Incorporating titanium atoms into the barium hexaferrite crystal lattice leads to the formation of an unexpected substructure, according to work carried out by researchers at the Moscow Institute of Physics and Technology and their colleagues at the Ural Federal University, and in Germany and the USA. They suggest that this novel material could have applications in ultrafast computer memory.

Writing in the journal Scientific Reports, the team explains how they have combined optical and acoustic approaches to study the properties of this novel multiferroic. Aside from the approach itself having novelty, the use of only readily sourced elements marks a potentially useful departure from the use of rare metals in electronic components. [Gudkov, V.V., et al. Sci Rep (2020); DOI: 10.1038/s41598-020-63915-7]

Multiferroic materials commonly present with more than one type of internal order in the crystal lattice and as such can exhibit both ferroelectric and ferromagnetic properties at different temperatures. In a given temperature range, the material might undergo spontaneous polarization but below another critical point, it displays magnetic properties even in the absence of an applied, external magnetic field. Tailoring this temperature-dependent response could open up multiferroic materials for various applications such as the aforementioned ultrafast magnetic memory, as anti-reflective coatings, and even fast data transmission at terahertz frequencies.

Team leader Liudmila Alyabyevaexplains that utilizing optical and acoustic techniques to study the material offers a complementary way to examine its properties. "The two channels together provide a more complete understanding of an object," she says. "Whenever two very different experimental techniques demonstrate certain phenomena occur at a particular temperature, that is a strong indication that something's happening in the sample at the microscopic level. So we need to identify the mechanism behind those effects."

The team has now explained the unusual optical and acoustic properties of their material by showing that the foreign titanium atoms disturb the iron atoms in the original lattice, shifting them to a different oxidation state giving rise to a Jahn-Teller sublattice.

"When a trivalent iron ion gets replaced by the smaller quadrivalent titanium ion, this distorts the lattice and violates electrical neutrality," explains terahertz team leader Boris Gorshunov, "But the electrical neutrality has to persist somehow, it's a fundamental rule." He adds that this leads to some of the neighboring iron atoms becoming divalent to offset the charge of the titanium ions. The emergence of the Jahn-Teller sublattice in the crystal allows the material to be re-magnetized by terahertz radiation.