Anomalous magnetoelastic behaviour near morphotropic phase boundary in ferromagnetic Tb_1-xNd_xCo₂ system

In this work, we report a morphotropic phase boundary (MPB) involved ferromagnetic system Tb_1-xNd_xCo₂ and reveal the corresponding structural and magnetoelastic properties of this system. With high resolution synchrotron X-ray diffractometry, the crystal structure of the TbCo₂-rich side is detected to be rhombohedral and that of NdCo₂-rich side is tetragonal below their respective Curie temperatures T_C. The MPB composition Tb_0.35Nd_0.65Co₂ corresponds to the coexistence of the rhombohedral phase (R-phase) and tetragonal phase (T-phase). Contrary to previously reported MPB involved ferromagnetic systems, the MPB composition of Tb_0.35Nd_0.65Co₂ shows minimum magnetization which can be understood as compensation of sublattice moments between the R-phase and the T-phase. Furthermore, magnetostriction of Tb_1-xNd_xCo₂ decreases with increasing Nd concentration until x = 0.8 and then increases in the negative direction with further increasing Nd concentration; the optimum point for magnetoelastic properties lies towards the rhombohedral phase. Our work not only shows an anomalous type of ferromagnetic MPB but also provides an effective way to design functional materials.