MAGNETIC STRUCTURE OF TMNI2B2C

Neutron-diffraction measurements have been carried out to determine the nature of the magnetic ordering in ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C. ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C exhibits superconductivity below 11 K and magnetic ordering below 1.5 K. The magnetic structure is incommensurate and consists of ferromagnetic (110) planes of Tm moments aligned along the c axis with the magnitude of the moments modulated sinusoidally along the (110) direction, the modulation wave vector having a magnitude of 0.241 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$. The magnetic structure as well as the modulation direction exhibited by ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C is different from that of other R${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C (R=Gd--Er) compounds. The modulated state formed by the Tm moments allows the superconductivity to coexist with the magnetic ordering below ${\mathit{T}}_{\mathit{N}}$. \textcopyright{} 1996 The American Physical Society.