Study On The Magnetic Properties Of Transition Metal Oxides Based On In Situ Magneto-electrochemical Technique

Electric-field control of magnetism（magnetoelectric-coupling）is one of the research hotspots for the new generation of low-power spintronics in recent years.It refers to the application of an electric field to change the magnetic state of the material（i.e,magnetic control）.However,so far,the magneto-electric effect in most materials is limited to very small material systems,usually the surface or interface of the material.The regulation mechanisms involved mainly include:carrier regulation,stress regulation,orbital reconstruction regulation,exchange coupling and electrochemical regulation.Therefore,researchers have been exploring new methods to achieve efficient electric field regulation of magnetism.In this paper,we have carried out research on electric field-regulated transition metal compound magnetic materials based on electrochemical energy storage devices-lithium-ion batteries and supercapacitor structures.During the operation of lithium-ion batteries,lithium-ions migrate in the battery structure and can penetrate into the electrode material,resulting in changes in the valence state of some elements in the electrode material or the surrounding environment,so it has a strong magnetoelectric coupling effect.As a device for electrochemical energy storage,supercapacitors can achieve reversible and effective magnetic regulation and exhibit long device life.Transition metal compounds are widely used in spintronic devices due to their unique extranuclear electronic structure and magnetic properties.Therefore,the magnetic regulation of 3d transition metal elements and their compounds by electrochemical mechanism is very attractive.Our self-built in-situ real-time magneto-electrochemical test system can accurately monitor the dynamic process of electron transfer under different electrochemical devices,assist us to understand the electrochemical mechanism from the perspective of physical essence,and further provide ideas for the design of magnetic control devices.In this paper,some key problems in transition metal compound materials are systematically studied by in-situ magneto-electrochemical testing.The specific research contents include the following three parts:1.In-situ magneto-electrochemical tests reveal the effect of electrolytes on the magnetic regulation of transition metal oxides.Electrolyte gating can regulate the migration of charge carriers(greater than or equal to 10^-15cm^-2),and the effect is much higher than that of commonly used diodes at only a few volts.Different electrolyte gating can achieve obvious magnetoelectric effect through electrostatic doping（electric double layer）mechanism,electrochemical redox reaction（pseudo-capacitance）mechanism or a combination of the two.Voltage-regulated magnetism（VCM）has been studied in liquid electrolyte supercapacitors.However,most studies have focused on the magnetic changes during charge and discharge in the same electrolyte.Considering the prominent role of electrolyte and the complex mechanism involved in this process,how to select the appropriate electrolyte to optimize the magnetoelectric coupling effect is still an open problem.Therefore,we take the typical magnetic material Fe₃O₄as an example to explore its magnetic changes under different electrolytes in the supercapacitor structure.In-situ magnetic tests and XPS tests show that in aqueous electrolytes（Na₂SO₄,KOH）,obvious magnetic changes are achieved mainly through redox reactions.In organic electrolyte（Li PF₆）,redox reactions and electrostatic doping work together to achieve magnetic regulation.This provides a certain reference value for the selection of appropriate electrolytes for magnetic regulation.2.Lithium-ion migration induces significant magnetic changes in transition metal oxide Co Fe₂O₄.Spinel oxides have rich physical and chemical properties and have great application value in data storage and electronic devices.For the application of hard magnetic materials,coercivity and magnetization are the key parameters.In this work,by controlling the lithium-ion migration,the coercivity of Co Fe₂O₄is regulated up to 0.14 T,which is an order of magnitude higher than the previously reported results.In addition,the saturation magnetization regulation based on redox mechanism and electrostatic doping mechanism can be achieved by accurately controlling the electrochemical reaction interval,and nearly 95%of the regulation effect can be achieved.This work opens up a new way for the voltage manipulation of multiple magnetic parameters of transition metal oxides.3.In-situ magneto-electrochemical measurements reveal the ferromagnetism of Fe O surface.In view of the long-standing controversy over whether Fe O has surface ferromagnetism,we prepared Fe O by in-situ magneto-electrochemical device,which effectively avoided additional influencing factors in the transfer process,and carried out a series of in-situ magnetic tests.Magnetic measurements show that it has strong ferromagnetism(0.71～0.96μ_BFe^-1 at room temperature)and Curie temperature as high as754 K,which can be attributed to surface frustration and surface-body phase exchange.We further achieved nearly 100%regulation of saturation magnetization（M_S）by lithium-ion migration.These results provide new insights into the design of new spintronic devices...