Anomalous pressure evolution of the axial ratio c∕a in hcp cobalt: Interplay between structure, magnetism, and lattice dynamics

We performed angle-dispersive x-ray diffraction measurements on hydrostatically compressed hcp cobalt to 90GPa. Near 75GPa, we document an inversion in the pressure derivative of the axial ratio c∕a with no discontinuity in the volume and lattice parameters compression curves. These results are also reproduced by ab initio calculations. Our study indicates significant interactions among structure, magnetism and elasticity, suggesting that the collapse of the magnetic moment is responsible for the observed anomaly in c∕a, as well as for the anomalies in the elastic and vibrational properties of hcp Co at high pressure.