Study On The Room-temperature Magnetoelectric Coupling Performance Of Z-type Hexaferrites

With the rapid development of 5G communication,Internet of Things and artificial intelligence,people have put forward higher requirements for miniaturization,multi-function,integration,and low energy consumption of electronic devices.Single-phase multiferroic materials have both ferroelectricity and ferromagnetism,and these two properties can be regulated mutually.Single-phase multiferroic materials have become a hot research direction in condensed matter physics and material science,and they have a broad application prospects in the fields of sensor detection,information storage,spintronics devices,etc.In this dissertation,the multiferroic Z-type hexaferrite Sr₃Co₂Fe₂₄O₄₁（SrCo₂Z）with room-temperature low magnetic field magnetoelectric coupling effect was taken as the research object.The room temperature magnetoelectric coupling effect of SrCo₂Z was tested and studied comprehensively by using the self-designed and built magnetoelectric test system.A kind of nonvolatile multi-states memory prototype was proposed based on its magnetoelectric coupling characteristics.Then,Al³⁺ion substitution and sintering with high oxygen atmosphere concentration were used to improve its magnetoelectric coupling performance,and the range of magnetic fields in which the ferroelectric phase occurs was extended to the high magnetic field range.Finally,the magnetoelectric coupling properties of Al³⁺doped samples sintered in high oxygen atmosphere concentration were studied.The details are described as follows:The iron-deficiency formula 3SrCO₃·Co₂O₃·10.8Fe₂O₃ was adopted to prepare polycrystalline samples by conventional solid-state ceramic processing methods.The calcining temperature was 1100℃,the calcining time was 16 hours,the calcining atmosphere was air,the sintering temperature was 1200℃,the sintering time was 16hours,and the sintering atmosphere was oxygen.The XRD showed that the sample was almost pure polycrystalline Z-type hexaferrite.The SEM showed that the grans were arranged closely in blocks,which is similar with the reported microstructure of multiferroic Y-type hexaferrite.M-T test showed that a stable transverse conical magnetic structure was formed below 400 K.The P-E test showed that the sample had ferroelectricity at room temperature,which proves that the prepared sample is a kind of multiferroic material at room temperature.A set of magnetoelectric testing system suitable for the room-temperature magnetoelectric effect of hexaferrites was designed and built based on quasi-static method.The magnetoelectric coupling effect of the prepared SrCo₂Z was tested comprehensively at room temperature,and the"hysteresis"characteristic was found.This feature was closely related to the remnant magnetization of the sample.The magnetoelectric coupling effect was larger when the magnetic field was increased along the direction of remnant magnetization,and the maximum magnetoelectric coupling effect occurred at a smaller magnetic field.The maximum magnetoelectric parameters J_m=-10.2（?）A/m²,P_m=-12.8（?）C/m²,andα_m=-601 ps/m were measured after SrCo₂Z sample was fully poled(（?）₀H_pole=-0.01T,E_pole=-1000 k V/m).It was found that the magnetoelectric coupling effect of the sample appeared in 0～0.4 T only,indicating that the magnetic-induced ferroelectric phase occurs in the range of 0～0.4 T only.When the magnetic field exceeded 0.4 T,the sample showed a paraelectric phase.In addition,it was found that the magnetic field range of 0～0.2 T is quite special.When the scanning magnetic field exceeded 0.2 T,the transverse conical magnetic structure would be destroyed,and the magnetoelectric coupling effect would be severely weakened when measured again.This indicates that the transverse conical magnetic structure can only maintains its integrity within the magnetic field range of0～0.2 T,and only when the scanning magnetic field does not exceed 0.2 T,the magnetoelectric coupling effect can be repeated without attenuation.Both（?）₀H_pole and E_pole in the poling process have significant regulatory effects on the magnetoelectric coupling effect of SrCo₂Z.Only when（?）₀|H_pole|≤0.2 T can the sample be fully poled,showing the maximum magnetoelectric coupling effect.When（?）₀|H_pole|>0.2T,the magnetoelectric coupling strength decreases with the increase of（?）₀H_pole.Only when|E_pole|≥600 k V/m can the sample be fully poled,showing the maximum magnetoelectric coupling effect.In addition,the magnetoelectric coupling effect of the sample is closely related to the direction of E_pole,and the symbol for ferroelectric polarization P is always the same as the symbol for E_pole.In other words,the sample’s magnetoelectric coupling effect can be reversed by E_pole.A nonvolatile multi-states memory prototype was proposed based on this memory characteristics.This memory can realize 128 kinds of storage states theoretically and has great application potential.The magnetic field range of magnetoelectric coupling effect of SrCo₂Z-Al was extended from 0～0.4 T（x=0.00）to 0～1 T（x=0.04）by Al³⁺doping.Al³⁺doping changes the magnetic crystal anisotropy of SrCo₂Z,improves the stability of its transverse conical magnetic structure,and makes its magnetically induced ferroelectric phase appears in the high magnetic field region.When x=0.06 or 0.08,the sample only exhibits a very weak magnetoelectric coupling effect.In addition,it was found that when x=0.08,the magnetoelectric coupling effect can be reversed by the scanning magnetic field,which is contradictory to the magnetoelectric coupling effect of Z-type hexaferrite,indicating that the magnetic structure of the sample has undergone essential changes,and this new magnetic structure needs to be further studied.The concentration of sintered oxygen atmosphere also has an obvious regulating effect on the magnetic field range of the magnetoelectric coupling effect of SrCo₂Z.The0.1 L/min sample exhibited magnetoelectric coupling effect only in the magnetic field range of 0～0.6 T,while the 0.3 L/min sample exhibited magnetoelectric coupling effect in the magnetic field range of 0～1.2 T,indicating that the increase of oxygen atmosphere concentration in the sintering process significantly increased the magnetic field range in which the ferrielectric phase occurs.The magnetoelectric coupling effect was not detected in 0.5 L/min sample due to the obvious impurity phase.High oxygen atmosphere concentration in the sintering process will reduce the leakage current and trapped states related to oxygen vacancy,leading to the higher resistivity,the relatively low magnetoelectric coupling effect and the faster capacity discharge process.The high sintered oxygen atmosphere concentration（0.3 L/min）makes the transverse conical magnetic structure of Al doped SrCo₂Z-Al（x=0.00,0.02,0.04）more stable,and it can still induce great ferroelectricity under the high magnetic field of～1 T.However,among the SrCo₂Z-Al samples sintered under low oxygen atmosphere concentration,only the transverse conical magnetic structure of the x=0.04 sample can induce obvious ferroelectricity under the high magnetic field of～1 T.In addition,the samples sintered in low oxygen atmosphere concentration have larger leakage current and trapped states related to the oxygen vacancy,resulting in an overestimation of electric polarization strength P.The concrete physical mechanism needs to be further studied.