Magnetic phase transitions in TbFe₂Al₁₀, HoFe₂Al₁₀and ErFe₂Al₁₀

The magnetic order of the orthorhombic aluminides TbFe 2 Al 10 , HoFe 2 Al 10 and ErFe 2 Al 10 (space group Cmcm ) has been studied by specific heat and magnetic measurements, as well as by neutron powder diffraction down to 100 mK and in external fields up to 5 T. Only the rare-earth ions carry a magnetic moment. At T = 1.5 K the terbium moments in TbFe 2 Al 10 show a square-wave modulated magnetic order with wavevector k = (0, 0.7977, 0) and a moment direction parallel to the a -axis. At a critical field of H c 1 = 0.9 T one of ten spins is forced to flip, going into an intermediate ferrimagnetic phase that is stable up to the critical field H c 2 = 1.8 T. Above this field finally all the rest of the spins flip, resulting in a ferromagnetic order of the terbium moments. ErFe 2 Al 10 orders antiferromagnetically below T N = 1.77(7) K with a similar magnetic structure characterized by a wavevector k = (0, ∼ 0.8, 0). In contrast, no signs of long range magnetic order could be observed for HoFe 2 Al 10 down to 100 mK.