  Accurate determination of carrier densities in ferromagnetic semiconductors
by Hall measurements is hindered by the anomalous Hall effect, and thus
alternative methods are being sought. Here, we propose that cyclotron resonance
(CR) is an excellent method for carrier density determination for InMnAs-based
magnetic structures. We develop a theory for electronic and magneto-optical
properties in narrow gap InMnAs films and superlattices in ultrahigh magnetic
fields oriented along [001]. In n-type InMnAs films and superlattices, we find
that the e-active CR peak field is pinned at low electron densities and then
begins to shift rapidly to higher fields above a critical electron
concentration allowing the electron density to be accurately calibrated. In
p-type InMnAs, we observe two h-active CR peaks due to heavy and light holes.
The lineshapes depend on temperature and line broadening. The light hole CR
requires higher hole densities and fields. Analyzing CR lineshapes in p-films
and superlattices can help determine hole densities.
