Study On Abnormal Thermal Expansion And Magnetostrictive Effects Of Multiferroic Materials And Their Devices

Multiferroic materials have attracted extensive research interest in both fundamental research and applications due to their complex magnetoelectric properties.In this doctoral dissertation,we focused on the controllability of thermal expansion and magnetostriction through magnetic field.Then we achieved the functions of non-volatile random-access memory and Boolean logic operation based on thin film magnetostrictive material/ferroelectric material heterostructures.The specific research contents are as follows:We first investigated the thermal expansion and magnetostriction behavior of polycrystalline z-DyCrO₄ sample synthesized at atmospheric pressure and polycrystalline s-DyCrO₄ sample synthesized at high pressure and found that the z-type phase has a negative thermal expansion coefficient below the ferromagnetic ordering temperature of 23 K at zero magnetic field.However,below this temperature,the sign of the coefficient of thermal expansion changes with increasing magnetic field,with the coefficient of thermal expansion changing from negative to positive around 1 T.The s-type phase has a similar thermal expansion behavior:below the antiferromagnetic ordering temperature of 24 K,a magnetic field can induce the conversion of the positive and negative coefficients of thermal expansion of the s-type samples.At the same time,both z-DyCrO₄ and s-DyCrO₄ exhibit large magnetostriction,reaching absolute values of～860 ppm and～760 ppm at 2 K and 10 T,respectively,and have not yet reached saturation.The physical origin of these phenomena is closely related to the strong spin-lattice coupling of the rare-earth ion Dy³⁺in the crystal.Then,based on the local spin interactions and symmetry of the lattice point group,we have calculated and analyzed in detail the origin of linear magnetoelectric phenomena and polarization in s-DyCrO₄.Due to its special lattice symmetry and magnetic moment arrangement,the single ion spin interactions between the metal ions Cr⁵⁺and Dy³⁺and the double ion spin interactions between Dy³⁺-Dy³⁺and Cr⁵⁺-Dy³⁺all result in polarization in the ab plane when s-DyCrO₄ has a ferromagnetic moment component in the c direction.This is consistent with experiments and previous research.This analytical method reveals all the possible microscopic origins of each non-zero electric polarization factor,and the contribution of each mechanism can be located to a specific spin or spin pair,so it is very efficient and accurate to explore the microscopic mechanism of magnetoelectric effect in multiferroic materials.Furthermore,we also fabricated multiferroic FeGa/PZT thin film memtranstor and studied the modulation of its magnetoelectric coupling voltage by external magnetic and electric fields.The magnetoelectric coupling voltage of the FeGa/PZT memtranstor can be reversed by flipping the magnetization direction of FeGa or the polarization direction of PZT.It is worth noting that the sign of the magnetoelectric coupling voltage can be switched several times by reversing the ferroelectric polarization direction of the PZT when the external DC bias magnetic field remains constant.Furthermore,the binary switch behavior can still be maintained at zero DC bias magnetic field.The magnetoelectric coupling voltage also does not change when the polarization direction is stable.This means that the magnetoelectric coupling voltage of FeGa/PZT is non-volatile.Additionally,we also studied the retention and durability characteristics of the FeGa/PZT thin film memtranstor.By applying pulsed electric fields of different magnitudes,multi-level state memory is again realized in FeGa/PZT thin film memtranstor.These findings demonstrate the potential of FeGa/PZT thin film memtranstor for non-volatile memory applications.Meanwhile,we have implemented two non-volatile Boolean logic operations,NOR and NAND,in the FeGa/PZT thin film memtranstor,indicating that the thin film memtranstor also has the potential to be used as a Boolean logic device.