Direct experimental evidence of a body-centered-cubic Co phase in radio-frequency-sputtered Co/Fe multilayers

Radio-frequency-sputtered Co/Fe multilayers with layer thicknesses varying in the 8–100 Å range were studied by extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge spectroscopy (XANES), and nuclear magnetic resonance (NMR) techniques. While the Fe environment is body-centered-cubic (bcc) in all the samples, the Co environment depends drastically on the Co layer thickness. Under ≂20 Å, investigation at the Co K edge by XANES provides a bcc fingerprint in both polarization directions. The XANES profile becomes typical of a full hcp stacking only for Co layer thicknesses larger than 98 Å. This behavior is confirmed by EXAFS spectra at the Co K edge and NMR analysis. For Co layer thickness between 6 and 18 Å, eight first neighbors at 2.48 Å and six second neighbors at 2.83 Å are detected which match the bcc phase. Resonance peak at a very low frequency is also observed (198 MHz). The second shell radius appears at 2.78 Å only for larger Co layer thicknesses. This effect goes along with the NMR peak shift to higher frequency (220 MHz). All these results are consistent with a Co-bcc-like structure which changes into a hcp phase when the Co layer thickness increases.