Magnetization measurements of antiferromagnetic domains in ${\mathrm{Sr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$

The ${\mathrm{Cu}}_3{\mathrm{O}}_4$ layer in ${\mathrm{Sr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$ is a variant of the square ${\mathrm{CuO}}_2$ lattice of the high-temperature superconductors, in which the center of every second plaquette contains an extra ${\mathrm{Cu}}^{2+}$ ion. Whereas the ordering of the spins in the ground-state and the spin-wave excitations of this frustrated spin system are both well understood, we find peculiar behavior resulting from antiferromagnetic domain walls. Pseudodipolar coupling between the two sets of ${\mathrm{Cu}}^{2+}$ ions results in a ferromagnetic moment, the direction of which reflects the direction of the antiferromagnetic staggered moment, allowing us to probe the antiferromagnetic domain structure. After an excursion to the high fields $(>\mathrm{}1\mathrm{}\hspace{0.5em}\mathrm{T}),$ as the field is lowered, we observe the growth of domains with ferromagnetic moment perpendicular to the field. This gives rise to a finite domain wall susceptibility at small fields, which diverges near 100 K, indicating a phase transition. We also find that the shape of the sample influences the domain-wall behavior.