Co／Co3O4／PZT多铁复合薄膜的交换偏置效应及其磁电耦合特性

Exchange bias effect and magnetoelectric coupling behaviors in multiferroic Co/Co_3O_4/PZT composite thin films

本文采用溶胶-凝胶工艺并结合脉冲激光沉积技术,在Pt/Ti/SiO2/Si衬底上制备了Co/Co3O4/PZT多铁复合薄膜.对复合薄膜的微结构和组分进行了表征,并系统研究了复合薄膜中的交换偏置效应及其对磁电耦合作用的影响.研究结果表明,复合薄膜在77 K具有明显的交换偏置效应,交换偏置场达到80 Oe,且交换偏置场及矫顽场均随温度降低而增大.当温度降低到10 K时,交换偏置场增至160 Oe.X射线光电子能谱(XPS)测试结果证实在Co和Co3O4界面处存在约5 nm厚的CoO层,表明77 K下的交换偏置效应源自反铁磁的CoO层对Co的钉扎作用.观察到复合薄膜的电容-温度曲线随着外加磁场大小和方向的改变而呈现出规律性的变化,表明复合薄膜存在磁电耦合效应.进一步研究发现,在低温下复合薄膜呈现出各向异性的磁电容效应,与磁场大小和方向密切相关.复合薄膜的这种磁电耦合特性主要与复合体系的交换偏置效应及基于界面应力传递的磁电耦合作用有关,本文对其中的物理机理进行了详细讨论与分析.

The multiferroic Co/Co3O4/PZT composite films are prepared on Pt/Ti/SiO2/Si wafers by sol-gel process combined with pulsed laser deposition method. The phase structures, microstructural topographies and element valence states of the composite films are characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and X-ray photoelectron spectrum （XPS）. The ferroelectric, electrical and magnetic properties as well as the magnetoelectric coupling behaviors are measured, and the exchange bias effect and its influence on the magnetoelectric coupling behavior of the composite film are studied systematically. The results show the composite films have well-defined ferroelectric hysteresis loops with a remanent polarization value of ～17 μC/cm2. The composite film exhibits evidently an exchange bias effect. Typically, a exchange bias field of ～80 Oe is observed at 77 K. Both the exchange bias field and magnetic coercive field increase with reducing the temperature. The exchange bias field increases to 160 Oe when the temperature decreases to 10 K. The XPS results confirm that an about 5 nm-thick CoO layer appears at the Co/Co3O4 interface due to the oxygen diffusion during the preparation, indicating that the exchange bias effect at 77 K is caused by the pinning effect of the antiferromagnetic CoO layer while the exchange bias effect at 10 K originates from the combining effect of antiferromagnetic CoO and Co3O4 layers. The measurement results of magnetocapacitance versus magnetic field curves at different temperatures show that the composite films have remarkable magnetoelectric coupling properties. The response of capacitance to temperature changes with the variation of external magnetic field. Further investigations show that the composite film possesses distinct anisotropic magnetocapacitance effect. When the direction of the magnetic field changes, the magnetocapacitance of the composite film changes from positive value to negative value. Moreover, the magnetocapacitance value changes with the variations of temperature and magnetic field magnitude. Typically, at 300K a maximum value of positive magnetocapacitance （5.49%） and a minimum value of negative magnetocapacitance of （1.85%） are obtained at -4000 and 400 kOe, respectively. When the temperature is reduced to 10 K, the positive magnetocapacitance decreases to a minimum value （0.64%） while the negative magnetocapacitance increases to a maximum value （5.4%）. We perform a detailed analysis on such a magnetoelectric coupling behavior, and elucidate its origin, which should be attributed to the exchange bias effect and interface-mediated magnetism-stress-electricity coupling process.