  This paper presents a compact low-temperature atomic vector magnetometer for
weak field measurements, using an atomic cell containing two orthogonal
multipass cavities. At the working temperature of 75 $^\circ$C, the magnetic
field sensitivities at all three axes are better than 45 fT/Hz$^{1/2}$ at 10~Hz
limited by photon noise, and 85 fT/Hz$^{1/2}$ at 0.1~Hz. This sensor also shows
measurement stabilities better than 1.5~pT at three axes for an integration
time of $10^4$ s, even with the laser frequency unlocked. The sensor response
to a rotation is demonstrated, which is also developed to measure the effective
gyromagnetic ratio of atoms in this sensor when the bias field is nulled. This
magnetometer makes an important step towards long-term stable measurements and
calibrations of ultra-low fields.
