Pressure-induced two magnetic collapses in the ferromagnetic L12-Fe3Pd alloy and related elasticity and lattice dynamics anomalies

Abstract

Pressure effect on the magnetic, mechanical and lattice dynamical properties of the ferromagnetic L12-Fe3Pd crystalline alloy was investigated by first-principles calculations. The calculated properties under ambient pressure are in good agreement with available theoretical results. A two-step pressure-induced collapse of magnetic moment was observed, accompanied by the collapse of the volume. The first one is at about 34.2 GPa, and the other is at about 67 GPa. While the magnetic moment disappears around 84 GPa. Thelattice dynamics calculations under high pressure revealed that the dynamically stable and unstable cases appear alternately. The system is dynamically unstable at low pressures due to the spontaneous magnetization of the system, and then the system is dynamically stable in the pressure range of 33.5 to 66.9 GPa, and this pressure range is between the first collapse pressure and second collapse pressure. The system becomes dynamically unstable again at pressure ranging from 67 to 83.7GPa, and afterwards becomes stable. Elastic constants and their dependence on pressure and related mechanical properties were investigated. Stability criteria show that ferromagnetic L12-Fe3Pd crystalline alloy is mechanically stable over the entire studied pressure range. The bulk modulus exhibits pronounced softening with pressure, and present minimum values at magnetic collapse pressures. The mechanism of elastic softening was analyzed with the magnetoelastic force. From Pugh’s ratio and Poisson’s ratio, the system exhibits brittleness at the vicinity of magnetic collapse pressures. The linear thermal expansion coefficient was calculated based on the quasi-harmonic approximation. The results reveal that the alloy does notexhibit thermal Invar behavior under zero pressure and high pressure. Finally, a high-anisotropy tetragonal structurewith space group symmetry P4/mbm was obtained by soft-mode phase transition theory, and this structure is dynamically and mechanically stable at zero pressure. Theoretical prediction suggested that the P4/mbm structure is energetically favorable than the previously reported structures at the pressure below 32 GPa and also showed that above 32 GPa the hexagonal D019 structure becomes the energetically most stable structure.