  The efficiency of a quantum metrology protocol can be considerably reduced by
the interaction of a quantum system with its environment, resulting in a loss
of purity and, consequently, a mixed state for the probing system. In this
paper we examine the potential of mixed spin-$j$ states to achieve sensitivity
comparable, and even equal, to that of pure states in the measurement of
infinitesimal rotations about arbitrary axes. We introduce the concept of mixed
optimal quantum rotosensors based on a maximization of the Fisher quantum
information and show that it is related to the notion of anticoherence of spin
states and its generalization to subspaces. We present several examples of
anticoherent subspaces and their associated mixed optimal quantum rotosensors.
We also show that the latter maximize negativity for specific bipartitions,
reaching the same maximum value as pure states. These results elucidate the
interplay between quantum metrology of rotations, anticoherence and
entanglement in the framework of mixed spin states.
