  Spin-exchange collisions in hot vapours are generally regarded as a
decoherence mechanism. In contrast, we show that linear and non-linear
spin-exchange coupling can lead to the generation of atomic coherence in a
Bell-Bloom magnetometer. In particular, we theoretically and experimentally
demonstrate that non-linear spin exchange coupling, acting in an analogous way
to a wave-mixing mechanism, can create new modes of coherent excitation which
inherit the magnetic properties of the natural Larmor coherence. The generated
coherences further couple via linear spin-exchange interaction, leading to an
increase of the natural coherence lifetime of the system. Notably, the
measurements are performed in a low-density caesium vapour and for non-zero
magnetic field, outside the standard conditions for collisional coherence
transfer. The strategies discussed are important for the development of
spin-exchange coupling into a resource for an improved measurement platform
based on room-temperature alkali-metal vapours.
