Hyperfine field distribution in the Heusler compound Co₂FeAl probed by⁵⁹Co nuclear magnetic resonance

The Heusler compound Co 2 FeAl is reported to occur in various structures ranging from the completely ordered L 2 1 to the completely disordered A 2 structure type. In this work, we use spin echo nuclear magnetic resonance (NMR) to probe the local structure of Co 2 FeAl bulk samples. The study of Co 2 FeAl bulk samples provides the unique possibility to verify the intrinsic generic structural properties. The 59 Co NMR measurements reveal a distribution of Fe and Al not only in the first neighbouring shells of the 59 Co nuclei but also in more distant shells. The analysis of 59 Co NMR main resonance lines with an underlying sub-structure confirms that the local structure of the as-cast Co 2 FeAl bulk samples consists of a B 2 type structure with contributions of the L 2 1 type structure of about 10%. The observed sub-lines, which are attributed to a distribution of Fe and Al atoms in more distant shells, were previously not resolved in NMR spectra of Co 2 FeAl thin films, pointing to better long range order in bulk material than in thin films. We also show that the individual contributions of the structure types can be influenced by annealing.