The Study Of Magnetoelectric Coupling Effect Based On Spin Capacitance Phenomenon

Modern technology has an increasing demand for high-density,stable,and low-energy nanoscale storage elements,as well as numerous basic research in the field of spintronic and energy storage,which has triggered a research enthusiasm based on the coupling of magnetic and electric fields.Magnetoelectric coupling is the magnetic response of a system to an external electric field,or equivalent to the electric polarization caused by an external magnetic field.It has extremely important application value in revealing the energy storage reaction mechanism and magnetoelectric control.Based on the principle and technology of electrochemistry,we can realize the effective regulation of magnetic function of materials,so as to design and develop new magneto-ionic devices.At the same time,through magnetic theory and testing technology,we can study the structural phase transition and local electronic distribution in energy storage materials,and obtain information that cannot be obtained by other traditional theoretical technologies.Combining the fields of electrochemistry and magnetism,the experimental approaches for electric-field magnetization control via ionic conductors mainly rely on supercapacitors and ion batteries.Studies have proposed that synergistic interfacial charge storage essentially simplifies the ideal conductive phase of ions and electrons into double layers,forming a spin-capacitance phenomenon,which combines the two major advantages of the intercalation depth of ion batteries and the rapid response of capacitors.So the magnetoelectric coupling caused by this phenomenon can not only effectively regulate the magnetism,but also reveal the complex interface reaction mechanism of electrode materials.In this paper,we put the self-made flexible packaging battery in the in-situ physical property measurement system for the convenience of real-time monitoring.We electrochemically reduced the?-Fe₂O₃ electrode to 0.01 V through the reaction principle of lithium-ion batteries to obtain a nano-scale Fe⁰/Li₂O matrix,and then,reversible charging/discharging was carried out in the low potential range that avoids Fe⁰ oxidation.The saturation magnetization changed accordingly and stably with an amplitude of up to 0.14?_B Fe^-1 due to the space charge effect,which formed by the spin-polarized electrons and the lithium ions on the surface of the ferromagnetic metal and lithium ion conductor.Moreover,the magnetic response remained reversible and stable after nearly 150 cycles of the fast pulse cycle.At the same time,we took advantage of the operando monitoring technology,the electrode material still shows magnetism after being fully charged to 3.0 V.Through a series of ex-situ structural and physicochemical characterizations,we have verified that the reconversion reaction of?-Fe₂O₃ is irreversible and the reconversion product is FeO.Magnetic monitoring shows that FeO surface is ferromagnetically ordered at room temperature.This result break the controversy about whether FeO has ferromagnetism,and also lay the research foundation for the development of the frontier of physics.In order to improve the tuning extent of ferromagnetic metals,we chose LiF with higher conductivity to form a space charge effect with Fe⁰ for magneto-ionic regulation.We prepared FeF₂ electrode,and completely reduced it to 0.01 V to obtain a nano-scale Fe⁰/LiF composites.Similarly,we selected a defined voltage window to reversibly regulate the spin-polarized electron arrangement on the surface of ferromagnetic metal and the change in saturation magnetization is increased to 0.17?_B Fe^-1.Additionally,we also selected Li₂Se with higher conductivity and larger layer spacing to further enhance the advantages of magneto-ionic control based on space charge effect in amplitude and stability.The FeSe₂ electrode was synthesized and Fe⁰/Li₂Se matrix was generated by electrochemical reduction of lithium ions to the lowest potential.The variation of saturation magnetization obtained by the same magneto-ionic control method was 0.34?_B Fe^-1,and the magnetic response was still very reversible and stable under faster pulse charge and discharge.We use the synergistic space charge effect of the combination of lithium ion batteries and spin capacitors to achieve ideal magneto-ionic control on ferromagnetic metals.By changing the conductivity and atomic radius of the lithium ion conductor,the amplitude and stability of magnetic regulation are greatly improved.Finally,the reaction mechanism between the solid electrolyte interface film and the transition metal carbide electrode is intuitively revealed by the magneto-electric coupling effect mediated by the spin capacitance.Operando magnetometry strongly and convincingly demonstrated that the initial Fe₃C@Fenanoparticles can store a large number of spin-polarized electrons at low voltage,resulting in a significant change in interface magnetization and a large excess capacity.Different from the traditional understanding of the lithium storage mechanism of transition metal carbides,the inorganic components contained in the catalytic SEI film are the important sources of Li⁺storage at the interface.These findings open up a new way for the further development of novel energy storage devices.