Understanding the atomic effect of magnetic interactions in perovskite materials
Perovskites are a class of compounds which have the same crystal structure as calcium titanate, in which different cations can be embedded, imparting properties such as superconductivity, magnetoresistance, ionic conductivity, and a multitude of dielectric properties, leading to the development of diverse engineering materials.
In this research, scientists from the Department of Materials Science and Engineering were studying the magnetic interactions within the structure of the double perovskite crystals found in a compound made up of barium, manganese, tungsten and oxygen containing either tellurium or tungsten ions to see how the configuration of the electrons in the ions could affect the magnetic structure of the overall material.
By using a range of investigative techniques available at the ISIS Neutron and Muon Source in Oxfordshire, including neutron diffraction, inelastic neutron scattering, muon spectroscopy and magnetic susceptibility, the research group led by Dr Eddie Cussen built up a comprehensive picture of the structure and magnetic behaviour of these materials. They found that although the two materials Ba2MnTeO6 and Ba2MnWO6 are isostructural, and have almost identical structures, exchanging the tellurium ion for tungsten has a dramatic effect on the magnetic behaviour. This comes down to their positions in the periodic table. As tellurium is in group 16, in its Te6+ form it has ten electrons occupying its outer d-orbitals, and can therefore be described as d10. Tungsten, however, is in group 6, leading to its W6+ form taking the configuration d0.
Dr Cussen comments, "We used the advanced diffraction facilities at ISIS to establish that the same crystal symmetry exists in the magnetic systems for two perovskite materials. This geometrically cancelled out many of the stronger magnetic effects and allowed the impact of the empty or full d orbitals of Te and W to take centre stage and 'choose' the most stable magnetic structure."
The researchers found that, in Ba2MnTeO6 at low temperature, the manganese ions (Mn2+) are arranged in a cubic structure, forming a Type I magnetic structure. Above 20 K, the group's inelastic neutron scattering and muon spectroscopy experiments showed that the material undergoes a transition to form a short-range correlated magnetic state similar to that seen in MnO.
However, when studying Ba2MnWO6, they found that it takes the Type II structure. This is likely to be because the empty d orbitals on the W6+ ion can interact with the orbitals on the oxygen ions, creating a mechanism for extending superexchange through the structure and allowing magnetic interactions between next-nearest Mn2+ cations to dominate. Inelastic neutron scattering was used to determine the magnetic interactions in the materials.
The group's studies not only inform the study of these perovskites, but the influence of the empty d orbitals could also be occurring in other magnetic systems, says Dr Cussen; “Many electronic applications such as sensing, data storage and information processing rely on exploiting transitions between states. The facilities at ISIS have allowed us to quantify the impact of chemical control arising from non-magnetic ions. We can use this as a tool to tweak materials composition so that they lie on the cusp of transition and may be switchable into new states."
He adds; “This work relies on materials synthesis, crystal structure measurements, heat capacity, magnetometry as well as multiple neutron scattering and muon relaxation measurements. The facilities at ISIS are key to the experiments and this work relied on a diverse team. The resources provided at ISIS: neutrons, muons and the materials characterisation lab, are key to almost every aspect of the work."
The details of the research can be found in:
Physical Review Materials characterising Ba2MnWO6, available at DOI: 10.1103/PhysRevMaterials.4.014408
Chemistry of Materials characterising Ba2MnTeO6, available at DOI: 10.1021/acs.chemmater.0c02971
Article adapted from https://www.isis.stfc.ac.uk/Pages/SH20_dorbital_occupation.aspx, acknowledging the work of Rosie de Laune
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