Frustrated pentagonal Cairo lattice in the non-collinear antiferromagnet Bi4Fe5O13F

TL;DR

This study investigates the complex magnetic behavior of Bi4Fe5O13F, revealing multiple magnetic transitions and a non-collinear antiferromagnetic structure in a pentagonal Cairo lattice, using advanced diffraction and computational techniques.

Contribution

It provides the first detailed analysis of the magnetic structure and interactions in Bi4Fe5O13F, a model system for the Cairo pentagonal spin lattice, highlighting its unique frustrated exchange couplings.

Findings

Multiple magnetic transitions at T1=62 K, T2=71 K, and TN=178 K.

Formation of a fully ordered non-collinear antiferromagnetic structure below T1.

Evidence of intricate magnetic transitions not previously observed in pentagonal Cairo lattices.

Abstract

The crystal and magnetic structures and underlying magnetic interactions of Bi4Fe5O13F, a model system for studying the physics of the Cairo pentagonal spin lattice, are investigated by transmission electron microscopy, low-temperature synchrotron x-ray and neutron powder diffraction, thermodynamic measurements, and density functional band-structure calculations. The crystal structure of Bi4Fe5O13F contains infinite rutile-like chains of edge-sharing FeO6 octahedra interconnected by the Fe2O7 groups of two corner-sharing FeO4 tetrahedra. The cavities between the chains are filled with the fluorine-centered Bi4F tetrahedra. The Fe3+ cations form pentagonal units that give rise to an unusual topology of frustrated exchange couplings and underlie a sequence of the magnetic transitions at T1= 62 K, T2 = 71 K, and TN = 178 K. Below T1, Bi4Fe5O13F forms a fully ordered non-collinear antiferromagnetic structure, whereas the magnetic state between T1 and TN may be partially disordered according to the sizable increase in the magnetic entropy at T1 and T2. Therefore, Bi4Fe5O13F shows the evidence of intricate magnetic transitions that were never anticipated for the pentagonal Cairo spin lattice. Additionally, it manifests a sillimanite (Al2SiO5)-based homologous series of compounds that feature the pentagonal magnetic lattice spaced by a variable number of octahedral units along the rutile-type chains.