Ab initio total-energy and phonon calculations of Co at high pressures

We present results of ab initio calculations on the high pressure phonon, magnetic anomalies, and structural instability in cobalt. Our calculations reproduce the magnetic collapse starting near 100 GPa and the hcp to fcc phase transition around this compression, agreeing with earlier calculations and with the experimental data. Our studies show anomaly in the E-2g mode phonon frequency variation near 75 GPa pressure, consistent with the measurements. We have demonstrated that phonon and magnetic anomalies are interrelated in cobalt and hence, the magnetoelastic coupling should be stronger in Co than in iron, which also undergoes a high pressure bcc to hcp transition with magnetic collapse but without showing any associated anomaly in the phonon frequency variation under compression.