The Mg-Rich Phase NdNiMg₁₅: Structural and Magnetic Properties

The intermetallic NdNiMg₁₅ is the Mg-richest phase (more than 88 atom % of Mg) discovered in the Mg-Nd-Ni system. Its structure was determined by X-ray diffraction on single crystal with the following crystal data: tetragonal system, P4/ nmm, Z = 2, a = 10.0602(1) Å, c = 7.7612(2) Å, d_calc = 2.40 g·cm^-3. Its structure is made of a three-dimensional framework of magnesium atoms showing channels filled by one-dimensional chain consisting of alternating Nd and Ni atoms along the c-axis. Anti-ferromagnetic ordering was observed with T_N = 9 K, which is remarkably high considering the long distances between magnetic atoms, that is, Nd atoms. The effective magnetic moment μ_eff is equal to 3.58 μ_B, which is consistent with magnetic Nd³⁺ ions and weakly or nonmagnetic Ni atoms. Below T_N, the M( H) curves show field-induced metamagnetic transitions at critical fields increasing with decreasing temperatures. The magnetic structure of NdNiMg₁₅ was determined from neutron powder diffraction data by considering the propagation vector k = (1/2 1/2 0). This magnetic structure consists in ferromagnetic chains along the c-axis of Nd atoms carrying moments, only separated by Ni atoms. The chains are ferromagnetically coupled within planes perpendicular to the [110] direction, and these planes are anti-ferromagnetically coupled to neighboring planes forming a checkerboard-like magnetic structure.