An ASI (artificial spin ice) is typically an array of small nanomagnets which interact with each other and with external magnetic fields. ASIs are a class of metamaterials (so-called materials of the future), engineered to exhibit unique electromagnetic properties through structured arrangements, differing significantly from the natural behavior of their constituent materials. Recently, ASIs have shown promise for device applications, such as substrates for computation.
These magnetic systems get their name from water ice, where the magnetic moments, or spins, align similar to the hydrogen bonds of ice molecules. ASI nanomagnets are typically blocked (frozen) at room temperature, as the thermal energy is not enough to change their magnetic state. Because there are many magnets, the overall system can have many different states which can be prepared using external magnetic fields.
However until now, these methods have been rather coarse, i.e. they were changing many magnets at a time, in an uncontrolled manner. Or the magnets are written individually, in a non-practical manner, using a scanning probe tip.
Now, scientists from the Norwegian University of Science and Technology have devised a new method called “astroid clocking” that uses a special external field sequence which is able to exactly switch only those elements which are at the border of two regions within the ASI with different states. Thus, it is possible to finely control the state of the ASI array.
The method takes its name from the Stoner–Wohlfarth astroid, a curve that characterizes the critical switching field of a nanomagnet as a function of the angle of the applied magnetic field. By using this information, the method introduced by the research team make use of the magnetic properties of the individual magnets (the Stoner-Wohlfarth astroid) and the interaction between them (dipolar interaction). This approach enables precise targeting of only the border magnets in a given clock cycle. As the border advances in each cycle, the whole array can be addressed.
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