We report the discovery of a very large tunneling anisotropic magnetoresistance in an epitaxially grown $(\mathrm{G}\mathrm{a},\mathrm{M}\mathrm{n})\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/(\mathrm{G}\mathrm{a},\mathrm{M}\mathrm{n})\mathrm{A}\mathrm{s}$ 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 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.
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.
It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently low. In such films, the easy axis assumes a standard in-plane orientation when the temperature is raised towards the Curie temperature or the hole concentration is increased by low temperature annealing. These findings are shown to corroborate quantitatively the predictions of the mean-field Zener model for ferromagnetic semiconductors. The in-plane anisotropy is also examined, and possible mechanisms accounting for its character and magnitude are discussed.
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 report a photoinduced change of the coercive field of a low doped Ga1‐xMnxAs ferromagnetic semiconductor under very low intensity illumination. This photocoercivity effect (PCE) is local and reversible, which enables the controlled formation of localized magnetization domains. The PCE arises from a light induced lowering of the domain wall pinning energy as confirmed by test experiments on high doped, fully metallic ferromagnetic Ga1‐xMnxAs.
We applied x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at the Mn $2p\text{\ensuremath{-}}3d$ resonances to study the Mn $3d$ electronic configuration and the coupling of Mn $3d$ magnetic moments in various ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films. The homogeneity of the Mn depth profile throughout the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ film was tested by additional structure-sensitive x-ray resonant reflectivity measurements. In all investigated ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films the electronic and magnetic configuration of the Mn impurities varies throughout the Mn-doped layer. This inhomogeneity is caused by the surface segregation of nonferromagnetic Mn in a ${d}^{5}$ configuration. X-ray resonant reflectivity data show that the accumulation of nonferromagnetic Mn near the surface is strongly enhanced by low-temperature annealing. By XMCD we identified the Mn species responsible for the long-range ferromagnetic coupling. It is characterized by an Mn $3{d}^{5}\text{\ensuremath{-}}3{d}^{6}$ mixed-valence acceptor state that is unchanged at all investigated Mn concentrations, ranging from 1% to 6%. Additional nonferromagnetic Mn occurs in the bulk of high-concentration samples. We discuss a model in which the latter is due to antiferromagnetic Mn-Mn nearest-neighbor pairs.
The lattice constant and the alloy composition of (Ga,Mn)As are investigated by high-resolution x-ray diffraction and secondary ion mass spectroscopy. The (Ga,Mn)As layers are grown by low-temperature molecular-beam epitaxy under various growth conditions. We find that, while the alloy composition is mainly determined by the Mn cell temperature (TMn), the substrate temperature (Tsub) and the arsenic to gallium flux ratio (As/Ga) strongly influence the lattice constant. In particular, layers which have the same composition but different growth parameters have quite different lattice constants, caused by the amount of excess As incorporation in the (Ga,Mn)As crystal. This implies that the lattice parameter of (Ga,Mn)As cannot even serve as a rough measure of the crystal composition. (Ga,Mn)As is therefore an example of a system which does not obey Vegard’s law.
This article reports on a spintronics device based on the ferromagnetic semiconductor (Ga,Mn)As. Our transport measurements on a Au∕AlOx∕(Ga,Mn)As tunnel junction yield the surprising result that it is possible to get a spin-valve-like signal using only one magnetic layer. The strong spin-orbit coupling in (Ga,Mn)As creates significant anisotropies in the density of states with respect to the magnetization orientation. This, together with a two-step magnetization reversal creates a bistable magnetoresistive device with properties unattainable in current metal based spin-valves.
