| A reversal of magnetization requiring only the application of an electric field can leadto low-power devices by replacing conventional magnetic switching methods. As such,there has been a tremendous revival of activity in the feld of magnetoelectrics,motivated largely by the entirely new device application promises that would beenabled by electric-feld control of magnetism. With remarkable large magnetoelectriccoupling effect and greater design flexibility, magnetoelectric composites have beenutilized to realize electrically control magnetization. In this work, research seeks toexplore the extent of electrically tuning magnetoelectrics, specially for lead-free novelsystems of LiNbO3-CoFe2O4(LNO-CFO) nanocomposite,NaNbO3-CoFe2O4(NNO-CFO) nanocomposite, NaNbO3-NiFe2O4(NNO-NFO)composite and (Co,Mg)(Al,Fe)2O4nanocomposite. Herein, we present discoveriesof new composite magnetoelectrics which reveal intriguing coupling betweenferroelectric and magnetic orders. The composite systems were characterizedmagnetoelectrically by studying the magnetic responses upon applying electric fields.Remarkable converse magnetoelectric coefficients were obtained.First, we attempt to demonstrate a magnetoelectric nanocomposite of(100-x)LNO-xCFO(x=20,30,50), consisting of rhombohedral R3c LNO and R3mCFO phases. The lead-free novel magnetoelectric composites were synthesized by a“one-step mixing” sol-gel and “two-step heat treatment” procedure. It is characterizedby a maximum inverse ME coefficient of6.5Oe cm·V-1measured in a106V/melectric field and displays a magnetic field dependent Vogel–Fulcher-like relaxationcharacterized by a relaxation time exp(H/k(T TH0mvf)),029.66s, H1.06ev,T Hvfrepresents the magnetic field-dependent Vogel-Fulchertemperature. The room temperature electric field-induced magnetization changerelaxes as18.05s, which is consistent with the dielectric relaxation. The activationenergy equals the activation energy of oxygen vacancy diffusion in niobate-basedcrystals, indicates that the dielectric and magnetoelectric relaxation are caused by themigration of positively charged oxygen vacancies migrating to the magnetoelectricinterface. In addition, we studied the temperature dependent magnetodielectric effectfor the LNO-CFO systems, and we attributed the MD effect to the extrinsic chargecarriers induced by the metallic ferromagnetic phase, probably to the intrinsic magnetic-polarization coupling at high frequencies.Second, we employed the magnetoelectric coupling in antiferroelectric/magnetic50NNO-50CFO nanocomposite. The composite displays well defined MH loop,dielectric responses and a magnetic field dependent Vogel–Fulcher-like dielectricrelaxation, from which the activation energy of1.11ev is extracted. By applyingelectric field, we can reversibly switch the magnetization and a maximum converseME coefficient of5.07Oe cm/V is obtained in a106V/m electric field. At roomtemperature, an electric control of magnetization relaxes as=14.09s, signifying amagnetoelectric relaxation, which shares the same mechanism with dielectricrelaxation (the dielectric relaxation frequency is0.071Hz0.0027Hz (1/f14.090.92s) at294K). Both the dielectric relaxation and magnetoelectric relaxatioincould be understood by considering the diffusion of the positively charged Oxygenvacancies. Additionally, temperature dependence of magnetodielectric effect was alsoinvestigated for the NNO-CFO composite. These results prove that NNO is a potentialferroelectric phase for fabricating lead-free multiferroic composites.Third, magnetodielectric effect and resonance magnetoelectric effect weredemonstrated in magnetoelectric composites of50NNO-50NFO and(100-x)NNO-xNFO (x=15,25,35,45). They were prepared by a standard solid-stateprocedure. Presence of Orthorhombic Pbma NaNbO3and Cubic Fd3m NiFe2O4phases were proved by X-ray diffraction analysis. The composites exhibitwell-defined magnetic and dielectric behaviors. A strong coupling between NNO andNFO phases has been revealed and a maximum room temperature MD coefficient of2.8%was obtained. The appearance of magnetodielectric effect in theferromagnetic/ferroelectric composite, NNO-NFO, indicates coupling between themagnetic and dielectric phases, which may be controllable by the application ofmagnetic fields. The converse magnetoelectric effect characteristics with drivingfrequency in a range of10k-15kHz were investigated at room temperature for(100-x)NNO-xNFO (x=15,25,35,45) systems. The converse magnetoelectriccouplings increase initially with increasing the driving frequencies and then decreasewith the further increase in the frequency. Overall enchancement of CME couplingeffect were obtained at resonance frequencies for all the investigated samples. LargeCME coefficients are obtained at electromechanical resonance frequency of12.8kHz.The composite of55NNO-45NFO exhibits a maximum CME coefficient of123Oe cm kV-1under3000Oe at12.8kHz. These lead-free multiferroic compositesexhibiting electrostatically induced magnetoelectric resonance provide greatopportunities for electric feld tunable microwave devices.Finally, effort has been made on the discovery of magnetoelectric coupling for a novel(Co,Mg)(Al,Fe)2O4nanocomposite. The comosite was synthesized by the “one-stepmixing” sol-gel and “two-step heat treatment” procedure. X-ray diffraction patternsindicate the formation of (Co,Mg)(Al,Fe)2O4solid solution instead of parent phases ofCFO and MAO. Consistent with the X-ray analysis, HRTEM and element mappinganalysis indicates the homogeneous distribution of Co, Fe, Mg, Al elements. Thiscomposite simultaneously exhibits dielectric and magnetic properties. The electricfeld-induced magnetization is shown to switch in response to the pulsed electric feld.With the assistance of a small magnetic bias feld of merely10G, one observesmagnetization switching. Large reversible electric-field induced magnetization changewas observed. A maximum magnetoelectric coefficient of1.8Oe cm·V-1wasobtained in a106V/m electric field and10Gauss bias magnetic field. Importantly, thisdemonstration of magnetization reversal in a multiferroic compound enables thedevelopment of a generation of magnetic devices dynamically controlled by electricfelds. Simplicity of our ceramic processing and strength of the observedmagnetoelectric coupling effects promise significant potential for future multifunctionapplications. |
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