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.     

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.     

Diffusion thermopower of (Ga,Mn)As/GaAs tunnel junctions

We report the observation of tunneling anisotropic magnetothermopower, a voltage response to a temperature difference across an interface between a normal and a magnetic semiconductor. The resulting voltage is related to the energy derivative of the density of states in the magnetic material, and thus has a strongly anisotropic response to the direction of magnetization in the material. The effect will have relevance to the operation of semiconductor spintronic devices, and may indeed already play a role in correctly interpreting the details of some earlier spin injection studies.     

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.     

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.     

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.     

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.     

Formation of the single-phase ferromagnetic semiconductor (Ga,Mn)As by pulsed laser annealing

It is shown that (Ga,Mn)As layers formed by Mn⁺ ion implantation into GaAs and subsequent annealing by an excimer laser pulse with an energy density to 200–300 mJ/cm² feature the properties of the p-type semiconductor and ferromagnetic properties. The threshold dose of implanted ions (~10¹⁵ cm^–2) for activating Mn acceptors is determined. The sheet hole concentration and the Curie temperature increase with further increasing Mn⁺ ion dose. Hysteresis loops in the magnetic field dependences of the Hall effect, the negative magnetoresistance, and magnetic and structural studies suggest that the layers are analogues of single-phase ferromagnetic compounds (Ga,Mn)As formed by low-temperature molecular beam epitaxy.     

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.     

Carrier spin polarization in ferromagnet-semiconductor (Ga,Mn)As/GaAs structures

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet-semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.     

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.     

Tunneling and injection in ferromagnetic structures InGaAs/GaAs/(Ga,Mn)As and InGaAs/<Emphasis Type="Italic">n</Emphasis> <Superscript>+</Superscript>-GaAs/(Ga,Mn)As

The spin light-emitting diodes based on InGaAs/GaAs heterostructures with a quantum well and an injector in the form of a (Ga,Mn)As ferromagnetic layer have been studied. It has been demonstrated that the efficiency of electron spin injection in the structure with a (Ga,Mn)As/n⁺-GaAs tunneling barrier can be controlled by varying the parameters of n⁺-GaAs. The spin injection control mechanisms associated with the thermal activation and tunneling of carriers have been discussed.     

磁性半导体/磁性d波超导结中的自旋极化输运

通过求解磁性d波超导中的能隙与磁交换能的自恰方程,利用推广的Blonder-Tinkham-Klapwijk 理论研究磁性半导体/磁性d波超导结中自旋极化准粒子输运系数与微分电导. 计算表明： 1) 磁性d波超导结中的磁交换能h₀可导致零偏压电导峰与能隙电导峰劈裂,劈裂的宽度为2h₀;2) 磁性半导体中的磁交换能h_FS可使零偏压电导峰劈裂的峰值变低. 而由能隙电导峰劈裂的两个子峰,当两种磁性材料的磁 关键词： 磁性半导体 磁性d波超导体 自旋极化输运     

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.     

Shallow donors in extended state GaAs/(Al,Ga) As superlattices

We have studied the effect of superlattice structure on the 1s to 2p⁺ transition energy of shallow donors. In a strong magnetic field along the growth direction the transition is inhomogeneously broadened with absorption features that can be correlated with the position of the donor with respect to the barriers and wells. The results compare well with recent theory of donors in superlattices and are potentially important for the determination of donor distributions in superlattices.     

Origin of the tunnel anisotropic magnetoresistance in Ga(1-x)Mn(x)As/ZnSe/Ga(1-x)Mn(x)As magnetic tunnel junctions of II-VI/III-V heterostructures

We investigated spin-dependent transport in magnetic tunnel junctions made of III-V Ga(1-x)Mn(x)As electrodes and II-VI ZnSe tunnel barriers. The high tunnel magnetoresistance (TMR) ratio up to 100% we observed indicates high spin polarization at the barrier/electrodes interfaces. We found anisotropic tunneling conductance having a magnitude of 10% with respect to the direction of magnetization to linearly depend on the magnetic anisotropy energy of Ga(1-x)Mn(x)As. This proves that the spin-orbit interactions in the valence band of Ga(1-x)M(x)As are responsible for the tunnel anisotropic magnetoresistance (TAMR) effect.