Speight, R. J. (Richard J.) and Wong, A. and Ellis, P. and Bishop, P. T. and Hyde, T. I. and Bastow, T. J. and Smith, Mark E. (2009) (59)Co NMR study of the allotropic phase transformation in small ferromagnetic cobalt particles. Physical review B, 79 (5): 054102. ISSN 1098-0121
Abstract
To demonstrate the potential of nuclear-magnetic-resonance (NMR) spectroscopy in investigating detailed structural properties in ferromagnetic materials, the allotropic phase transformation of polycrystalline cobalt with mu m particle size (<2 mu m) is characterized by internal-field Co-59 NMR. The Co-59 NMR spectra show distinct resonance bands corresponding to the different Co sites: face-centered cubic (fcc), hexagonal-close packed (hcp), and stacking faults (sfs), in Co metal powder. The hcp -> fcc phase-transition temperature is determined by systematically monitoring the signal intensity of each Co environment in a series of heat-treated Co powders. The potential limits to which absolute quantification of the different sites can be pushed are mentioned, with relative changes in intensity giving unequivocal evidence of the structural evolution. For example, the phase-transition temperature is observed to be 500 +/- 25 degrees C, and above this temperature, the sf Co sites were reduced by more than 10%.