Rich magnetic phase diagram of putative helimagnet Sr3Fe2O7

The cubic perovskite ${\text{SrFeO}}_{\text{3}}$ was recently reported to host hedgehog- and skyrmion-lattice phases in a highly symmetric crystal structure which does not support the Dzyaloshinskii-Moriya interactions commonly invoked to explain such magnetic order. Hints of a complex magnetic phase diagram have also recently been found in powder samples of the single-layer Ruddlesden-Popper analog ${\text{Sr}}_{\text{2}}{\text{FeO}}_{\text{4}}$, so a reinvestigation of the bilayer material ${\text{Sr}}_{\text{3}}{\text{Fe}}_{\text{2}}{\text{O}}_{\text{7}}$, believed to be a simple helimagnet, is called for. Our magnetization and dilatometry studies reveal a rich magnetic phase diagram with at least six distinct magnetically ordered phases and strong similarities to that of ${\text{SrFeO}}_{\text{3}}$. In particular, at least one phase is apparently multiple-$\mathbf{q}$, and the $\mathbf{q}\mathrm{s}$ are not observed to vary among the phases. Since ${\text{Sr}}_{\text{3}}{\text{Fe}}_{\text{2}}{\text{O}}_{\text{7}}$ has only two possible orientations for its propagation vector, some of the phases are likely exotic multiple-$\mathbf{q}$ order, and it is possible to fully detwin all phases and more readily access their exotic physics.