Lithographic engineering of anisotropies in (Ga,Mn)As

Abstract

The focus of studies on ferromagnetic semiconductors is moving from material issues to device functionalities based on novel 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 we present a method which provides patterning induced anisotropy which not only can be applied locally, but also dominates over the intrinsic material anisotropy at all temperatures.The coupling of transport and magnetic properties in ferromagnetic semiconductors gives rises to many interesting anisotropy related transport phenomena such as strong anisotropic magnetoresistance (AMR), inplane hall [1], tunneling anisotropic magnetoresistance (TAMR) [2,3] and Coulomb blockade AMR [4]. Studies on all of these effects so far have primarily made use of the intrinsic anisotropy present in the host (Ga,Mn)As layer. Before they can be harnessed to their full potential, a means of engineering the anisotropy locally is needed, such that multiple elements with different anisotropies can be integrated, and their interactions can be properly investigated.One successful approach to local anisotropy control in metallic ferromagnets has been to make use of shape anisotropy. The same approach has been tried in the prototypical ferromagnetic semiconductor (Ga,Mn)As with lackluster results. In Ref. 5, the authors reported the observation of shape induced anisotropy in (Ga,Mn)As wires of 100 nm thickness x 1.5 x 200 µm 2 , but only over a limited temperature range. Moreover, our own experience in attempting to use wires of similar dimensions have yielded sporadic results with the wires having irreproducible anisotropy, with either biaxial or uniaxial easy axes in inconsistent directions.Furthermore, a simple calculation of the expected shape anisotropy term in such wires indicates that it should not play a significant role. While the infinite rod model used in [5] does predict an appreciable shape anisotropy field given by µ 0 M S /2, where M S is the sample magnetization, it is not applicable to structures which are much thinner than their lateral dimensions. A more exact rectangular prism calculation [6] gives a 5 times weaker shape anisotropy with an anisotropy energy density of 80 J/m 3 which is much too small to compete with the typical crystalline anisotropy of 3000 J/m 3 [5,7] in this material.Growth strain reduces the cubic symmetry of the (Ga,Mn)As zinc-blend crystal structure creating a uniaxial anisotropy with an easy/hard magnetic axis in growth direction when the (Ga,Mn)As layer is tensile/compressively strained.This growth strain is known[8] to influence the strength of the perpendicular component of the anisotropy of the whole layer. Here we discuss (Ga,Mn)As grown on a (001) oriented GaAs substrate, whose out-of-plane hard magnetic axis confines the magnetization in the plane. Phenomenologically, the net in-plane magnetic anisotropy is known to result from a competition of two primary contributions...

Ferromagnetic GaAs/GaMnAs Core−Shell Nanowires Grown by Molecular Beam Epitaxy

“…As shown for lithographically generated GaMnAs stripes of similar width, shape anisotropy terms do not play a significant role as the saturation magnetization is small. 54,56 Two basic scenarios are thought to be possible. Either a uniform magnetization forms with a well-defined magnetic anisotropy or the different side facets develop separate magnetic domains which add up to a net magnetization.…”

“…For the uniform magnetization, although no simulation of strain induced anisotropy of (211) GaMnAs stripes exist, we expect the reason for the observed easy axis along the NW axis to be the same as in the case of lithographically defined stripes, namely, anisotropic strain relaxation. 54,55 Further studies are required to fully understand the magnetic anisotropy of the GaMnAs nanotubes.…”

GaMnAs-based hybrid multiferroic memory device

“…Lithographically induced unidirectional lateral relaxation can be used to select the easy axis. 16,17 In addition to the large in-plane crystalline anisotropy, there is a uniaxial anisotropy between the ͓110͔ and the ͓110͔ directions which is probably due to the underlying anisotropy of the reconstructed GaAs surface. 18 We use this magnetocrystalline anisotropy in combination to the magnetostrictive effect to demonstrate a bistable memory device.…”

Magnitude and sign control of lithography-induced uniaxial anisotropy in ultra-thin (Ga,Mn)As wires

“…4 Lithography-induced uniaxial anisotropy due to the magnetostriction effect has been observed in relatively thick ͑Ga,Mn͒As wires on GaAs. [10][11][12][13][14][15] Since the lithographyinduced anisotropy can be externally modulated by changing the wire width 15 after the crystal growth, it enables the switching of the magnetization of ͑Ga,Mn͒As by an electric field with adjusted uniaxial anisotropy in combination with lithography-induced uniaxial anisotropies.In this Letter, we prove the presence of the lithographyinduced uniaxial anisotropy in 1-m-wide ultrathin ͑Ga,Mn͒As wires and also propose that this effect can assist in the electrical manipulation of magnetization.Devices were fabricated from a single wafer consisting of 5-nm-thin ͑Ga 0.94 ,Mn 0.06 ͒As grown on a semi-insulating GaAs substrate. Since the lattice constant of ͑Ga,Mn͒As is larger than that of GaAs, a compressive strain is built into ͑Ga,Mn͒As, which induces an in-plane magnetic easy axis.…”

“…4 Lithography-induced uniaxial anisotropy due to the magnetostriction effect has been observed in relatively thick ͑Ga,Mn͒As wires on GaAs. [10][11][12][13][14][15] Since the lithographyinduced anisotropy can be externally modulated by changing the wire width 15 after the crystal growth, it enables the switching of the magnetization of ͑Ga,Mn͒As by an electric field with adjusted uniaxial anisotropy in combination with lithography-induced uniaxial anisotropies.…”