  Nuclear magnetic resonance (NMR) is particularly relevant for studies of
internuclear spin coupling at zero and ultra-low fields (ZULF), where spin-spin
interactions dominate over Zeeman ones. Here we report on ZULF NMR in CdTe. In
this semiconductor all magnetic isotopes have spin $I = 1/2$, so that
internuclear interactions are never overshadowed by quadrupole effects. Our
experiments rely on warm-up spectroscopy, a technique that combines optical
pumping, additional cooling via adiabatic demagnetisation, and detection of the
oscillating magnetic field-induced warm-up of the nuclear spin system via Hanle
effect. We show that NMR spectra exhibit a rich fine structure, consistent with
the low abundance of magnetic isotopes in CdTe, their zero quadrupole moments,
as well as direct and indirect interactions between them. A model assuming that
the electromagnetic radiation is absorbed by nuclear spin clusters composed of
up to 5 magnetic isotopes allows us to reproduce the shape of a major part of
the measured spectra.
