Effect of geometrical frustration on the magnetic properties of the triangular-layer system Tb2C2I2:: a neutron diffraction investigation

Powder neutron diffraction on Tb2C2I2 reveals strong resolution-limited magnetic Bragg reflections below 60 K due to commensurate antiferromagnetic (afm) long range ordering. Between 60 and 85 K the magnetic reflections broaden beyond instrumental resolution and the propagation vector becomes incommensurate. Above approximate to95 K magnetic scattering is only seen as a diffuse ridge which we ascribe to short-range correlations within the triangular Tb atom planes. With decreasing temperature an additional Lorentzian shaped reflection grows on top of the decaying edge of this diffuse ridge. We analyse the diffuse scattering by fitting a Warren-type lineshape and a Lorentzian shaped Bragg reflection. The results of the fits indicate that, with diverging intra-plane correlations, significant inter-plane afm correlations build up close to 85 K. Above 95 K inter-plane correlations are essentially limited to nearest-neighbour Tb atom double layers only.