Stability of ferromagnetism in Fe, Co, and Ni metals under high pressure

The stability of the ferromagnetic state in Fe, Co, and Ni metals under high pressure is studied using X-ray magnetic circular dichroism (XMCD) at the K-edge and X-ray powder diffraction. It is found that the ferromagnetic state under pressure is very different for Fe, Co, and Ni metals, and is closely associated with the crystal structure. In the case of Fe, the XMCD spectrum disappears with increasing pressure from 14.2 to 17.8 GPa, indicating the transition from a ferromagnetic to a nonmagnetic state. This transition results in the bcc --> hcp martensitic transformation accompanied by a decrease in the c/a ratio of the hcp phase. In contrast to Fe, the dichroic amplitude in Co and Ni decreases slightly under the applied pressure. The ferromagnetic states in Co and Ni metals are maintained even at 39 and 48 GPa, respectively. The stability of ferromagnetism is interpreted from the viewpoint of electronic states.