Observation of Coexisting Weak Localization and Superconducting Fluctuations in Strained Sn1-xInxTe Thin Films.

Topological superconductors have attracted tremendous excitement as they are predicted to host Majorana zero modes that can be utilized for topological quantum computing. Candidate topological superconductor Sn1-xInxTe thin films (0 < x < 0.3) grown by molecular beam epitaxy and strained in the (111) plane are shown to host quantum interference effects in the conductivity coexisting with superconducting fluctuations above the critical temperature Tc. An analysis of the normal state magnetoresistance reveals these effects. A crossover from weak antilocalization to localization is consistently observed in superconducting samples, indicating that superconductivity originates dominantly from charge carriers occupying trivial states that may be strongly spin-orbit split. A large enhancement of the conductivity is observed above Tc, indicating the presence of superconducting fluctuations. Our results motivate a re-examination of the debated pairing symmetry of this material when subjected to quantum confinement and lattice strain.

Exchange bias in ferromagnetic bilayers with orthogonal anisotropies: the case of GaMnAsP/GaMnAs combination.

We report the observation of exchange bias in a ferromagnetic Ga0.94Mn0.06As0.77P0.23/ Ga0.94Mn0.06As bilayer, in which the easy axis in one layer is oriented out-of-plane, and in the other in-plane. Magnetization reversal in this system is explored using planar Hall effect (PHE) measurements under various initial conditions and with various field-cooling orientations. Our results show that the two magnetic layers are ferromagnetically exchange-coupled, and that such coupling results in pronounced exchange-bias-like shifts of magnetic hysteresis loops during reversal of in-plane magnetization. The presence of exchange bias in this system can be understood on the basis of magnetic closure domains formed in the layer with the out-of-plane easy axis.

Interlayer exchange coupling in ferromagnetic semiconductor trilayers with out-of-plane magnetic anisotropy.

We report the observation of ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange coupling (IEC) in GaMnAsP-based trilayer structures with out-of-plane magnetic anisotropy. Magnetization and anomalous Hall effect (AHE) measurements show well-resolved magnetization transitions corresponding to the two GaMnAsP layers. Minor loop measurements reveal a characteristic shift caused by IEC in all trilayer samples investigated. Interestingly, the FM IEC changes to AFM IEC for a trilayer with the thinnest (7 nm) top GaMnAsP layer as the temperature increases. The observation of temperature-induced transition of FM and AFM IEC in the same sample suggests the possibility of device applications by controlling the type of IEC in such GaMnAsP-based multilayers.

Magnetization reversal and interlayer exchange coupling in ferromagnetic metal/semiconductor Fe/GaMnAs hybrid bilayers.

We report a detailed study of magnetization reversal in Fe/GaMnAs bilayers carried out by magnetotransport measurements. Specifically, we have used planar Hall resistance (PHR), which is highly sensitive to the direction of magnetization, and is therefore ideally suited for tracking magnetization as it reorients between successive easy axes in the two magnetic layers during reversal. These reorientations take place separately in the two magnetic layers, resulting in a series of different magnetization alignments (parallel or orthogonal) during reversal, providing a series of stable PHR states. Our results indicate that the magnetic anisotropy of the structure is dominated by cubic symmetry of both layers, showing two in-plane easy axes, but with significantly different energy barriers between the easy orientations. Importantly, a careful analysis of the PHR results has also revealed the presence of strong ferromagnetic interlayer exchange coupling (IEC) between the two magnetic layers, indicating that although magnetization reorients separately in each layer, this process is not independent, since the behavior of one layer is influenced by its adjacent magnetic neighbor. The ability to design and realize multiple PHR states, as observed in this investigation, shows promise for engineering Fe/GaMnAs bilayer structures for multinary magnetic memory devices and related multinary logic elements.

Magnetization reversal in trilayer structures consisting of GaMnAs layers with opposite signs of anisotropic magnetoresistance.

Magnetization reversal in a GaMnAs trilayer system consisting of two GaMnAs layers separated by a Be-doped GaAs spacer was investigated by magnetotransport measurements. The rotation of magnetization in the two GaMnAs layers is observed as two abrupt independent transitions in planar Hall resistance (PHR). Interestingly, one GaMnAs layer manifests a positive change in PHR, while the other layer shows a negative change for the same rotation of magnetization. Such opposite behavior of the two layers indicates that anisotropic magnetoresistance (AMR) has opposite signs in the two GaMnAs layers. Owing to this opposite behavior of AMR, we are able to identify the sequence of magnetic alignments in the two GaMnAs layers during magnetization reversal. The PHR signal can then be decomposed into two independent contributions, which reveal that the magnetic anisotropy of the GaMnAs layer with negative AMR is predominantly cubic, while it is predominantly uniaxial in the layer with positive AMR. This investigation suggests the ability of engineering the sign of AMR in GaMnAs multilayers, thus making it possible to obtain structures with multi-valued PHR, that can be used as multinary magnetic memory devices.

Field-free manipulation of magnetization alignments in a Fe/GaAs/GaMnAs multilayer by spin-orbit-induced magnetic fields.

We investigate the process of selectively manipulating the magnetization alignment in magnetic layers in the Fe/GaAs/GaMnAs structure by current-induced spin-orbit (SO) magnetic field. The presence of such fields manifests itself through the hysteretic behavior of planar Hall resistance observed for two opposite currents as the magnetization in the structure switches directions. In the case of the Fe/GaAs/GaMnAs multilayer, hystereses are clearly observed when the magnetization switches direction in the GaMnAs layer, but are negligible when magnetization transitions occur in Fe. This difference in the effect of the SO-field in the two magnetic layers provides an opportunity to control the magnetization in one layer (in the presence case in GaMnAs) by a current, while the magnetization in the other layer (i.e., Fe) remains fixed. Owing to our ability to selectively control the magnetization in the GaMnAs layer, we are able to manipulate the relative spin configurations in our structure between collinear and non-collinear alignments simply by switching the current direction even in the absence of an external magnetic field.

Non-volatile logic gates based on planar Hall effect in magnetic films with two in-plane easy axes.

We discuss the use of planar Hall effect (PHE) in a ferromagnetic GaMnAs film with two in-plane easy axes as a means for achieving novel logic functionalities. We show that the switching of magnetization between the easy axes in a GaMnAs film depends strongly on the magnitude of the current flowing through the film due to thermal effects that modify its magnetic anisotropy. Planar Hall resistance in a GaMnAs film with two in-plane easy axes shows well-defined maxima and minima that can serve as two binary logic states. By choosing appropriate magnitudes of the input current for the GaMnAs Hall device, magnetic logic functions can then be achieved. Specifically, non-volatile logic functionalities such as AND, OR, NAND, and NOR gates can be obtained in such a device by selecting appropriate initial conditions. These results, involving a simple PHE device, hold promise for realizing programmable logic elements in magnetic electronics.

Observation of uniaxial anisotropy along the [100] direction in crystalline Fe film.

We report an observation of uniaxial magnetic anisotropy along the [100] crystallographic direction in crystalline Fe film grown on Ge buffers deposited on a (001) GaAs substrate. As expected, planar Hall resistance (PHR) measurements reveal the presence of four in-plane magnetic easy axes, indicating the dominance of the cubic anisotropy in the film. However, systematic mapping of the PHR hysteresis loops observed during magnetization reversal at different field orientations shows that the easy axes along the and are not equivalent. Such breaking of the cubic symmetry can only be ascribed to the presence of uniaxial anisotropy along the direction of the Fe film. Analysis of the PHR data measured as a function of orientation of the applied magnetic field allowed us to quantify the magnitude of this uniaxial anisotropy field as Oe. Although this value is only 1.5% of cubic anisotropy field, its presence significantly changes the process of magnetization reversal, revealing the important role of the uniaxial anisotropy in Fe films. Breaking of the cubic symmetry in the Fe film deposited on a Ge buffer is surprising, and we discuss possible reason for this unexpected behavior.