Exploring the properties of mixed cobalt/iron/manganese-tin clusters

Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO₃ (R=Tb, Dy) systems. We have performed Raman spectroscopy on ¹⁶O and ¹⁸O-substituted TbMnO₃ single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn³⁺ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions are seen. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on ¹⁶O and ¹⁸O-substituted DyMnO₃ powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm. These results indicate that the structure is actively involved in the emergence of ferroelectricity in these materials.