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We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field- dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1-xMnxAs and observe different Mn constituents: ferromagnetic Mn with an n(d) > 5 lineshape and paramagnetic Mn with distinct n(d) = 5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency. (c) 2005 Elsevier B.V. All rights reserved

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.

The incorporation of carbon in low-temperature (LT) GaAs and GaMnAs layers deposited by molecular-beam epitaxy under various growth conditions has been investigated. The lattice parameter depends on Mn content, C content, and the growth conditions which strongly affect Mn, C, and point defect incorporation. We found optimum growth conditions (Tsub=270°C, the beam equivalent pressure ratio is 5, growth rate is 0.1nm∕s) at which high-quality GaMnAs layers with moderate Mn content as well as LT-GaAs layers were deposited, which were efficiently p-doped by carbon. LT GaAs:C revealed the same electrical activation of carbon of about 50% at a high doping level p=5×1019cm−3 as observed in high-temperature GaAs:C. The Curie temperature as well as the saturation magnetization of GaMnAs decreases with C doping. This suggests a reduced amount of Mn contributing to the ferromagnetic phase in GaMnAs:C.

The lattice constant and the alloy composition of (Ga,Mn)As are investigated by high-resolution x-ray diffraction and secondary ion mass spectroscopy. The (Ga,Mn)As layers are grown by low-temperature molecular-beam epitaxy under various growth conditions. We find that, while the alloy composition is mainly determined by the Mn cell temperature (TMn), the substrate temperature (Tsub) and the arsenic to gallium flux ratio (As/Ga) strongly influence the lattice constant. In particular, layers which have the same composition but different growth parameters have quite different lattice constants, caused by the amount of excess As incorporation in the (Ga,Mn)As crystal. This implies that the lattice parameter of (Ga,Mn)As cannot even serve as a rough measure of the crystal composition. (Ga,Mn)As is therefore an example of a system which does not obey Vegard’s law.

We report a photoinduced change of the coercive field of a low doped Ga1‐xMnxAs ferromagnetic semiconductor under very low intensity illumination. This photocoercivity effect (PCE) is local and reversible, which enables the controlled formation of localized magnetization domains. The PCE arises from a light induced lowering of the domain wall pinning energy as confirmed by test experiments on high doped, fully metallic ferromagnetic Ga1‐xMnxAs.

We perform high resolution X-ray diffraction on GaMnAs mixed crystals as well as on GaMnAs/GaAs and GaAs/MnAs superlattices for samples grown by low temperature molecular beam epitaxy under different growth conditions. Although all samples are of high crystalline quality and show narrow rocking curve widths and pronounced finite thickness fringes, the lattice constant variation with increasing manganese concentration depends strongly on the growth conditions: For samples grown at substrate temperatures of 220 and 270 degrees C the extrapolated relaxed lattice constant of Zincblende MnAs is 0.590 nm and 0.598 nm respectively. This is in contrast to low temperature GaAs, for which the lattice constant decreases with increasing substrate temperature., Comment: pdf only

The recently demonstrated photoinduced reduction of the coercive field in (Ga,Mn)As is shown to display a pronounced increase in efficiency when triggered by short laser pulses. This is due to the relaxation timescale of the effect that is measured to be about 1.5 ns. In addition, a single 100-fs-pulse with only 80 pJ of energy is found to be sufficient to write a magnetic domain.

We applied x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at the Mn $2p\text{\ensuremath{-}}3d$ resonances to study the Mn $3d$ electronic configuration and the coupling of Mn $3d$ magnetic moments in various ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films. The homogeneity of the Mn depth profile throughout the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ film was tested by additional structure-sensitive x-ray resonant reflectivity measurements. In all investigated ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films the electronic and magnetic configuration of the Mn impurities varies throughout the Mn-doped layer. This inhomogeneity is caused by the surface segregation of nonferromagnetic Mn in a ${d}^{5}$ configuration. X-ray resonant reflectivity data show that the accumulation of nonferromagnetic Mn near the surface is strongly enhanced by low-temperature annealing. By XMCD we identified the Mn species responsible for the long-range ferromagnetic coupling. It is characterized by an Mn $3{d}^{5}\text{\ensuremath{-}}3{d}^{6}$ mixed-valence acceptor state that is unchanged at all investigated Mn concentrations, ranging from 1% to 6%. Additional nonferromagnetic Mn occurs in the bulk of high-concentration samples. We discuss a model in which the latter is due to antiferromagnetic Mn-Mn nearest-neighbor pairs.

We observe the occurrence of an Efros-Shklovskii gap in (Ga,Mn)As based tunnel junctions. The occurrence of the gap is controlled by the extent of the hole wave-function on the Mn acceptor atoms. Using k.p-type calculations we show that this extent depends crucially on the direction of the magnetization in the (Ga,Mn)As (which has two almost equivalent easy axes). This implies one can reversibly tune the system into the insulating or metallic state by changing the magnetization.

We report the discovery of a super-giant tunneling anisotropic magnetoresistance in an epitaxially grown (Ga,Mn)As/GaAs/(Ga,Mn)As structure. The effect arises from a strong dependence of the electronic structure of ferromagnetic semiconductors on the magnetization orientation rather than from a parallel or antiparallel alignment of the contacts. The key novel spintronics features of this effect are: (i) both normal and inverted spin-valve like signals; (ii) a large non-hysteretic 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. PACS numbers: 75.50.Pp, 85.75.Mm The emerging field of semiconductor based spintronics, which explores the spin and charge degrees of freedom on an equal footing, is expected to lead to novel information technologies that will overcome current key obstacles in the microelectronics roadmap [1]. A main component needed to realize the full potential of this technology is a device with similar behavior as current metal-based spin-valves [2], and with novel spintronics features unattainable in their metal counterparts. Previous attempts in this direction have yielded promising spin-valve results [3] apparently mimicking the functionality of the metal devices. However, our recent discovery of tunneling anisotropic magnetoresitance (TAMR) in a single (Ga,Mn)As layer structure [4] suggests that the moderate magnetoresistance (MR) effects observed so far in structures such as the one in Ref. 3 may originate from TAMR rather than the traditional metal tunneling MR (TMR). If this is the case, the device behavior should be much richer than for TMR, and could offer many new functionalities not possible in metal based devices. To investigate this hypothesis, we have fashioned a tunnel structure based on the ferromagnetic semiconductor (Ga,Mn)As. We report the existence of a huge TAMR effect exceeding 100 000% in these structures. The full layer structure of our device, shown in Fig. 1a, consists of a Ga0.94Mn0.06As (10 nm)/GaAs (2 nm) /Ga0.94Mn0.06As (100 nm) trilayer grown by low temperature molecular beam epitaxy (LT-MBE) on a semiinsulating GaAs substrate and an undoped GaAs buffer layer. The (Ga,Mn)As layers are intrinsically highly ptype due to the Mn and have metallic transport character. The undoped LT-GaAs layer on the other hand is insulating and forms an epitaxial tunnel barrier between the two ferromagnetic layers. The ferromagnetic transition temperature of the (Ga,Mn)As is � 65 K.

It is demonstrated by SQUID magnetization measurements that (Ga,Mn)As films can exhibit rich characteristics of magnetic anisotropy depending not only to the epitaxial strain but being strongly influenced by the hole and Mn concentration, and temperature. This behavior reflects the spin anisotropy of the valence subbands and corroborates predictions of the mean field Zener model of the carrier mediated ferromagnetism in III-V diluted magnetic semiconductors with Mn. At the same time the existence of in-plane uniaxial anisotropy with [110] the easy axis is evidenced. This is related to the top/bottom symmetry breaking, resulting in the lowering of point symmetry of (Ga,Mn)As to the C_{2v} symmetry group. The latter mechanism coexists with the hole-induced cubic anisotropy, but takes over close to T_C., 4 pages, Latex, 4 eps figures. Extended and upgraded version appeared in cond-mat/0410549

Giant magnetic linear dichroism (MLD) is observed in the ferromagnetic semiconductor ${\mathrm{Ga}}_{0.98}{\mathrm{Mn}}_{0.02}\mathrm{As}$. The contribution to this effect induced by the spontaneous magnetization can be clearly identified by azimuthal dependencies. The spectral dependence of the effect in the range from 1.4 to 2.4 eV shows that the MLD induced by the spontaneous magnetization is strongly enhanced for excitations from the electronic states that are responsible for the ferromagnetism in this material. This spectral sensitivity and the size of the effect makes MLD a powerful tool for the study of $(\mathrm{III},\mathrm{Mn})\mathrm{V}$ alloys and similar novel ferromagnetic semiconductors.

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.

Static and time-resolved magneto-optical spectra of the ferromagnetic semiconductor (Ga,Mn)As show that a pulsed photoexcitation with a fluence of 10 microJ/cm(2) is equivalent to the application of an external magnetic field of about 1 mT, which relaxes with a decay time of 30 ps. This relaxation is attributed to the spin relaxation of electrons in the conduction band and is found to be not affected by interactions with Mn ions.

This article reports on a spintronics device based on the ferromagnetic semiconductor (Ga,Mn)As. Our transport measurements on a Au∕AlOx∕(Ga,Mn)As tunnel junction yield the surprising result that it is possible to get a spin-valve-like signal using only one magnetic layer. The strong spin-orbit coupling in (Ga,Mn)As creates significant anisotropies in the density of states with respect to the magnetization orientation. This, together with a two-step magnetization reversal creates a bistable magnetoresistive device with properties unattainable in current metal based spin-valves.

We demonstrate complete reversal of a full magnetic hysteresis loop of the magnetic semiconductor (Ga,Mn)As by ultrashort optical excitation with a single subpicosecond light pulse, with obvious implications for ultrafast magneto-optical recording. Our approach utilizes the fourfold magnetic anisotropy of (Ga,Mn)As, in combination with the magnetic linear dichroism of the material.