| 稀土基合金的磁卡效应及氧化薄膜和纳米结构的磁-电性质 | |
| 胡卫进 | |
| Subtype | 博士 |
| Thesis Advisor | 张志东 ; 李奇 |
| 2012 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 北京 |
| Degree Discipline | 材料物理与化学 |
| Keyword | 磁卡效应 交换偏置 隧道磁电阻 隧道电致电阻 垂直自旋扭矩 Magnetocaloric Effect Exchange Bias Tunnel Magneto-resistance Tunnel Electro-resistance Field-like Torque |
| Abstract | "本文采用固态反应烧结、电弧熔炼技术制备了DySb、Dy2Cr2C3化合物。利用脉冲激光沉积法制备了NdMnO3/Mn3O4纳米复合薄膜。采用脉冲激光沉积和光刻技术制备了La0.7Ca0.3MnO3/Ba0.5Sr0.5TiO3/La0.7Ca0.3MnO3 铁电性隧道结。通过X射线衍射、透射电镜 (TEM)、原子力显微镜 (AFM) 等探测样品的形貌和微观结构。用超导量子干涉仪 (SQUID) 和多物性测量系统 (PPMS) 测量样品的磁性能、电性能以及磁电耦合效应。 在DySb反铁磁体中，我们发现了磁熵变在奈尔温度附近从正值(反常磁卡效应)随温度增加变成负值(正常磁卡效应)，这种转变是由于场诱导的反铁磁到铁磁的一级磁转变。正的磁熵变来源于场诱导的HoP型亚铁磁中间过渡态。5 T磁场变化下，DySb多晶在9 K下获得最大的磁熵变为15.8 J/kg K，和工作在同一温区的其它磁卡材料相当。Dy2Cr2C3化合物同样具有场诱导的一级磁转变导致的反常和正常磁卡效应的转变，5 T磁场变化下在25 K获得的最大磁熵变为13.5 J/kg K。其磁致冷效率为425 J/kg, 比大部分工作在相同温区的磁卡材料要大。由于反铁磁致冷材料基本没有因磁场反向而导致的磁滞问题，因此将会是非常有潜力的低温致冷材料。 采用单相自分解技术制备的NdMnO3/Mn3O4纳米复合薄膜具有垂直型界面。这种具有垂直界面的纳米结构对于研究界面交换偏置现象很有意义。微观结构观察表明NdMnO3和STO基片有两种外延结构，而多晶取向Mn3O4纳米颗粒则弥散分布在NdMnO3相当中。我们发现该复合体系具有交换偏置现象，且交换偏置随温度在20 K出现一个峰值。这种异常的交换偏置峰可以归结为NdMnO3相中Nd晶格以及Mn晶格和Mn3O4相的相互竞争的界面耦合行为，是一种新的引起交换偏置峰的物理机制。 我们详细研究了La0.7Ca0.3MnO3/Ba0.5Sr0.5TiO3/La0.7Ca0.3MnO3 铁电性隧道结的磁电阻、电致电阻和界面磁电耦合效应。发现Ba0.5Sr0.5TiO3势垒层在1.5 nm厚度情况下仍然具有铁电性，铁电温度在100 K左右。铁电极化方向的翻转导致了隧道电致电阻；电致电阻和磁电阻的结合使我们获得了一个四阻态，这对于提高磁存储密度很有意义。另外，发现铁磁层电极的磁矩在外加电场的作用下可以提前翻转。随着外加电场的增加，所需要的翻转磁场减小，和电压的方向无关。采用电压诱导的非平衡层间交换耦合理论可以合理解释这一现象。我们发现电压诱导的双二次性层间交换耦合在磁矩翻转过程中起着重要的作用。从应用角度讲，我们显示了因施加电场而引起的磁矩的提前翻转和由此导致的隧道电阻在不同阻态之间的转化。这种转换意味着我们可以采用电场的方式来调控磁矩和磁电阻。由于相应的磁矩翻转临界电流(~ 6´103 A/cm2)远小于目前研究广泛的电流相关自旋扭矩的临界电流(~ 107-109 A/cm2), 这种铁电性隧道结将会是一种非常低能耗的高效磁存储器件。" |
| Other Abstract | "The DySb and Dy2Cr2C3 compounds were prepared by solid state reaction and arc-melting appropriate metals respectively, followed by annealing at a certain temperature in the purified argon atmosphere. The NdMnO3/Mn3O4 nanocomposite thin films (105 nm) were prepared by pulsed laser deposition (PLD) method by using the self-decomposition of a single-phase target NdMn2O5. The micro-size La0.7Ca0.3MnO3/Ba0.5Sr0.5TiO3/La0.7Ca0.3MnO3 ferroelectric tunnel junctions were prepared by PLD and photolithography technology. The microstructures of the crystalline compounds were studied by X-ray diffraction (XRD), while the microstructures of films were investigated by means of XRD, atomic force microscope (AFM) and transmission electron microscope (TEM). The magnetic, electronic, magneto-transport and magnetocaloric properties were measured by a superconducting quantum interference device (SQUID) magnetometer and physical property measurement system (PPMS). In DySb compound, we found a crossover of magnetic-entropy change (DSM) from positive value to negative value with increasing temperature through the Neel point, which is associated with the magnetic-field-induced first-order meta-magnetic transition from antiferromagnetic state (AFM) to ferromagnetic state (FM). The positive MCE at low temperature is due to the formation of a sub-stable ferrimagnetic state (FIM) with the HoP-type magnetic structure in the external field. The combining of negative and positive MCE has also been observed in Dy2Cr2C3 compound, which has the same mechanism as that of DySb, the magnetic structure of the sub-stable state is however not clear. The maximum magnetic entropy changes for DySb and Dy2Cr2C3 are of -15.8 J/kg K at 5.5 K and -13.5 J/kg K at 25 K respectively, under an external magnetic field change (DB) of 5 T. These values are comparable with other materials having giant magnetocaloric effect (MCE) that works in the same temperature range. The relative cooling power (RCP) value for Dy2Cr2C3 of 425 J/kg is larger than most other MCE materials working in the same temperature range, indicating it could be a potential magnetic refrigerant. These AFM-MCE materials have no obvious magnetic hysteresis, which will be a benefit for their application in magnetic cooling. Exchange bias (EB) phenomenon has been widely studied in the nano-particles and the plane-multilayer structures. However the mechanism in detail is still unclear due to the complexity of the interface magnetic structure. To study hetero-structures with vertical interface will help us to better understand the EB phenomenon. NdMnO3/Mn3O4 nano-composite thin films prepared by PLD allow us to study the effect of consecutive magnetic orders of NdMnO3 compound on the Mn3O4 ferrimagnet. The EB has been observed below the Curie point of Mn3O4 at 45 K. It behaves as a peak at 20 K and then decreases at low temperature. Such a EB peak is ascribed to the existence of two different magnetic couplings at the interface: the FM coupling of Mn-sublattice and Mn3O4 below 45 K, and the AFM alignment of Nd-sublattice and Mn3O4 below 15 K. The EB phenomenon has also been observed after zero-field cooling, which is due to the local field produced by the short range interactions of Mn3O4 at the paramagnetic state. Nowadays, the demand of high density storage needs to explore multi-states storage and controlling magnetization vectors by methods other than magnetic field. Magnetic tunnel junctions with ferroelectric barrier (FE-MTJs) could be a potential candidate for the next generation of magnetic storage devices. We observed the electric-field induced tunnel electo-resistance (TER) and magnetic-field induced tunnel magneto-resistance (TMR) in La0.7Ca0.3MnO3 (50 nm)/Ba0.5Sr0.5TiO3 (1.5 nm)/La0.7Ca0.3MnO3 (50 nm) junctions with size of several micrometers. The Ba0.5Sr0.5TiO3 barrier is confirmed to be ferroelectric up to 100 K by the resistance-electric field loops. The TER is then ascribed to the changing of the barrier height induced by the reversal of the FE-polarization vector under the external electric field. TER and TMR allow us to obtain a 4-resistance-state which obviously improves the density of magnetic storage at least two times larger. TER at magnetization-antiparallel state (AP) is larger than TER at magnetization-parallel state (P), which also indicates the existence of magneto-electric coupling in the junctions as predicted by the theory. We also found the field-induced magnetization reversal both for the top and bottom FM electrode. The TMR curves shift to the lower field almost linearly with increasing the electric field, independent of the polarity of the electric field. This phenomenon could be explained by the voltage-dependent perpendicular spin torque (also as field-like torque, FLT), which suggests that the direction oft FLT changes with changing the polarity of the voltage. We found that the FLT is closely related with the voltage-induced interlayer exchange coupling between the two ferromagnetic electrodes. The voltage-induced biquadratic term is responsible for the sign reversal of the FLT. The critical current density for the switching of magnetization in present junctions is about 6´103 A/cm2, much smaller than that of using current-dependent STT (107-109 A/cm2), indicating the lower power consumption of the FE-MTJs devices." |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/64419 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 胡卫进. 稀土基合金的磁卡效应及氧化薄膜和纳米结构的磁-电性质[D]. 北京. 中国科学院金属研究所,2012. |
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