In-gap states and band-like transport in memristive devices

The creation of point defects in matter can profoundly affect the physical and chemical properties of materials. If appropriately controlled, these modifications can be exploited in applications promising advanced and novel functionalities. Redox-based memristive devices – one of the most attractive emerging memory technologies – provide one of the most striking examples for the potential exploitation of defects. Applying an external electric field to an initially insulating oxide layer is known to induce a non-volatile, voltage-history dependent switching between a low resistance state and a high resistance state, also named memristive device. This switching occurs through the creation and annihilation of the so-called conductive filaments, which are generated at the nanoscale by assembly of donor-type point defects such as oxygen vacancies.
To date, the exact relationship between concentration and nanoscale distribution of defects within the filament on the one hand and the electronic transport properties of the devices on the other hand is still elusive. Due to limitations in sensitivity or spatial resolution of most characterization methods, the electronic structure of conductive filaments has not yet been characterized in detail. However, this knowledge is crucially needed as input for the development of electronic transport models with high predictive power.

>Read more on the Elettra website

Image: (a) Ti3+ map based on the Ti 3p3/2 spectrum. (b) Ti 2p 3/2 spectra for the filament and the surrounding. (c) Spatial map of the in-gap state distribution. (d) Valence band spectrum extracted from the filament at a photon energy of 463.3 eV with a fit of the valence band maximum and the in-gap states (red lines). (e) Band diagram of the device calculated based on the position of the in-gap states. The blue line shows the conduction band and the dashed green lines shows position of the defect states obtained by PEEM in respect to the conduction band 

Tunable ferromagnetism in a 2D material at room temperature

Breakthroughs in next-generation spintronic logic and memory devices could hinge on our ability to control spin behavior in two-dimensional materials—stacks of ultrathin layers held together by relatively weak electrostatic (van der Waals) forces. The reduced dimensionality of these so-called “van der Waals materials” often leads to tunable electronic and magnetic properties, including intrinsic ferromagnetism. However, it remains a challenge to tune this ferromagnetism (e.g. spin orientation, magnetic domain phase, and magnetic long-range order) at ambient temperatures.

In this work, researchers performed a study of Fe3GeTe2, a van der Waals material that consists of Fe3Ge layers alternating with two Te layers. The material’s magnetic properties were characterized using a variety of techniques, including x-ray absorption spectroscopy (XAS) with x-ray magnetic circular dichroism (XMCD) contrast at Beamline 6.3.1 and photoemission electron microscopy (PEEM) at Beamline 11.0.1.

>Read more on the Advanced Light Source (ALS) at LBNL website

Image: PEEM images for unpatterned and patterned Fe3GeTe2 samples at 110 K and 300 K. The unpatterned samples formed stripe domains at 110 K, which disappeared at 300 K. The patterned samples formed out-of-plane stripe domains at 110 K and transitioned to in-plane vortex states at 300 K, demonstrating control over magnetism at room temperature and beyond.

Magnetization ratchet in cylindrical nanowires

A team of researchers from Materials Science Institute of Madrid (CSIC), University of Barcelona and ALBA Synchrotron reported on magnetization ratchet effect observed for the first time in cylindrical magnetic nanowires (magnetic cylinders with diameters of 120nm and lengths of over 20µm).

These nanowires are considered as building blocks for future 3D (vertical) electronic and information storage devices as well as for applications in biological sensing and medicine. The experiments have been carried out at the CIRCE beamline of the ALBA Synchrotron. The results are published in ACS Nano.

The magnetic ratchet effect, which represents a linear or rotary motion of domain walls in only one direction preventing it in the opposite one, originates in the asymmetric energy barrier or pinning sites. Up to now it has been achieved only in limited number of lithographically engineered planar nanostructures. The aim of the experiment was to design and prove the one-directional propagation of magnetic domain walls in cylindrical nanowires.

>Read more on the ALBA website

Image: (extract) Unidirectional propagation of magnetization as seen in micromagnetic simulations and XMCD-PEEM experiments. See entire image here.

Subfilamentary Networks in Memristive Devices

Redox-based memristive devices are one of the most attractive emerging memory technologies.

…in terms of scaling, power consumption and speed. In these devices, external electrical stimuli cause changes of the resistance of an oxide layer sandwiched between two metal electrodes. In the simplest application, the device can be set into a low resistance state (LRS) and reset into a high resistance state (HRS), which may encode a logical one and zero, respectively. The major obstacle delaying large-scale application, however, is the large cycle-to-cycle (C2C) and device-to-device (D2D) variability of both LRS and HRS resistance values. These variabilities describe the stochastic nature of the switching process within one cell, resulting in different resistances obtained for each switching cycle and different resistances obtained for different cells on the same chip.

Read more on the Elettra website.

Image:(a) Schematic of the device geometry. A SrTiO3 layer (blue) is sandwiched between a Nb:SrTiO3 bottom electrode (dark grey) and graphene top electrode (grey honeycomb lattice). The graphene electrode is contacted through a metal lead, which is electrically separated from the continuous bottom electrode, allowing for biasing inside PEEM instruments. (b) Quasistatic I-V curve of a representative graphene/SrTiO3/Nb:SrTiO3 device. The bottom electrode serves as virtual ground, while the bias is applied to the graphene top electrode. (c) PEEM image of a graphene/Al2O3/SrTiO3 device in the LRS at an electron energy E – EF of 3.4 eV. Scale bar, 5 µm. (d) PEEM image of the same device after Reset. (e) and (f) PEEM images after one additional Set and Reset operation, respectively. Insets: magnified photoemission threshold map of the area around the conductive filament. The maps were obtained by fitting the threshold spectrum for each pixel.