  We report first results on a method aimed at simultaneously characterising
atmospheric parameters and magnetic properties of M dwarfs from high-resolution
nIR spectra recorded with SPIRou in the framework of the SPIRou Legacy Survey.
Our analysis relies on fitting synthetic spectra computed from MARCS model
atmospheres to selected spectral lines, both sensitive and insensitive to
magnetic fields. We introduce a new code, $\texttt{ZeeTurbo}$, obtained by
including the Zeeman effect and polarised radiative transfer capabilities to
$\texttt{Turbospectrum}$. We compute a grid of synthetic spectra with
$\texttt{ZeeTurbo}$ for different magnetic field strengths and develop a
process to simultaneously constrain $T_{\rm eff}$, $\log{g}$, [M/H],
[$\alpha$/Fe] and the average surface magnetic flux. In this paper, we present
our approach and assess its performance using simulations, before applying it
to six targets observed in the context of the SPIRou Legacy Survey (SLS),
namely AU Mic, EV Lac, AD Leo, CN Leo, PM J18482+0741, and DS Leo. Our method
allows us to retrieve atmospheric parameters in good agreement with the
literature, and simultaneously yields surface magnetic fluxes in the range 2-4
kG with a typical precision of 0.05 kG, in agreement with literature estimates,
and consistent with the saturated dynamo regime in which most of these stars
are.
