Large piezomagnetism appears in an antiferromagnet

Observing large piezomagnetism in the antiferromagnetic Weyl semimetal Mn3Sn at room temperature. (Courtesy: S Nakatsuji)

Researchers at the University of Tokyo in Japan, Cornell and Johns Hopkins Universities in the US and the University of Birmingham in the UK have observed large piezomagnetism in an antiferromagnetic material, manganese-tin (Mn3Sn). The finding could allow this material and others like it to be employed in next-generation computer memories.

Antiferromagnetic materials are promising candidates for future high-density memory devices for two main reasons. The first is that electron spins (which are used as the bits or data units) in antiferromagnets flip quickly, at frequencies in the terahertz range. These rapid spin flips are possible because spins in antiferromagnets tend to align antiparallel to each other, leading to strong interactions among the spins. This contrasts with conventional ferromagnets, which have parallel electron spins.

The second reason is that while antiferromagnets have an internal magnetism created by the spin of their electrons, they have almost no macroscopic magnetization. This means that bits can be packed in more densely as they do not interfere with each other. Again, this contrasts with the ferromagnets employed in conventional magnetic memory, which do generate sizable net magnetization.

Researchers use the well-understood Hall effect (in which an applied magnetic field induces a voltage in a conductor in a direction perpendicular to both the field and the flow of current) to read out the values of antiferromagnetic bits. If the spins in the antiferromagnetic bit all flip in the same direction, the Hall voltage changes sign. One sign of the voltage, therefore, corresponds to a “spin up” direction or “1” and the other sign to a “spin down” or “0”.

Strain controls sign change

In the new work, a team led by Satoru Nakatsuji of the University of Tokyo used equipment developed by Clifford Hicks and colleagues at Birmingham to place a sample of Mn3Sn under strain. Mn3Sn is an imperfect (Weyl) antiferromagnet with a weak magnetization, and it is known to display a very strong anomalous Hall effect (AHE), in which charge carriers acquire a velocity component perpendicular to an applied electric field even without an applied magnetic field.

The researchers found that, by placing different degrees of strain on the sample, they could control both the magnitude and the sign of the material’s AHE. “Since the discovery of the AHE by Edwin Hall in 1881, no report has been made on the continuous tuning of the AHE sign by strain,” Nakatsuji tells Physics World. “At first sight, it may appear that the Hall conductivity, a quantity that is odd under time reversal, cannot be controlled by strain, which is even under time reversal. However, our experiment and theory clearly demonstrate that a very tiny strain in the order of 0.1% can control not only the size but also the sign of the AHE.”

Important for antiferromagnetic spintronics

The team says that being able to control AHE using strain will be important for so-called “spintronics” applications involving antiferromagnetic materials. Since the Weyl semimetal state of Mn3Sn can also be switched electrically, the new discovery makes the material even more attractive for spintronics, and a number of groups around the world are now working on fabricating it in thin-film form.

The present work is detailed in Nature Physics.