Normally a material can be either magnetically or electrically polarized, but not both. Now researchers at the Niels Bohr Institute at University of Copenhagen have studied a material that is simultaneously magnetically and electrically polarizable. This opens up new possibilities, for example, for sensors in technology of the future.
“We have studied the rare, naturally occurring iron compound, TbFeO3, using powerful neutron radiation in a magnetic field. The temperature was cooled down to near absolute zero, minus 271 C. We were able to identify that the atoms in the material are arranged in a congruent lattice structure consisting of rows of the heavy metal terbium separated by iron and oxygen atoms. Such lattices are well known, but their magnetic domains are new. Normally, the magnetic domains lie a bit helter-skelter, but here we observed that they lay straight as an arrow with the same distance between them. We were completely stunned when we saw it,” explains Kim Lefmann, Associate Professor at the Nano-Science Center, University of Copenhagen.
The ‘8-armed candlestick’ in this unusual image of the measurements is proof that the ‘walls’ of the domains in TbFeO3 repel each other at certain temperatures and therefore lie at a fixed distance from each other. The signal from the ‘ordinary’ chaotic domain walls would more resemble a fly swatter.
The random fluctuations of spins give rise to many interesting physical phenomena, such as the ‘order-from-disorder’ arising in frustrated magnets and unconventional Cooper pairing in magnetic superconductors. Here we show that the exchange of spin waves between extended topological defects, such as domain walls, can result in novel magnetic states. We report the discovery of an unusual incommensurate phase in the orthoferrite TbFeO3 using neutron diffraction under an applied magnetic field. The magnetic modulation has a very long period of 340 Å at 3 K and exhibits an anomalously large number of higher-order harmonics. These domain walls are formed by Ising-like Tb spins. They interact by exchanging magnons propagating through the Fe magnetic sublattice. The resulting force between the domain walls has a rather long range that determines the period of the incommensurate state and is analogous to the pion-mediated Yukawa interaction between protons and neutrons in nuclei.
“What the models are describing is that the terbium walls interact by exchanging waves of spin (magnetism), which is transferred through the magnetic iron lattice. The result is a Yukawa-like force, which is known from nuclear and particle physics. The material exhibits in a sense the same interacting forces that hold the particles together in atomic nuclei,” explains Heloisa Bordallo, Associate Professor at the Niels Bohr Institute.
It is precisely this interaction between the transition metal, iron, and the rare element, terbium, that plays an important role in this magneto-electrical material. The terbium’s waves of spin cause a significant increase in the electric polarization and the interaction between the ions of the elements creates one of the strongest magneto-electrical effects observed in materials.
“Through these results we found a new pathway to discover and develop new multiferroics”, emphasize the researchers in the group.