  The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the most
sensitive radio telescope for pulsar observations. We make polarimetric
measurements of a large number of faint and distant pulsars using the FAST. We
present the new measurements of Faraday rotation for 134 faint pulsars in the
Galactic halo. Significant improvements are also made for some basic pulsar
parameters for 15 of them. We analyse the newly determined rotation measures
(RMs) for the Galactic magnetic fields by using these 134 halo pulsars,
together with previously available RMs for pulsars and extragalactic radio
sources and also the newly determined RMs for another 311 faint pulsars which
are either newly discovered in the project of the Galactic Plane Pulsar
Snapshot (GPPS) survey or previously known pulsars without RMs. The RM
tomographic analysis in the first Galactic quadrant gives roughly the same
field strength of around 2~$\mu$G for the large-scale toroidal halo magnetic
fields. The scale height of the halo magnetic fields is found to be at least
2.7$\pm$0.3~kpc. The RM differentiation of a large number of pulsars in the
Galactic disk in the Galactic longitude range of $26^{\circ}<l<90^{\circ}$
gives evidence for the clockwise magnetic fields (viewed from the north
Galactic pole) in two interarm regions inside the Scutum arm and between the
Scutum and Sagittarius arm, and the clockwise fields in the Local-Perseus
interarm region and field reversals in the Perseus arm and beyond.
