A neutron diffraction study of the hexagonal Laves phases, Ho(Co0.667Ga0.333)2 and Er(Co0.667Ga0.333)2: Co/Ga site preferences and magnetic structure

Abstract

RE(Co0.667Ga0.333)2 (RE = Ho, Er) were characterized by neutron powder diffraction (NPD). Rietveld refinement of the NPD data at 280K confirms the hexagonal MgZn2-type structure (P63/mmc), in accordance with the previous XRD results. Co/Ga occupancies on the 2a and 6h sites were refined to be 0.46/0.54(2) and 0.74/0.26(2), respectively, indicating a preference of Ga for the 2a site and of Co for the 6h site. Both materials are ferromagnetic, k = (000), with measureable moments only on the RE sites. The magnetic (Shubnikov) space group is P63/mm′c′ for the Er phase. Co moments refined to values of 0.1–0.3 μB and are effectively zero within 3σ. The Er moment, 6.07(8) μB at 3.5K is parallel to the c axis and this orientation persists up to TC = 18.5K, while the Ho moment has components along both the a and c axes with a total moment of 6.3(1) μB at 3.5K. Both moments are much reduced from the free ion values, 9 μB (Er) and 10 μB (Ho), which is attributed to crystal field effects. In addition, the Ho moment angle with the c axis is strongly temperature dependent, being roughly constant, ~ 24(6)° from T = 29K to ~25K, then increasing with decreasing temperature to 44(1)° at 3.5K. Magnetic small angle neutron scattering (MSANS) was observed over a Q-range from 0.14Å−1 to 0.50Å−1 in both samples. The integrated MSANS of the Er phase peaks near TC, then decreases monotonically with decreasing temperature, behavior typical for a simple ferromagnet. On the other hand, the integrated MSANS for the Ho phase exhibits a weak peak around TC = 31K, followed by a plateau to 25K and a strong increase with decreasing temperatures, which tracks the evolution of the Ho moment angle with the c axis.