Pressure-induced structural and magnetic phase transitions in ordered and disordered equiatomic FeCo

The magnetic and structural phase diagram of equiatomic FeCo has been studied up to 45 GPa using $K$-edge x-ray magnetic circular dichroism, x-ray absorption near edge spectroscopy, x-ray diffraction, and supporting density-functional-theory-based calculations. FeCo foils with different degrees of chemical order were obtained by magnetron sputtering. Our results show that Fe${}_{0.5}$Co${}_{0.5}$ undergo the bcc ferromagnetic to hcp nonferromagnetic transition in the 30\char21{}45 GPa pressure range. Interestingly, the chemical order, i.e., the relative arrangements of Fe and Co atoms, plays a major role in affecting the high-pressure structural and magnetic phase diagram of these alloys. This result is confirmed by first-principles modeling of different structures of equiatomic FeCo alloy. Moreover, the total-energy analysis reveals a strong competition between different magnetic hcp states upon compression. A possible emergence of antiferromagnetism is emphasized and requires further experimental investigation.