  Magnetic fields are an energetically important component of star-formation
galaxies, but it is often difficult to measure their properties from
observations. One of the complexities stems from the fact that the magnetic
fields, especially in spiral galaxies, have a two-scale nature: a large-scale
field, coherent over ${\rm kpc}$ scales and a small-scale, random field with a
scale of $\lesssim$ $100~{\rm pc}$. Moreover, it is known that the strength of
small- and large-scale fields are comparable and this makes it even harder to
find their imprints in radio polarisation observations such as the Faraday
rotation measure, ${\rm RM}$, which is the integral over the path length of the
product of the thermal electron density and the parallel component of the
magnetic field to the line of sight. Here, we propose and demonstrate the use
of second-order structure functions of ${\rm RM}$ computed with multiple
higher-order stencils as a powerful analysis to separate the small- and
large-scale magnetic field components. In particular, we provide new methods
and calibrations to compute the scale and the strength of the large-scale
magnetic field in the presence of small-scale magnetic fluctuations. We then
apply the method to find the scale of large-scale magnetic fields in the nearby
galaxies M51 and NGC 6946, using archival data and further discuss the need for
computing the ${\rm RM}$ structure functions with higher-order stencils. With
multiple modern radio polarisation observatories and eventually the Square
Kilometre Array, ${\rm RM}$ observations will significantly improve in quantity
and quality, and the higher-order stencil structure function techniques
developed here can be used to extract information about multiscale magnetic
fields in galaxies.
