59th Annual Report on Research 2014

            The concluding fiscal year of 2013-2014 of our ACS Petroleum Fund grant has resulted in many insights to the multiferroic dugganites.  We have had several new publications, an invited talk at the University of California Santa Barbara, and have trained many students within this timeframe.   The work has also led to numerous additional research avenues which are now being explored.                    

            As noted in the report from the last fiscal year, the dugganites, with chemical formula A₃BC₃D₂O₁₄, were explored as potential multiferroics with the series Pb₃TeA₃X₂O₁₄ (A= Co, Mn; X = P, V).   The magnetic properties of these compounds are still being elucidated.  We now have an understanding of the role that structure plays in the multiferroic properties.  Large distortions of the unit cells result in complicated magnetically ordered phases at low temperatures.  Through physical property measurements, including low temperature magnetometry, heat capacity, and neutron scattering experiments, we now have a deeper understanding of the multiferroic properties and their origins.  The next step in this process is to use this information to raise the transition temperature for technological devices.

Figure 1:  Unit cell of the compound Pb₃TeMn₃P₂O₁₄.  The old unit cell has been denoted (of P321 symmetry) in the upper left hand corner.  The new unit cell is much larger and of P3 symmetry.

Figure 2: Magnetic phase diagram of Pb₃TeMn₃P₂O₁₄.  PM denotes the paramagnetic phase, while AFM and AFM II denote two distinct antiferromagnetic phases.

            Significant progress has been made across the series of compounds, but perhaps the greatest breakthrough is with Pb₃TeMn₃P₂O₁₄, which orders in a large magnetic unit cell at T_N ~ 6.9 K (similar to the multiferroic Ba₃NbFe₃Si₂O₁₄).  In addition, there is a large pseudohexagonal incommensurate supercell that develops which strongly affects the magnetism.  Detailed physical property measurements, neutron and x-ray scattering experiments, and analysis have been completed on this compound.  The large supercell has been finally elucidated (see figure 1) through very careful diffraction work.  We have also mapped out the high field phase diagram using the resources at the National High Magnetic Field Laboratory in Tallahassee, Florida.

Figure 3:  Magnetic structure of Pb₃TeMn₃P₂O₁₄ in the AFM phase

            Studies are continuing in the series of dugganites with the Cu²⁺ species (which being S=1/2 should have exotic magnetic properties due to enhanced quantum fluctuations).   Considerable progress has been made towards this goal using the Mn species as a template, which will facilitate the future synthesis of new members of this series.  We now believe that we have an understanding of the reaction pathway for the dugganites, which should facilitate the discovery of other compounds in the future.