Influence of Magnetic Anisotropy on the Ground State of [CH$_3$NH$_3$]Fe(HCOO)$_3$: Insights into the Improper Modulated Magnetic Structure

TL;DR

This study investigates how magnetic anisotropy influences the ground state and magnetic structure of [CH₃NH₃]Fe(HCOO)₃, revealing incommensurate structural modulations and a unique antiferromagnetic order through comprehensive experimental and theoretical analysis.

Contribution

It provides the first detailed characterization of the incommensurate magnetic and structural modulations in [CH₃NH₃]Fe(HCOO)₃, highlighting the role of magnetic anisotropy in its ground state.

Findings

Identification of incommensurate structural modulations below 170 K.

Observation of 3D antiferromagnetic order below 17 K.

Discovery of perpendicular spin orientations in the magnetic structure.

Abstract

The hybrid perovskites [CH$_3$NH$_3$]Co$_x$Ni$_{x-1}$(HCOO)$_3$ with $x$ = 0, 0.25, 0.5, 0.75 and 1.0 possess multiple phase transitions including incommensurate structures. [CH$_3$NH$_3$]Ni(HCOO)$_3$ has also been found to have a proper magnetic incommensurate structure in its ground state. We have carried out a detailed structural characterization of the isomorphous [CH$_3$NH$_3$]Fe(HCOO)$_3$ (1) to investigate whether it also has incommensurate structural and magnetic modulations. We confirm that 1 crystallizes in the $Pnma$ space group at room temperature (RT) with a perovskite structure. Upon cooling, at about 170 K, the occurrence of new satellite reflections in the diffraction pattern show a phase transition to a modulated structure, which could be refined in the $Pnma(00\gamma)0s0$ super space group with $q_1~=~0.1662(2)c^\ast$. On further cooling to 75 K the satellite reflections become closer to the main reflections, indicating a new phase transition that keeps the super space group invariant but changes the modulation wave vector, $q_2~=~0.1425(2)c^\ast$. The structure then does not change structural phase down to base temperature (2 K). Magnetic susceptibility measurements collected under field-cooled and zero-field-cooled reveal a 3D antiferromagnetic order below 17 K. The overlapping in temperature between structural modulation and long-range magnetic order presents a unique opportunity to study magneto-structural coupling. Our results point to an improper modulated structure where interestingly the spins oriented strictly antiferromagnetic are perpendicular to those of previously reported compounds. In the present work, 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 1.