Magnetics Business & Technology - July/August 2022 - 22

RESEARCH & DEVELOPMENT
Lighting Up Ultrafast Magnetism in a Metal Oxide
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.)
Understanding how magnetic correlations change over very
short timescales could be harnessed to control magnetism for
applications including data storage and superconductivity
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 Proceedings of the National Academy of Sciences, 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 in Switzerland. " 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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Magnetics Business & Technology * July/August 2022
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 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. "
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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 (trRIXS).
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.
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Magnetics Business & Technology - July/August 2022

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