  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.
