Neutron diffraction studies on an exotic magnetic system, Nd₇Rh₃

Abstract

Binary intermetallic compound Nd₇Rh₃, crystallizes in a Th₇Fe₃ type hexagonal structure in space group P6₃mc and has been reported to show two antiferromagnetic (AFM) phase transitions at 32 K and 10 K from magnetic susceptibility, and a field induced first-order magnetic transition at a field strength of 1 Tesla at 2 K from magnetization measurements. These magnetic properties are different from the isostructural counterparts like Tb₇Rh₃, Ho₇Rh₃, and Nd₇Ni₃. In order to understand the differences in the magnetic behaviour between Nd₇Rh₃ and other isostructural compounds, we have carried out neutron diffraction (ND) studies on polycrystalline Nd₇Rh₃, at various temperatures between 2 and 45 K. ND patterns were also recorded at T = 2K in the presence of applied magnetic fields from 0 to 1.5 Tesla. ND spectra on Nd₇Ni₃ and Ho₇Rh₃, for example, at 2 K in zero magnetic field exhibit a strong AFM peak at a Q ( = 4πSinθ/λ) value of 0.32 and 0.38 Å^-1, respectively. However, ND spectra on Nd₇Rh₃, carried out at Focusing Crystal Diffractometer (FCD-Dhruva-Mumbai), diffractometers E6 and E9 (HZB-Berlin) using wavelengths λ = 1.48Å, 2.45Å and 2.8Å respectively, at T = 2K in zero field do not exhibit any AFM peaks in the entire Q-range studied. Instead, long-range ferromagnetic order is established as evidenced by the intensity enhancement of the nuclear Bragg peaks. The lack of extra peaks in the ND patterns would imply that the magnetic order seen in magnetization could be of a k = 0 type which changes from one type to another on the application of magnetic field. The results of nuclear and magnetic structure refinement are discussed to explain the magnetic behaviour of Nd₇Rh₃, at low temperatures.