Close M-dwarf binaries and higher multiples allow the investigation of
rotational evolution and mean magnetic flux unbiased from scatter in
inclination angle and age since the orientation of the spin axis of the
components is most likely parallel and the individual systems are coeval.
Systems composed of an early (M0.0 -- M4.0) and a late (M4.0 -- M8.0) type
component offer the possibility to study differences in rotation and magnetism
between partially and fully convective stars. We have selected 10 of the
closest dM systems to determine the rotation velocities and the mean magnetic
field strengths based on spectroscopic analysis of FeH lines of Wing-Ford
transitions at 1 μm observed with VLT/CRIRES. We also studied the quality
of our spectroscopic model regarding atmospheric parameters including
metallicity. A modified version of the Molecular Zeeman Library (MZL) was used
to compute Land\'e g-factors for FeH lines. Magnetic spectral synthesis was
performed with the Synmast code. We confirmed previously reported findings that
less massive M-dwarfs are braked less effectively than objects of earlier
types. Strong surface magnetic fields were detected in primaries of four
systems (GJ 852, GJ 234, LP 717-36, GJ 3322), and in the secondary of the
triple system GJ 852. We also confirm strong 2 kG magnetic field in the primary
of the triple system GJ 2005. No fields could be accurately determined in
rapidly rotating stars with \vsini>10 \kms. For slow and moderately rotating
stars we find the surface magnetic field strength to increase with the
rotational velocity \vsini which is consistent with other results from
studying field stars.Comment: Accepted by MNRAS, 10 pages, 4 figures, 4 table
