Neutron powder diffraction study on the magnetic structure of NdPd₅Al₂

The magnetic structure of NdPd₅Al₂ has been studied by neutron powder diffraction. We observed the magnetic reflections with the modulation vector q = (1/2, 0, 0) below the ordering temperature T_N. We found a collinear magnetic structure with a Nd moment of 2.7(3) μ_B at 0.5 K parallel to the c-axis, where the ferromagnetically ordered a-planes stack with a four-Nd-layer period having a ++-- sequence along the a-direction with the distance between adjacent Nd layers equal to a/2 (magnetic space group P_anma). This "stripe"-like modulation is very similar to that in CePd₅Al₂ with q = (0.235, 0.235, 0) with the Ce moment parallel to the c-axis. These structures with in-plane modulation are a consequence of the two-dimensional nature of the Fermi surface topology in this family, originating from the unique crystal structure with a very long tetragonal unit cell and a large distance of >7 Å between the rare-earth layers separated by two Pd and one Al layers.