Crystal structure and magnetism of MnO under pressure

Manganese oxide is a prototype of an antiferromagnetic Mott insulator. Here, we investigate the interplay of magnetic ordering and lattice distortion across the Néel temperature $T_N$ under pressure using neutron and x-ray diffraction. We find an increase in $T_N$ with a rate of $d{T}_N/dP=+4.5\left(5\right)\mathrm{K}/\mathrm{GPa}$, an increase in the rhombohedral distortion $\alpha$ by $d\alpha /dP=+0.{018}^{\circ }/\mathrm{GPa}$, as well as a volume striction which is insensitive to pressure. These results allow retrieving the dependence of the coupling constants $J_1$ and $J_2$ on interatomic distances and compare it to first-principles predictions. Antiferromagnetic diffuse scattering was observed up to $\approx 1.2T_N$, and long-range magnetic order appears at room temperature at 42 GPa.

Figure 1

Neutron-diffraction patterns of MnO at (top) 2.1 GPa and (bottom) 8.5 GPa at various temperatures across the Néel temperature. The lines through the data (the circles) are fits from Rietveld refinements. The insets give enlarged views over the $2.3–2.8\text{−}\AA$ range. Note the splitting of the 111 reflection below the Néel temperature. The asterisks and arrows mark, respectively, the strongest reflections from diamond (anvils) and lead (pressure marker).

Figure 2

(Upper) Magnetic moment and (bottom, defined as positive) distortion angle of MnO under pressure. The horizontal line in the upper panel indicates $m=1.5{\mu }_B$ (i.e., 1/3 of the value at 0 K). The lines are guides to the eye.

Figure 3

Pressure dependence of (upper) $T_N$, (middle) $\alpha$, and (lower) $\delta V/V$. The data are compared to experimental findings using a strain gauge technique to 1.7 GPa ($T_N$ defined as the midpoint) [6], x-ray spectroscopy [3], and ab initio results by Schrön et al. [8].

Figure 4

Temperature dependence of the unit-cell volume of MnO at various pressures. The dotted lines below the Néel temperatures are guides to the eye. The dashed lines describe the behavior in the absence of magnetic order derived from a Grüneisen model as explained in the text. The right and left arrows in each panel indicate temperatures where the magnetic signal is first visible and where the refined moment reaches $1.5{\mu }_B$, respectively.

Figure 5

Pressure dependence of exchange parameters $J_1$ and $J_2$ derived from this paper (the solid lines) and compared to calculations by Fischer et al. [7].

Figure 6

X-ray-diffraction patterns of MnO recorded at 295 K and increasing pressure. Note the progressive splitting (see arrows) of the cubic 111 and 220 reflections above 42.7 GPa which signals magnetic ordering. Reflections of solid neon (pressure transmitting medium) are marked. $\lambda =0.486\AA$.