Power/Performance Bits: Jan. 10

Antiferromagnetic magnetoelectric RAM; solar under the skin; new anode material for li-ion batteries.

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Antiferromagnetic magnetoelectric RAM

Researchers at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Swiss Nanoscience Institute, and the University of Basel developed a concept for a new, low power memory chip.

In particular, the group focused on finding an alternative to MRAM using magnetoelectric antiferromagnets, which are activated by an electrical voltage rather than by a current.

“These materials cannot be easily controlled,” said HZDR group leader Dr. Denys Makarov. “It is difficult to write data to them and read them out again.” So far it has been assumed that these magnetoelectric antiferromagnets can only be read indirectly via ferromagnets, which negates many of the advantages. The goal, then, was to produce a purely antiferromagnetic magnetoelectric memory (AF-MERAM).

The AF-MERAM prototype was based on a thin layer of chromium oxide inserted between two nanometer-thin electrodes. If a voltage is applied to these electrodes, the chromium oxide “flips” into a different magnetic state and the bit is written. The key is that a few volts are sufficient. “In contrast to other concepts, we could reduce the voltage by a factor of fifty,” said Tobias Kosub, post-doctoral researcher at HZDR. “This allows us to write a bit without excessive energy consumption and heating.”

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The prototype of an antiferromagnetic magnetoelectric memory chip, which was invented by researchers from Dresden and Basel. It consists of a thin layer of chromium oxide (Cr2O3) for saving data, on top of which the physicists attached a nanometer-thin platinum layer for read out. (Source: T. Kosub/HZDR)

A particular challenge was the ability to read out the written bit again.

In order to do so, the physicists attached a nanometer-thin platinum layer on top of the chromium oxide. The platinum enables the readout through a special electrical phenomenon, the Anomalous Hall Effect. The actual signal is very small and is superimposed by interference signals. “We could, however, develop a method that suppressed the storm of interference, allowing us to obtain the useful signal,” Makarov describes. “This was, in fact, the breakthrough.”

Vitally, according to Makarov, such memory chips could be produced using standard methods employed by computer manufacturers.

So far, only a single element was realized, which can store merely one bit. The next step is to construct an array of several elements.

Solar under the skin

Researchers from the University of Bern in Switzerland and Swiss Federal Laboratories for Materials Science and Technology tested the feasibility of using solar cells inserted under the skin to recharge implanted medical devices.

The cells were only 3.6 square centimeters in size, making them small enough to be implanted if needed. For the test, each of the ten devices was covered by optical filters to simulate how properties of the skin would influence how well the sun penetrates the skin. These were worn on the arm of 32 volunteers in Switzerland for one week during summer, autumn and winter.

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(a) Cross-sectional view of the measurement device. The solar cells (1) are located directly below the optical filters (2). Furthermore, the PCB (3) and battery (5) are enclosed in the housing (4). (b) Measurement device fixated on the upper arm. (Source: Lukas Bereuter)

Regardless of season, the cells were always found to generate much more than the 5 to 10 microwatts of power that a typical cardiac pacemaker uses. The participant with the lowest power output still obtained 12 microwatts on average.

“The overall mean power obtained is enough to completely power for example a pacemaker or at least extend the lifespan of any other active implant,” said Lukas Bereuter of Bern University Hospital. “By using energy-harvesting devices such as solar cells to power an implant, device replacements may be avoided and the device size may be reduced dramatically.”

The team believes that the results of this study can be scaled up and applied to any other mobile, solar powered application on humans.

New anode material for li-ion batteries

A research team at the Ulsan National Institute of Science and Technology (UNIST) in South Korea developed a new type of anode material for lithium-ion batteries that would be used in place of a conventional graphite anode, which they claim will lead to lighter and longer-lasting batteries.

In the study, the team demonstrated the feasibility of a next-generation hybrid anode using silicon-nanolayer-embedded graphite/carbon. They report that this architecture allows compatibility between silicon and natural graphite and addresses the issues of severe side reactions caused by structural failure of crumbled graphite dust and uncombined residue of silicon particles by conventional mechanical milling.li-ion-anode-material-unist-jan10

Cross-sectional schematic view showing the detailed structural characteristics of a SGC hybrid particle. (Source: UNIST)

This newly-developed anode material was manufactured with an increase in graphite content in composite by 45%. The research team has also developed new equipment, which is capable of producing 300kg in 6 hours per batch using a small amount of silane gas (SiH4).

According to the researchers, the silicon/graphite composite is mass-producible and it has superior battery performances with industrial electrode density, high areal capacity, and low amounts of binder.

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