Layer by layer – How reducing the thickness of layered magnetic materials can change tomorrows electronics

By Daniel Čavlović


What if I told you that nearly all electronic devices like phones or computers waste incredible potential in the way they are designed to work? All currently used electronic devices move electrons from one place to another creating a current relying solely on the particle’s negative charge. Very old-school physics known for centuries. But electrons are more than just charges! According to quantum mechanics – yes bear with me – an electron always possesses an additional intrinsic property, its spin, which can be either spin-up or spin-down (often depicted with arrows). These spins are slowly moving into the attention of applied research even though the quantum mechanics behind it is known for one century. The next-generation of electronics will leverage the full potential of the disregarded information stored in spins. But to build a so-called spin transport electronic (spintronic) device we need to find materials that are able to store, transport, and switch the spin information.

A group of researchers has synthesized and studied a novel material TaFe1.14Te3 which could be perfect for spintronic applications. By pressing elemental tantalum (Ta), iron (Fe), tellurium (Te), and some tellurium chloride into a pellet, sealing in a fused glass tube under vacuum and heating between 600 – 700 °C for a week they obtained needle-like crystals of this material. What makes this newly forged material so special is the way of how the elements are connected to each other. Even though the crystals resemble three dimensional needles, the underlying fundamental structure shows stacked layers of two dimensional sheets. Just like a stack of papers, each sheet is firmly connected and requires scissors to cut the sheet. On the other hand only loose forces keep the stack of sheets together and as little as a bit of wind blows the stack of paper away). Amusingly, the same way that you can remove the top sheet from a paper stack with tape, this quite literally works the same way with TaFe1.14Te3 (see Figure 1 below). This Nobel Prize winning technique is known as mechanical exfoliation and led to the isolation of the first in-depth studied 2D material, graphene. Graphene is a single layer of graphite (mistakenly called ‘lead’ in pencils). In graphite each layer of graphene interacts with the layers in between it’s sandwiched in. Thus, all defects or inconsistencies are diffused into and covered up by the mountain of layers. Like picking out the best singer from the choir for a solo, by reducing the number of layers of the material the properties start to change and become more featured and outstanding. In the case of graphite (the pen material) one single layer of it—graphene—becomes incredibly conductive and mechanically robust. Cool properties but not very useful for electronic devices because graphene has no band gap. It’s pretty much just a “wire” which always conducts electricity. In contrast a band gap is like a wire with a switch, where two states can be addressed (for binary information). Single layers of TaFe1.14Te3 indeed do possess a band gap and are magnetic too, both properties required for spintronics. Its magnetism is especially interesting: The studies revealed that in each single layer of this material there are quasi-one dimensional chains of iron atoms decorated with unpaired electrons whose spins give rise to its magnetism. In a single layer all neighboring spins align and point in the same direction, a property called ferromagnetism or 2D magnet.

Figure 1: Schematic side-view of a TaFe1.14Te3 layer being pulled off as an illustration of mechanical exfoliation.

You can imagine each of those spins in iron atoms like a compass needle always aligning with the earth’s magnetic field. But what happens if you stack another layer on top of it? The top layer’s spins align in the opposite direction with respect to the layer below forming an antiferromagnetic bilayer. This shows that the spins between two layers strongly interact with each other. While a single layer interacts strongly with an external magnetic field from let’s say a magnet closeby the bilayer does not care about external magnetic fields because of the much stronger interaction with its neighboring layer. You may have heard that a magnet can damage or wipe your computer’s hard drive, which is true for older technology which relied on ferromagnetic layers. One elegant solution to this is to store information in antiferromagnetic bilayers which further enables minimizing the size and amount of materials needed for electronic devices.

The researchers have studied TaFe1.14Te3 and its electronic structure in even more detail. They found that the material shows metallic behavior, further supported by calculations, and itinerant magnetism but at the same time local moments which makes the exact electronic structure complicated to determine. What this means is that as a whole it shows local sites where the magnetism arises from but is spread out and averaged as it never resides fully localized. The underlying origin of this special magnetism is still subject of further studies of this material which is probably being deciphered at this very moment.

The most important property–stability under ambient conditions–may sound boring but is essential for practicable applications. Even though there are other 2D materials known which show similar remarkable properties but their translation from fundamental research towards application is not really feasible because they react with trace water or oxygen from the air we breathe!

Even though spintronic devices are far from being commercialized, one day you might hold a superior and more energy efficient spintronic phone in hand and admire the success of technological advancement. Remember, that all originated from fundamental research which managed to leverage the hitherto elusive spin information with 2D materials like TaFe1.14Te3


Original paper

Reviewed by: Trang Nguyen


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