  The valence and spin state evolution of Mn and Co on TbMn$_{\rm 1-x}$Co$_{\rm
x}$O$_3$ series is precisely determined by means of soft and hard x-ray
absorption spectroscopy (XAS) and K$\beta$ x-ray emission spectroscopy (XES).
Our results show the change from Mn$^{3+}$ to Mn$^{4+}$ both high-spin (HS)
together with the evolution from Co$^{2+}$ HS to Co$^{3+}$ low-spin (LS) with
increasing $\rm x$. In addition, high energy resolution XAS spectra on the K
pre-edge region are interpreted in terms of the strong charge transfer and
hybridization effects along the series. These results correlate well with the
spin values of Mn and Co atoms obtained from the K$\beta$ XES data. From this
study, we determine that Co enters into the transition metal sublattice of
TbMnO$_3$ as a divalent ion in HS state, destabilizing the Mn long range
magnetic order since very low doping compositions (${\rm x} \le 0.1$). Samples
in the intermediate composition range ($0.4 \le {\rm x} \le 0.6$) adopt the
crystal structure of a double perovskite with long range ferromagnetic ordering
which is due to Mn$^{4+}$-O-Co$^{2+}$ superexchange interactions with both
cations in HS configuration. Ferromagnetism vanishes for ${\rm x} \ge 0.7$ due
to the structural disorder that collapses the double perovskite structure. The
spectroscopic techniques reveal the occurrence of Mn$^{4+}$ HS and a
fluctuating valence state Co$^{2+}$ HS/Co$^{3+}$ LS in this composition range.
Disorder and competitive interactions lead to a magnetic glassy behaviour in
these samples.
