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Published online: 22 July 2009 | doi:10.1038/nchina.2009.149

Condensed matter physics: Mixed moments

Felix Cheung

Introduction

© (2009) Nature Physics

Magnetism plays an important role in superconductivity, and a better understanding of the magnetic properties in iron-arsenide (FeAs) superconductors may help to reveal the origin of high-temperature superconductivity in unconventional systems. Pengcheng Dai at the Chinese Academy of Sciences in Beijing, the University of Tennessee in Knoxville and Oak Ridge National Laboratory, and co-workers¹ have used inelastic neutron scattering to probe the microscopic nature of magnetism in CaFe₂As₂, a parent (undoped) compound of the FeAs superconductor family.

Magnetism in solids arises through quantum mechanical exchange interactions between electron spins. In CaFe₂As₂ crystals (see image), the spins of Fe atoms are aligned antiferromagnetically (in opposite directions) at temperatures below 170 K. It was previously unclear, however, whether the magnetic order originates from electrons residing in the vicinity of the Fe atom (localized electrons) or those that hop from Fe atom to Fe atom (itinerant electrons).

By studying the spin waves in CaFe₂As₂ crystals, the researchers found that the exchange energy between spins in the Brillouin zone (the unit cell marked by nearest-neighbour Fe atoms) can be described by a Heisenberg Hamiltonian operator — the hallmark of a localized electron system.

The finding seems to suggest that the magnetism in CaFe₂As₂ originates from localized electrons. However, the localized electron model is unable to explain the two-dimensional properties of CaFe₂As₂, or how Fe atoms acquire their half-integer spins (a Fe atom has an even number of electrons). The researchers believe that electrons responsible for the magnetism in CaFe₂As₂ are neither purely localized nor purely itinerant, but rather a complicated mix of both.

The authors of this work are from:
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, USA; ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxford, UK; Department of Physics, Purdue University, West Lafayette, Indiana, USA; Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China; Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, China; Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.

Reference

1. Zhao, J. et al. Spin waves and magnetic exchange interactions in CaFe₂As₂. Nature Phys. doi:10.1038/nphys1336 (2009). | Article | ChemPort |