Magnetic properties of a TbFe_3.5Al_8.5 single crystal

We have studied the magnetic behaviour of a (tetragonal) TbFe_3.5Al_8.5 single crystal extending earlier work on a TbFe_4.4Al_7.6 one. At 5 K, the magnetisation curve measured along the [0 0 1] direction points to easy-plane behaviour, whereas that along the [1 0 0] direction shows a strong hysteresis, coercivity field of about 0.5 T and a remanence of 5.0 μ_B/f.u. A similar behaviour is observed when the field is applied along the [1 1 0] direction, the remanence now being 3.7 μ_B/f.u., which corresponds closely to ½√2 of the remanence measured along the [1 0 0] direction. It is concluded that the orthorhombically distorted ferrimagnetic state, found to be field-induced for TbFe_4.4Al_7.6, is formed spontaneously in TbFe_3.5Al_8.5. The magnetisation was measured at increasing temperature in a field of 0.05 T applied along the [1 0 0] direction. After field cooling (FC), the magnetisation decreases from about 4.8 μ_B/f.u. at 5 K down to about zero at 110 K. After zero-field cooling (ZFC), the magnetisation starts with a very small (consistently negative) value, passes through a maximum (at about 60 K) and then joins the FC curve. From a closer examination of the low-temperature ZFC virgin magnetisation curves, it was concluded that 180° walls play an important role in the initial stage.