Purely electrical memory chips commonly used today are volatile and their state must be continuously refreshed, which requires a lot of energy. An alternative to these electrical memory chips is magnetic random access memory (MRAM), which saves data magnetically and does not require constant refreshing. They do, however, require relatively large electrical currents to write the data to memory, which reduces reliability.

The AF-MERAM chip consists of a thin layer of chromium oxide for saving data, on top of which is attached a nanometer-thin platinum layer for readout. (Source: T. Kosub, Helmholtz-Zentrum Dresden-Rossendorf)

MRAM alternatives include a material class called magnetoelectric antiferromagnets that is activated by an electrical voltage rather than by a current. The material, however, cannot be easily controlled, and can only be read indirectly via ferromagnets, which negates many of the advantages. A purely antiferromagnetic magnetoelectric memory (AF-MERAM) was produced as a solution by scientists at Helmholtz-Zentrum Dresden-Rossendorf in Germany.

The AF-MERAM prototype is based on a thin layer of chromium oxide that is inserted between two nanometer-thin electrodes. When a voltage is applied to these electrodes, the chromium oxide “flips” into a different magnetic state, and the bit is written. Only a few volts are sufficient, which allows writing a bit without excessive energy consumption and heating.

To read out the written bit again, a nanometer-thin platinum layer was attached 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. A method was developed that suppressed interference, allowing the useful signal to be obtained.

The material works at room temperature, but only within a narrow window, which may be expanded by selectively altering the chromium oxide. To achieve this, a method was designed to provide images of the magnetic properties of the chromium oxide with nanoscale resolution. Several memory elements can be integrated on a single chip. 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.

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NASA Tech Briefs Magazine

This article first appeared in the March, 2017 issue of NASA Tech Briefs Magazine.

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