We obtain control of magnetic anisotropy in epitaxial (Ga,Mn)As by anisotropic strain relaxation in patterned structures. The strain in the structures is characterized using sophisticated X-ray techniques. The magnetic anisotropy before patterning of the layer, which shows biaxial easy axes along [100] and [010], is replaced by a hard axis in the direction of large elastic strain relaxation and a uniaxial easy axis in the direction where pseudomorphic conditions are retained. This strong anisotropy can not be explained by shape anisotropy and is attributed solely to lattice strain relaxation. Upon increasing the uniaxial strain anisotropy in the (Ga,Mn)As stripes, we also observe an increase in magnetic anisotropy.PACS numbers: 75.50. Pp, 75.30.Gw The (Ga,Mn)As material system has been the focus of many studies over the last years. As the understanding of its complex transport and magnetic properties increases, the focus of interest shifts from basic research towards its application in devices. For this, it is necessary to understand how different parameters influence the ferromagnetic material in structures at the device level. In this letter we present a systematic study of the role of strain relaxation as the dominating factor contributing to the magnetic anisotropy in (Ga,Mn)As nanostructures. A (Ga,Mn)As layer grown epitaxially on a GaAs substrate is subject to compressive strain in the plane of the sample and typically exhibits biaxial in-plane easy axes along [100] and [010], at temperatures around 4 K [1, 2]. In earlier studies, control of the magnetic anisotropy has been achieved by modifying the strain in the layer. Tensile strain can be imposed on the (Ga,Mn)As by growing it on a thick, plastically relaxed (In,Ga)As buffer with a larger lattice parameter, and results in an out-of-plane easy axis [3,4].Here, we follow an alternative approach in modifying the lattice strain of (Ga,Mn)As on GaAs by lithography, which allows us to locally control the magnetic anisotropy of the material. By structuring a fully pseudomorphic 70 nm (Ga,Mn)As layer into thin, elongated stripes, we allow anisotropic, elastic strain relaxation perpendicular to the long axis of the stripe. To increase the strain in the structure compared to the case of (Ga,Mn)As on GaAs, a second sample is processed which includes a highly compressively strained layer acting as an extra stressor to the overlying (Ga,Mn)As layer. The uniaxial strain relaxation in the structures is investigated by grazing incidence X-ray diffraction (GIXRD) and high-resolution X-ray diffraction (HRXRD). To determine the influence of patterning on the magnetic anisotropy, a series of magnetometric and magnetotransport studies are performed. We also present finite-element simulations of anisotropic strain relaxation and k ·p calculations which confirm the relationship between the structural and magnetic behavior observed in our samples.The samples are grown in a dedicated III-V MBE chamber with effusion cells for Ga, In, Mn, and a valved As 4 cell. A 200 nm thic...

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.PACS numbers: 75.50. Pp, 75.30.Gw, 73.50.Jt, The most pressing issues of modern information technologies are power consumption and heat dissipation. As such, a proper understanding of the relationship between electrical and thermal effects in devices is essential to their design and operation. When these devices are magnetic in nature, the study of this relationship is the topic of the field of spin caloritronics [1], which was strongly inspired by the initial reports of the spin Seebeck effect in metals [2] and subsequently in magnetic semiconductors [3].As a simple example of the importance of thermal effects on device operation, consider the problem of the injection of spin polarized current into a nonmagnetic semiconductor. Spin injection is often measured using a four contact nonlocal technique [4,5], where current is passed between two adjacent contacts, and the chemical potential resulting from spin accumulation is detected at two further contacts which are situated next to, but not between the current contacts. Ideally, the signal from such measurements should produce a voltage at the detection leads, which is equal but opposite in sign depending on the relative orientations of the contacts. In practice this is almost never the case, as the measurements typically include a constant voltage offset, which it has become customary to neglect as an unimportant background contribution [5].This background voltage is a thermoelectric effect [6], which in most cases is isotropic to magnetic field and thus independent of the magnetization direction of the contacts. The subtraction of such a constant background is thus inconsequential to the experimental conclusions.As we show in this Letter however, in materials where strong spin-orbit coupling links the magnetic properties to the density of states [7], a spin caloritronic phenomenon, which we dub tunneling anisotropic magnetotermopower (TAMT), appears. This not only may have interesting implications for device applications, but can also lead to misinterpretations if one fails to take it into consideration. Indeed, we show that thermal voltages detected in such materials, e.g. the ferromagnetic semiconductor (Ga,Mn)As, closely resemble typical nonlocal spin-valve signals [8], even when no spin injector is used.The configuration we study looks at the thermopower across a tunnel junction between ferromagnetic and nonmagnetic regions [9]. Specificly, our experiment examines diffusion thermop...

We report the realization of a read-write device out of the ferromagnetic semiconductor (Ga,Mn)As as the first step to fundamentally new information processing paradigm. Writing the magnetic state is achieved by current-induced switching and read-out of the state is done by the means of the tunneling anisotropic magneto resistance (TAMR) effect. This one bit demonstrator device can be used to design a electrically programmable memory and logic device.PACS numbers: 75.50. Pp, 75.30.Gw, At present memory and logic fabrication are two fully separated architectures [1,2]. While bulk information storage traditionally builds on metallic ferromagnets, logic makes use of gateability of charge carriers in semiconductors. Combining storage and processing in a single monolithic device not only would solve current technical issues such as the heat dissipation generated by transferring information between the two architectures, but also offer the possibility of a fully non-volatile information processing system. Here we present a read-write device which can be used as one element of an electrically programmable logic gate. Our structure is made from the ferromagnetic semiconductor (Ga,Mn)As, which exhibits carrier-induced ferromagnetism at low temperatures [3][4][5]. The 70 nm thick (Ga,Mn)As layer is grown by lowtemperature molecular beam epitaxy (MBE) on a GaAs buffer and substrate. Due to the lattice mismatch to the GaAs buffer the (Ga,Mn)As layer is compressively strained and therefore has its magnetic easy axes in the plane perpendicular to the growth direction [6]. After growth of the MBE layers, and without breaking vacuum, the sample is transferred to a UHV evaporation chamber, and 3x0.9 nm of aluminum is deposited on top of the (Ga,Mn)As layer. After deposition, each of the three Al layers is oxidized by keeping it for 8 hours in a 200 mBar oxygen atmosphere. The wafer is then covered by 5 nm Ti and 30 nm Au. The ferromagnetic transition temperature of the (Ga,Mn)As layer is 61 K as determined by SQUID (superconducting quantum interference device). Figure 1 shows the read-write device. It consists of four nanobars which are connected to a circular center region. The structure is defined using electron beam lithography and chemical assisted ion beam etching (CAIBE). The nanobars are 200 nm wide and 2 µm long. After patterning, the Ti/Au and aluminum oxide (Alox) layer are removed from the bars and each nanobar is contacted by Ti/Au contacts using a lift-off technique. The Alox/Ti/Au layer on top of the 650 nm central disk remains on the structure and acts as a read-out tunnel contact. For this purpose, the Au layer on the central disk is contacted by a metallic air-bridge [7]. Small notches are patterned at the transition from the nanobars to the central disk and serve to pin down domain walls.Thin films of unpatterened compressively strained (Ga,Mn)As exhibit an in-plane biaxial magnetic anisotropy at low temperatures. The bars connected to the central disk are aligned with their length parallel to the magnetic easy...

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We demonstrate the ability to release the growth-induced strain in (Ga,Mn)As layers and (In,Ga)As/(Ga,Mn)As bilayers by lifting them from the GaAs substrate. The lifted (bi)layers are then deposited back onto various substrates. The change in strain before and after processing has been studied by means of x-ray diffraction. Magnetic characterization demonstrates the efficiency of our lift-off process to reorient the magnetization to the direction normal to the layer plane.

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Miniaturizing a tunneling anisotropic magneto resistance contact allows local sensing of the density of states of (Ga,Mn)As. This offers the possibility to locally read-out magnetization states in nanoscale devices without the need to add disruptive metal contacts. Using this technique, we show that the behavior of (Ga,Mn)As at a sub micron scale is closer to that of an ideal macrospin than is the case for macroscopic (Ga,Mn)As layers.