Seeing Magnetic Excitations With X-rays
An artist's impression of a magnetic disturbance or excitation as it
propagates through a material.
Mark P. M. Dean
Magnetic materials have fascinated scientists since
antiquity, but it was only with the advent of quantum
mechanics in the 20th century that we could
properly understand simple magnetic materials such as iron
and nickel. This understanding provided the conceptual
foundation that engineers used to develop magnetic storage
technologies such as computer hard drives.
Today, scientists are looking to discover and perfect new
materials from which we can build future technologies.
Transition metal oxide compounds are showing considerable
potential in this regard. Unlike ordinary metals, these
materials cannot be understood by considering electrons that
move independently from one another; rather the electrons
interact with each other strongly. This gives rise to exotic
new magnetic and electronic properties, which can be
exploited in new technologies.
The properties of the oxides are much more sensitive to their exact
chemical and structural configuration than traditional magnetic materials.
And, although this makes transition metal oxides more difficult to
understand, it opens up many more opportunities to tweak these materials in
order to optimize their properties. In this blog post, I describe how new
x-ray techniques are providing us powerful insights into the magnetic
properties of these fascinating new oxide materials.
Several powerful, widely-used methods to determine the structure of
materials can be collectively referred to as scattering techniques. In these
techniques, particles such as neutrons or x-rays are fired at the material
of interest and the scattered particles are collected. When a particle
scatters it can transfer some of its energy to the material, creating a
disturbance called an excitation, as shown in the figure above.
By measuring the energy loss of the
particle we can determine the energy of these excitations and how this
varies with scattering direction. These excitations can have many different
characters: structural, magnetic, electronic etc. and by studying them we
can infer all the intrinsic properties of our material of interest.
Researchers interested in magnetism usually scatter neutrons from their
material of interest, because neutrons are sensitive to both the magnetic,
as well as the structural, properties of materials. This sensitivity to
magnetism arises because neutrons themselves are little magnets, which
allows the neutrons to see the magnetic properties of the materials in a
simple, well-understood way. Unfortunately, however, neutrons are very
difficult to focus into a spot smaller than a few cm in diameter and they
usually travel several cm in a material before they scatter. This can be an
advantage when researchers want to study the whole volume of a large piece
of material, but small samples are usually very difficult to study with
neutron scattering. This is
unfortunate because new materials are typically only available as small
crystals providing a bottleneck for scientific progress. More crucially
still, we are often interested in very thin layers of materials, as future
electronic devices will inevitably be very small in order for them to
With the advances in x-ray sources, highly intense x-rays beams can
be routinely focused to a few microns in size. Indeed, x-rays have played a
vital role in determining the crystal structure of metals and oxides, going
on to solve the far more complicated structures such as proteins and even
chocolate! Unfortunately for researchers interested in magnetism, an x-ray
photon has no intrinsic magnetic moment so under usual circumstances x-rays
do not provide information about magnetism. Fortunately, a trick can be
played to get around this restriction. This involves using the core
electrons, which live deep within the atoms as an intermediate step in the
scattering process. By picking a special initial energy for the x-ray, we
can make the x-ray kick an electron out of the core of the atom and up into
the valance states were this electron can interact magnetically with the
valence state, making the x-rays sensitive to the magnetic properties of the
material. An electron then fills the hole left by ejecting the core electron
and, in order to conserve energy, it emits a photon. We then measure the
direction and energy loss of the emitted photon. The name for this technique
is resonant inelastic x-ray scattering, abbreviated RIXS.
In order to use RIXS for practical purposes, we require instruments
that are capable of measuring tiny changes in the energy of the x-rays. One
of the leading instruments for this technique is called SAXES. This
instrument was designed and built at the Politechnico di Milano  and is
installed at the Swiss Light Source, near Zurich . RIXS studies at SAXES
have allowed several breakthroughs in the study of transition metal oxides.
The magnetic excitation spectrum of a single atomic layer of the compound La2CuO4
was measured last year highlighting the extreme sensitivity of this new
technique . In a similar way, the
magnetic excitations in several classes of copper-oxide superconductors have
also been measured, extending and complementing decades of work using
neutron scattering [4-6]. Such a characterization may prove a key part of
our ongoing attempts to understand the mechanism of high temperature
superconductivity in these novel copper-oxide based compounds.
The recent successes of the RIXS technique have generated
considerable enthusiasm to design new, improved instruments. There are
projects at the European Synchrotron Radiation Facility, Grenoble, France,
the Diamond Light Source, Didcot, UK and the National Synchrotron Light
Source II, Brookhaven, USA. These new instruments promise factors of 10
improvements in precision, which will doubtless provide many more insights
into the complex array of magnetic behaviors exhibited by transition metal
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M. P. M. Dean et al., Nature Materials
L. Braicovich et al., Phys. Rev. Lett. 104,
Le Tacon et al. Nature Physics 7, 725730 (2011)
M. P. M. Dean et al., Phys Rev. Lett.
In Press (2013)