Rich Magnetic Phase Diagram of Putative Helimagnet Sr$_3$Fe$_2$O$_7$

TL;DR

This study uncovers a complex magnetic phase diagram in Sr$_3$Fe$_2$O$_7$, revealing at least six magnetic phases, some with exotic multiple-q order, challenging previous assumptions of its simple helimagnetic nature.

Contribution

The paper provides the first detailed magnetic phase diagram of Sr$_3$Fe$_2$O$_7$, identifying multiple phases and potential exotic magnetic orders not previously understood.

Findings

At least six distinct magnetic phases identified.

Presence of multiple-q magnetic order in some phases.

Strong similarities to the magnetic behavior of SrFeO$_3$.

Abstract

The cubic perovskite SrFeO$_3$ 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 Sr$_2$FeO$_4$, so a reinvestigation of the bilayer material Sr$_3$Fe$_2$O$_7$, believed to be a simple helimagnet, is called for. Our magnetization and dilatometry studies reveal a rich magnetic phase diagram with at least 6 distinct magnetically ordered phases and strong similarities to that of SrFeO$_3$. In particular, at least one phase is apparently multiple-$\mathbf{q}$, and the $\mathbf{q}$s are not observed to vary among the phases. Since Sr$_3$Fe$_2$O$_7$ 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.