Neutron Scattering Study of Antiferromagnet U₂Pt₆Al₁₅

We have performed the neutron scattering study of antiferromagnet U2Pt6Al15 with the honeycomb lattice to determine the magnetic structure. U2Pt6Al15 exhibits two phase transitions at Tm1 = 9 K and Tm2 = 26 K. In phase I at Tm1 < T < Tm2, the magnetic reflections described by a propagation vector kI = (1/3, 0, 0) were observed. In phase II at T < Tm1, magnetic reflections of kI = (1/3, 0, 0) and kII = (1/2, 0, 0) were observed and the magnetic reflection intensity of kI = (1/3, 0, 0) is weak but finite. The hysteresis in the temperature dependent intensities is observed across the phase transition at Tm1, suggesting that the transition at Tm1 is of first order.