X-Ray Magnetic Circular Dichroism Measurements in Ni up to 200 GPa: Resistant Ferromagnetism

The structural stability of fcc Ni over a very large pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by simple compression. $K$-edge x-ray magnetic circular dichroism (XMCD) shows that fcc Ni is ferromagnetic up to 200 GPa, contradicting recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Density functional theory calculations point out that the pressure evolution of the $K$-edge XMCD closely follows that of the $p$ projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa.

Figure 1

Upper panel: Combined XANES (left) and XMCD (right) spectra in pure Ni up to 200 GPa ($E_0=8323\mathrm{eV}$). Lower panel: selected angle-dispersive XRD patterns up to 200 GPa.

Figure 2

Calculated orbital magnetization density of $4p$ states (right axis) and $K$-edge differential XMCD for bulk Ni (left axis). The calculations are performed for a zero-pressure unit cell using the PY-LMTO code. The curves are broadened with a Lorentzian function of 0.8 eV width.

Figure 3

Volume evolution of (a) the calculated total spin moment, (b) the $p$ orbital moment, and (c) the normalized XMCD integral ($A_{\mathrm{XMCD}}$). The nonlinear pressure scale in the top axis is calculated with the equation of state given in 7.