  Multiferroics is a class of functional materials that simultaneously exhibit
ferroelectricity and ferromagnetism in a single structure. We report here the
integration of anti-ferroelectricity and ferromagnetism in a new M-type
hexaferrite compound, in which 0.3 Sr ions was substituted by 0.2 La3+ ions in
SrFe12O19 so as to keep the charge balance. This doped compound is expressed as
a molecular formula of La0.2Sr0.7Fe12O19, which was confirmed by XRD to have
the same structure as SrFe12O19 with a lattice contraction of 0.59%.
Surprisingly, the doping effect turns La0.2Sr0.7Fe12O19 from ferroelectric to
antiferroelectric phase, which displays double hysteresis loops with a maximum
polarization of 154 uC/cm2 and a remnant one of 38uC/cm2. The dielectric
constant is also greatly improved from 1462 to 1451778 at 10.2Hz by this
substitution. This compound simultaneously demonstrates strong ferromagnetism,
the remnant magnetic moment and coercive field are 52 emu/g and 5876 Oe,
respectively. In addition, La0.2Sr0.7Fe12O19 also exhibits strong
magnetodielectric (MD) response, in which a applying magnetic field B lifts the
whole e1-f spectra up and right shifts e2-f spectra to higher frequency side.
Both real and imaginary parts of the dielectric constant varies with frequency
and B field and obey Debye relaxation model. The maximum MD increment in e1
reaches as high as 540830 upon a B field of 926mT. The capacitance of the
La0.2Sr0.7Fe12O19 ceramics could be much more enhanced by applying a magnetic
field B, which could induce an additional polarization P(H) upon the
conventional P(E) by the cycloid conical spin in the intermediate phases. These
results suggest that La0.2Sr0.7Fe12O19 exhibits a strong interplay between
magnetic ordering and ferroelectricity, which makes it a good magnetoelectric
coupling candidate.
