"We were able to take the theory and, through targeted synthesis and measurement, prove that the FeTiO3 has both weak ferromagnetism and ferroelectricity, just as predicted," Argonne scientist John Mitchell said. Researchers from Argonne's materials science division and its Center for Nanoscale Materials, along with colleagues from Pennsylvania State University, the University of Chicago and Cornell University, used piezoresponse force microscopy, optical second harmonic generation, and magnetometry to show ferroelectricity at and below room temperature and weak ferromagnetism below 120 K for polycrystalline FeTiO3 synthesized at high pressure.
Through targeted synthesis and measurement, Argonne and university researchers have proven that the FeTiO3 material they produced has both weak ferromagnetism and ferroelectricity. (Source: Argonne National Laboratory)
Multiferroic materials show both magnetism and polar order, which are seemingly contradictory properties. Magnetic ferroelectrics may have applications in memory, sensors, actuators and other multifunctional devices by acting as magnetic switches when their electrical fields are reversed.
"The key to this was not the synthesis, the prediction, or measurement, but the fact that all of these things were sort of together in one place at the right time," Mitchell said. "Craig had been working here on these materials trying to predict their properties and structures for quite some time. By using what I call high-performance thinking and high-performance computing, he predicted that this particular structural polymorph of FeTiO3 would have the right atoms in the exactly right place to exhibit the coexistence of weak ferromagnetism and ordered ferromagnetic components in a same single phase as ferroelectricity." Mitchell added that everything Fennie predicted was "almost embarrassingly precise" in terms of the quantitative prediction of the size of the moment and the temperatures at which these things should occur. "This is a real triumph for what I call materials by design," he added.
The main advantage that this development has over other single-phase multiferroics is that there is an ordered moment that is not an anti-ferromagnet; it actually has a magnetic polarization. This could be used to switch magnetization with an electric field in devices or fundamental physics. For example, it would be possible to write a bit electrically and read it magnetically, and switch it electrically rather than using currents or magnetic fields that may cause heat or stray fields. Such a capability would use voltage switching of a memory state.
There are still many open questions about the new material. "The first big stumbling block is that it must be made using high-pressure synthesis," Mitchell said. "If it eventually shows promise, we'll have to figure out how to make it in some sort of thin film form — no small task. Another thing that remains a technological, as well as fundamental, open question is whether or not this coupling that Craig predicts really happens in real life."
The prediction was that there is a coexistence of the ferromagnetic and ferroelectric components. Using the polycrystalline samples produced, the researchers were unable at this time to definitively show the coupling. This requires an aligned structure, like a film or a crystal. Subsequently, they made some progress in getting crystals and are working on proving that coupling; however, this is something yet to be demonstrated from a technological standpoint.
If the coupling is proven, it could potentially open up the way for multiferroic memories, or device structures like TMR cells or exchange couplings could be tuned in this sort of structure, possibly leading to multistate memory elements in multiferroics. "I don't think that this is the case with this material," Mitchell said. "To have a multistate memory element you must be able to individually address the magnetic and ferroelectric polarizations, but in the material they're coupled. But the idea of being able to independently write and read using electrical switching and magnetic sensing is a huge opportunity."
The next step is to demonstrate definitively that Fennie's prediction of switchability is correct. At this point in the research, it would be extremely surprising if it were to be incorrect, but it still must be proven. "We need to engage some film growers to try to make this stuff in thin-film structure — that is the necessary step before attempting to go into device structure," Mitchell said. "This could be the first step in a road leading toward new device structures. We were able to develop this by this high-pressure surface as a predicted structure — it cannot be the only one of its kind, there must be others, and this materials-by-design paradigm is just coming into its own. It's a step forward to finding others that might be more amenable to production-line needs." |
