Investigation On The Converse Magnetoelectric Effects In Composite Multiferroic Materials

Multiferroic magnetoelectric materials have drawn much interest because of their potential application in sensors,molectron,high-density information storage media,spintronics,and so on.Multiferroic magnetoelectric composites,which simultaneously exhibit ferroelectricity and ferromagnetism,can be made by different methods combining single ferroelectric and ferromagnetic phase together and produce remarkable magnetoelectric?ME?effect at room temperature.ME effect can be classified as direct magnetoelectric?DME?effect and converse magnetoelectric?CME?effect.Compared with DME effect,there are few reports about the CME effect.Moreover,the giant magnetostrictive material Terfenol-D is mostly selected as the ferromagnetic phase in the research of CME.In this dissertation,based on the reported results in this field,we use the alternative ferromagnetic materials in the ME heterostructures and investigate the CME effect in these composites.The main results are listed below:1.The CME effect in Metglas/ferroelectric laminated composite.The CME effect was investigated by induction method in Metglas/PMN-PT laminated composite consisting of high-magnetic-permeability Metglas as magnetic layer.The largest CME coefficient reaches 3.05 G/V at the resonance frequency of 76.5 kHz under the low magnetic bias field of 50 Oe.On the other hand,we also investigated the CME effect in Metglas/PZT composites with different thickness of PZT.The experimental results show that,with different thickness,the composites exhibit the same resonance frequency and magnetic bias field,while the maximum CME coefficients vary.Additionally,the CME effect of both composite can be significantly influenced by the applied dc electric field bias.2.The CME effect in Ni₄₉Fe₁₈Ga₂₇Co₆/ferroelectric laminated composite.The large magnetic field induced strain has been observed in ferromagnetic shape memory alloy Ni₄₉Fe₁₈Ga₂₇Co₆?NFGC?,which originates from the motion and reorientation of the martensite variants.We selected this alloy as the ferromagnetic layer and investigated the CME effect in NFGC/PZT laminated composite.The largest CME coefficient is 1.05 G/V,indicating that this composite has a valuable potential in applications.The motions of the domain wall and martensitic variant under the strain transferred from the PZT would be responsible for the large CME effect in the composite.In addition,the CME effect can be effectively tuned by applying dc electric voltage bias on PZT plate.The maximum values of CME coefficient at 1 kHz can be tuned from 8.7 to 25 mG/V by varying voltage bias from-100 to 100 V.Therefore,the dc electric voltage bias can be used to tune the CME effect of the composite.3.The CME effect in Ni/PMN-PT/Ni laminated composite without magnetic bias field.A sandwich Ni/Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃/Ni composite was prepared by electrochemical deposition,in which a remanent magnetization?Mr?of 1.10 emu and coercivity?Hc?of 70 Oe were observed.Without magnetic bias field,the large CME coefficient of 0.45 G/V was obtained at the resonance frequency of 102 kHz.Moreover,the magnetism can be obviously tuned by dc electric voltage?Edc?,and the M-Edc loop exhibits a butterfly shape.A stable change of magnetization exhibits an analogous on-off behavior in the composite with applied Edc switching on and off alternatively.The magnetization of the composite can be reversibly and reproducibly controlled by an applied Edc.Therefore,the CME effect without magnetic bias field was realized in Ni/PMN-PT/Ni composite.The large CME effect without magnetic bias field in the composite would offer potential applications in electrically controlled high-density magnetic memory devices with power consumption.4.The exchange bias effect,magnetic entropy change,and magnetoresistance in Ni-Mn based ferromagnetic shape memory alloys with high content of Mn.Comparing with the traditional Ni-Mn-X?X = In,Sn,Sb?alloys,the content of Mn in these alloys was further increased,and the content of Mn is higher than that of Ni,even reaches more than 50 at.%.In these alloys,the excess Mn atoms would occupy not only the Sn sites but also the Ni sites,and their moments are coupled antiferromagnetically to those of Mn atoms on the regular Mn sites.The largest exchange bias field?HE?is up to 910 Oe for Mn50Ni40Sn10 alloy at 2 K,which is larger than HE of most of the previous reports.The martensitic transformation?MT?temperature of the Mn_47+xNi_43-xSn₁₀ alloys decreases with increasing content of Mn.The largest magnetic entropy change is 14.1 J/kg K for Mn₄₉Ni₄₁Sn₁₀ under the low magnetic field of 12 kOe.Moreover,the magnetoresistance of this alloy reaches-26%under the field of 50 kOe around its MT temperature.In addition,the MT temperature of Mn₅₀Ni_50-xIn_x decreases by increasing the content of In,and these alloys also exhibit large magnetic entropy around their MT temperatures.Additionally,a unique MT from the ferromagnetic austenite to the weak-magnetic martensite phase was obtained in Co-doped Ni_50-xCo_xMn₃₂Al₁₈ Heusler alloys,in which the reverse MT can be induced by magnetic field.A change of magnetization up to 45 emu/g was obtained across the MT for Ni₄₂Co₈Mn₃₂Al₁₈ alloy.In the same quaternary alloy,the large magnetoresistance effect of-67%was observed at 225 K and the field of 90 kOe.The large magnetization change and the magnetoresistance effect originated from the magnetic field-induced MT.