Pressure-induced collapse of ferromagnetism in cobalt up to 120 GPa as seen via x-ray magnetic circular dichroism

The magnetism and structure of compressed Co has been studied up to 120 GPa using K-edge x-ray magnetic circular dichroism (XMCD) and x-ray absorption near edge spectroscopy (XANES). The XMCD signal decreases linearly with pressure and is totally suppressed around 120 GPa at ambient temperature. We detect local fcc-like atomic configurations starting from 80 GPa and at 120 GPa these represent at least 40% of the total. Supporting theoretical calculations indicate that the mixed hcp/fcc phase above 120 GPa is nonmagnetic. Comparison between the pressure evolution of magnetism and structure highlights major differences between the Fe and Co cases.

Figure 1

Normalized K-edge XANES (left) and XMCD (right) spectra of cobalt as a function of pressure (ID24). Inset: XANES integral evolution under pressure.

Figure 2

Simulations of XANES spectra using the FEFF code (Ref. 26). The atomic clusters for Co are built using the crystallographic parameters of the structurally distorted hcp and fcc phases found in Refs. 10 and 13. The solid lines are models of our XANES data while the dashed lines show the spectra expected for pure hcp or fcc phases. Above 80 GPa some combined hcp and fcc character is observed in our data.

Figure 3

Integral of the K-edge XMCD signal of Co (rescaled to the value of the ambient magnetic moment) in scatter from this work. Solid and dashed lines are fit of data and extrapolation from Ref. 7. The inset shows theoretical calculations of the evolution of the magnetic moment of hcp and fcc cobalt with pressure.