Act in Concert: PANDA complements PUMA

PUMA-PANDA PUMA-PANDA-results © B. Keimer, MPI

PUMA-PANDA-results © B. Keimer, MPI

Recently the Solid State Spectroscopy group in Max-Planck Institute (Stuttgart, led by Prof. Bernhard Keimer) published in Physical Review Letters their results of inelastic neutron scattering on stoichiometric Sr₃Fe₂O₇ under collaboration with scientists at PANDA and PUMA. The research paper shows a good example of where experiments both at PANDA and PUMA equally contribute to a common research.

The bilayer pervoskite Sr₃Fe₂O₇, which shows a spiral arrangement of spins below 115 K and a metal-insulator transition at 340 K, is a subject for long-standing theoretical debates about its origin of noncollinear magnetic order. One scenario is a competition between superexchange (antiferromagnetic) interaction and double-exchange (ferromagnetic) interaction, which can lead to the formation of spiral magnetism. The other theory predicts that a sole double-exchange interaction can stabilize a spiral state of magnetic order. To clarify this issue, one needs to map out spin-wave dispersion in the order state, then extract precise values of spin-spin interaction using a plausible model for given spectrum. Since the spin-wave dispersion in the transition-metal oxide Sr₃Fe₂O₇spans from very low (close to zero) to a few tens of milli-electron volts (meV), several experiments have been done at PANDA for low-energy (0 – 6 meV) and PUMA for high-energy (3 – 16 meV) ranges to clearly show full spin-excitation spectrum (see the figure). By modelling the obtained magnon dispersion, authors have proven that the sizes of superexchange and double-exchange interaction is quite comparable, which confirms a decades-old theoretical prediction of competing exchange interactions near a boundary of metal-insulator transition in Sr₃Fe₂O₇. In addition, such clean inelastic neutron scattering data could also be served as a model spectrum hosting magnetically disordered systems, where multiple exchange interactions are competing in a similar scale. Without combining strength of PANDA and PUMA spectrometers, it would not be possible to describe spin dynamics in Sr₃Fe₂O₇at such quantitative level.
In spite of sharing the same technique for inelastic neutron scattering measurement at PANDA (cold neutron) and PUMA (thermal neutron), these three-ax spectrometers differ in area of energy and momentum transfer they can cover. Thus, a subject of study at each spectrometer usually varies depending on a characteristic energy scale in given systems, for instance, frustrated magnets at PANDA or high-Tc superconductor at PUMA. Yet, sometimes it requires use of both instruments to cover from low- to high-energy transfer range.