Spin noise spectroscopy is an optical technique which can probe spin
resonances non-perturbatively. First applied to atomic vapours, it revealed
detailed information about nuclear magnetism and the hyperfine interaction. In
solids, this approach has been limited to carriers in semiconductor
heterostructures. Here we show that atomic-like spin fluctuations of Mn ions
diluted in CdT e (bulk and quantum wells) can be detected through the Kerr
rotation associated to excitonic transitions. Zeeman transitions within and
between hyperfine multiplets are clearly observed in zero and small magnetic
fields and reveal the local symmetry because of crystal field and strain. The
linewidths of these resonances are close to the dipolar limit. The sensitivity
is high enough to open the way towards the detection of a few spins in systems
where the decoherence due to nuclear spins can be suppressed by isotopic
enrichment, and towards spin resonance microscopy with important applications
in biology and materials science
