Cobalt substitution and phosphorus defect engineering ferromagnetic ordering in well-dispersed CoxNi2-xP nanocrystals

Abstract

Heteroatoms substitution and defect design are efficient strategies to fabricate multi-metallic phosphides and manipulate the properties. Herein well-dispersed multi-metallic Co_xNi_2-xP (0 ≤ x ≤ 0.55) nanocrystals were synthesized through a thermal decomposition and Co substitution and phosphorus defect on the structure morphology and magnetic properties were studied from both experimental and theoretical views. As the substituting Co content or phosphorus source increases Co_xNi_2-xP nanocrystals change from large hollow structure to small solid. Atomic high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and elemental mapping reveal that Ni and Co atoms are evenly distributed at the tetrahedral and pyramidal sublattices. Co substitution and phosphorus defect are favored to induce ferromagnetic ordering in Co_xNi_2-xP nanocrystals different from the nonmagnetic bulk Co₂P and Ni₂P. By increasing the substituting Co the saturation magnetization (Ms) values increase first and then decrease among which Co_0.36Ni_1.64P shows the largest Ms value of 14.11 memu/g. The nonlinear variation tendency of Co-related magnetism is ascribed to the selective occupancy of Co atoms. Co substituted at tetrahedral site show lower formation energy and low spin state but Co at pyramidal site behave high spin state with larger magnetic moment. Moreover, phosphorus defect can stably exist and induce spin polarization boosting the room-temperature ferromagnetism. The Ms values decrease from 14.39 to 2.87 memu/g by increasing the phosphorus sources. X-ray magnetic circular dichroism (XMCD) indicates the existence of ferromagnetic coupling and the total magnetic moments are reduced by decreasing phosphorus defects. This work provides a new insight on the origination and regulation of magnetic properties for multi-metallic phosphides.