  Results of magnetic field and temperature dependent neutron diffraction and
magnetization measurements on oxy-arsenate Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$
are reported. The crystal structure of this compound contains
pseudo-one-dimensional [Fe$_{2}$O$_{6}$]$^\infty$ sawtooth-like chains, formed
by corner sharing isosceles triangles of $Fe^{3+}$ ions occupying two
nonequivalent crystallographic sites. The chains extend infinitely along the
crystallographic $b$-axis and are structurally confined from one another via
diamagnetic (AsO$_{4}$)$^{3-}$ units along the $a$-axis, and Rb$^+$ cations
along the $c$-axis direction. Neutron diffraction measurements indicate the
onset of a long range antiferromagnetic order below approximately 25 K. The
magnetic structure consists of ferrimagnetic chains which are
antiferromagnetically coupled with each other. Within each chain, one of the
two Fe sites carries a moment which lies along the \emph{b}-axis, while the
second site bears a canted moment in the opposite direction. Externally applied
magnetic field induces a transition to a ferrimagnetic state, in which the
coupling between the sawtooth chains becomes ferromagnetic. Magnetization
measurements performed on optically-aligned single crystals reveal evidence for
an uncompensated magnetization at low magnetic fields that could emerge from to
a phase-segregated state with ferrimagnetic inclusions or from
antiferromagnetic domain walls. The observed magnetic states and the
competition between them is expected to arise from strongly frustrated
interactions within the sawtooth chains and relatively weak coupling between
them.
