We report the realization of a read-write device out of the ferromagnetic semiconductor (Ga,Mn)As as the first step to a fundamentally new information processing paradigm. Writing the magnetic state is achieved by current-induced switching and readout of the state is done by the means of the tunneling anisotropic magnetoresistance effect. This 1 bit demonstrator device can be used to design an electrically programmable memory and logic device.

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.

The prototypical ferromagnetic semi-conductor (Ga,MnAs) is interesting for spintronics devices, largely because of its rich magnetic and transport anisotropy. The lack of local anisotropy control has until recently limited device options to structures where all elements have the anisotropy characteristic of the parent layer. We describe here a novel approach to anisotropy control using lithographically engineered strain relaxation. By patterning the layer to create local strain relaxation, we allow for a controlled deformation of the crystal. The strong spin orbit coupling then leads to a new anisotropy term that can be independently tuned for each element of a compound device. We use this technique to demonstrate a novel non-volatile memory element.

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 nonvolatile resistance states.

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 the 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.

We report control of magnetic anisotropy in epitaxial (Ga,Mn)As by anisotropic strain relaxation in patterned structures. The strain in the structures is characterized using reciprocal space mapping by 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.

A numerical technique is developed to solve the Luttinger-Kohn equation for impurity states directly in k space and is applied to calculate bound-hole wave functions in a ferromagnetic (Ga,Mn)As host. The rich properties of the band structure of an arbitrarily strained, ferromagnetic zinc-blende semiconductor yields various features which have direct impact on the detailed shape of a valence band hole bound to an active impurity. The role of strain is discussed on the basis of explicit calculations of bound-hole states.

In this Review, we discuss the rich anisotropic properties of the ferromagnetic semiconductor (Ga, Mn)As, and their implications in transport studies. We review the various sources and types of anisotropy seen in the material, discuss its magnetization reversal process, and demonstrate how basic transport properties, such as resistivity and Hall measurements, can be used as very sensitive tools to investigate the magnetization properties of the material. We also discuss how the magnetic anisotropy, coupled with large spin-orbit coupling, leads to an anisotropy in the transport density of states, which in turn leads to fundamentally novel behavior such as tunneling anisotropic magnetoresistance (TAMR).

The focus of studies on ferromagnetic semiconductors is moving from material issues to device functionalities based on phenomena often associated with the anisotropy properties of these materials. This is driving a need for a method to locally control the anisotropy in order to allow the elaboration of devices. Here the authors present a method which provides patterning induced anisotropy that not only can be applied locally but also dominates over the intrinsic material anisotropy at all temperatures.

The rich magnetic anisotropy of compressively strained (Ga,Mn)As has attracted great interest recently. Here we discuss a sensitive method to visualize and quantify the individual components of the magnetic anisotropy using transport. A set of high resolution transport measurements is compiled into color coded resistance polar plots, which constitute a fingerprint of the symmetry components of the anisotropy. As a demonstration of the sensitivity of the method, we show that these typically reveal the presence of both the [-110] and the [010] uniaxial magnetic anisotropy component in (Ga,Mn)As layers, even when most other techniques reveal only one of these components.

The rich anisotropic transport behaviour shown by ferromagnetic semiconductors arises from a complex interplay of their electronic density of states and magnetic response. Such behaviour promises to enable devices whose ability to manipulate information in the form of electronic spin goes well beyond the now ubiquitous spin-valve read heads of magnetoelectronics, and on a platform that is compatible with conventional complementary metal oxide semiconductor technology. Most ferromagnetic semiconductor devices so far have relied on the bulk anisotropic behaviour of their constituent layers. Recent improvements in lithographic patterning enable the fabrication of a novel class of devices in which the anisotropy of many individual elements can be independently engineered. Here we demonstrate the first such device consisting of two nanobars with mutually orthogonal easy axes linked by a constriction. It behaves as a non-volatile memory element, where information can be written by setting the relative orientation of the magnetization of the nanobars, and read by measuring the constriction resistance.

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.

Experimental studies of nanostructured devices are carried out. The devices are fabricated using e-beam lithography and thin film deposition techniques with critical dimensions in the 10 to 100 nm range. To optimize the fabrication process and increase the sample throughput a new electrical test system was set up. The system is based on a programmable matrix multiplexer combined with a surface probe card system equipped with a micropositioning XYZ sample stage. The test system is computer controlled using Labview programming environment. It provides fast and accurate electrical test results for multiple devices built into standard 16-pad test cages, essentially eliminating the test from the overall processing time. The test system was used in optimizing the process for the fabrication of ferromagnetic single electron transistors (FM-SETs). These are novel mesoscopic devices relying on quantum charge tunneling trough a double tunnel junction formed by a small metallic island and two ferromagnetic electrodes. Working devices with feature sizes down to 35 nm have been fabricated. The electrodes are designed to produce parallel and antiparallel magnetic states in specific externally applied field intervals. In the antiparallel state (spin-valve configuration) a non-equilibrium spin accumulation on the island is expected to yield large changes in the resistance of the device. This effect should be useful in controlling spin states in magnetic nano-circuits, which is the subject of the rapidly developing field of spintronics.