Paving the way to six-state magnetic memory

The researchers, Yevgeniy Telepinsky et al., from Bar-Ilan University in Israel and New York University in the US, have published a paper on the new magnetic structure in a recent issue of Applied Physics Letters.

This isn't the first time that researchers have designed memory cells with more than two states, or bits. The best-known example is multi-level flash memory cells, which can store up to four bits per cell. While multi-level flash cells have advantages such as a higher density and lower cost, they also suffer intrinsic drawbacks such as lower writing speeds and higher power consumption.

The new six-state memory element presented here is different because it is magnetic, whereas flash memory is electronic. Although electronic memories are currently the most commonly used type of memory, various types of magnetic random access memory (MRAM) are being actively researched due to advantages in low power consumption, fast operation, and long lifetime.

Realising the six-state magnetic element does not require any significant increase in complexity, such as adding layers, but rather involves simply structuring one of the magnetic layers differently—specifically, arranging the magnetic film into a pattern of three crossing ellipses.

In the middle region where all three ellipses overlap, the researchers found that there are six different stable magnetic orientations. The orientations are parallel to the long axis of each ellipse, and can run in two opposite directions.

If such a pattern with six magnetic orientations can be controlled and incorporated in a magnetic memory element, then the number of memory states can be increased from two to six.

The researchers showed that such control is possible by using a technique called spin-orbit torque switching, which uses spin-polarised electric current to switch between magnetic states. This demonstration shows that the spin-orbit torques can write data onto the magnetic structure, showing the potential for using the structure as a memory element.

The main advantage of having six states is that it would increase the memory density while avoiding the problems inherent in miniaturisation. Currently the primary strategy for increasing memory density is to miniaturise each memory element so that more of them can fit on a chip.

However, at these small scales, the memory elements are so close together that they begin to interfere with each other's states. The new design can avoid this problem, and also offers other advantages.

The researchers expect that it may be possible to design patterns with even more magnetic states. For example, their simulations show that a pattern of four crossing ellipses would yield a memory element with eight magnetic memory states.