The magnetic ordering of the novel binary compound ( structure type, space group Cmcm) has been studied by neutron diffraction, specific heat and magnetic measurements. Antiferromagnetic ordering of sets in below K and is associated with two wave vectors and . The major magnetic phase with has a planar canted antiferromagnetic structure. Group theory was used to determine the possible magnetic structures. The two Tm sites ( and ) order simultaneously with mutually perpendicular orientations. Both sites split into two independent sets under the action of the wave vector . The sites have a uniaxial arrangement along the b-axis with the same moment values. The sites have a uniaxial moment arrangement along the a-axis but two different moment values. At 1.7 K the ordered magnetic moment values are 4.38(5) , 3.1(2) and 0.7(2) . The minor magnetic phase with has a planar canted antiferromagnetic structure. The and sites order with a uniaxial antiferromagnetic moment arrangement and with mutually perpendicular orientations: along the b- and c-axis, respectively. At 1.7 K the ordered magnetic moment values are 1.5(1) and 0.9(2) .