This paper reports on a detailed magnetotransport investigation of the magnetic anisotropies of (Ga,Mn)As layers produced by various sources worldwide. Using anisotropy fingerprints to identify contributions of the various higher order anisotropy terms, we show that the presence of both a [100] and a [110] uniaxial anisotropy in addition to the primary ([100] + [010]) anisotropy is common to all medium doped (Ga,Mn)As layers typically used in transport measurement, with the amplitude of these uniaxial terms being characteristic of the individual layers.PACS numbers: 75.50. Pp,75.30.Gw, An extensive comparison of anisotropies in MBE grown (Ga,Mn)As material.
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...
We report the discovery of an effect where two ferromagnetic materials, one semiconductor ((Ga,Mn)As) and one metal (permalloy), can be directly deposited on each other and still switch their magnetization independently. We use this independent magnetization behavior to create various resistance states dependent on the magnetization direction of the individual layers. At zero magnetic field a two layer device can reach up to four non-volatile resistance states.PACS numbers: 75.50. Pp, 75.30.Gw, 75.70.Cn, Devices whose functioning is based on the relative magnetization state of two controllable magnetic elements, such as GMR (giant magneto resistance) [1] [2] based read heads [3] and TMR (tunnel magneto resistance) [4] based MRAM [5] are crucial to the modern information technology industry. So far, all such devices have been comprised of at least three layers: the two magnetic layers and a spacer layer to break the direct coupling between them and allow them to reorientate their relative magnetization. In this letter we show that, unlike the case of two ferromagnetic (FM) metals, the bringing together of a FM metal with a FM semiconductor (SC) can allow the layers to remain magnetically independent and thus permit the fabrication of devices without the need of a non magnetic interlayer. We demonstrate a first such device, which because of the strong anisotropies in the FM semiconductor layer has not only two, but up to four stable resistance states in the absence of a magnetic field.To prepare these structures, a 100 nm (Ga,Mn)As layer is grown by low-temperature molecular beam epitaxy on a GaAs buffer and substrate. Subsequently, without breaking the vacuum, the sample is transferred to a UHV magnetron sputtering chamber, and a permalloy (Ni 80 Fe 20 ) film with a thickness of 7 nm (and in some cases a 3 nm thick magnesium oxide (MgO) capping film) is deposited on top of the (Ga,Mn)As layer ( fig. 1a). Using optical lithography and chemically assisted ion beam etching (CAIBE), this layer stack is patterned into a 40 µm wide Hall bar oriented along the (Ga,Mn)As [010] crystal direction. Ti/Au contacts are established through metal evaporation and lift-off.For an initial study of the layer system, we include an MgO film on top of the permalloy layer to prevent the permalloy from naturally oxidizing in air. Without a cap layer the natural oxide consisting of NiO on an Fe oxide layer [6] produces an exchange bias coupling [7] with a magnetic field dependent anisotropy [8] [9] below its Néel temperature [10] which unduly complicates the layer characterization. We find that sputtering MgO on Py creates a well-defined uniform antiferromagnetic layer which couples antiferromagnetically to the Py film. • (light gray) and 20• (red) relative to the (Ga,Mn)As [010] easy axis after cooling the sample to 4.2 K in a field of 300 mT. The measurements exhibit clear double-step switching of the (Ga,Mn)As layer and a shifted magnetization contribution of the Py/MgO system due to exchange bias. Inset: projection of the magnetizat...
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.
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.
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