BaTiO_3/La_(0.67)Sr_(0.33)MnO_(3-δ)复合薄膜的磁致电极化和磁介电特性研究

采用脉冲激光沉积法制备了BaTiO_3(BTO)与缺氧的铁磁绝缘态La_(0.67)Sr_(0.33)MnO_(3-δ)(LSMO)构成的磁电复合薄膜,研究了20—300K温度区间内磁场对电极化特性和介电特性的影响.研究发现,施加磁场使得电滞回线的剩余极化强度和矫顽场均增大,其变化率峰值分别为111.9%和89.6%,峰值温度分别为40K和60K.异质结具有显著的磁介电效应,在测量温度区间内,磁场使得介电常数增大,介电损耗减小.在0.8T场强下,介电常数的最大磁致变化率出现在60K,达到了300%,而介电损耗也在此温度实现了最大变化,减小为零场时的50.9%.该磁电复合薄膜的磁致电极化和磁介电特性的极值均出现在LSMO层的磁电阻峰值温度附近,这说明磁场对电滞回线和介电参数的调制应该源自电荷相关的耦合作用.其可能的机理是磁场使得锰氧化物中的Mn离子局域磁矩趋于有序排列,并通过自旋-轨道耦合以及界面效应间接影响了BTO的电极化特性.研究结果对于多铁器件的开发和应用具有重要意义.

Magneto-induced polarization enhancement and magneto-dielectric properties in oxygen deficient La0.67Sr0.33MnO3-δ/BaTiO3 composite film

Magnetoelectric composite film is an important type of multiferroic materials, which is usually composed of typical ferromagnetic and ferroelectric materials. For the ferroelectric layer, BaTiO₃ (BTO) attracts much attention due to its lead-free characteristic. For the ferromagnetic layer, doped manganite (R_1-xA_xMnO₃) has been a good candidate for designing the advanced multiferroic films. Multiple interactions among the freedom degrees of charge, orbital, spin and lattice inside the doped manganite bring many additional properties into the manganite based composite films. At present, most of researches of manganite/BTO focus on the stoichiometric oxygen ion in manganite. Considering the fact that the oxygen deficiency can remarkably adjust the properties of manganite itself and relevant heterostructure by the interface effect, abnormal magnetoelectric properties are expected in an oxygen deficient manganite/BTO composite film. In this work, a composite film composed of BTO and oxygen deficient La_0.67Sr_0.33MnO_3-δ (LSMO) is deposited on LaAlO₃ 001 substrate by the pulsed laser deposition method, and the effects of magnetic field on the properties of polarization and dielectric in a temperature range of 20-300 K are investigated. The X-ray diffraction pattern reveals good epitaxial growth of this bilayer film. The upper LSMO film exhibits semiconductive characteristic (dR/dT < 0) in a temperature range of 20-300 K. Magnetization curves indicate that the LSMO keeps ferromagnetic state without any magnetic phase transition in this temperature range. When applying a magnetic fields of 0.8 T, the resistance in LSMO is observed to decrease. The changing rate MR=|R_{0.8 T}-R₀ T|/R₀ T decreases from 45.28% at 30 K to 0.15% at 300 K. This composite film exhibits remarkable temperature-dependent magneto-induced ferroelectric and dielectric change. It is found that the remanent polarization (P_r) and coercive electric field (E_c) are enhanced by the 0.8 T magnetic field. The maximum changing rates of P_r and E_c are 111.9% and 89.6% at the temperatures of 40 K and 60 K, respectively. The magnetic field enhances the dielectric constant ε, but suppresses the dielectric loss tan θ. The maximum changing rates of ε and tan θ both occur at 60 K with the values of 300% and 50.9%. The temperature at which appear the maximum magneto-induced relative changes of polarization and dielectric parameters is accordant with the temperature at which occurs the peak value of magnetoresistance, which indicates a charge-based coupling in this heterojunction. A potential mechanism is that the magnetic field promotes the degree of parallelism of local spin magnetic moment of Mn ion, and produces an indirect effect on BTO layer by the spin-obital coupling and interface effect. Our findings make the oxygen deficient LSMO/BTO heterojunction promising for the design of multiferroic devices.