![]() ![]() Nevidomskyy said this can be understood as if the spins on the corresponding nickel ions had different “mass.” Those atoms have a tetrahedral arrangement of oxygens around them, and the electric field effect is nearly 20-fold stronger, meaning the excitations are that much harder to create.” ![]() But it is most puzzling to see them coming from nickel atoms that are subject to the second type. “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. “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. From those experiments, the researchers concluded that two kinds of crystal field environments occurred in the layered nickel molybdate, and the two affected nickel ions very differently. Morosan’s group probed the thermal response of the crystals to changes in temperature using specific heat measurements. “The collaboration between experimental groups and theory is paramount to painting a full picture and understanding the unusual spin excitations observed in this compound,” said Rice co-author Emilia Morosan. Probing crystal field effects in the nickel molybdate crystals required additional experiments and theoretical interpretation of the data from the experiments. 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. For instance, electromagnetic forces from atoms in crystals can compete with the magnetic field and affect electrons inside neighboring atoms. To understand the waves’ origins, it was necessary to delve into the atomic details of the magnetic crystals. Credit: Jeff Fitlow/Rice Universityĭai and his collaborators were therefore surprised when instruments in the neutron-scattering experiments detected not one, but two families of propagating waves, each at dramatically different energies. Pengcheng Dai is the Sam and Helen Worden Professor of Physics and Astronomy at Rice University. ![]() In a new study, Rice University physicists and their collaborators have discovered dramatically different excitations called “spin excitons” that can also “ripple” through a nickel-based magnet as a coherent wave. Perturbing electron spins in a magnet usually results in excitations called “spin waves” that ripple through the magnet like waves on a pond that’s been struck by a pebble. 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 lattices. Neutron scattering reveals coherent waves of ‘spin excitons’ in nickelate crystal. Left and right halves of the panel show different model calculations of these patterns. (Right panel) 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. (Left panel) 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. ![]()
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