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

## TL;DR

This study explores the magnetic and structural properties of a hybrid perovskite material, revealing unique phase transitions and magnetic order.

## Contribution

The paper identifies an improper modulated magnetic structure with perpendicular spin orientation in [CH3NH3]Fe(HCOO)3.

## Key findings

- The compound undergoes two modulated structural transitions below 170 K.
- Magnetic anisotropy is found to influence the improper magnetic structure and spin orientation.
- The material exhibits 3D antiferromagnetic order at 17 K with a longer modulation period.

## Abstract

The hybrid perovskites [CH3NH3]CoxNix–1(HCOO)3 with x = 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 (1). 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. At room temperature, it crystallizes in
the Pnma space group with a perovskite structure.
Below 170 K, new satellite reflections indicate a transition to a
modulated structure, refined in the Pnma(00γ)0s0 with q1 = 0.1662(2)c*. 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 q2 = 0.1425(2)c*, 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.

The present study investigates the phase transitions and
magnetic properties of [CH3NH3]Fe(HCOO)3, highlighting its two modulated structural phases and improper
magnetic order with perpendicular spin orientations compared to analogous
compounds. Noncollinear DFT calculations reveal significant magnetic
anisotropy, which may explain the distinct magnetic behavior.