Proportionality between the magnetic exchange energy and Néel temperature in pyrochlore ruthenates*

Pyrochlore oxides provide a rich platform for exploring diverse states of matter due to the unique crystal structure and chemical versatility. They are one of the most widely known systems to inherently host magnetic frustrations, setting the ground for exotic quantum states and diverse magnetic structures. Specifically, pyrochlore ruthenates, a family of strongly correlated 4d materials, houses a wide range of interaction between orbital, spin, and lattice degrees of freedom. Rare earth pyrochlore ruthenates are generally known to be spin-1 Mott insulators that display antiferromagnetic ordering of Ru ions below the Néel temperature (T_N), with T_N strongly depending on the size of the rare earth ions. Also, they have a linear relation between rare earth ionic radii, lattice constant, Ru-O-Ru bond angle, bond length, and Ru 4d t_2g bandwidth which is believed to be the origin of the systematic metal to insulator transition shown throughout these compounds.

In our study, a series of polycrystalline pyrochlore powder samples RE₂Ru₂O₇ (RE= Y, Nd, Sm, Eu, Ho, and Er) was synthesized and temperature dependent Raman spectroscopy measurements were performed. Our results concisely show a linear correlation between the one-magnon excitation energy within range of 15 - 35 meV and T_N of each compound, varying from T_N = 80K - 150 K. This strongly suggests the system is predominantly dependent on the superexchange interaction. This indicates that we have found a family of compounds in which we can freely tune the magnon energy. Therefore, changing the A site components only tunes the magnetic exchange energies while everything else is the same.