The research team of Zhang Lei, a researcher at the High Magnetic Field Science Center of the Chinese Academy of Sciences' Hefei Institute of Material Science, and Tian Zhaoming, a professor at Huazhong University of Science and Technology, have made new progress in the magnetic field-induced phase transition in the iridium-based double perovskite material La2ZnIrO6. The related research, titled Tricritical phenomenon and H-T phase diagram in a single crystal of the double-perovskite iridate La2ZnIrO6, was published in the American Physical Society journal "Physical Review B".
La2ZnIrO6 is a typical bi-perovskite structure. Zn and Ir ions occupy the center position of an octahedron formed by O. Ir atoms with 5d electrons and Zn atoms with 3d electrons form an octahedral structure with oxygen atoms. Then alternately nested to form a double perovskite structure. Because La2ZnIrO6 has strong spin-orbit coupling and rich phase transitions, it has attracted widespread attention and in-depth research by researchers. In La2ZnIrO6, due to the strong spin-orbit coupling effect, it behaves as a Mott insulator with effective angular momentum Jeff = 1/2. As the temperature decreases, La2ZnIrO6 undergoes an orderly transition from paramagnetic to antiferromagnetic at a temperature of TN ~ 7.5K under a zero magnetic field. Theoretical calculations show that the magnetic ground state of La2ZnIrO6 is an A-type antiferromagnetic, that is, a ferromagnetic arrangement in the layer, and an antiferromagnetic arrangement between the layers.
The researchers conducted a detailed study of La2ZnIrO6 single crystal samples. By studying the magnetic properties of different crystal orientations and rotation angles of the sample, it is found that La2ZnIrO6 is isotropic in the ab direction, but has strong magnetic anisotropy in the c direction. Under zero magnetic field, La2ZnIrO6 behaves as an A-type antiferromagnetic ordered ground state. A magnetic field is applied along the a or b direction. As the strength of the external magnetic field increases, the type A antiferromagnetism gradually evolves into a tilted antiferromagnet. Further increasing the magnetic field, the external magnetic field destroys the antiferromagnetic order, thereby inducing an antiferromagnetic to ferromagnetic phase transition. This phase change is different from the orderly polarization change of the magnetic field to the magnetic field, but a first-order magnetic phase change. When an external magnetic field was applied in the c direction, no magnetic field-induced phase transition was found. By studying the magnetic behavior of the magnetic field along the b direction, the H-T phase diagram of La2ZnIrO6 near the phase transition was constructed. The phase diagram shows the rich phase transitions in La2ZnIrO6 in the a or b direction. In the low magnetic field region, the external magnetic field causes the A-type antiferromagnetic (A-AFM) to transition to the inclined antiferromagnetic (C-AFM). When the magnetic field increases to a certain threshold, an antiferromagnetic to ferromagnetic first-order phase transition (AFM-FM) is induced. The phase diagram shows that a triple critical point (T ~ 8K, H ~ 165Oe) appears at the intersection of the ferromagnetic phase, the antiferromagnetic phase, and the paramagnetic phase.