Tuning the antiferromagnetic helical pitch length and nanoscale domain size in Fe$_3$PO$_4$O$_3$ by magnetic dilution

TL;DR

This study demonstrates how magnetic dilution in Fe3PO4O3 tunes the helical pitch length and domain size, revealing the relationship between magnetic interactions and nanoscale magnetic structures in an unconventional antiferromagnet.

Contribution

It introduces a method to control magnetic structure in Fe3PO4O3 through non-magnetic Ga substitution, providing new insights into the magnetic frustration and nanoscale domain formation.

Findings

Helical pitch length decreases with increasing Ga dilution.

Nanoscale domain size reduces and long-range order is lost at higher dilution.

The ratio of magnetic interactions J2/J1 correlates with the observed magnetic changes.

Abstract

The insulating magnetic material Fe3PO4O3 features a non-centrosymmetric lattice composed of Fe^{3+} triangular units. Frustration, due to competing near neighbor ($J_1$) and next nearest neighbor ($J_2$) antiferromagnetic interactions, was recently suggested to be the origin of an antiferromagnetic helical ground state with unusual needle-like nanoscale magnetic domains in Fe3PO4O3. Magnetic dilution is shown here to tune the ratio of these magnetic interactions, thus providing deeper insight into this unconventional antiferromagnet. Dilution of the Fe^{3+} lattice in Fe3PO4O3 was accomplished by substituting non-magnetic Ga^{3+} to form the solid solution series Fe_{3-x}Ga_xPO4O3 with $x = 0.012, 0.06, 0.25, 0.5, 1.0, 1.5$. Magnetic susceptibility and neutron powder diffraction data from this series are presented. A continuous decrease of the both the helical pitch length and the domain size is observed with increasing dilution up to at least $x = 0.25$, while for $x \ge 0.5$, the compounds lack long range magnetic order entirely. The decrease in the helical pitch length with increasing $x$ can be qualitatively understood by reduction of the ratio of $J_2/J_1$ in the Heisenberg model, consistent with mean field considerations. Intriguingly, the magnetic correlation length in the $ab$ plane remains nearly equal to the pitch length for each value of $x \le 0.25$, showing that the two quantities are intrinsically connected in this unusual antiferromagnet.