Neutron scattering reveals coherent waves of ‘spin excitons’ in nickelate crystal. Rice University physicists discovered 'spin excitons' in nickel molybdate crystals, a new type of magnetic excitation that can propagate as coherent waves, offering insight into magnetic frustrations in triangular
In nickel molybdate crystals made of two parts nickel, three parts molybdenum and eight parts oxygen, nickel ions are subject to both tetrahedral and octahedral crystalline environments, and the ions are locked in triangular lattices in each environment. Crystal electric field spin excitons from tetrahedral sites in nickel molybdate crystals form a dispersive, diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.
“Such a substance should not be a magnet at all,” said Rice’s Pengcheng Dai, corresponding author of the study. “And if a neutron scatters off a given nickel ion, the excitations should remain local and not propagate through the sample.” To understand the waves’ origins, it was necessary to delve into the atomic details of the magnetic crystals. For instance, electromagnetic forces from atoms in crystals can compete with the magnetic field and affect electrons inside neighboring atoms. This is called the crystal field effect, and it can force electron spins to orient themselves along directions distinct from the orientation of the magnetic field.
“In one, the field effect is rather weak and corresponds to a thermal energy of about 10 Kelvin,” said study co-author Andriy Nevidomskyy, a theoretical physicist at Rice who helped interpret the experimental data. “It is perhaps not surprising to see, at few-Kelvin temperatures, that neutrons can excite magnetic spin waves from nickel atoms that are subject to this first type of crystal field effect.
The resulting effect on the nickel spin is called a spin exciton, and one would normally expect the effect of the exciton-producing “kick” to be confined to a single. But measurements from the experiments indicated “basketballs” were moving in unison, creating an unexpected sort of wave. Even more surprising, the waves appeared to persist at relatively high temperatures where the crystals no longer behaved as magnets.
In magnetism on triangular lattices, frustration refers to the difficulty in aligning all the magnetic moments anti-parallel with respect to their three immediate, nearest neighbors.
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