A Magnetic Compass Inside a Crystal express
The tiny magnetic arrows inside a crystal of terbium-iridium oxide behave strangely: instead of lying along their preferred axis, they stand across it. This state fully forms when cooled to 1.5 Kelvin — colder than outer space.
Researchers probed the crystal with neutrons and observed how these flipped arrows oscillate and exchange energy. Calculations showed that to explain what they saw, pairwise interactions between terbium ions alone are not enough — one must also account for the influence of the iridium sublattice, which acts like an external magnetic field.
Such materials are interesting not only for fundamental physics. Unusual magnetic states could find use in spintronics — electronics where information is carried not by charges but by the directions of magnetic moments.
Original abstract
as written by the authors arXiv:2601.00749
Magnetic-rare-earth pyrochlore iridates exhibit a rich variety of unconventional phases, driven by the complex interactions within and between the rare-earth and the iridium sublattices. In this study, we investigate the peculiar magnetic state of Tb$$_{2}$$Ir$$_{2}$$O$$_{7}$$, where a component of the Tb$$^{3+}$$ moment orders perpendicular to its local Ising anisotropy axis. By means of neutron diffraction and inelastic neutron scattering down to dilution temperatures, complemented by specific heat measurements, we show that this intriguing magnetic state is fully established at 1.5 K and we characterize its excitation spectrum across a broad range of energies. Our calculations reveal that bilinear interactions between Tb$$^{3+}$$ ions subjected to the Ir molecular field capture several key features of the experiments, but need to be supplemented to fully reproduce the observed behavior.
A temperature of 1.5 Kelvin is about minus 271.65 degrees Celsius. At such temperatures, thermal motion nearly freezes, and quantum effects come to the fore.