts devoted to increasing the Curie temperature(T C) of(Ga,Mn)As,which is most widely considered as the prototype ferromagnetic semiconductor.Heavy Mn doping,nanostructure engineering and post-growth annealing which increase T C are described in detail.     

Current-driven magnetization reversal in a ferromagnetic semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction

Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 microm2 device revealed that magnetization switching occurs at low critical current densities of 1.1-2.2 x 10(5) A/cm2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.     

Velocity of domain-wall motion induced by electrical current in the ferromagnetic semiconductor (Ga,Mn)As

Current-induced domain-wall motion with velocity spanning over 5 orders of magnitude up to 22 m/s has been observed by the magneto-optical Kerr effect in (Ga,Mn)As with perpendicular magnetic anisotropy. The data are employed to verify theories of spin transfer by the Slonczewski-like mechanism as well as by the torque resulting from spin-flip transitions in the domain-wall region. Evidence for domain-wall creep at low currents is found.     

Fabrication of ferromagnetic (Ga,Mn)As by ion irradiation

Using Mn⁺ implantation following ion beam-induced epitaxial crystallization (IBIEC) annealing, high Curie temperature ferromagnetic (Ga,Mn)As thin film was fabricated. The crystalline quality of the Mn⁺ implanted layer was identified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A clear ferromagnetic transition at T_c 253 K was observed by magnetization vs. temperature measurement. We infer that IBIEC treatment is a useful method not only for the low-temperature annealing of (Ga,Mn)As thin films but also for other dilute magnetic semiconductor (DMS) samples.     

Domain-wall resistance in ferromagnetic (Ga,Mn)As

A series of microstructures designed to pin domain walls (DWs) in (Ga,Mn)As with perpendicular magnetic anisotropy has been employed to determine extrinsic and intrinsic contributions to DW resistance. The former is explained quantitatively as resulting from a polarity change in the Hall electric field at DW. The latter is 1 order of magnitude greater than a term brought about by anisotropic magnetoresistance and is shown to be consistent with disorder-induced mistracking of the carrier spins subject to spatially varying magnetization.     

Photoinduced precession of magnetization in ferromagnetic (Ga,Mn)As

Precession of magnetization induced by pulsed optical excitation is observed in a ferromagnetic semiconductor (Ga,Mn)As by time-resolved magneto-optical measurements. It appears as complicated oscillations of a polarization plane of linearly polarized probe pulses, but is reproduced by gyromagnetic theory incorporating an impulsive change in an effective magnetic field due to a change in the magnetic anisotropy. The shape of the impulse suggests a significant nonthermal contribution of photogenerated carriers to the change in anisotropy through spin-orbit interaction.     

Dilute moment n-type ferromagnetic semiconductor Li(Zn,Mn)As

We propose to replace Ga in (Ga,Mn)As with Li and Zn as a route to high Curie temperature, carrier mediated ferromagnetism in a dilute moment n-type semiconductor. Superior material characteristics, rendering Li(Zn,Mn)As a realistic candidate for such a system, include high solubility of the isovalent substitutional Mn impurity and carrier concentration controlled independently of Mn doping by adjusting Li-(Zn,Mn) stoichiometry. Our predictions are anchored by ab initio calculations and comparisons with the familiar and directly related (Ga,Mn)As, by the physical picture we provide for the exchange interaction between Mn local moments and electrons in the conduction band, and by analysis of prospects for the controlled growth of Li(Zn,Mn)As materials.     

Investigation of photoluminescence in GaAs quantum well coupled with a ferromagnetic semiconductor layer of (Ga,Mn)As

We have investigated circular-polarized photoluminescence (CPL) from a novel quantum structure in which a ferromagnetic semiconductor (Ga,Mn)As is placed adjacent to the GaAs quantum well. By eliminating the contribution of the magneto-circular dichroism effect of the (Ga,Mn)As top layer from the observed CPL, we found a small but nonnegligible contribution of quantum mechanical coupling between the GaAs quantum well states and the spin-polarized states in (Ga,Mn)As.     

Direct measurement of the spin polarization of the magnetic semiconductor (Ga,Mn)As

We have carried out a direct measurement of the degree of spin polarization (P) of the magnetic semiconductor Ga1-xMnxAs using Andreev reflection spectroscopy. Analyses of the conductance spectra of high transparency Ga(0.95)Mn(0.05)As/Ga junctions consistently yield an intrinsic value for P greater than 85%. Our experiments also revealed an extreme sensitivity of the measured spin polarization to the nature and quality of the interface for this material.     

Curie温度附近稀磁半导体(Ga,Mn)As的电学噪声谱性质

建立了自发噪声谱测量系统来研究稀磁半导体（Ga,Mn）As的电学噪声性质.通过测量（Ga,Mn）As材料的自发噪声谱,发现（Ga,Mn）As的自发涨落会随温度升高而逐渐增大,同时,外加磁场会降低（Ga,Mn）As的自发涨落,这来源于外加磁场导致的（Ga,Mn）As磁畴部分有序化.此外,不同频率的噪声随温度的变化规律有很大差异：当频率低于30 kHz的时候,噪声谱和温度的变化关系和热噪声很相似,但数值上明显大于热噪声的值;当频率在30 kHz左右的时候,噪声大小和温度成线性关系;当频率大于30 kHz以后,在相变点附近噪声大小和温度的关系出现了明显的转折,高频高温噪声的大小和热噪声的理论值非常接近.这些结果有助于深入理解（Ga,Mn）As磁性起源的物理机制. 关键词： 自旋电子学 稀磁半导体 自发涨落谱     

Magnetic properties of (Ga,Mn)As

A summary of experimental findings and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductor (Ga,Mn)As is presented. It is shown that the Zener p–d model explains quantitatively observed Curie temperatures in compensation free samples and that major strain-related effects are correctly accounted for, including the presence of the magnetization reorientation transition, observed as a function of hole concentration and temperature. It is evidenced that a presence of a small trigonal distortion could account for both the presence and properties of uniaxial in-plane magnetic anisotropy.     

Very large tunneling anisotropic magnetoresistance of a (Ga,Mn)As/GaAs/(Ga,Mn)As stack

We report the discovery of a very large tunneling anisotropic magnetoresistance in an epitaxially grown (Ga,Mn)As/GaAs/(Ga,Mn)As structure. The key novel spintronics features of this effect are as follows: (i) both normal and inverted spin-valve-like signals; (ii) a large nonhysteretic magnetoresistance for magnetic fields perpendicular to the interfaces; (iii) magnetization orientations for extremal resistance are, in general, not aligned with the magnetic easy and hard axis; (iv) enormous amplification of the effect at low bias and temperatures.     

(Ga,Mn)As光调制反射光谱

室温下我们研究了稀磁半导体(Ga,Mn)As的光调制反射(PR)光谱,观测到来自样品的Franz-Keldysh振荡(FKO)信号。随着Mn原子浓度的增加,PR线形展宽,但是临界点E₀和E₀+Δ₀没有明显的移动。根据FKO振荡数据,计算得到样品表面电场强度随Mn原子掺杂浓度的增加而增强。测量到与Mn原子掺杂相关的杂质带,其能量位置离GaAs价带边~100 meV。根据样品的表面电场强度和表面耗尽层模型,估算样品的空穴浓度为~10¹⁷cm^-3,较低的空穴浓度可能与样品具有较低的居里温度有关,或测量的PR信号来自于样品中外延层的部分耗尽区域。     

Magneto-optical and micromagnetic simulation study of the current-driven domain wall motion in ferromagnetic (Ga,Mn)As

We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved polar magneto-optical Kerr effect microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1-2×10⁵ A/cm². Considering the spin-transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current-driven domain wall motion in this material.     

Diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As: the effect of a charge state

Migration barriers for diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As are studied using first-principles calculations. The diffusion pathway goes through two types of interstitial sites: As coordinated and Ga coordinated. The energy profile along the path is found to depend on the ratio of concentrations between substitutional and interstitial Mn in GaAs. Two regions of distinctly different behavior, corresponding to n-type and p-type (Ga,Mn)As, are identified. The difference in mobility is a reflection of the change in the charge state of Mn interstitials (double donors) that occurs in the presence of substitutional Mn impurities (acceptors). In addition, substitutional Mn impurities are shown to act as traps for interstitial Mn. The effective migration barrier for the positively doubly charged Mn interstitials in p-type (Ga,Mn)As is estimated to vary from 0.55 to about 0.95 eV.     

Weak localization in ferromagnetic (Ga,Mn)as nanostructures

We report on the observation of weak localization in arrays of (Ga,Mn)As nanowires at millikelvin temperatures. The corresponding phase coherence length L phi is typically between 100 and 200 nm at 20 mK. Strong spin-orbit interaction in the material is manifested by a weak antilocalization correction around zero magnetic field.     

Anisotropic magnetoresistance components in (Ga,Mn)As

We explore the basic physical origins of the noncrystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. The sign of the noncrystalline AMR is found to be determined by the form of spin-orbit coupling in the host band and by the relative strengths of the nonmagnetic and magnetic contributions to the Mn impurity potential. We develop experimental methods yielding directly the noncrystalline and crystalline AMR components which are then analyzed independently. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. Generic implications of our findings for other dilute moment systems are discussed.