Magnetic anisotropy, unusual hysteresis and putative “up-up-down” magnetic structure in EuTAl(4)Si(2) (T = Rh and Ir)

We present detailed investigations on single crystals of quaternary EuRhAl(4)Si(2) and EuIrAl(4)Si(2). The two compounds order antiferromagnetically at T(N1) = 11.7 and 14.7 K, respectively, each undergoing two magnetic transitions. The magnetic properties in the ordered state present a large anisotropy despite Eu(2+)being an S-state ion for which the single-ion anisotropy is expected to be weak. Two features in the magnetization measured along the c-axis are prominent. At 1.8 K, a ferromagnetic-like jump occurs at very low field to a value one third of the saturation magnetization (1/3 M(0)) followed by a wide plateau up to 2 T for Rh and 4 T for Ir-compound. At this field value, a sharp hysteretic spin-flop transition occurs to a fully saturated state (M(0)). Surprisingly, the magnetization does not return to origin when the field is reduced to zero in the return cycle, as expected in an antiferromagnet. Instead, a remnant magnetization 1/3 M(0) is observed and the magnetic loop around the origin shows hysteresis. This suggests that the zero field magnetic structure has a ferromagnetic component, and we present a model with up to third neighbor exchange and dipolar interaction which reproduces the magnetization curves and hints to an “up-up-down” magnetic structure in zero field.