Anisotropic putative "up-up-down" magnetic structure in EuTAl$_4$Si$_2$ (T = Rh and Ir)

TL;DR

This study investigates the complex magnetic behavior of EuRhAl4Si2 and EuIrAl4Si2, revealing an anisotropic 'up-up-down' magnetic structure with multiple magnetic phases and unusual hysteresis effects in their magnetization properties.

Contribution

The paper reports the discovery of an anisotropic 'up-up-down' magnetic structure in EuTAl4Si2 compounds, supported by experimental data and a theoretical model including exchange and dipolar interactions.

Findings

EuRhAl4Si2 and EuIrAl4Si2 order antiferromagnetically with two magnetic transitions.

Magnetization exhibits a 1/3 plateau and hysteresis, indicating complex magnetic states.

Model suggests an 'up-up-down' magnetic structure in zero field.

Abstract

We present detailed investigations in single crystals of two recently reported quaternary intermetallic compounds EuRhAl$_4$Si$_2$ and EuIrAl$_4$Si$_2$ employing magnetization, electrical resistivity in zero and applied fields, heat capacity and $^{151}$Eu M\"{o}ssbauer spectroscopy measurements. The two compounds order antiferromagnetically at $T_{\rm N1}$ = 11.7 and 14.7\,K, respectively, each undergoing two magnetic transitions: the first from paramagnetic to incommensurate modulated antiferromagnetic, the second at lower temperature to a commensurate antiferromagnetic phase as confirmed by heat capacity and M\"{o}ssbauer spectra. 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 T = Rh and 4\,T for T = Ir. 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.