Resonant magnetic
x-ray scattering
on artificial spin ice
We probed the magnetic signatures of an
artificial spin ice with soft x-ray resonant
magnetic scattering at the SEXTANTS
beamline. Exploiting the extended view
of reciprocal space accessible with a CCD
detector, we could identify pure magnetic
Bragg peaks associated with long-range
ordering in the dipolar-coupled nanomagnet
arrays. In addition, the evolution of the
magnetization as a function of the applied
field was determined from the magnetic
circular dichroic contrast at the Bragg
peak positions. These measurements
provide an important step for the future
study of frustrated nanomagnet systems
with unprecedented spatial and temporal
resolution.
Frustration is an important concept
in physics, which arises in different
systems when all interactions cannot be
satisfied at the same time. Artificial
spin ice, consisting of arrays of closely
spaced nanomagnets placed at the sites
of a square or kagome lattice [1],
is an example of a magnetic system that
is frustrated by the competition between
the dipolar interactions.
Until now, microscopy methods have
mainly been used to investigate artificial
spin ice with the lateral dimensions
of the nanomagnets typically of the order
of hundreds of nanometres, which results
in static magnetic moments [2]. By using
superparamagnetic elements with smaller
volumes, the magnetization dynamics
in systems with magnetic fluctuations
can be accessed [3], so providing a new
route to understand the mechanisms
driving artificial spin ice. For studying
such dynamics, scattering techniques
offer an invaluable alternative to imaging,
thanks to the significantly better spatial
resolution and the promise of improved
temporal resolution.
In this work, carried out at the SEXTANTS
beamline, we studied artificial square
ice (Figure
➊
) using soft x-ray resonant
magnetic scattering (SXRMS). Using
an in-vacuum CCD detector, we mapped
a large fraction of the two dimensional
reciprocal space at once, which allowed
us to clearly distinguish all the relevant
features of the scattering patterns,
facilitating the interpretation of the data.
Artificial square ice has a magnetic
ground state which can be observed in
as-grown samples [4]. In this long-range
ordered state, the magnetic moments form
alternating vortices of opposite chirality,
as depicted in the inset of Figure
➊
. Here
the magnetic unit cell is bigger than the
structural one, and the corresponding
Bragg peaks in reciprocal space should
appear at intermediate positions.
Tuning the photon energy to the Fe L
3
absorption edge, we measured scattering
patterns sensitive to both the charge and
the magnetic scattering (Figure
➋
), while
going to off resonant photon energies gave
pure charge scattering. Comparing the two
measurements, we could unambiguously
identify the pure magnetic peaks (shown
in red) corresponding to the magnetic
ground state ordering.
Introduction
Ordering revealed in as-grown artificial square ice
PHYSICS AND CHEMISTRY OF CONDENSED MATTER, EARTH SCIENCES
➊
Scanning electron microscopy image of an artificial square ice array. The insert
shows the magnetic ground-state ordering with the unit cell highlighted in grey.
106
SOLEIL
HIGHLIGHTS
2013
