Two wrongs make a right for novel multiferroic material

Multiferroics – materials that exhibit both magnetic and electric order – are of interest for next-generation computing, but are difficult to create because the conditions conducive to each of these states are usually mutually exclusive. And in most multiferroics found to date, their properties emerge only at extremely low temperatures.

Two years ago, researchers in the labs of Darrell Schlom and Dan Ralph at Cornell University, in collaboration with Ramamoorthy Ramesh at the University of California, Berkeley, published a paper announcing a breakthrough in multiferroics. This involved the only known material in which magnetism can be controlled by applying an electric field at room temperature: the multiferroic bismuth ferrite.

Schlom’s group has now partnered with David Muller and Craig Fennie, also at Cornell University, to take this research a step further. By combining two non-multiferroic materials, the researchers have managed to create a new room-temperature multiferroic.

A paper on this work is published in Nature. The lead authors are: Julia Mundy, a former doctoral student working jointly with Muller and Schlom who’s now a postdoctoral researcher at UC Berkeley; Charles Brooks, a visiting scientist in the Schlom group; and Megan Holtz, a doctoral student in the Muller group. Collaborators hailed from the University of Illinois at Urbana-Champaign, the US National Institute of Standards and Technology, the University of Michigan and Penn State University.

The group engineered thin films of hexagonal lutetium iron oxide (LuFeO₃), a material known to be a robust ferroelectric but not strongly magnetic, which consists of alternating single monolayers of lutetium oxide and iron oxide. In contrast, a strong ferrimagnetic form of lutetium iron oxide (LuFe₂O₄) consists of alternating monolayers of lutetium oxide with double monolayers of iron oxide.

The researchers found that they could combine these two materials at the atomic-scale to create a new compound that was not only multiferroic but had better properties than either of the individual constituents. In particular, adding just one extra monolayer of iron oxide to every 10 atomic repeats of LuFeO₃ dramatically changed the properties of the system.

That precision engineering was done via molecular-beam epitaxy (MBE), a specialty of the Schlom lab. A technique Schlom likens to “atomic spray painting”, MBE let the researchers design and assemble the two different materials in layers, a single atom at a time.

The combination of the two materials produced a strongly ferrimagnetic material near room temperature. Tests of this new material at the Lawrence Berkeley National Laboratory (LBNL) Advanced Light Source, in collaboration with co-author Ramesh, revealed that the ferrimagnetic atoms followed the alignment of their ferroelectric neighbors when switched by an electric field.

“It was when our collaborators at LBNL demonstrated electrical control of magnetism in the material that we made that things got super exciting,” Schlom said. “Room-temperature multiferroics are exceedingly rare and only multiferroics that enable electrical control of magnetism are relevant to applications.”

In electronics devices, the advantages of multiferroics include their reversible polarization in response to low-power electric fields – as opposed to heat-generating and power-sapping electrical currents – and their ability to hold their polarized state without the need for continuous power. High-performance memory chips make use of ferroelectric or ferromagnetic materials.

“Our work shows that an entirely different mechanism is active in this new material,” Schlom said, “giving us hope for even better – higher-temperature and stronger – multiferroics for the future.”

This story is adapted from material from Cornell University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.