Magnetoelectric Properties Of Y-Type Hexaferrites

The magnetoelectric?ME?multiferroics,wherein magnetic and ferroelectric?FE?orders coexist,have attracted extensive research interests for decades due to their potential technological applications.Y-type hexaferrites with tunable conical magnetic structures are promising single-phase multiferroics to exhibit large magnetoelectric effects.In this thesis,we investigated the magnetoelectric effects of Y-type hexaferrites.The main results are listed as follows:1.We have investigated the influence of Co substitution on the magnetoelectric properties in the Y-type hexaferrites Ba_0.3Sr_1.7Co_xMg_2-xFe₁₂O₂₂.The spininduced electric polarization can be reversed by applying a low magnetic field for all the samples.The magnetoelectric phase diagrams of Ba_0.3Sr_1.7Co_xMg_2-xFe₁₂O₂₂ are obtained based on the measurements of magnetic field dependence of dielectric constant at selected temperatures.It is found that the substitution of Co ions can preserve the ferroelectric phase up to a higher temperature,and thus is beneficial for achieving single-phase multiferroics at room temperature.2.The influence of substitution of Sr and Ni on the magnetic and magnetoelectric properties of Y-type hexaferrite BaSrCo₂Fe₁₁AlO₂₂ has been investigated.Sr doping does not change much the Curie temperature T_C near 760 K and the saturation magnetization at 300 K.In contrast,the saturation magnetization is reduced by Ni doping.The spin-induced electric polarization can be reversed by a low magnetic field for all samples,and the doping has almost no influence on the magnetoelectric coefficient at room temperature.The magnetoelectric phase diagrams of all samples have been obtained by the measurements of magnetic field dependence of the dielectric constant at selected temperatures.Both Sr and Ni doping broaden the region of ferroelectric phase by driving the phase boundary to higher magnetic fields.3.We show that the aE of some magnetoelectric multiferroics are very sensitive to external dc magnetic fields,leading to a giant magnetotranstance?GMT?effect at room temperature.A low magnetic field of 500 Oe applied on the multiferroic hexaferrites BaSrCo₂Fe₁₁AlO₂₂ and Ba_0.9Sr_1.1Co₂Fe₁₁AlO₂₂ is able to cause a huge change in a_E by several orders.The GMT effect which is parallel to the well-known giant magnetoresistance?GMR?effect,has a great potential for applications in magnetic sensors and spintronic devices.