  We present a single-beam atomic magnetometer operating at room temperature
for the measurement of ac magnetic fields. The magnetometer functions in the
non-linear regime of magneto-optical rotation of $^{85}$Rb atomic vapour. We
demonstrate a sensitivity of $\sim 0.9$ pT$/ \sqrt{Hz}$ at 2 kHz and a large
bandwidth of 24 kHz. The dynamic range of measurement is $10^6$, making the
sensor effective even in Earth's field. We present the signal-to-noise and
bandwidth characteristics of the system for both shielded and unshielded modes
of operation. Moreover, we perform theoretical analysis for the atom-light
system for the single laser beam configuration. The effect of light intensity
and detuning on the magnetometer are studied theoretically as well as
experimentally to understand the strengths and limitations of the technique.
