Magnetic phase transitions in TbFe2Al10, HoFe2Al10 and ErFe2Al10

The magnetic order of the orthorhombic aluminides TbFe2Al10, HoFe2Al10 and ErFe2Al10 (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 TbFe2Al10 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 Hc1 = 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 Hc2 = 1.8 T. Above this field finally all the rest of the spins flip, resulting in a ferromagnetic order of the terbium moments. ErFe2Al10 orders antiferromagnetically below TN = 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 HoFe2Al10 down to 100 mK.