A crankshaft converts linear motion of a piston, for example in an internal combustion engine into rotational motion of a wheel. It was known to Romans, played a crucial role in the industrial revolution and still is an essential part of many vehicles and devices. A team of researchers from the Vienna University of Technology, Istituto Italiano di Tecnologia, Beijing Institute of Technology, Rutgers University and University of Groningen found a magnetic material Gd2Mn2O5 that works similarly to crankshaft. Half of all Mn spins in this material rotate as a magnetic field applied along a fixed direction increases and subsequently decreases to zero. In contrast to the usual magnetization precession, spins in Gd2Mn2O5 only rotate when the magnetic field varies. The magnetic crankshaft works like a four-stroke engine, the full circle rotation requires ramping up and down the magnetic field two times.
Gd2Mn2O5 belongs to a family of multiferroic materials, in which magnetic moments can be controlled by applying voltage and the direction of electric dipoles can changed by an applied magnetic field. Remarkably, in one half of the 4-stroke cycle the electric polarization is nearly constant, whereas in another half its direction is reversed. This behavior can be traced back to the microscopic interaction between magnetic and electric dipoles and can be used in magnetic memory and data processing devices.
Another interesting property of this magnetic crankshaft is the unidirectional rotation of spins. To switch the rotation direction (to switch from forward to reverse gear), one has to change the orientation of the magnetic field by about 20 degrees. At first glance, this ratchet-like motion seems to defy physical laws. During one rotation cycle, the magnetic state of the system changes 4 times in two possible sequencies: 1->2->3->4->1... and 1->4->3->2->1... and since the energies of states 2 and 4 are equal and it was initially unclear how the system in state 1 makes the choice between the two. Theoretical studies of magnetism in Gd2Mn2O5 revealed the trick: the energy barriers separating states 1 and 2, and states 1 and 4 are not the same. As the magnetic field strength varies, one of the two barriers disappears first, which is how the rotation direction is selected.
The realization of the crank mechanism in Gd2Mn2O5 is very complex. It involves two spin chains of Mn ions as well as magnetic Gd ions located between the chains. However, numerical simulations suggest that this mechanism can also work in magnets with simple structures, showing that this unusual behavior can be found in many other materials.
Reference: Ponet, L., Artyukhin, S., Kain, T., Wettstein, J., Pimenow, A., Shuvaev, A., Wang, X., Cheong, S.W., Mostovoy, M., & Pimenov, A. Topologically protected magnetoelectric switching in a multiferroic. Nature 607, 81–85 (2022). https://doi.org/10.1038/s41586-022-04851-6
The paper was also reviewed in Nature News&Views: https://www.nature.com/articles/d41586-022-01786-w
Contact: Prof. Maxim Mostovoy
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