Neutrons, possessing a magnetic dipole moment, are highly sensitive to the magnetic fields generated by unpaired electrons within materials. This makes neutron-based techniques exceptionally effective in probing atomic-scale magnetic structures. The latest study on layered perovskites underscores both the importance of fundamental research in technological innovation and the effectiveness of neutron-based analysis.
A recent breakthrough has revealed the spiral magnetic structure of YBaCuFeO5, a layered perovskite, finally clarifying the origin of its unique magneto-electric properties observable at room temperature. The experiments, conducted entirely at the Institut Laue-Langevin (ILL), utilized five advanced neutron instruments from its extensive suite of over 40, alongside state-of-the-art sample environment technologies.
"This study removed essential ambiguities, covering the gap created by the lack of single-crystal investigations," stated J. Alberto Rodriguez-Velamazan, ILL researcher and D3 instrument specialist. He further emphasized, "All the study was done with neutrons alone, relying on the combination of different diffraction techniques and capabilities available at the ILL."
A key feature in some multiferroics is the strong coupling between electric and magnetic properties. In such cases, the alignment of magnetic moments directly influences charge distribution. One of the most prominent examples of this coupling is spiral magnetic order, where neighboring spins arrange in a helical pattern, thereby generating electric dipoles.
This intertwined behavior allows manipulation of magnetic properties using electric fields and vice versa. This makes multiferroic materials ideal candidates for energy-efficient data storage and spin-based electronic devices, as electric fields require significantly less energy than magnetic fields for switching operations. Moreover, these materials exhibit enhanced stability and miniaturization potential, as they are less susceptible to external magnetic perturbations.
A theoretical mechanism, termed 'spiral order by disorder,' was proposed to explain the unusual thermal stability of the presumed spiral magnetic structure in these materials. However, previous investigations using powder neutron diffraction on polycrystalline samples could not definitively confirm whether the magnetic order was truly spiral or merely sinusoidal. The latter would not support ferroelectric properties, raising critical questions about the underlying physical principles at play.
The newly published study in Communications Materials has resolved this uncertainty through two key advancements.
First, researchers transitioned from polycrystalline samples to high-quality single crystals. These crystals, synthesized and characterized at the Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC) in Spain, were analyzed extensively using neutron scattering at ILL. The Orient Express instrument was employed to verify crystal quality and orientation, while the Laue diffractometer Cyclops provided full spatial mapping at cryogenic temperatures. The most promising samples were then further examined using the monochromatic diffractometers D10 and D9.
Second, the study employed polarised neutrons, a crucial innovation that enabled precise identification of magnetic structures. By utilizing spherical neutron polarimetry (SNP) at the hot neutron diffractometer D3, the researchers were able to definitively establish the presence of spiral magnetic order. Further experiments explored the magnetoelectric response under applied electric fields.
"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," concluded Rodriguez-Velamazan.
Research Report:Evidence of high-temperature magnetic spiral in YBaCuFeO5 single-crystal by spherical neutron polarimetry
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