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 normalmetal/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.PACS numbers: 75.50. Pp, 85.75.Mm Devices relying on spin manipulation are hoped to provide low-dissipative alternatives for microelectronics. Furthermore, spintronics is expected to lead to full integration of information processing and storage functionalities opening attractive prospects for the realization of instant on-and-off computers. A primary goal of current spintronics research is to realize a device with metal spin-valve like behavior [1] in an all semiconductor-based structure enhancing integration of spintronics with existing microelectronics technologies. An oft proposed scheme for such a device consists of a tunnel barrier between two ferromagnetic semiconductors. As such, (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As structures have previously been studied [2,3] with some promising results. However, realizing the full potential of these systems will require a complete understanding of the physics of tunneling into (Ga,Mn)As, which we have found to be rather different than previously thought.In this spirit, we investigate transport in a structure consisting of a single ferromagnetic (Ga,Mn)As layer fitted with a tunnel barrier and a non-magnetic metal contact. We report some of the rich experimental properties of such a tunneling structure and provide an interpretation of the measured spin-valve like effect as a tunneling anisotropic magnetoresistance (TAMR) due to a two-step magnetization reversal and a magnetization dependent density of states (DOS) in the (Ga,Mn)As layer.The magnetic layer in our sample is a 70 nm thick epitaxial (Ga,Mn)As film grown by low temperature (270• C) molecular beam epitaxy onto a GaAs (001) substrate [4]. High-resolution x-ray diffraction showed that the sample had high crystalline quality comparable to that of the substrate. From the measured lattice constant and the calibration curves of Ref.[5], the Mn concentration in the ferromagnetic layer is roughly 6%. Etch capacitance-voltage control measurements yielded a hole density estimate of ∼ 10 21 cm −3 and the Curie temperature of 70 K was determined from SQUID measurements.After growth, the sample surface was Ar sputtered to remove any potential oxides, and a 1.4 nm Al layer was deposited at a rate of 0.

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.

We have fabricated (Ga,Mn)As nanostructures in which domain walls can be pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can switch the regions on either side of the constriction to either parallel or antiparallel magnetization. All samples exhibit a positive magnetoresistance, consistent with domain-wall trapping. For metallic samples, we find a magnetoresistance up to 8%, which can be understood from spin accumulation. In samples where, due to depletion at the constriction, a tunnel barrier is formed, we observe a magnetoresistance of up to 2000%.

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.PACS numbers: 71.30.+h, 75.30.Hx, 75.50.Pp A very direct way to observe the Efros-Shklovskii (ES) gap, the soft gap induced by Coulomb correlations near the Fermi level of a Mott insulator [1,2], is by means of tunnel spectroscopy. Such experiments were, e.g., performed on the (three-dimensional) nonmetallic doped semiconductor Si:B [3, 4] and on thin (two-dimensional) Be films [5]. While both of these experiments employed large area tunnel junctions and a metallic counter electrode, a more recent study employed Ge:As break junctions[6]. This latter approach avoids possible screening of the Coulomb correlations, but the mesoscopic character of the contact may complicate extraction of bulk Coulomb gap behaviour [7].We have recently investigated the physics of a novel type of magnetoresistance, dubbed TAMR (tunneling anisotropic magnetoresistance) [8,9]. TAMR results from the dependence of the density of states (DOS) in strongly spin-orbit coupled ferromagnetic semiconductors, such as (Ga,Mn)As, on the direction of the magnetization of the material. In [9] we reported a drastic (> 10 4 ) increase of the spin-valve signal in a (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel structure on lowering the sample temperature from 4.2 to 1.7 K, and speculated that this behaviour might result from the opening of an ES gap. Here, we provide evidence that the high resistance state of the sample indeed corresponds to a soft-gapped Mott insulator. In these samples, the metalto-insulator transition (MIT) is driven by a large variation of the Bohr radius of a hole bound to a Mn-impurity when the magnetization of the layer is switched from one easy axis to the other. This assignment is supported by a k · p-type calculation of a hydrogen-like impurity in a ferromagnetic GaAs host, extending the successful mean field model for (Ga,Mn)As [11,12].Our (Ga,Mn)As tunnel structure is shown in Fig. 1b. From bottom to top, the Ga 0.94 Mn 0.06 As (100 nm)/ GaAs (2 nm) /Ga 0.94 Mn 0.06 As (10 nm) trilayer stack has been grown by low temperature molecular beam epitaxy (LT-MBE) on a semi-insulating GaAs substrate and a 120 nm undoped GaAs buffer layer. Both (Ga,Mn)As layers are ferromagnetic with an as-grown Curie temperature of ∼ 65 K and highly p-type due to the intrinsic doping arising from the Mn atoms.As seen in the optical micrograph of Fig. 1a, the layer stack is patterned into a square mesa of 100×100 µm 2 by positive optical lithography, metal evaporation, liftoff and wet etching. The top contact is in-situ Ti/Au. Contact to the lower (Ga,Mn)As layer is established by a W/...

We report the growth of the half-Heusler alloy NiMnSb on InP ͑001͒ by molecular-beam epitaxy using a lattice-matched ͑In,Ga͒As buffer. High-resolution x-ray diffraction confirms a high crystalline quality. Spot-profile analysis low-energy electron diffraction measurements show well-defined surface reconstructions. The samples show the expected high Curie temperature and an uniaxial anisotropy.

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The structural and magnetic properties of NiMnSb films, 5-120 nm thick, grown on InGaAs/ InP͑001͒ substrates by molecular-beam epitaxy, were studied by x-ray diffraction, transmission electron microscopy ͑TEM͒, and ferromagnetic resonance ͑FMR͒ techniques. X-ray diffraction and TEM studies show that the NiMnSb films had the expected half-Heusler structure, and films up to 120 nm were pseudomorphically strained at the interface, greater than the critical thickness for this system, about 70 nm ͑0.6% mismatch to InP͒. No interfacial misfit dislocations were detected up to 85 nm, however, relaxation in the surface regions of films thicker than 40 nm was evident in x-ray reciprocal space maps. TEM investigations show that bulk, planar defects are present beginning in the thinnest film ͑10 nm͒. Their density remains constant but they gradually increase in size with increasing film thickness. By 40 nm these defects have overlapped to form a quasicontinuous network aligned closely with ͗100͘ in-plane directions. The associated strain fields and or compositional ordering from these defects introduced a reduction in crystal symmetry that influenced the magnetic properties. The in-plane and perpendicular FMR anisotropies are not well described by bulk and interface contributions. In thick films, the in-plane uniaxial and fourfold anisotropies increased with increasing film thickness. The lattice defects resulted in a large extrinsic magnetic damping caused by two-magnon scattering, an increase in the coersive field with increasing film thickness, and a lower magnetic moment ͑3.6 Bohr magnetons͒ compared to the expected value for the bulk crystals ͑4 Bohr magnetons͒.

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