From DOE’s Brookhaven National Laboratory (US) : “Lighting Up Ultrafast Magnetism in a Metal Oxide”

From DOE’s Brookhaven National Laboratory (US)

June 7, 2021

Ariana Manglaviti
amanglaviti@bnl.gov
(631) 344-2347

Peter Genzer
genzer@bnl.gov
(631) 344-3174

Understanding how magnetic correlations change over very short timescales could be harnessed to control magnetism for applications including data storage and superconductivity.

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Scientists struck a crystalline material with ultrafast pulses of laser light and then used x-rays to probe how its magnetic order changes. Image credit: Cameron Dashwood, University College London (UK).

What happens when very short pulses of laser light strike a magnetic material? A large international collaboration led by the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory set out to answer this very question. As they just reported in the PNAS, the laser suppressed magnetic order across the entire material for several picoseconds, or trillionths of a second. Understanding how magnetic correlations change on ultrafast timescales is the first step in being able to control magnetism in application-oriented ways. For example, with such control, we may be able to more quickly write data to memory devices or enhance superconductivity (the phenomenon in which a material conducts electricity without energy loss), which often competes with other states like magnetism.

The material studied was strontium iridium oxide (Sr3Ir2O7), an antiferromagnet with a bilayer crystal structure and a large magnetic anisotropy. In an antiferromagnet, the magnetic moments, or electron spins, align in opposite directions to neighboring spins. Anisotropy means the spins need to pay an energetic cost to rotate in any random direction; they really want to sit pointing upwards or downwards in the crystal structure. The X-ray Scattering Group of Brookhaven Lab’s Condensed Matter Physics and Materials Science (CMPMS) Division has previously studied this material (and a single-layer sister compound, Sr2IrO4), so they entered this study with a good understanding of its equilibrium state.

“The very short laser pulses disturb the system, destroying its magnetic order,” said first author Daniel Mazzone, former group member and now an instrument scientist at the Continuous Angle Multiple Energy Analysis (CAMEA) spectrometer at the Paul Scherrer Institute [Paul Scherrer Institut](PSI) (CH). “In this study, we were interested in seeing how the system relaxes back to its normal state. We knew the relaxation occurs on a very fast timescale, and to take a picture of something that moves very fast, we need very short pulses of illumination. With an x-ray free-electron laser source, we can generate pulses short enough to see the movement of atoms and molecules. Such sources only exist at five places around the world—in the United States, Japan, Korea, Germany, and Switzerland.”

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A schematic of the resonant inelastic x-ray scattering (RIXS) and resonant elastic x-ray scattering (REXS) setups. The square in the middle represents the sample, which is struck with a laser (pump) and then x-rays (probe) almost immediately after. For the RIXS experiments, the team built a motorized x-ray spectrometer (copper-colored circle) to see how spins are behaving locally.

In this study, the team ran experiments at two of the five facilities. At the SPring-8 Angstrom Compact free-electron Laser (SACLA) in Japan, they conducted time-resolved resonant elastic x-ray scattering (tr-REXS).

At the x-ray pump-probe instrument of the Linac Coherent Light Source—a DOE Office of Science User Facility at SLAC National Accelerator Laboratory—the scientists performed time-resolved resonant inelastic x-ray scattering (tr-RIXS).

In both scattering techniques, x-rays (probe) strike the material almost immediately after the laser pulse (pump). By measuring the energy and angle of scattered particles of light (photons), scientists can determine the material’s electronic structure and thus magnetic configuration. In this case, the x-ray energy was tuned to be sensitive to the electrons around iridium atoms, which drive magnetism in this material. While tr-REXS can reveal the degree of long-range magnetic order, tr-RIXS can provide a picture of local magnetic interactions.

“In order to observe the detailed behavior of spins, we need to measure the energy change of the x-rays with very high precision,” explained co-corresponding author Mark Dean, a physicist in the CMPMS Division X-ray Scattering Group. “To do so, we built and installed a motorized x-ray spectrometer at SLAC.”

Their data revealed how magnetic interactions are suppressed not just locally but everywhere. This suppression persists for picoseconds before the magnetic order returns to its initial antiferromagnetic state.

“The bilayer system does not have energetically low-cost ways to deform the magnetic state,” explained Dean. “It gets stuck in this bottleneck where the magnetism is out of equilibrium and is not recovering, at least not as quickly as in the monolayer system.”

“For most applications, such as data storage, you want fast magnetic switching,” added Mazzone. “Our research suggests systems where spins can point whichever direction are better for manipulating magnetism.”

Next, the team plans to look at related materials and hopes to manipulate magnetism in more targeted ways—for example, changing how strongly two neighboring spins “talk to” each other.

“If we can change the distance between two spins and see how that affects their interaction, that would be really cool,” said Mazzone. “With an understanding of how magnetism evolves, we could tweak it, maybe generating new states.”

The complexity of setting up and operating the spectrometer required a large collaboration including former and current Brookhaven X-ray Scattering Group members Daniel Mazzone, Derek Meyers, Yue Cao, Jiaqi Lin, Vivek Thampy, Hu Miao, Tadesse Assefa, John Hill, Ian Robinson, and Xuerong Liu. James Vale, Cameron Dashwood, and Desmond McMorrow of University College London; Diego Casa and Jungho Kim of DOE’s Argonne National Laboratory (US); laser experts Alan Johnson and Roman Mankowsky of the Paul Scherrer Institut, Michael Först of the MPG Institute for the Structure and Dynamics of Matter [MPG Institut für Struktur und Dynamik der Materie] (DE), and Simon Wall of Aarhus University [Aarhus Universitet] (DK); and the beamline teams from SLAC and SACLA were also crucial to the success of the experiments. Theoretical collaborations included Robert Konik of Brookhaven and Neil Robinson and Andrew James, both formerly at Brookhaven.

The other collaborating institutions are Oklahoma State University (US), Chinese Academy of Sciences [中国科学院](CN), The Open University (UK), University of Amsterdam [Universiteit van Amsterdam] (NL), ShanghaiTech University [上海科技大学] (CN), Riken [理研](JP), Barcelona Institute of Science and Technology [Instituto de Ciencia y Tecnología de Barcelona](ES), and University of Tennessee (US).

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One of ten national laboratories overseen and primarily funded by the DOE(US) Office of Science, DOE’s Brookhaven National Laboratory (US) conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University(US), the largest academic user of Laboratory facilities, and Battelle(US), a nonprofit, applied science and technology organization.

Research at BNL specializes in nuclear and high energy physics, energy science and technology, environmental and bioscience, nanoscience and national security. The 5,300 acre campus contains several large research facilities, including the Relativistic Heavy Ion Collider [below] and National Synchrotron Light Source II [below]. Seven Nobel prizes have been awarded for work conducted at Brookhaven lab.

BNL is staffed by approximately 2,750 scientists, engineers, technicians, and support personnel, and hosts 4,000 guest investigators every year. The laboratory has its own police station, fire department, and ZIP code (11973). In total, the lab spans a 5,265-acre (21 km^2) area that is mostly coterminous with the hamlet of Upton, New York. BNL is served by a rail spur operated as-needed by the New York and Atlantic Railway. Co-located with the laboratory is the Upton, New York, forecast office of the National Weather Service.

Major programs

Although originally conceived as a nuclear research facility, Brookhaven Lab’s mission has greatly expanded. Its foci are now:

Nuclear and high-energy physics
Physics and chemistry of materials
Environmental and climate research
Nanomaterials
Energy research
Nonproliferation
Structural biology
Accelerator physics

Operation

Brookhaven National Lab was originally owned by the Atomic Energy Commission(US) and is now owned by that agency’s successor, the United States Department of Energy (DOE). DOE subcontracts the research and operation to universities and research organizations. It is currently operated by Brookhaven Science Associates LLC, which is an equal partnership of Stony Brook University(US) and Battelle Memorial Institute(US). From 1947 to 1998, it was operated by Associated Universities, Inc. (AUI) (US), but AUI lost its contract in the wake of two incidents: a 1994 fire at the facility’s high-beam flux reactor that exposed several workers to radiation and reports in 1997 of a tritium leak into the groundwater of the Long Island Central Pine Barrens on which the facility sits.

Foundations

Following World War II, the US Atomic Energy Commission was created to support government-sponsored peacetime research on atomic energy. The effort to build a nuclear reactor in the American northeast was fostered largely by physicists Isidor Isaac Rabi and Norman Foster Ramsey Jr., who during the war witnessed many of their colleagues at Columbia University leave for new remote research sites following the departure of the Manhattan Project from its campus. Their effort to house this reactor near New York City was rivalled by a similar effort at the Massachusetts Institute of Technology (US) to have a facility near Boston, Massachusettes(US). Involvement was quickly solicited from representatives of northeastern universities to the south and west of New York City such that this city would be at their geographic center. In March 1946 a nonprofit corporation was established that consisted of representatives from nine major research universities — Columbia University(US), Cornell University(US), Harvard University(US), Johns Hopkins University(US), Massachusetts Institute of Technology(US), Princeton University(US), University of Pennsylvania(US), University of Rochester(US), and Yale University(US).

Out of 17 considered sites in the Boston-Washington corridor, Camp Upton on Long Island was eventually chosen as the most suitable in consideration of space, transportation, and availability. The camp had been a training center from the US Army during both World War I and World War II. After the latter war, Camp Upton was deemed no longer necessary and became available for reuse. A plan was conceived to convert the military camp into a research facility.

On March 21, 1947, the Camp Upton site was officially transferred from the U.S. War Department to the new U.S. Atomic Energy Commission (AEC), predecessor to the U.S. Department of Energy (DOE).

Research and facilities

Reactor history

In 1947 construction began on the first nuclear reactor at Brookhaven, the Brookhaven Graphite Research Reactor. This reactor, which opened in 1950, was the first reactor to be constructed in the United States after World War II. The High Flux Beam Reactor operated from 1965 to 1999. In 1959 Brookhaven built the first US reactor specifically tailored to medical research, the Brookhaven Medical Research Reactor, which operated until 2000.

Accelerator history

In 1952 Brookhaven began using its first particle accelerator, the Cosmotron. At the time the Cosmotron was the world’s highest energy accelerator, being the first to impart more than 1 GeV of energy to a particle.


The Cosmotron was retired in 1966, after it was superseded in 1960 by the new Alternating Gradient Synchrotron (AGS).

The AGS was used in research that resulted in 3 Nobel prizes, including the discovery of the muon neutrino, the charm quark, and CP violation.

In 1970 in BNL started the ISABELLE project to develop and build two proton intersecting storage rings.

The groundbreaking for the project was in October 1978. In 1981, with the tunnel for the accelerator already excavated, problems with the superconducting magnets needed for the ISABELLE accelerator brought the project to a halt, and the project was eventually cancelled in 1983.

The National Synchrotron Light Source (US) operated from 1982 to 2014 and was involved with two Nobel Prize-winning discoveries. It has since been replaced by the National Synchrotron Light Source II (US) [below].

After ISABELLE’S cancellation, physicist at BNL proposed that the excavated tunnel and parts of the magnet assembly be used in another accelerator. In 1984 the first proposal for the accelerator now known as the Relativistic Heavy Ion Collider (RHIC)[below] was put forward. The construction got funded in 1991 and RHIC has been operational since 2000. One of the world’s only two operating heavy-ion colliders, RHIC is as of 2010 the second-highest-energy collider after the Large Hadron Collider(CH). RHIC is housed in a tunnel 2.4 miles (3.9 km) long and is visible from space.

On January 9, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of Energy Office of Science, that the BNL eRHIC design has been selected over the conceptual design put forward by DOE’s Thomas Jefferson National Accelerator Facility [Jlab] (US) as the future Electron–ion collider (EIC) in the United States.

In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 (mission need) from the Department of Energy. BNL’s eRHIC design proposes upgrading the existing Relativistic Heavy Ion Collider, which collides beams light to heavy ions including polarized protons, with a polarized electron facility, to be housed in the same tunnel.

Other discoveries

In 1958, Brookhaven scientists created one of the world’s first video games, Tennis for Two. In 1968 Brookhaven scientists patented Maglev, a transportation technology that utilizes magnetic levitation.

Major facilities

Relativistic Heavy Ion Collider (RHIC), which was designed to research quark–gluon plasma and the sources of proton spin. Until 2009 it was the world’s most powerful heavy ion collider. It is the only collider of spin-polarized protons.
Center for Functional Nanomaterials (CFN), used for the study of nanoscale materials.
BNL National Synchrotron Light Source II(US), Brookhaven’s newest user facility, opened in 2015 to replace the National Synchrotron Light Source (NSLS), which had operated for 30 years.[19] NSLS was involved in the work that won the 2003 and 2009 Nobel Prize in Chemistry.
Alternating Gradient Synchrotron, a particle accelerator that was used in three of the lab’s Nobel prizes.
Accelerator Test Facility, generates, accelerates and monitors particle beams.
Tandem Van de Graaff, once the world’s largest electrostatic accelerator.
Computational Science resources, including access to a massively parallel Blue Gene series supercomputer that is among the fastest in the world for scientific research, run jointly by Brookhaven National Laboratory and Stony Brook University.
Interdisciplinary Science Building, with unique laboratories for studying high-temperature superconductors and other materials important for addressing energy challenges.
NASA Space Radiation Laboratory, where scientists use beams of ions to simulate cosmic rays and assess the risks of space radiation to human space travelers and equipment.

Off-site contributions

It is a contributing partner to ATLAS experiment, one of the four detectors located at the Large Hadron Collider (LHC).


It is currently operating at CERN near Geneva, Switzerland.

Brookhaven was also responsible for the design of the SNS accumulator ring in partnership with Spallation Neutron Source at DOE’s Oak Ridge National Laboratory (US), Tennessee.

Brookhaven plays a role in a range of neutrino research projects around the world, including the Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China.