EuTiO3薄膜的制备及磁性的研究

EuTiO3 (ETO)是一种立方结构的多铁材料，其在低温下表现为反铁磁 序和量子顺电序共存，奈尔温度为 5.1 K。在 ETO 顺电-反铁磁序共存的温 度范围内，其块材中的 Eu 原子间存在着较强的自旋晶格耦合作用，通过合 适的双轴应变，可以使 ETO 发生磁序的转变，从反铁磁有序性相变为铁磁 有序性，铁磁居里温度为 4.2 K。此外，由于低氧压生长而引入的氧空位也 可以使其发生同样的磁序变化。这些现象说明 ETO 材料中同时存在复杂的 电荷、自旋、晶格和轨道自由度之间的耦合作用。因此本文详细研究了氧 空位及双轴应变对薄膜磁性的影响。 本文选用 SrTiO3（STO）（100）和 KTaO3（KTO）（100）作为衬底， 首先，通过脉冲激光沉积（PLD）技术在这两种衬底上生长不同氧压下的 单相 ETO 外延薄膜。通过 X 射线衍射（XRD），RSM 等手段详细研究了薄 膜的应变情况，并研究氧空位对薄膜磁性的影响。然后在不可避免氧空位 存在的情况下进一步研究了双轴拉应变对薄膜磁性的调控作用。 ETO 薄膜的结构表征结果表明，所有薄膜与衬底都呈现良好的外延关 系。在 STO 衬底上表现为未受应变的状态，在 KTO 衬底上表现为受拉应 变的状态。通过对磁性进行表征发现在 STO 上生长的薄膜出现了铁磁有序 现象。这是由于在薄膜沉积过程中氧压较低导致了氧空位的产生，氧空位 的引入使 ETO 的晶胞体积增大和体对角线上 Eu-Ti-Eu 的键角变小。因此减 少了 Eu 和 Ti 之间的轨道重叠，并且减弱了 Eu-Ti-Eu 的反铁磁超交换作用。 最终，引入并增强铁磁相互作用，使薄膜呈现铁磁态。随着氧压的升高， 薄膜中的氧空位减少，导致薄膜的磁性减弱。在相同的生长条件下，KTO 衬底上生长的 ETO 薄膜的磁化强度比在 STO 衬底上生长的薄膜都要更大。 并且在相同氧压下，受双轴拉应变的薄膜铁磁居里温度也更高，可以达到 3.1 K，这说明施加双轴拉应变可以增强 ETO 薄膜的磁性。此外，通过调节 氧压，发现其对 KTO 上的 ETO 薄膜磁性的影响与对 STO 衬底上的类似， 即随着氧压的升高，薄膜的饱和磁化强度和自发磁化强度均减小。对 ETO 薄膜磁性的研究可以丰富对其多铁性的认识以及薄膜磁性的调控方式。

EuTiO3 (ETO) is a cubic multiferroic material that exhibits coexistence of antiferromagnetic and quantum paraelectric orders with the Néel temperature of 5.1 K at low temperatures. There is a strong spin-lattice coupling in the bulk material in the temperature range where the paraelectric and antiferromagnetic order of ETO coexist. With suitable biaxial strain, the magnetic order of ETO can be transformed from an antiferromagnetic order to a ferromagnetic order with the ferromagnetic Curie temperature of 4.2 K. On the other hand, the oxygen vacancies introduced by the growth conditions at low oxygen pressure can change the magnetic order. These results indicate that the complex coupling between charge, spin, lattice and orbital degrees of freedom exists simultaneously in ETO materials. Therefore, the effects of oxygen vacancies and biaxial strain on the magnetic properties of thin films were studied in detail in this thesis. Firstly, the single-phase ETO epitaxial thin films were grown on SrTiO3 （ STO ）（ 100 ） and KTaO3 （ KTO ）（ 100 ） substrates by pulsed laser deposition (PLD) under different oxygen pressures. To probe the microstructure, the strains of the films were studied in detail using X-ray diffraction (XRD), RSM, etc. The effects of oxygen vacancies on the magnetic properties of the thin films were investigated. Then the regulation of the biaxial tensile strain on the magnetic properties of the thin films was further studied in the presence of unavoidable oxygen vacancies. The structural characterization results of the ETO films show that all films exhibit epitaxy relationships with the substrates. The ETO films on STO substrates are not subject to epitaxial strains, while those on KTO substrates are subject to tensile strains. Through the characterization of magnetic properties, it is found that the films grown on STO show ferromagnetic order, which is due to the oxygen vacancies introduced by low oxygen pressure during films deposition. The introduction of the oxygen vacancies induces the increase of the unit cell volume and the reduction of the bond angle of Eu-Ti-Eu in ETO films. Abstract III Therefore, the orbital overlap between Eu and Ti is reduced and the antiferromagnetic super-exchange interactions of Eu-Ti-Eu are weakened. As a result, the ferromagnetic interaction is introduced and enhanced, making the films acquire a ferromagnetic state. With the increase of oxygen pressure, the oxygen vacancies decrease in the films, which weakens the magnetic properties of thin films. Under the same growth conditions, the magnetization of ETO films grown on KTO substrates are larger than those grown on STO substrates. Under the same oxygen pressure, the ferromagnetic Curie temperature of the film subjected to biaxial tensile strain is also higher and able to reach 3.1 K. It indicates that the application of biaxial tensile strain can enhance the magnetic properties of the ETO films. In addition, by adjusting the growth oxygen pressure, it is found that the effects of the oxygen pressure on the magnetic properties of ETO films on the KTO substrate are consistent with those on the STO substrate. Both the saturation magnetization and the spontaneous magnetization of the film decrease with the increase of the oxygen pressure. The study of the magnetic properties of ETO thin films will enrich the understanding of its multiferroic properties and the modulation of the magnetic properties of thin films

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