  Recent neutron diffraction and NMR studies suggest that the incommensurately
modulated spin cycloid structure of BiFeO3 is stable down to 4.2 K, whereas DC
[M(T)] and AC [\c{hi} ({\omega}, T)] magnetization, and caloric studies have
revealed several magnetic transitions including a spin glass transition around
25 K. The two sets of observations are irreconcilable and to settle this, it is
important to first verify if the low temperature magnetic transitions are
intrinsic to BiFeO3 or some of them are offshoots of oxygen vacancies and the
associated redox reaction involving conversion of Fe3+ to Fe2+. We present here
the results of M (T) and \c{hi} ({\omega}, T) measurements on pure and 0.3 wt%
MnO2 doped BiFeO3 samples in the 2 to 300 K temperature range. It is shown that
MnO2 doping increases the resistivity of the samples by three orders of
magnitude as a result of reduced oxygen vacancy concentration. A comparative
study of the M (T) and AC \c{hi} ({\omega}, T) results on two types of samples
reveals that the transitions around 25 K, 110 K and 250 K may be intrinsic to
BiFeO3. The widely reported transition at 50 K is argued to be defect induced,
as it is absent in the doped samples. We also show that the spin glass
transition temperature TSG is less than the spin glass freezing temperature
(Tf), as expected for both canonical and cluster glasses, in marked contrast to
an earlier report of TSG > Tf which is unphysical. We have also observed a cusp
corresponding to the spin glass freezing at Tf in ZFC M (T) data not observed
so far by previous workers. We argue that the ground state of BiFeO3 consists
of the coexistence of the spin glass phase with the long range ordered AFM
phase with a cycloidal spin structure.
