Total Scattering Study of Local Structural Changes in MnAs1-xPx Influenced by Magnetic Interactions

Abstract

The interplay between fluctuations of the local structure and magnetic interactions is of great importance for phenomena like superconductivity, colossal magnetoresistance, and frustrated magnetism. Such local distortions are often subtle and difficult to probe. The metallic MnAs1–xPx (x = 0.06, 0.12, 0.18) solid solution represents a sensitive model system, currently analyzed by variable-temperature X-ray total scattering (TS). A second-order transition (Pnma to P63/mmc) occurs on heating, intimately coupled to magnetic ordering at low temperatures and a temperature-induced low- to high-spin transition for manganese. Intrinsic compositional fluctuations along with a particular magneto-volumetric coupling to low- and high-spin like states triggers formation of local high-symmetry nanosized domains in an orthorhombically deformed matrix that eventually converts globally into a high-temperature hexagonal arrangement. These features are only traced by TS analyses, whereas diffraction provides a regular second-order transition with, e.g., cationic displacements defined by Rietveld analysis acting as order parameter. The coexistence model provides good fits of the TS data and, on average, translates nicely into a continuous V(T) thermal expansion relation with maximum expansivity in the range where the TS data shows the largest difference between local (<20 Å) and intermediate-range (20–80 Å) structures. The degree of local distortions, Mn(As, P)6 octahedra inclusive, is a result of volume-dependent electronic properties as well as magnetic interactions. This study demonstrates how X-ray TS can conveniently give essential insights into the local symmetry fluctuations from the perspective of strong magnetic interactions.