外延反铁磁间金属Mn3Pt薄膜制备及反常霍尔效应研究

Recently, as a consequence of the Berry phase of electrons in the reciprocal momentum space, both theoretical calculations on non-collinear antiferromagnetic (AFM) Mn3Ir [Chen et al., Phys. Rev. Lett.112, 017205(2014)] and experimental studies on bulk Mn3Sn [Nakatsuji et al., Nature 527,212(2015)] have demonstrated the topological Anomalous Hall effect (AHE) in non-collinear antiferromagnets, which has pioneered the investigation of the AHE in AFM materials. Up to now, most of studies on the AHE of antiferromagnets have been performed on bulk materials. However, to utilize the exotic AHE of antiferromagnets for high-density memory device applications, the realization and manipulation of the AHE in AFM thin films are essential. Mn3Pt, an AFM intermetallic alloy similar to Mn3Ir, exhibits a unique spin structure phase transition at 360 K, below which the spins of adjacent Mn atoms are 120 degree relative to one another, forming a triagular AFM lattice, and above which it has a collinear AFM spin structure. This project is to first realize successful epitaxial growth of Mn3Pt thin films on regular oxide substrates and then investigate the AHE across the spin structure phase transition; subsequently, epitaxial integration of Mn3Pt films on function ferroelectric oxides would be performed for electric switching of the AHE via an electric field around the phase transition. Overall, a novel mechanism for electrically controlling the AHE in AFM materials will be explored for low energy-consuming multiferroic memory device applications.

由于动量空间电子Berry相的存在，关于非共线型反铁磁Mn3Ir的理论计算[Phys. Rev. Lett.112, 017205(2014)]和块体Mn3Sn的实验研究[Nature 527,212(2015)]都发现了非共线反铁磁中会出现拓扑序的反常霍尔效应,开创了反铁磁体系中拓扑反常霍尔效应研究的新领域。要将此新奇效应进一步应用于高密度磁性存储器件，薄膜材料中此效应的实现和操控势在必行。Mn3Pt，一种类似于Mn3Ir和Mn3Sn的反铁磁间金属，但它缺拥有独特的自旋结构相变：360K以下，相邻Mn原子的自旋互成120度，为三角形非共线反铁磁；360K以上为共线型反铁磁。本项目拟于前期实现Mn3Pt外延单晶薄膜在普通氧化物基片上的优化生长，并研究相变前后反常霍尔效应的变化；进而将外延Mn3Pt薄膜集成在功能铁电氧化物上，通过多铁途径对Mn3Pt的反常霍尔效应在其相变附近进行电场调控。

（1）实现了间金属化合物Mn3Pt在氧化物单晶基底上外延生长及电场操控反常霍尔效应。利用磁控溅射系统优化生长参数，将Mn3Pt薄膜外延沉积到氧化物铁电单晶基底BaTiO3上，并进行了系统的磁输运实验，观测到了显著的反常霍尔效应。通过施加电场，在压电基片上诱发压电应力产生了0.35%的面内应变作用，使Mn3Pt薄膜的相变温度提升了约25K，在360K附近实现了Mn3Pt薄膜非共线反铁磁相-共线反铁磁相的可逆转变，成功有效地调控了Mn3Pt的反常霍尔效应的开关。.（2）开展了电场操控共线反铁磁MnPt薄膜电阻的研究。通过磁控溅射系统将MnPt薄膜沉积到氧化物单晶基底PMN-PT上，施加电场诱发压电应力，改变MnPt薄膜中反铁磁自旋轴的取向，利用反铁磁材料的各项异性磁电阻效应产生不同的电阻态，在60T的强磁场下依然可以保持稳定。构建了基于MnPt的反铁磁隧道结器件,室温下实现了11.2%的电阻调控，极大提升了电信号输出。利用电场可使压电基片产生裂纹的方式，探究了不同频率的电场与裂纹生长的关系。MnPt薄膜的裂纹在周期性电场的作用下能够打开与闭合，高低阻态的开关比超过108，器件在107的循环次数以及60天的工作时间里保持稳定。.（3）进行了非共线反铁磁Mn3X（X=Sn、Ge、Ga）新奇物理性质以及电场操控薄膜电阻和反常霍尔效应的研究工作。通过插入LaAlO3缓冲层克服了Mn3Sn/PMN-PT异质结构在电场作用下不稳定的难题，利用施加电场有效调控了Mn3Sn薄膜的反常霍尔效应。在Mn3Ge和Mn3Sn中观测到了平行负磁阻，为非共线反铁磁Mn3X中存在外尔费米子提供了实验证据。利用电场调控了Mn3Ge薄膜的电阻，并且在3T的磁场下保持稳定的高低阻态。利用外加电场实现了Mn3Ga薄膜反常霍尔电阻超过30倍的调控，并构建基于非共线反铁磁Mn3Ga的隧道结器件，提升室温信号输出。