Researchers at the Czech Academy of Sciences, Institut Polytechnique de Paris, Vienna Technical University (TU Wien), Charles University, Malvern Panalytical B.V., Nuclear Physics Institute CAS and the European Commission's Joint Research Centre (JRC) recently made an important step that could advance the field of spintronics: they managed to switch the spins in an antiferromagnetic material using surface strain.
"There are different types of magnetism," explains Sergii Khmelevskyi from the Vienna Scientific Cluster Research Center, Vienna Technical University. "The best known is ferromagnetism. It occurs when the atomic spins in a material are all aligned in parallel. But there is also the opposite, antiferromagnetism. In an antiferromagnetic material, neighboring atoms always have opposite spins." Their effects therefore cancel each other out and no magnetic force can be detected from the outside.
"In 2010, however, scientists at the TU Wien and the Czech Academy of Science came up with the idea that such antiferromagnetic materials have promising properties for spintronic applications," says Sergii Khmelevskyi. This was the start of the new research field of “antiferromagnetic spintronics”, which has developed quickly ever since.
Intensive work was done recently by TU Wien, the Institute of Physics of Czech Academy of Sciences and Ecole Polyitechnique (Paris). The biggest challenge was that the spins in antiferromagnetic materials are difficult to manipulate – but finding a way to manipulate them in a reliable and precise way is crucial. Only if magnetic states can be switched from one state to another in a targeted manner it becomes possible to produce computer memory cells (e.g. MRAM).
Manipulating ferromagnets is easy: It is enough to simply apply an external magnetic field to influence its internal magnetic properties. This is not possible with antiferromagnets – but there here is where surface strain comes in.
It does, however, require very specific types of crystals: Depending on the geometry and the arrangement of the atoms in the crystal, several different antiferromagnetic spin arrangements may be possible. The crystal assumes the state with the lowest energy. But it might be an situation when several different spin orders have the same energy. This phenomenon is called "magnetic frustration". "In that case, tiny interactions, which would otherwise play no role, can decide which magnetic state the crystal assumes," says Sergii Khmelevskyi.
Experiments with uranium dioxide have shown that mechanical stress can be used to compress the crystal lattice a tiny bit, and this is enough to switch the magnetic order of the material.
"We have now shown that antiferromagnets can actually be switched by utilizing the properties of the magnetic frustration existing in many known materials”, says Sergii Khmelevskyi. “That opens the door to many exciting further developments in the direction of functional antiferromagnetic spintronics."