Magnetic Ordering and Frustration in Hexagonal UNi4B

We have determined unusual magnetic ordering of the hexagonal intermetallic uranium compound U${\mathrm{N}}_{\mathrm{i}4\mathrm{}}$B via neutron diffraction. In the easy basal plane the U moments have triangular symmetry with antiferromagnetic interactions. Along the hard $c$ axis ferromagnetic coupling occurs. Below $T_N=20\mathrm{}$ K only two out of every three U moments of $1.2{\mu }_B$ order in vortexlike arrangements around the third paramagnetic spin. This novel magnetic structure is related to the occurrence of a crystallographic superstructure. Previously observed anomalies in bulk properties below $T_N$ are attributed to unconventional spin-wave excitations associated with this type of ordering.