Red arrows represent electron spin orientations in a portion of the YbMgGaO4 crystal structure, where antiferromagnetic interactions between groups of magnetic moments cause neighboring spins to align anti-parallel to one another. This mechanism is partially responsible for the quantum spin liquid behavior observed in the neutron scattering data, illustrated on the hexagonal tiles.Credit: ORNL/Jill Hemman
Neutron scattering studies of a rare earth metal oxide have identified fundamental pieces to the quantum spin liquid puzzle, revealing a better understanding of how and why the magnetic moments within these materials exhibit exotic behaviors such as failing to freeze into an ordered arrangement even near absolute zero temperatures.
In a paper published in Nature Physics, a team of researchers from the Georgia Institute of Technology, the University of Tennessee and the Department of Energy’s Oak Ridge National Laboratory used neutrons to examine the origins of unusual magnetic behavior in a rare earth-based metal oxide, ytterbium-magnesium-gallium-tetraoxide (YbMgGaO4). The material, discovered in 2015, is known to have strange magnetic properties, putting it in a unique category of materials classified as quantum spin liquids.
“A quantum spin liquid is an exotic state of matter characterized by the entanglement of particles over long distances across the atomic scale,” said lead investigator Martin Mourigal, an assistant physics professor at the Georgia Institute of Technology.
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Think of Schrödinger’s cat, the thought experiment, he said: Many particles participate in a quantum superposition, where multiple quantum states combine to form a new quantum state, and cannot be characterized by the behavior of individual particles.
By definition, he said, “it’s something we can’t explain with classical physics.”
In a series of experiments at ORNL’s Spallation Neutron Source, the researchers revealed three key features underpinning the material’s exotic properties:
antiferromagnetic interactions, where groups of electron spins have an antiparallel alignment with their respective neighbors; spin space anisotropy, meaning that individual magnetic moments strongly prefer aligning themselves alongside specific directions in the material; and chemical disorder between the material’s magnetic layers that randomizes the interactions between electron spins.
Neutrons are well suited for studying magnetism because their lack of electric charge allows them to penetrate through materials, even when the neutrons’ energy is low. The neutrons also have magnetic moments, allowing researchers to directly probe the behavior of spins within materials.
“Neutron scattering is the only technique that allows us to study the dynamics of quantum spin liquids at the lowest temperatures,” Mourigal said.
However, quantum spin liquids present a challenge because their magnetic moments are constantly changing. In typical materials, researchers can lock the spins into certain symmetric patterns by lowering the temperature of the sample, but this approach doesn’t work on spin liquids.
In the team’s first neutron scattering measurements of an YbMgGaO4 single-crystal sample at the SNS’s Cold Neutron Chopper Spectrometer, CNCS, the researchers observed that, even at a temperature of 0.06 kelvins (approximately negative 460 degrees Fahrenheit), magnetic excitations remained disordered or “fuzzy.” This fluctuating magnetic behavior, known to occur to quantum spin liquids, runs counter to the laws of classical physics.
“The material screamed spin liquid when we put it in the beam,” Mourigal said.
To overcome this fuzziness, the team used an 8 Tesla magnet to create a magnetic field that locked the spins into an ordered and partly frozen arrangement, allowing for better measurements.
“Once we applied the magnetic field, we were able to measure coherent magnetic excitations in the material that propagate sort of like sound waves,” said CNCS instrument scientist Georg Ehlers. “When a neutron comes into the material, it flies by a magnetic moment and shakes it. The nearby magnetic moments see this happening, and they all begin to vibrate in unison. The frequency of these vibrations is determined by the energy between neighboring spins.”
Those magnetic field measurements enabled the team to directly validate theoretical expectations and provided a physical understanding of the spin behavior and the system as a whole.
“A quantum spin liquid is an intrinsically collective state of matter,” said Mourigal. “But if you want to understand the society, you need to understand the individuals as well.”
The team then turned to another SNS instrument, the Fine-Resolution Fermi Chopper Spectrometer instrument, SEQUOIA, to understand the individual properties of the magnetic moments.
“In rare earth magnets, rich physics, like what was observed at the CNCS instrument, can emerge from the fact that the individual spins can prefer to point along certain directions in a crystal,” said SEQUOIA instrument scientist Matthew Stone. “SEQUOIA examined the localized higher energy states to confirm the individual pieces of the model used to describe the CNCS data were correct.”
Mourigal says the information gleaned from the experiments will enable researchers to develop better theoretical models to further study these quantum phenomena.
“While the exact nature of the quantum state hosted by this material has not been fully established yet, we’ve discovered that chemical disorder and other effects are important here,” said Mourigal. “With these experiments, we’ve really been able to nail down what ingredients need to be taken into the recipe for a quantum spin liquid in this material.”
Joseph A. M. Paddison, Marcus Daum, Zhiling Dun, Georg Ehlers, Yaohua Liu, Matthew B. Stone, Haidong Zhou, Martin Mourigal. Continuous excitations of the triangular-lattice quantum spin liquid YbMgGaO4. Nature Physics, 2016; DOI: 10.1038/nphys3971