Structure and magnetism of cobalt at high pressure and low temperature

The magnetic and structural properties of cobalt were investigated under high pressure (160 GPa) and low temperature (50 K), by synchrotron K-edge x-ray magnetic circular dichroism and x-ray diffraction. A quasihydrostatic equation of state was measured up to 160 GPa. We found that uniaxial stress plays a role in the hexagonal close packed-face centered cubic (hcp-fcc) structural transition pressure. Also, our data provide the first experimental evidence that changes of the $c/a$ ratio pressure derivative are related to the magnetic behavior. The complete extinction of ferromagnetism is observed above 130 GPa in a mixed hcp-fcc phase with no recovery upon cooling to 50 K, indicating that cobalt at 150 GPa is very likely nonmagnetic, i.e., characterized by zero local spin polarization. Density functional theory calculations point out that the K-edge x-ray magnetic circular dichroism (XMCD) signal is related to the $4p$ orbital moment rather than to the total spin moment and allow us to get a deeper insight into the K-edge XMCD measurements interpretation. The combination of novel theoretical results and experimental outputs provides a detailed scenario of the structural and magnetic properties of cobalt at these extreme conditions answering some previously unsolved issues.

Figure 1

Compression data on a hcp Co grain measured in quasihydrostatic conditions (empty red circles) and fit to a Vinet EoS (black line) of these data merged to data from Ref. [28] (empty black squares); previous EoS from Refs. [4] and [10] are shown from comparison.

Figure 2

Selection of XRD patterns acquired at room temperature during the nonhydrostatic combined XRD-XMCD run. Reflections from the hcp and fcc phases are indicated as h and f, respectively. Diffraction peaks from the Co fcc phase appear around 90 GPa; small vertical lines indicate reflections from the Re gasket.

Figure 3

Volume compression of hcp and fcc Co.

Figure 4

Schematic overview of hcp and fcc Co phases domains found in this study and in previous literature as a function of pressure (room temperature) and hydrostatic conditions.

Figure 5

$c/a$ value as a function of compression for the Co grain compressed in Ne (quasihydrostatic run, red circles) and Co powder (nonhydrostatic run, black squares) and comparison to previous trend reported in Ref. [10] (green line) and Ref. [4] (blue dashed line).

Figure 6

XMCD evolution as a function of pressure: all spectra are acquired at room temperature (RT) except the one on the top (black) which was acquired at 150 GPa and 50 K.

Figure 7

Top panel: Calculated total spin moment and the $p$-projected orbital moment in the mixed hcp-fcc phase, according to the fraction found from XRD, and assuming that the magnetization in both phases is aligned parallel to each other. Central panel: K-edge XMCD integrals from the present work (full circles) and the previous study (empty circles [17]) and data from Ishimatsu et al. [22]. Bottom panel: hcp/fcc phase fraction obtained from the XRD analysis and inversion of $c/a$ ratio pressure derivative as a function of compression in this run.

Figure 8

Calculated spin and orbital components of $l$-projected magnetic moments in hcp and fcc Co as a function of compression. Note that absolute values are plotted; ${\langle S\rangle }_p$ is antiparallel to ${\langle S\rangle }_d$. $V_0$ is the calculated equilibrium volume for a given crystal structure