Discovery of coherent spin excitons challenges traditional magnetism in nickel molybdate

In a recent study published in Nature Communications, physicists from Rice University and their collaborators have made a fascinating discovery regarding spin excitations in magnets. Typically, when electron spins in a magnet are perturbed, it results in the formation of spin waves that propagate through the magnet like ripples on a pond. However, in this study, the researchers observed a different type of excitation called spin excitons that behave as coherent waves in a nickel-based magnet.

The scientists focused their investigation on a layered magnetic crystal called nickel molybdate. Electrons, which can be thought of as tiny magnets, usually align their spins like compass needles in response to magnetic fields. However, the researchers found that the two outermost electrons of each nickel ion in the crystal exhibited an intriguing behavior. Instead of aligning their spins, the two electrons canceled each other out, creating a spin singlet.

According to Pengcheng Dai, the corresponding author of the study, such a substance should not exhibit magnetism at all, and any excitations generated by scattering neutrons off the nickel ions should remain localized and not propagate through the sample. However, contrary to their expectations, the neutron-scattering experiments revealed the presence of two distinct families of propagating waves, each with vastly different energies.

To comprehend the origins of these waves, the researchers delved into the atomic details of the magnetic crystals. They found that electromagnetic forces from atoms in the crystal could compete with the magnetic field and influence the behavior of electrons in neighboring atoms. This effect, known as the crystal field effect, caused the spins of the electrons to align in directions different from that of the magnetic field. Unraveling the crystal field effects in nickel molybdate required additional experiments and theoretical interpretations.

Emilia Morosan, a co-author of the study, emphasized the importance of collaboration between experimental and theoretical groups in understanding the peculiar spin excitations observed in this compound. Morosan’s team conducted specific heat measurements to investigate the thermal response of the crystals to temperature changes. Based on their experiments, the researchers concluded that two distinct crystal field environments existed in the layered nickel molybdate, affecting the nickel ions in different ways.

This study sheds light on the complex behavior of electron spins in magnets and uncovers the existence of spin excitons as coherent wave-like excitations in nickel molybdate. The findings highlight the significance of combining experimental observations with theoretical analyses to gain a comprehensive understanding of such unique phenomena.

Andriy Nevidomskyy, a theoretical physicist at Rice University and co-author of the study, explained that there are two types of crystal field effects observed in the nickel molybdate crystals. The first type is relatively weak, corresponding to a thermal energy of around 10 Kelvin. At temperatures in the range of a few Kelvin, it is expected that neutrons can excite magnetic spin waves from nickel atoms affected by this type of crystal field effect. However, the second type of crystal field effect is much stronger, around 20 times stronger, due to a tetrahedral arrangement of oxygens around the nickel atoms. This makes it more difficult to create excitations from the nickel atoms subject to this effect, requiring more energetic neutrons.

Nevidomskyy used an analogy of heavy basketballs and intermixed tennis balls to explain the situation. The spins on the nickel ions subject to the second type of crystal field effect can be thought of as heavy basketballs, while the surrounding magnetic excitons (affected by the first type of effect) can be thought of as lighter tennis balls. To excite the spins of the heavier basketballs, a stronger “kick” is required, analogous to using more energetic neutrons. The resulting effect, called a spin exciton, is typically expected to be confined to a single atom. However, measurements from the experiments indicated that the basketballs (heavier spin excitons) were moving collectively, forming an unexpected type of wave. Moreover, these waves persisted at relatively high temperatures where the crystals no longer exhibited magnetic behavior.

The explanation provided by Nevidomskyy and co-author Leon Balents, a theorist from the University of California, Santa Barbara, is that the heavier spin excitons respond to the fluctuations of the surrounding lighter magnetic excitons. If the interactions between these two types of excitations are strong enough, the heavier spin excitons participate in coherent motion resembling a wave.

One intriguing aspect highlighted by Pengcheng Dai, another co-author of the study, is that the two types of nickel atoms form a triangular lattice, and the magnetic interactions within this lattice lead to frustration. In magnetism on triangular lattices, frustration arises from the challenge of aligning all the magnetic moments anti-parallel (up-down) with respect to their three immediate nearest neighbors.

Dai, Emilia Morosan, and Andriy Nevidomskyy are members of the Rice Quantum Initiative, with Dai being the Sam and Helen Worden Professor of Physics and Astronomy, Morosan a professor of physics and astronomy and of chemistry, and Nevidomskyy an associate professor of physics and astronomy. The neutron scattering experiments were conducted by Bin Gao and Tong Chen in Dai’s group, in collaboration with instrument scientists at Oak Ridge National Laboratory and ISIS Neutron and Muon Source at Rutherford Appleton Laboratory. The specific heat measurements and analysis were performed by Chien-Lung Huang, a research scientist in Morosan’s group.

Source: Rice University

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