  Radio emission in jets from young stellar objects (YSOs) in the form of
nonthermal emission has been seen toward several YSOs. Thought to be
synchrotron emission from strong shocks in the jet, it could provide valuable
information about the magnetic field in the jet. Here we report on the
detection of synchrotron emission in two emission knots in the jet of the
low-mass YSO DG Tau A at 152 MHz using the Low-Frequency Array (LOFAR), the
first time nonthermal emission has been observed in a YSO jet at such low
frequencies. In one of the knots, a low-frequency turnover in its spectrum is
clearly seen compared to higher frequencies. This is the first time such a
turnover has been seen in nonthermal emission in a YSO jet. We consider several
possible mechanisms for the turnover and fit models for each of these to the
spectrum. Based on the physical parameters predicted by each model, the Razin
effect appears to be the most likely explanation for the turnover. From the
Razin effect fit, we can obtain an estimate for the magnetic field strength
within the emission knot of $\sim 20\ \mu \mathrm{G}$. If the Razin effect is
the correct mechanism, this is the first time the magnetic field strength along
a YSO jet has been measured based on a low-frequency turnover in nonthermal
emission.
