Non-collinear spin-density wave antiferromagnetism in FeAs

Abstract:

The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity. We present the magnetic structure of the itinerant monoarsenide, FeAs, with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wavevector q = (0.395 ± 0.001)c ∗ at 4 K, but cannot distinguish between a simple spiral and a collinear spin-density wave structure. Polarized single crystal diffraction confirms that the structure is best described as a non-collinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the b-axis direction being (15 ± 5)% larger than in the a-direction. Furthermore, the propagation vector is temperature dependent, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The magnetic structures of closely related systems are discussed and compared to that of FeAs.

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http://arxiv.org/pdf/1012.5311.pdf

I. INTRODUCTION 

The exact treatment of localized electrons in materials with Fermi surfaces remains a substantial challenge in condensed matter science and has recently come to prominence with the close association of itinerant magnetism and superconductivity in iron based superconductors.

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Fundamental to the coexistence of magnetic order and metallic conductivity in this compound is the presence of incommensurate magnetic ordering, which can be interpreted in terms of either a spin density wave (SDW) or a spiral magnetic structure. The importance of spiral phases in itinerant magnets has been highlighted by recent work on MnSi which unveiled a novel A-phase where a unique skyrmion lattice, similar to the vortex phases in superconductors, has been proposed to exist.6,7 Crucial to this phase is the presence of both a spiral magnetic phase and itinerant electrons, providing yet another motivation to studying itinerant magnets such as FeAs.

The B31 structure of FeAs, commonly referred to as the MnP-type structure, can be thought of as a distorted form of the hexagonal NiAs-type structure (See Figure 1).8,9 Found in over 400 compounds, the NiAstype structure occurs frequently for intermetallics combining a transition metal and a metalloid such as Si, As, Se, or Te.