Magnetic interactions in NiO at ultrahigh pressure

Magnetic properties of NiO have been studied in the multimegabar pressure range by nuclear forward scattering of synchrotron radiation using the 67.4 keV Mössbauer transition of ${}^{61}\mathrm{Ni}$. The observed magnetic hyperfine splitting confirms the antiferromagnetic state of NiO up to 280 GPa, the highest pressure where magnetism has been observed so far, in any material. Remarkably, the hyperfine field increases from 8.47 T at ambient pressure to $\sim 24$ T at the highest pressure, ruling out the possibility of a magnetic collapse. A joint x-ray diffraction and extended x-ray-absorption fine structure investigation reveals that NiO remains in a distorted sodium chloride structure in the entire studied pressure range. Ab initio calculations support the experimental observations, and further indicate a complete absence of Mott transition in NiO up to at least 280 GPa.

Figure 1

Time evolution of the nuclear forward scattering for NiO at room temperature at the following pressures: (a) ambient, (b) 39 GPa, (c) 150 GPa, and (d) 280 GPa. The red solid lines show the fit according to the model described in the text. Gray rectangles mark the areas that were not fitted due to insufficient statistics. Blue straight lines are used as a guide for the eyes to track the position shift of the major beats under compression.

Figure 2

Pressure dependence of the magnetic hyperfine field in NiO obtained by experiment and ab initio calculations within various approximations. Solid olive circles: NiO by NFS; solid black squares: by $\text{LSDA}+U$ for $U=7.1$ eV and ${\alpha }_R={60}^{\circ }$; open black squares: by $\text{LSDA}+U$ for $U=7.1$ eV and $\alpha ={62}^{\circ }$; blue circles: by $\text{LSDA}+U$ for $U=2.7$ eV and $\alpha ={62}^{\circ }$. Solid blue triangles represent the dependence of the magnetic hyperfine field in Ni by NFS from [18], for comparison. The lines are guides for the eye.

Figure 3

(a) Diffractograms at 120 and 210 GPa. The arrows show the reflections (111), (202), (213), and (222) that split under pressure due to the rhombohedral distortions. (b) XAS spectra measured at ambient pressure and 280 GPa.

Figure 4

Pressure dependence of the unit cell volume of NiO. Olive circles: single-crystal XRD measurements; blue diamonds: powder XRD measurements; gray triangles: data calculated from XAS spectra. The black curve is the EOS extracted from single-crystal measurements. For comparison, we have plotted two equations of state found in the literature: dashed red line from [22], and blue dash-dot-dot line from [11]. The top inset shows the volume dependence of the rhombohedral angle $\alpha$, as defined in the sketch of the unit cell in the bottom inset. The black dots are values from powder XRD measurements, and red triangles are calculated from [22].