  We study analytically the non-Markovianity of a spin ensemble, with arbitrary
number of spins and spin quantum number, undergoing a pure dephasing dynamics.
The system is considered as a part of a larger spin ensemble of any geometry
with pairwise interactions. We derive exact formulas for the reduced dynamics
of the system and for its non-Markovianity as assessed by the witness of
Lorenzo \emph{et al.} [Phys.~Rev.~A \textbf{88}, 020102(R) (2013)]. The
non-Markovianity is further investigated in the thermodynamic limit when the
environment's size goes to infinity. In this limit and for finite-size systems,
we find that the Markovian's character of the system's dynamics crucially
depends on the range of the interactions. We also show that, when the system
and its environment are initially in a separable state, the appearance of
non-Markovianity is independent of the entanglement generation between the
system and its environment.
