Research On The Magnetoelectric Performance Of Several Single-phase Materials

In the past decades,single-phase magnetoelectrics(MEs)have been attracting considerable interests,not only due to plentiful physics behind but also for potential application.The strong coupling between spin,charge,orbital and lattice degree of freedom in these materials provides us an appropriate frame to research for studying the coupling effect of strongly correlated system,as well as fertile soil for observation of new physical phenomenon.Investigation of single-phase ME materials is a result motivated by the proceeding desire of modern information storage?detection processing technology,and so on.Multifunctional materials,in which the electric order and magnetic order coexist and mutually couple,would supply possibilities for designing new devices.The coupling effect between electric and magnetic order allows electric control of magnetism and magnetic modification of electricity,showing potential application.New type of memory devices is an application fields of ME coupling materials,such as four state memories in single device embed by ME material with magnetic and electric order coexists,new devices with the mode of electric write and magnetic read,and so on.This means the tendency of future development.Based on this research background,we carried out some studies on exploring single-phase ME.These details are as follows:1.Magnetism,electricity and ME coupling effects in Co4Nb2O9We synthesize polycrystalline sample Co4Nb2O9 by solid-state reaction method and study its magnetism,electricity,and the mutual control of them.Co4Nb2O9 has the same crystalline structure as ?-Al2O3.At low temperature of the ordered magnetic phase,the moments of Co2+ ions align along c axis with a ???? arrangement(magnetic point 3'm').The magnetic-field-induced polarization has been observed in Co4Nb2O9,since both the space-inversion and time-reversal symmetries are broken under magnetic field.The experiment results reveal that the electric polarization exists in the magnetic field and increases with increasing magnetic field.Moreover,electric control of magnetism is also observed in Co4Nb2O9.In this compound,there are two kinds of antiferromagnetic domains with ?? and ?? configurations,which have opposite ME coefficients,respectively.In zero magnetic field,the materials don,t show ME behavior due to the cancellation of the opposite ME coefficients.However,when the sample are cooled through their Neel temperature in electric and magnetic fields,the ME susceptibility can be strongly enhanced by favoring the growth of one of the antiferromagnetic domains,resulting in a macroscopic ME effect.Since tihe electric and magnetic ordered regions are coupled,the magnetism would be tuned when electric ordered region is modified by the electric field,showing ME effect.2.Magnetism,electricity and ME coupling effect in Cr2WO6The magnetic,electric and ME properties of Cr2WO6,which is synthesized by the solid-state reaction method,are investigated in this part.Since the magnetic order of this sample is non-centrosymmetry in magnetic field and the magnetic and chemical unit cells of Cr2WO6 are identical,the sample should be polarized below its Neel temperature.The experimental results show that the compound undergoes a phase transition from low.dimensional magnetic correlation to a collinear magnetic phase.At the transition temperature,the magnetic-field-induced dielectric anomaly and polarization are observed.Moreover,with the increase of magnetic field,the magnetic structure is destroyed more seriously.In the compound Cr2WO6,there are two kinds of antiferromagnetic domain with ?? and ?? spin configuration.This compound has different antiferromagnetic domains with equal volume fraction in the absence of external magnetic field,which leads to zero ME response due to cancellation of ME effect of the domains.However,after ME annealing,one of the domains would dominate over the other,which would result in the ME signals in Cr2WO6.3.Magnetic,ferroelectric and ME properties of MnW1-xMoxO4(x=0,0.05,0.1,0.2)We synthesize a series of polycrystalline MnW1-xMoxO4(x=0,0.05,0.1,0.2)by solid-state reaction method and study their magnetism,ferroelectricity and multiferrocity.It is evidenced from the experiments that MnWO4 experiences paraelectric(AF3)?ferroelectric(AF2)?paraelectric(AF1)phase with the temperature decreasing.At the doping content of 0.05,the AF2 phase extends to low-temperature AF1 phase.When x reaches up to 0.1,the AF1 phase is well suppressed and the sample shows ferroelectric property in the measured temperature regime.With the increase of the doping content,the polarization of MnWO4 decreases.After 20%of W is substituted,the pyroelectric current can't be observed in the apparatus resolution.When Mo ions are doped in this compound,the ferroelectricity is modified,as well as the magnetism.Moreover,the magnetic phase transition temperature and the magnetism are also enhanced.Since the electronegativity of Mo6+ ion is smaller than that of the W6+ion,the exchange interaction between Mn and Mn becomes stronger after substituting Mo6+ for W6+ ions,resulting in enhancing the magnetic phase transition temperature.Besides,when MnWO4 is doped,the magnetism of this compound can be tuned more easily,as well as the ferroelectricity due to the intimate coupling between them.4.Magnetism,ferroelectricity and multiferrocity of Y2MnCrO6We synthesize Y2MnCrO6 by solid-state reaction and measure its crystalline structure,magnetism,electricity and ME properties.The results of crystal structure refinement suggest that Y2CrMnO6 is monoclinic and the Mn3+and Cr3+ ions align orderly.This compound shows antiferromagnetism and ferroelectric polarization below its TN.Since the crystal structure and magnetic properties of Y2CrMnO6 are similar as those of Y2CoMnO6,it is assumed that a ???? magnetically ordered phase would exist in the former one.As the space-inversion broken by the ???? spin structure,the compound Y2CrMnO6 presents ferroelectric polarization.While the magnetic domains with ?? spin configuration are instable,they can be easily driven to parallel state,yielding the degree of broken space-inversion suppressed,as well as the ferroelectric polarization.