Magnetic And Magnetoelectric Coupling Properties Of Co/Al Doped Y-Type/Z-Type Hexagonal Ferrites

With continuous development of information technology,a large number of functional materials with various physical properties have attracted more attentions because of higher requirements for information storage devices with high density,low power consumption,miniaturization and multi-function.Magnetoelectric multiferroics are one kind of multiferroic materials with magnetic and ferroelectric orders,which can realize the control of electric polarization P（magnetization M）by magnetic field H（electric field E）.At present magnetoelectric coupling effects of most magnetoelectric multiferroics generally occurs either under high magnetic fields or at low temperatures,and its coupling coefficientαis small,both of which limit their application significantly.As a result,exploring magnetoelectric materials with higher transition temperatures（above or room temperture）,lower magnetic fields and larger magnetoelectric coupling coefficient has been a research hotspot in this field.Among them,spin-induced multiferroics have rich physical properties and show a promising application in spintronic devices.Y and Z-type hexagonal ferrites belong to this family.They have tunable conical magnetic structures and magnetoelectric coupling in a wide range of temperatures.Meanwhile,electric polarization can be controlled by low magnetic fields or magnetization by electric fields at room temperature in them which are considered as one of the most promising single-phase multiferroics for application.In this thesis,the tuning mechanism of magnetoelectric coupling in Y and Z-type hexagonal ferrites by element doping is studied,which provides guidance for seeking magnetoelectric materials with high temperature,low magnetic fields and large magnetoelectric coupling coefficient.The main contents of this thesis include the following four parts:1)crystal structure,magnetic properties and magnetoelectric coupling effects of Co²⁺-doped Ba_0.5Sr_1.5(Co_xMg_1-x)₂Fe₁₂O₂₂（Co-Mg₂Y）polycrystalline samples;2)magnetic anisotropy and magnetoelectric coupling effects of Co²⁺-doped Mg₂Y-type hexagonal ferrite single crystal of Ba_0.5Sr_1.5Co_0.4Mg_1.6Fe₁₂O₂₂（Co04-Mg₂Y）;3)magnetic anisotropy,magnetic dielectric properties,and magnetoelectric coupling as well as magnetocaloric effects of Co²⁺-doped Zn₂Y-type hexagonal ferrite single crystal of Ba_0.7Sr_1.3Co Zn Fe₁₁Al O₂₂（Co-Zn₂Y）;4)magnetic properties and mangnetoelectric coupling effects of Al³⁺-doped Z-type hexagonal ferrite single crystal of Sr₃Co₂Fe_23.04Al_0.96O₄₁(Al-Fe₂₄Z).1)With the increase of Co²⁺doping,the lattice parameters of Y-type hexagonal ferrites Co-Mg₂Y samples increase.Magnetization of Co-Mg₂Y also increases due to the large magnetic moments of Co²⁺.The magnetic phase transition and P reversal temperatures of Co-Mg₂Y system also increase by the introduction of Co²⁺.There is a spontaneous P near zero H.The P andαof Co04-Mg₂Y polycrystalline are 75μC/m²and 2939 ps/m,respectively at T=50 K.The P of Co-Mg₂Y polycrystalline is reversed and controlled by a sawtooth-shape periodic H.2)Based on the excellent performance of Co²⁺-doped Co04-Mg₂Y polycrystalline sample,single-crystal of Co04-Mg₂Y is successfully synthesized using Na₂CO₃-Fe₂O₃ flux.When magnetic fields are applied by H⊥c and H//c,the Co04-Mg₂Y single crystal exhibits different magnetic configurations in ab-plane and out-of ab-plane,respectively.The direction of the applied magnetic field H has a great influence on the spin configuration.The P andαare 100μC/m²and 13,300ps/m,respectively at T=30 K.Based on magnetoelectric phase diagram,the P is reversed,which is controled by the H near zero magnetic field below 150 K.The P with repeatability and stability is periodically changed after many cycles under sawtooth-shape periodic H at 50 K.Therefore,Co04-Mg₂Y is demonstrated considerable importance for potential applications as novel non-volatile storage devices.3)Co²⁺doping enhances the magnetic transition temperature(T_N2)to～356 K in the ab plane of Co-Zn₂Y single crystal.The P andαare 130μC/m²and 16,667ps/m,respectively,under the condition of T=30 K and E=880 k V/m.With increasing poling electric field E,P andαincrease.The P is changed periodically,which can be controlled by the H under a sawtooth-shape periodic H.Therefore,Co-Zn₂Y has potential application in novel non-volatile storage devices.4)With Al³⁺doping,the transverse conical magnetic phase of Z-type hexagonal ferrites Al-Fe₂₄Z can be stabilized up to 359 K.The magnetization increases by two steps before reaching saturation at 300 K in the M-H hysteresis curve,implying the existence of a magnetic-field-induced magnetic phase transition.Based on the relative dielectric diagram,the magnetoelectric coupling effect of the Al-Fe₂₄Z single crystal occurs at room temperature.The magnetoelectric current curves show butterfly shape between 200 and 275 K.The magnitude of the P is periodically with changed with the H and without any decay after many cycles under a sawtooth-shape periodic H.However,the sign of P is not changed.Al-Fe₂₄Z single crystal has potential applications in resistive switching memory and phase change memory.