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 perform efficiently.

 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 [1] and is installed at the Swiss Light Source, near Zurich [2]. 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 [3].  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 oxide materials.

References

[1] G. Ghiringhelli et al. Rev. Sci. Instrum. 77, 113108 (2006)

[2] http://www.psi.ch/sls/adress/adress

[3] M. P. M. Dean et al., Nature Materials 11, 850–854 (2012)

[4] L. Braicovich et al., Phys. Rev. Lett. 104, 077002 (2010)

[5] Le Tacon et al. Nature Physics 7, 725–730 (2011)

[6] M. P. M. Dean et al., Phys Rev. Lett. In Press (2013)

 

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