Neutron Diffraction and Magneto‐Optical Study of the Effect of Chloride Doping. On the Incommensurate Magnetic Phase of NiBr2

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AbstractThe magnetic structures of crystals of NiBr2 doped with 2, 5, and 10 mol % NiCl2 are investigated by neutron diffraction and optical spectroscopy, with special reference to the incommensurate helical phase. Behaviour of the magnetic satellite reflections around (003/2) shows that the propagation vector of the helical phase is reduced from 0.15 nm−1 in NiBr2 to 0.12 nm−1 in the crystal doped with 5% NiCl2. The commensurate‐incommensurate transition temperature Tic is similarly reduced, from 22.5 to 17 K. The propagation direction is unaffected by the dopant, remaining along [110]. Although no resolved magnetic satellite reflections are found in the 10% Cl− ‐doped crystal, a difference plot between 7 and 10 K suggests that the incommensurate phase exists below 10 K. The Néel temperatures of the samples show no significant variation with dopant concentration. The intensity of the “cold” exciton‐magnon combination band between 6050 and 6100 Å, which is associated with the incommensurate phase is used to determine Tic and its behaviour in a magnetic field is used to plot magnetic phase diagrams in the Hab‐T plane. No cold band is observed in the 10% Cl− ‐doped crystal. A hot band. resulting from the ferromagnetic component of the intraplanar coupling, is also studied.