NMR study of Nd2(Fe1−xCox)14B for low Co content (abstract)

Abstract

In order to improve the understanding of Co site preference and 59Co hyperfine fields in Nd2Fe14B, a NMR study has been carried out on Nd2(Fe1−xCox)14B for low Co contents, 0.0015≤x≤0.15. Using a high sensitivity NMR spectrometer, good quality 59Co spectra for frequencies from 100 to 300 MHz have been recorded at 4.2 K, even for very low Co concentration −0.15%. The identical spectrum shapes for the concentrations 0.0015 and 0.015 show that these spectra are free from the satellite lines due to local environment effects and are representative of the isolated Co atoms in the Nd2Fe14B lattice. A high frequency triplet structure in these spectra, with the maxima of intensity at 249, 260, and 273 MHz, has been identified as arising from a single crystallographic position. With increasing Co content the triplet progressively evolves towards a doublet and shifts rapidly to lower frequencies: 235, 247 MHz for x=0.15. This doublet has been traced through the whole Co concentration range down to pure Nd2Co14B,1 and has been assigned to 16k2 sites based on an NMR spin reorientation study.2 Independent evidence for single site origin of the triplet structure is based on NMR line shape simulation in the special case of an NMR signal, with strong hyperfine field anisotropy, originating from conical domain walls in the canted magnetic structure of Nd2(FeCo)14B. The 16k2 structure, for the lowest Co concentration, contains 60% of the overall spectrum intensity and slowly decreases to around 50% for x=0.15. the rest of the spectral intensity is divided amongst four other lines located at the frequencies 156, 190, 202, and 213 MHz. The ratio of the respective intensities of these lines is close to 1:4:2:1, indicating that they originate from the 4e, 16k1, 8j, and 4c sites, respectively. The results obtained confirm that Co enters preferentially the 16k2 sites and avoids one of 8j sites in Nd2Fe14B structure.