New processes in modern ReRAM memory cells decoded

Resistive memory cells or ReRAMs for short are deemed to be the new super information-storage solution of the future. At present, two basic concepts are being pursued, which, up to now, were associated with different types of active ions. But this is not quite correct, as Jülich researchers working together with their Korean, Japanese and American colleagues were surprised to discover. In valence change memory (VCM) cells, not only are negatively charged oxygen ions active, but – akin to electrochemical metallization memory (ECM) cells – so too are positively charged metal ions. The effect enables switching characteristics to be modified as required and makes it possible to move back and forth from one concept to the other, as reported by the researchers in the journals ("Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems") and ("Graphene-Modified Interface Controls Transition from VCM to ECM Switching Modes in Ta/TaOx Based Memristive Devices"). Formation of metallic Tantalum filament Formation of metallic Tantalum (Ta) filament within Ta/TaOx/Pt ReRAM memory cell. Positively charged Ta5+-ions and oxygen vacancies (VO) contribute to the process. (Image: Forschungszentrum Jülich / RWTH Aachen / Pössinger) ReRAM cells have a unique characteristic: their electrical resistance can be altered by applying an electric voltage. The cells behave like a magnetic material that can be magnetized and demagnetized again. In other words, they have an ON and an OFF state. This enables digital information to be stored, i.e. information that distinguishes between “1” and “0”. The most important advantages of ReRAMs are that they can be switched rapidly, consume little energy, and maintain their state even after long periods of time with no external voltage. The memristive behaviour of ReRAMs relay on mobile ions. These ions move in a similar manner to in a battery, flowing back and forth between two electrodes in a metal oxide layer no more than a few nanometres thick. For a long time, researchers believed that VCMs and ECMs functioned very differently. In ECMs, the ON and OFF states are achieved when metal ions move and form whisker-like filaments. This happens when an electric voltage is applied, causing such filaments to grow between the two electrodes of the cell. The cell is practically short-circuited and the resistance decreases abruptly. When the process is carefully controlled, information can be stored. The switching behaviour of VCMs, in contrast, were primarily associated with the displacement of oxygen ions. Contrary to metal ions, they are negatively charged. When a voltage is applied, the ions move out of an oxygen-containing metal compound. The material abruptly becomes more conductive. In this case as well, the process needs to be more carefully controlled. Jülich researchers working together with their partners from the Chonbuk National University, Jeonju, the National Institute for Materials Science in Tsukuba and the Massachusetts Institute of Technology (MIT) in Boston discovered an unexpected second switching process in VCMs: metal ions also help to form filaments in VCMs. The process was made visible because the scientists suppressed the movement of the oxygen ions. To do so, they modified the surface by applying a thin carbon layer directly at the interface of the electrode material with the solid electrolyte. In one case, they used the “miracle material” graphene, which comprises only one single layer of carbon. “Graphene was used to suppress the transport of oxygen ions through the phase boundary and to slow down the oxygen reactions. Suddenly, we observed a switching characteristic similar to that of an ECM cell and therefore assume that free metal ions are also active in VCMs. This was additionally verified using scanning tunnelling microscopy (STM) and diffusion experiments. It appears that the metal ions provide additional support for the switching process,” says Dr. Ilia Valov, electrochemist at Jülich’s Peter Grünberg Institute (PGI-7). A look into the Oxide Cluster A look into the Oxide Cluster at Forschungszentrum Jülich in which resistive cells and other layers of material are produced and examined in an ultrahigh vacuum. (Image: Forschungszentrum Jülich) Incorporating such a carbon interlayer would make it possible to jump from one switching process to the other in VCMs. This would lead to new options for designing ReRAMs. “Depending on the application, our findings could be exploited and the effect purposely enhanced or intentionally suppressed,” says Valov. The scientists’ findings give rise to several questions. “Existing models and studies will have to be reworked and adapted on the basis of these findings,” says the Jülich scientist. Further tests will clarify how such novel components behave in practice.
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Hopes of improved brain implants with nanowire structures

Neurons thrive and grow in a new type of nanowire material developed by researchers in Nanophysics and Ophthalmology at Lund University in Sweden ("Support of Neuronal Growth Over Glial Growth and Guidance of Optic Nerve Axons by Vertical Nanowire Arrays"). In time, the results might improve both neural and retinal implants, and reduce the risk of them losing their effectiveness over time, which is currently a problem. By implanting electrodes in the brain tissue one can stimulate or capture signals from different areas of the brain. These types of brain implants, or neuro-prostheses as they are sometimes called, are used to treat Parkinson’s disease and other neurological diseases. They are currently being tested in other areas, such as depression, severe cases of autism, obsessive-compulsive disorders and paralysis. Another research track is to determine whether retinal implants are able to replace light-sensitive cells that die in cases of and other eye diseases. However, there are severe drawbacks associated with today’s implants. One problem is that the body interprets the implants as foreign objects, resulting in an encapsulation of the electrode, which in turn leads to loss of signal. “Our nanowire structure prevents the cells that usually encapsulate the electrodes – glial cells – from doing so”, says Christelle Prinz, researcher in Nanophysics at Lund University in Sweden, who developed this technique together with Maria Thereza Perez, a researcher in Ophthalmology. “I was very pleasantly surprised by these results. In previous in-vitro experiments, the glial cells usually attach strongly to the electrodes”, she says. To avoid this, the researchers have developed a small substrate where regions of super thin nanowires are combined with flat regions. While neurons grow and extend processes on the nanowires, the glial cells primarily occupy the flat regions in between. “The different types of cells continue to interact. This is necessary for the neurons to survive because the glial cells provide them with important molecules.” So far, tests have only been done with cultured cells but hopefully they will soon be able to continue with experiments . The substrate is made from the semiconductor material gallium phosphide where each outgrowing nanowire has a diameter of only 80 nanometres.
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Electric field control of magnetic moment in palladium

Researchers at the University of Tokyo and Central Research Institute of Electric Power Industry have successfully induced a magnetic moment in palladium (Pd), usually a non-magnetic material, and demonstrated the ability to reversibly control the strength of the magnet by applying an electric field ("Electric-field control of magnetic moment in Pd"). This research has demonstrated the possibility of electrically inducing magnetism in non-magnetic materials. Induced magnetic moment in palladium (Pd) by application of voltage Induced magnetic moment in palladium (Pd) by application of voltage. One possible mechanism to explain the increased magnetic moment is that, when a voltage is applied to the structure shown in the figure, charges within the palladium layer accumulate near the surface because the surface is covered by ions. (Image: Chiba Lab) If the properties of a material could be electrically tuned after production, it would be possible to easily obtain the desired functions when needed, further increasing the range of materials that could be used in magnetic devices. In fields that employ magnetic materials, tuning of magnetic force and control of magnetization direction (together, these properties are termed the “magnetic moment”) has been demonstrated by applying a voltage to a capacitor containing a magnetic film as one electrode and charging and discharging charge carriers (electrons) from the electrode. It is expected that this method will dramatically reduce power consumption compared to conventional means of controlling magnetic moment (heating, magnetic field or electric current application). Prior studies have reported that it is possible to erase the magnetic properties of a material by the application of an electric field. However, there are no reports of successfully inducing and cancelling magnetic properties in a non-magnetic material by the same method. The research group of Associate Professor Daichi Chiba at the University of Tokyo Graduate School of Engineering and the Central Research Institute of Electric Power Industry has shown that the strength of a magnetic moment induced in palladium, a metal which is usually non-magnetic, is electrically controllable, and that application of a positive voltage induces a stronger magnetic moment than a negative voltage. The research group fabricated an ultra-thin cobalt/palladium structure in which a ferromagnetically ordered magnetic moment was induced in the top palladium layer by the ferromagnetic proximity effect. The magnetic moment in this Pd layer was reversibly controlled by applying a voltage. “This offers a new avenue for making non-magnetic materials ferromagnetic,” says Associate Professor Chiba of this latest research. He continues, “If it becomes possible to easily and reversibly induce magnetic properties in a non-magnetic material by applying an electric voltage, we may be able to make use of many materials currently not used in the field of magnetic engineering and further increase the range of materials available for use in magnetic devices.”
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