Magnetic properties of Th3P4-type Tb4Sb3−X{Si, Ge, Bi}X solid solutions

The magnetic properties of Tb4Sb2Bi, Tb4Sb2.75Ge0.25 and Tb4Sb2.3Si0.7 solid solutions (Th3P4-type, cubic; cI28, space group I4¯3d, No. 220) have been investigated by means of magnetization and neutron diffraction studies. The magnetization measurements indicate that the Tb4Sb2Bi compound orders magnetically at 120 K (TN) and 50 K (TC). The magnetization vs. field isotherm at 5 K shows signature of ferromagnetic order. Neutron diffraction experiment in zero applied magnetic field shows that below TN = 125(4) K Tb4Sb2Bi exhibits a antiferromagnetic flat spiral type ordering with wave vector K1 = [±KX, ± KX, ± KX] (KX value changes from 0.1033 up to 0.158 in the temperature interval down to 2 K). Below Tm = 70(4) K the decreasing of antiferromagnetic component coincides with increasing of the ferromagnetic component and about TCN = 55(4) K, Tb4Sb2Bi shows ferromagnetic transition. At 2 K, the magnetic structure of Tb4Sb2Bi is ferromagnetic cone and the cone axis is along [1 1 1] direction.Neutron diffraction study of Tb4Sb2.75Ge0.25 indicate an antiferromagnetic ordering at 155(4) K (ferromagnetic cone with K1 = [±0.136(2), ±0.136(2), ±0.136(2)]) and ferromagnetic ordering (sharp increasing of ferromagnetic component) at 145(4) K. The KX value is nearly constant from 155 K down to 2 K. At 2 K, the magnetic structure of Tb4Sb2.75Ge0.25 is ferromagnetic cone with a weak antiferromagnetic component and the cone axis is along [1 1 1] direction.Low field magnetisation measurements on Tb4Sb2.3Si0.7 indicate a ferromagnetic ordering at TC ∼ 180 K. Thus small substitutions of Bi, Ge and Si at Sb-site of Tb4Sb3 are found to increase the magnetic ordering temperature of the parent compound.