  We show that room temperature resistivity of Ba0.5Sr1.5Zn2Fe12O22 single
crystals increases by more than three orders of magnitude upon being subjected
to optimized heat treatments. The increase in the resistivity allows the
determination of magnetic field (H)-induced ferroelectric phase boundaries up
to 310 K through the measurements of dielectric constant at a frequency of 10
MHz. Between 280 and 310 K, the dielectric constant curve shows a peak centered
at zero magnetic field and thereafter decreases monotonically up to 0.1 T,
exhibiting a magnetodielectric effect of 1.1%. This effect is ascribed to the
realization of magnetic field-induced ferroelectricity at an H value of less
than 0.1 T near room temperature. Comparison between electric and magnetic
phase diagrams in wide temperature- and field-windows suggests that the
magnetic field for inducing ferroelectricity has decreased near its helical
spin ordering temperature around 315 K due to the reduction of spin anisotropy
in Ba0.5Sr1.5Zn2Fe12O22.
