  The extensive control of spin makes spintronics a promising candidate for
future scalable quantum devices. For the generation of spin-superfluid systems,
a detailed understanding of the build-up of coherence and relaxation is
necessary. However, to determine the relevant parameters for robust coherence
properties and faithfully witnessing thermalization, the direct access to
space- and time-resolved spin observables is needed. Here, we study the
thermalization of an easy-plane ferromagnet employing a homogeneous
one-dimensional spinor Bose gas. Building on the pristine control of
preparation and readout we demonstrate the dynamic emergence of long-range
coherence for the spin field and verify spin-superfluidity by experimentally
testing Landau's criterion. We reveal the structure of the emergent
quasi-particles: one 'massive'(Higgs) mode, and two 'massless' (Goldstone)
modes - a consequence of explicit and spontaneous symmetry breaking,
respectively. Our experiments allow for the first time to observe the
thermalization of an easy-plane ferromagnetic Bose gas; we find agreement for
the relevant momentum-resolved observables with a thermal prediction obtained
from an underlying microscopic model within the Bogoliubov approximation. Our
methods and results pave the way towards a quantitative understanding of
condensation dynamics in large magnetic spin systems and the study of the role
of entanglement and topological excitations for its thermalization.
