Neutron diffraction study of magnetically ordered TbAsO4

TbAsO₄ undergoes a magnetic phase tranfition at T_N = 1.5 K into a helical antiferromagnetic moment configuration. The spiral is planar, with the propagation vector along [001] and the moments perpendicular to [001]. The propagation period is $\text{197 ± 12}\text{A}\text{?}$, equivalent to 31 ± 2 unit cell constants. The neutron diffraction results are discussed in connection with previously measured macroscopic data.