  We aim to characterise the surface magnetic fields of a sample of 8 T Tauri
stars from high-resolution near-IR spectroscopy. Some stars in our sample are
known to be magnetic from previous spectroscopic or spectropolarimetric
studies. Our goals are 1) to apply Zeeman broadening modelling to T Tauri stars
with high-resolution data, 2) to expand the sample of stars with measured
surface magnetic field strengths, 3) to investigate possible rotational or
long-term magnetic variability by comparing spectral time series of given
targets, and 4) to compare the magnetic field modulus <B> tracing small-scale
magnetic fields to those of large-scale magnetic fields derived by Stokes V
Zeeman Doppler Imaging. We modelled the Zeeman broadening of magnetically
sensitive spectral lines in the near-IR K-band from high-resolution spectra by
using magnetic spectrum synthesis based on realistic model atmospheres and by
using different descriptions of the surface magnetic field. We developped a
Bayesian framework that selects the complexity of the magnetic field
prescription based on the information contained in the data. We obtain
individual magnetic field measurements for each star in our sample using four
different models. We find that the Bayesian Model 4 performs best in the range
of magnetic fields measured on the sample (from 1.5 kG to 4.4 kG). We do not
detect a strong rotational variation of <B> with a mean peak-to-peak variation
of 0.3 kG. Our confidence intervals are of the same order of magnitude, which
suggests that the Zeeman broadening is produced by a small-scale magnetic field
homogeneously distributed over stellar surfaces. A comparison of our results
with mean large-scale magnetic field measurements from Stokes V ZDI show
different fractions of mean field strength being recovered, from 25-42% for
relatively simple poloidal axisymmetric field topologies to 2-11% for more
complex fields.
