Visualization of ferromagnetic domains in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C single crystals: Weak ferromagnetism and its coexistence with superconductivity

TL;DR

This study visualizes magnetic domains in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C crystals, revealing weak ferromagnetism and its coexistence with superconductivity through high-resolution Bitter decoration.

Contribution

It provides detailed imaging of magnetic domain boundaries in these compounds, linking domain structures to weak ferromagnetism and antiferromagnetic order, and discusses their interpretation as Bloch domain walls.

Findings

0 boundaries coincide with weak ferromagnetic ordering.

0 twin boundaries are antiferromagnetic domains observed below T_N.

Magnetic domain structures relate to coexistence of ferromagnetism and superconductivity.

Abstract

The magnetic flux structure in the basal plane, (001), of single crystals of superconducting (R = Er) and non-superconducting (R = Tb) RNi$_2$B$_2$C was studied by high resolution Bitter decoration at temperatures below $T_c$ (superconducting transition) and/or $T_N$ (antiferromagnetic transition). For both materials two sets of domain boundaries, in \{110\} and \{100\} planes, were observed. The temperature ranges in which the \{100\} domain boundaries were observed in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C coincide with the weak ferromagnetic (WFM) ordering in these materials. On the other hand, the \{110\} twin boundaries - the antiferromagnetic domain boundaries - were observed in both compounds below $T_N$. The possibility of interpretation of \{100\} boundaries as Bloch domain walls in the weakly ferromagnetic phase, for $T < T_{WFM} < T_N$ (TbNi$_2$B$_2$C) or $T < T_{WFM} < T_N < T_c$ (ErNi$_2$B$_2$C) is discussed.