ÌÇÐÄÊÓƵ

Multiferroic materials, in which electric and magnetic properties are combined in promising ways, will be the heart of new solutions for data storage, data transmission, and quantum computers. Meanwhile, understanding the origin of such properties at a fundamental level is key for developing applications, and neutrons are the ideal probe.

Neutrons possess a magnetic dipole moment which makes them sensitive to magnetic fields generated by unpaired electrons in materials. This makes neutron scattering techniques a powerful tool to probe the magnetic behavior of materials at atomic level.

The story of the so-called layered perovskites and the breakthrough results now published are a paradigmatic example highlighting both the role of fundamental studies in the development of applications and of the power of neutrons. Being a promising class of materials exhibiting coupled magnetic and electric ordering properties at ambient temperatures, the magnetic structure of the layered perovskites YBaCuFeO₅—and thus the origin of their interesting magneto-electric behavior—was still to be unambiguously determined.

The results, now in Communications Materials, pinpoint the spiral magnetic structure of these materials, finally establishing the common origin of its promising magnetic and electric properties up to room temperatures. The experiments were fully conducted at the ILL, using five instruments out of a state-of-the-art suite of over 40, and taking advantage of advanced sample environment technologies.

"This study removed essential ambiguities, covering the gap created by the lack of single-crystal investigations," J. Alberto Rodríguez-Velamazán, ILL researcher and D3 instrument responsible. "All the study was done with neutrons alone, relying on the combination of different diffraction techniques and capabilities available at the ILL."

Tiny spiraling magnets

Magneto-electric multiferroics are materials where electric and magnetic orders coexist. The combination of ferroelectricity (characterized by a net electrical polarization) and long-range magnetic order (due to the alignment of magnetic moments arising from non-coupled electron spins) is highly sought after from a technological perspective.

In some multiferroics, electric and magnetic properties are strongly coupled: the alignment of the magnetic moments induces the charge separation. A well-established case of strongly coupled electric and magnetic order is spiral magnetic order—neighboring spins arrange themselves in a spiral pattern, which in turn is able to create electric dipoles.

Coupled magnetic and electric orders make it possible to act on the magnetic properties using an electric field, and to act on the electric properties using a magnetic field. Coupled multiferroics are thus promising materials to design new devices.

In particular, using an electric (rather than magnetic) field to act on the magnetic order—for example, to change the state of a bit in a storage device, or to manipulate spin states—is much less energy consuming. Moreover, such materials are usually less volatile (less perturbed by external magnetic fields), which increases stability in devices and allows for further miniaturization.

Spiral multiferroics are scarce. In fact, rather severe constrains on the symmetry and geometry of the material's microscopic structure are imposed for such peculiar properties to arise. In most multiferroic materials, the characteristic ordering only subsists at very low temperatures. In practice, this strongly limits the possibility of implementation in devices.

Keeping 'cool' at high temperatures: Unveiling perovskite mysteries with neutrons

Layered perovskites (RBaCuFeO₅) are a rare case exhibiting coupled magnetic and electric ordering properties at ambient temperatures, and thus a promising class of materials for applications. While their multiferroic behavior at high temperatures was well established, the underlying magnetic structure—and thus the origin of their interesting magneto-electric behavior—was still to be unambiguously determined.

In fact, a non-conventional mechanism (named "spiral order by disorder") was devised that could account for the extraordinary thermal stability of their presumed spiral magnetic order. Nevertheless, there was no conclusive data supporting the existence of spiral order in these materials.

Indeed, the available results, obtained with polycrystalline samples using powder neutron diffraction measurements, were compatible with spiral order but also with sinusoidal spin modulation—an arrangement that would not give rise to ferroelectricity. A study able to disentangle the two possibilities was still lacking.

While the interesting observed macroscopic properties of the material would still be there, the absence of spiral order would mean we did not really understand their microscopic origin, as the existing explanation for what was really happening in the material wouldn't hold—certainly not a good starting point for developing applications based on this material.

The study now in Communications Materials filled in this gap, essentially by taking two very important steps forward.

The first important step was to go from a polycrystalline powder sample to high-quality single crystals. The crystals were grown and characterized at Institut de Ciència de Materials de Barcelona (ICMAB-CSIC, Spain). Their magnetic structure was then extensively analyzed with neutrons at the ILL.

Instrument Orient Express was used to take snapshots of the crystal, allowing to assess its quality and orientation. The Laue diffractometer Cyclops then extended these measurements to cryogenic temperatures and quickly surveyed the full reciprocal space, which allowed researchers to select the most promising sample for the further monochromatic experiments. Extensive measurements where then performed with the monochromatic single-crystal diffractometers D10 and D9.

The second decisive step was the use of polarized neutrons. Indeed, the possibility of producing beams of polarized neutrons (with all their spins parallel) permits to pinpoint magnetic information much more precisely, facilitating the deciphering of complex magnetic structures. Spherical neutron polarimetry (SNP) experiments were conducted at the hot neutron diffractometer D3. The magnetoelectric response was explored by means of an electrical field.

"Our findings not only confirm that the magnetic order in our crystal is spiral, but also demonstrate that cationic disorder is responsible for stabilizing this spiral structure. This insight extends to samples of the perovskite family, where similar ordering has been observed well above room temperature in powder samples," concludes Rodríguez-Velamazán.