Influence of Magnetic Anisotropy on the Ground State of [CH₃NH₃]Fe(HCOO)₃: Insights into the Improper Modulated Magnetic Structure

The hybrid perovskites [CH₃NH₃]Co<i>_x</i>Ni_<i>x</i>-1(HCOO)₃ with <i>x</i> = 0, 0.25, 0.5, 0.75, and 1.0 possess multiple phase transitions, including incommensurate structures. Notably, [CH₃NH₃]Ni(HCOO)₃ features a proper magnetically incommensurate structure ground state. To explore similar behavior, we investigated the isomorphous [CH₃NH₃]Fe(HCOO)₃ (<b>1</b>). A combination of magnetometry measurements, single crystal and powder neutron diffraction, and density functional theory calculations have been used to accurately determine and understand the sequence of nuclear and magnetic phases present in compound <b>1</b>. At room temperature, it crystallizes in the <i>Pnma</i> space group with a perovskite structure. Below 170 K, new satellite reflections indicate a transition to a modulated structure, refined in the <i>Pnma</i>(00γ)0<i>s</i>0 with <b>q₁</b> = 0.1662(2)<i>c</i>*. At 75 K, the satellite reflections become closer to the main reflections, indicating a second transition, which maintains the superspace group symmetry but decreases the modulation wave vector to <b>q₂</b> = 0.1425(2)<i>c</i>*, i.e., with a longer modulation period. This modulation persists to 2 K, overlapping with the onset of 3D antiferromagnetic order at 17 K, offering a unique opportunity to study magneto-structural coupling. Our results point to an improper magnetic modulated structure where, interestingly, the spins are perpendicular to those of previously reported compounds.