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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 report a photoinduced change of the coercive field, i.e., a photocoercivity effect (PCE), under very low intensity illumination of a low-doped (Ga,Mn)As ferromagnetic semiconductor. We find a strong correlation between the PCE and the sample resistivity. Spatially resolved dynamics of the magnetization reversal rule out any role of thermal heating in the origin of this PCE, and we propose a mechanism based on the light-induced lowering of the domain wall pinning energy. The PCE is local and reversible, allowing writing and erasing of magnetic images using light.

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.

Current induced magnetization switching and resistance associated with domain walls pinned in nanoconstrictions have both been previously reported in (Ga,Mn)As based devices, but using very dissimilar experimental schemes and device geometries . Here we report on the simultaneous observation of both effects in a single nanodevice, which constitutes a significant step forward towards the eventual realization of spintronic devices which make use of domain walls to store, transport, and manipulate information.

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 introduce a new class of spintronics devices in which a spin-valve like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normal-metal/insulator/ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction, and the two-step magnetization reversal process in this material., 4 pages, 4 figures, submitted to PRL

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.

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.