Scientists in Thomas Jung’s research groups at the Paul Scherrer Institute (PSI) and at the University of Basel in Switzerland have fabricated the first two-dimensional ferrimagnetic material that consists of only two layers of material.
Two-dimensional magnetic structures have been hotly pursued in the research community because the magnetic properties of single molecules in these structures can be indivdually addressed and modified. This is especially important in spintronics, where the aim is to use the spins of electrons to encode information.
In a three-dimensional magnetic material, it is difficult to determine, or change, the spin of an electron when there are lots of other layers of material on top or below it. Developing a 2D material that exhibits ferrimagnetism promises a much more effective way to perform spintronics for things like data storage or to use electrons’ spins as quantum bits in quantum computing.
With that in mind, researchers at Lawrence Berkeley Laboratory demonstrated last month that multi-layered examples of the 2D material chromium germanium telluride did have an intrinsic ferrimagnetism. However, these multilayered crystals (whose layers are held together by van der Waal forces) do not provide the advantages that ferrimagnetism would give in a monolayer.
Jung and his colleagues did not, strictly speaking, make a monolayer ferrimagnet either—the existence of which is impossible according to Heisenberg’s model of magnetic systems. Instead, in research described in the journal Nature Communications, they fabricated a monolayer comprising molecules called porphyrins. These organic structures, used in biochemistry and sensing applications, are about one nanometer in size. Right in the middle of each porphyrin molecule sits a magnetic atom—like iron. The molecules assemble themselves on top of a gold monolayer, forming a checkerboard pattern in which the spin of the magnetic atoms at the center of the molecules alternate between up and down.
Jung explained in an interview with IEEE Spectrum that without the gold surface—which is not magnetic but highly conductive—the system wouldn't be ferrimagnetic.
The gold’s conduction electrons couple themselves to the magnetic atoms at the center of the molecules, a phenomenon called the Kondo effect. Since the gold is a conductor but not magnetic it serves just as a sea of electrons below the magnetic molecules. The electrons in the gold surface are attracted to the opposite spins of the magnetic molecules above. This attracting of opposite spins leads to the kind of coupling in which one neighboring molecule knows about the situation of the next. This is critical for a magnetic system.
“Without the gold the magnetic molecules wouldn't know what the neighbor is doing; it wouldn't be magnetic at all,” says Jung. “Once we put the gold into this system the electrons below either of the spins recognize what is there and they tell their neighbor spins so that the neighboring atom knows from the electrons how it should behave.”
Jung is quick to caution that this is still fundamental research and should not really be considered a technology—say, a new data storage system—quite yet. In order for something like a data storage device to be developed out of this material, some kind of sensor would need to be developed to read the surface of the magnetic molecules. For the research described in the paper, a scanning tunneling microscope was able to go to one or the other of these spin centers and detect the Kondo effect locally.
In continuing research, Jung and his colleagues are looking at similar molecular architectures. But the metal atom at the center of their molecule will be manganese or cobalt instead of iron.
Jung adds: “In material science, you like to play with the ‘Legos’ and see if by combining different pieces we get something stronger with even more exciting properties.”