Magnetoelectric (ME) materials are a class of functional materials which present coupling effect between magnetic and electric orders. M-type hexaferrites could embrace both ferroelectricity and ferromagnetism, but the ME effect is weak. Here, it is expected to enhance the magnetoelectric response in SrFe 12 O 19 by substitution of 0.3 Sr 2þ with 0.2 La 3þ to form La 0.2 Sr 0.7 Fe 12 O 19 . X-ray diffraction results show a lattice contraction of 0.59%, while the remnant magnetic moment of La 0.2 Sr 0.7 Fe 12 O 19 is improved by 45.3% in comparison with SrFe 12 O 19 . Strong magnetodielectric (MD) response is observed in La 0.2 Sr 0.7 Fe 12 O 19 , a magnetic field lifts the whole e 0 -f spectra up and right shifts the e 00 -f spectra (ω m ) to higher frequency side. Both real and imaginary parts of the dielectric constants obey the Debye relaxation model and vary with magnetic field B at a large amplitude. The maximum MD variation ratio in e 0 is 83.4% at B ¼ 926mT. The applied magnetic field greatly enhances the capacitance of the La 0.2 Sr 0.7 Fe 12 O 19 through imposing an additional polarization P(H) induced from the cycloid spin upon the conventional P(E). The physical mechanism for the lift of e 0 -f spectra and right shift of e 00 -f spectra with B field is discussed in detail from the point of view of Debye relaxation formula and magnetoelectric coupling theory.
Experimental ProcedureFirstly, La 0.2 Sr 0.7 Fe 12 O 19 fine powders were prepared by a polymer precursor procedure. Strontium acetate (Sr(CH 3 COO) 2 Á 3H 2 O),