Hyperfine Splitting and Room-Temperature Ferromagnetism of Ni at Multimegabar Pressure

Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mössbauer transition of ${}^{61}\mathrm{Ni}$. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mössbauer factor increases from 33 meV at ambient pressure to 60 meV at 100 GPa. The change of this energy over volume compression is well described by a Grüneisen parameter of 2.09.

Figure 1

Time evolution of the nuclear forward scattering for Ni measured along the DAC axis at room temperature. The solid lines show the fit according to the model described in the text.

Figure 2

Time evolution of the nuclear forward scattering for Ni measured perpendicular to the DAC axis at room temperature. The solid lines show the fit according to the model described in the text.

Figure 3

Pressure dependence of the hyperfine magnetic field in Ni obtained from the experiment (filled circle) and calculated using scalar-relativistic calculations (open circle) and fully relativistic ab initio calculations, explicitly including spin-orbit coupling (square). The inset shows the $3d$ electron orbital field arising in the fully relativistic calculations.

Figure 4

Dependence of the calculated magnetic moment $M$ (circle) and the effective exchange interaction parameter $J_0$ (square) on the unit cell volume $V$ normalized with respect to the equilibrium volume $V_0$ of ferromagnetic fcc Ni. The experimental value of the magnetic moment of Ni at zero pressure [38] is shown for comparison (diamond).

Figure 5

Pressure dependence of the Lamb-Mössbauer factor. The inset shows the logarithm of the Debye energy as a function of the logarithm of the volume. The linear fit shown by the line gives the Grüneisen parameter $\gamma$.