磁性纳米颗粒中的交换偏置和磁卡效应，磁性薄膜中的交换耦合效应和铁磁、铁电薄膜体系中的隧道电阻效应

IMR OpenIR

	磁性纳米颗粒中的交换偏置和磁卡效应，磁性薄膜中的交换耦合效应和铁磁、铁电薄膜体系中的隧道电阻效应
	刘雄华
学位类型	博士
导师	刘伟 ; 张志东
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	机械合金化交换偏置磁卡效应交换耦合隧道磁电阻 Mechanical-alloying Exchange Bias Magnetocaloric Effect Exchange Coupling Tunneling Magnetoresistance
摘要	"采用机械合金化及真空和不同气氛中退火的方法研究了铁磁-反铁磁纳米颗粒中的交换偏置效应。采用溶胶-凝胶方法制备反铁磁性的氢氧化物纳米颗粒研究氢氧化钴在低温下的可逆巨磁卡效应和氢氧化镍中的纳米尺寸效应和交换偏执效应。采用磁控溅射制备不同组成的铁磁-反铁磁-铁磁三层结构研究薄膜的界面交换耦合与层间交换耦合效应。用脉冲激光沉积（PLD）和磁控溅射生长铁磁-铁电-铁磁三层膜，用光刻技术（photolithography）和离子刻蚀(Ion milling)制备隧道结样品来研究隧道磁电阻和电致电阻效应。利用X射线粉末衍射（XRD）、扫描电镜（SEM）、透射电镜（TEM）、X光电子能谱（XPS）、原子力显微镜（AFM）和磁力显微镜（MFM）等测试分析技术，系统研究了各种材料的相组成、样品的表面、界面及微观结构及元素价态等。利用超导量子干涉仪（SQUID）和综合物性测量系统（PPMS）研究样品的磁、电性能。反铁磁的α-Fe2O3与NiO粉末通过机械合金化及退火后转变成为亚铁磁的γ-Fe2O3（或Fe3O4）与反铁磁的NiO磁性纳米颗粒。发现了反铁磁交换耦合效应及低温下较大的负交换偏置场，由于界面的非补偿自旋磁矩与钉扎的反铁磁自旋磁矩的作用使得磁滞回线形状出现一些异常，并且在反铁磁-反铁磁体系中也出现了比较明显的交换偏置效应。在亚铁磁Fe3O4与反铁磁Cr2O3中发现比较明显的反铁磁耦合的交换偏置现象。反铁磁的α-Fe2O3在球磨过程中会逐渐发生相变过程，球磨20小时后全部转变为亚铁磁的Fe3O4，在两相共存的情况下出现非常明显的交换偏置效应。通过溶胶-凝胶法制备的反铁磁性β-Co(OH)2纳米颗粒中发现了可逆的巨磁卡效应。在奈耳温度以下，由于反铁磁磁矩间较弱的相互作用，足够高的外场将导致从反铁磁到铁磁相的场致相变，这种相变将产生非常大的磁卡效应。通过溶胶-凝胶法制备反铁磁性Ni(OH)2纳米颗粒具有很强的尺寸效应。由于类自旋冻结态的形成，在低温低场作用下样品的磁矩随着温度的下降而增加。由于纳米颗粒界面的非补偿磁矩使得Ni(OH)2 中出现比较明显的交换偏置效应。通过磁控溅射方法，我们在Si基片上制备了铁磁/反铁磁/铁磁三层结构不同成分的薄膜。在Co/反铁磁/Fe结构中发现了非常明显的与温度相关的铁磁-反铁磁界面耦合与铁磁-铁磁层间耦合之间的竞争效应。当反铁磁层为绝缘体的NiO和Cr2O3时，由于界面耦合效应随着温度的上升而下降，而层间耦合效应随着温度的升高而上升，使得在零场冷的情况下磁滞回线在中间温度出现非常明显的台阶，而在温度两端由于界面耦合与层间耦合分别占主导地位使得样品耦合的很好。当反铁磁层为金属的Cr时，由于两种耦合作用都随着温度的上升而下降，使得样品在高温下出现了明显的脱耦合的现象。在铁磁/Cr2O3/铁磁体系中不同的铁磁层对薄膜的界面耦合与层间耦合效应都有着非常重要的影响。通过铁磁层分别为Fe、Co、Fe20Ni80时薄膜随温度的变化的研究，发现铁磁层的磁晶各向异性和跟Cr2O3接触的自旋非对称性反射系数在系统中起着非常重要的作用。同时我们研究了在多孔氧化铝模板上生长的Co/NiO/Fe三层膜体系中的交换耦合效应，发现相对于Si基片上的连续薄膜，多孔模板能强烈影响铁磁/反铁磁层之间的界面耦合强度，在Si基片上的薄膜表现为负磁电阻效应而在模板上发现了正的磁电阻效应。通过脉冲激光沉积，磁控溅射，光刻和离子刻蚀技术制备了铁磁/铁电/铁磁三层膜结构的隧道结。因为隧道结中隔离层为铁电的BaTiO3，铁电层不同的自旋电极化取向（正自旋极化和负自旋极化）和铁磁层之间不同的磁矩取向（平行和反平行磁矩）使得该隧道结中观察到明显的四阻态。在La0.67Sr0.33MnO3/BaTiO3/Co三层膜体系中，由于Co的自旋极化为负，而La0.67Sr0.33MnO3是自旋极化为正的半金属，因此在该体系中观察到明显的负磁电阻效应和四阻态。此外，在La0.67Sr0.33MnO3-BaTiO3/Co体系中，不同的生长条件使得一些样品中观察到了电阻切换效应，这种效应随着温度的下降而减小，而BaTiO3的铁电效应随着温度的下降而增大，即这两种效应随着温度的变化出现一种竞争关系。低温下铁电效应占主导地位，体系出现正电致电阻效应，而高温下电阻切换效应起主要作用，体系中观察到负电致电阻效应，在临界温度处电致电阻效应为零。在La0.67Sr0.33MnO3/BaTiO3/Co中观察到了不同隧道结中的隧道磁电阻从负到正的变化过程，是由于外加偏压界和温度改变了界面处铁磁层的费米能级。"
其他摘要	"The exchange bias effect in ferromagnetic-antiferromagentic (FM-AF) nanoparticles prepared by mechanical-alloying plus annealing in vacuum or in oxygen are investigated. AF Co(OH)2 and Ni(OH)2 nanoparticles are fabricated by a sol-gel method. The giant reversible magnetocaloric effect in Co(OH)2 and size effect and exchange bias in Ni(OH)2 are observed. The interfacial exchange coupling between AF/FM and interlayer exchange coupling between FMs across AF layer are investigated in FM/AF/FM trilayers prepared by magneto-sputtering. The tunneling magnetoresistance (TMR) and electroresistance (TER) effect are investigated in FM/ferroelectric(FE)/FM trilayers grown by pulsed laser deposition (PLD) and magneto-sputtering and made by photolithography and Ion milling. Component, surface, interface and microstructure were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), atomic force microscope (AFM) and magnetic force microscopy (MFM). Magnetic and magnetotransport properties were characterized by superconducting quantum interference device (SQUID) and physical property measurement system (PPMS). Antiferromagnetic α-Fe2O3 and NiO powders transform into ferrimagentic (FI) γ-Fe2O3 (or Fe3O4) and NiO nanoparticles by mechanical-alloying and post-annealing. A negative exchange bias field and AF coupling are observed at low temperature in this system. The anomalous shape of hysteresis loop and exchange bias in AF/AF system are observed, which may be related to uncompensated and pinned AF spins at the interface between FI and AF phases in the powder. The clear AF exchange coupling is found in Fe3O4 and Cr2O3 nanoparticles. The largest exchange bias field is observed in cooling field (Hcf) of 1 T, and the Zeeman interaction plays very important role for Hcf > 1 T. Antiferromagnetic α-Fe2O3 will be gradually transformed into FI Fe3O4, the pure Fe3O4 is obtained when the powder milled for 20 h, and the obvious exchange bias phenomenon is observed in coexistence of two phases. Furthermore, the exchange bias is found in either α-Fe2O3 or Fe3O4 single phase coated by an amorphous layer, indicating the interfacial status and domain structure may play important roles in the system. The giant reversible magnetocaloric effect is found in cobalt hydroxide nanoparticles fabricated by a sol-gel method. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a field-induced transition from the AF to FM below the Néel temperature. The large reversible magnetic-entropy change -ΔSm (20.9Jkg-1K-1) indicates that Co(OH)2 is a potential candidate for application in magnetic refrigeration in the low-temperature range. The magnetic properties of 10 nm size Ni(OH)2 nanoparticles prepared by sol-gel method have been studied. The magnetic moments increase with decreasing temperature in a low applied field, which is due to the spin-frozen-like state at low temperatures, and the metamagnetic transition is not clearly observed even in an applied field of 70 kOe due to the size effect. A clear transition from paramagnetic to AF occurs in large applied field due to the beaten of spin-frozen-like state. The FM/AF/FM trilayers are prepared by magnetron sputtering. The competition betweeninterlayer and interfacial exchange couplings is found to be temperature dependent in Co(3 nm) / AF / Fe(10 nm) trilayers with AF º antiferromagnetic NiO, Cr2O3 or Cr. The temperature dependence in trilayers with AF insulating NiO or Cr2O3 spacer layer differs from that with AF metallic Cr. In the insulator case, the enhancement of the interlayer exchange coupling and the reduction of interfacial exchange coupling with increasing temperature results in dominating interlayer exchange coupling at high temperature. In the metallic spacer case, both the couplings decrease with increasing temperature, leading to decoupling at high temperatures. The strong effects of ferromagnetic (FM) materials on exchange couplings are observed at different temperatures in FM1(3 nm)/Cr2O3(6 nm)/FM2(10 nm) trilayers with FM º Co, Fe or Ni80Fe20. The changes of anisotropy of FM and spin-asymmetry of the reflection coefficients for FM contacted antiferromagnetic layer greatly influence the strengths of interfacial and interlayer couplings of the trilayers. The reduction of interfacial coupling and the enhancement of interlayer coupling with increasing temperature result in quite different magnetic behaviors for different trilayers. The formation of an interconnected nanostructured network of Co/NiO/Fe trilayers on an anodic aluminum oxide (AAO) template strongly affects the interfacial exchange coupling between antiferromagneti and ferromagnetic layers in comparison with a continuous film of trilayers on a silicon substrate. Anomalous magnetic and magnetotransport properties, in particular a positive magnetoresistance (MR), are observed for the network on AAO, but a negative MR is found for the film on Si. The FM/FE/FM trilayers is grown by PLD and magnetro-sputtering, and the tunnelling junctions are made by photolithography and Ion milling. As ferroelectric BaTiO3 as a barrier, so it can be known as four resistance states due to two different polarization of BaTiO3 (positive and negative polarization) and two different magnetic direction (parallel and antiparallel). In La0.67Sr0.33MnO3/BaTiO3/Co system, the negative TMR and four resistance states can be found due to the negative spin polarization of Co and positive spin polarization of half-metallic LSMO. Furthermore, obvious TMR can be observed when the BTO is very thin (about 1.7nm), but the I-V curve and TMR are asymmetric due to the different FM layer, which varies with temperature. However, in La0.67Sr0.33MnO3/BaTiO3/Co system, the resistance switching effect can be observed in some samples prepared at different condition. At low temperature, the sign of TER is different from that at room temperature, and there are two main effects (resistance switching and ferroelectric effects) in this system, in which the resistance switching effect dominants the properties at room temperature, and the ferroelectric BTO takes main role at low temperatures. The two effects compete with temperature, and there is an critical temperature at which the balance between two effects with the zero TER. Moreover, the transition from positive to negative TMR for different tunneling junctions can be found, which may be due to the change of Fermi lever of Co at interface for different junctions."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64421
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘雄华. 磁性纳米颗粒中的交换偏置和磁卡效应，磁性薄膜中的交换耦合效应和铁磁、铁电薄膜体系中的隧道电阻效应[D]. 北京. 中国科学院金属研究所,2012.