Most technologies that store data, including hard drives or credit cards, achieve their functionality through the use of ferromagnetic (FM) materials like iron or cobalt. However, these materials are vulnerable to disruption by external disturbances. Placing a credit card near a magnet may, for example, result in the loss of data stored in its magnetic stripe, rendering the card useless.

To address this, researchers are turning to a unique class of materials known as antiferromagnets. Unlike FM materials, the magnetic poles in antiferromagnetic (AFM) systems are aligned in opposite directions, resulting in a net magnetic moment of zero. Such structures are robust against disturbances, can be scaled to very small sizes, and have very fast response times.

Of particular importance to the functionality of AFM materials is the stabilisation of topological spin textures, which are hurricane-like whirling magnetic patterns. These topological spin textures arise from the intricate arrangement of spins within the material, resulting in complex magnetic patterns. They have been predicted to possess whirlwind-like motion with speeds of up to kilometres per second, and could therefore be used as information carriers for the next generation of computing applications, which will require super-fast information processing.

Despite their potential to revolutionise our computing capabilities, producing well-defined and controlled topological textures in AFM is difficult and this has limited their practical use. At present, topological textures in AFM are rare and can only be created on rigid crystal templates, but these are incompatible with conventional silicon wafers.

Building free-standing layers

In an effort to integrate topological AFM textures into traditional silicon-based substrates, Professor Ariando’s group from NUS Physics, together with Dr Hariom Jani and Professor Paolo Radaelli’s group from the University of Oxford, created ultra-thin crystalline membranes of the mineral form of Iron(III) Oxide, α-Fe₂O₃, which is commonly known as hematite. Hematite is the most abundant AFM, and is the main component of rust.

To do this, the hematite layer was grown on a crystal substrate made of STO (SrTiO₃), which was chosen to reduce interlayer lattice mismatch, which would otherwise lower the quality of hematite layers produced. The growth layer was coated with a water soluble ‘sacrificial layer’ (SAO) made from (111)-oriented Sr₃Al₂O₆ – a component of cement. Once the sacrificial layer was dissolved in water, the hematite could easily be separated from the growth layer. The free-standing hematite membrane could then be transferred onto Si or Si₃N₄ substrates. This approach minimised the introduction of defects or strain at the AFM-substrate interface, which is crucial in optimising the magnetic properties of the final heterostructure.

Hematite membranes are relatively new in the world of crystalline quantum materials, and combine advantageous characteristics of both bulk 3D ceramics and 2D materials. They have similar magnetic and electronic properties to bulk materials, but are also flexible and durable like 2D materials. This means they can be twisted, bent, or curled into various shapes without fracturing.

To visualise the topological textures within these membranes, the team developed a novel imaging technique using linearly polarised X-rays which was implemented at the Swiss Light Source. This method revealed that the free-standing layers were able to host a robust family of magnetic patterns. Coupled with their ability to operate at a higher frequency compared to topological spin textures in FM, this finding opens the possibility for topological AFM spin textures in ultra-fast information processing.

These results have recently been published in Nature Materials.

This is the first time that detachable AFM materials with desired topological textures have been produced. This breakthrough opens up new possibilities in producing topological AFM textures for computing, data storage, and spintronics-based devices.

The work builds upon earlier research where the team demonstrated topological textures in an AFM (iron oxide crystals) with robust and ultra-fast properties for the first time.