National Demonstration Center for Experimental Physics Education (Sun Yat-sen University), School of Physics, Sun Yat-Sen University, Guangzhou 510275, China
Fund Project:Project supported by the Fundamental Research Fund for the Central Universities, China (Grant No. 19lgpy260)
Received Date:07 January 2021
Accepted Date:19 February 2021
Available Online:16 June 2021
Published Online:20 June 2021
Abstract:The study of two-dimensional (2D) magnetic materials has driven the development of modern nano-electronic devices. Exploration of novel intrinsic layered materials with 2D magnetic order will provide a material candidate pool for fabricating 2D devices and searching for new quantum phases. Recently the layered antiferromagnetic (AF) topological insulators have aroused the great interest of researchers. As one of the proposed axion insulators, EuIn2As2 exhibits a layered structure and 2D AF order. It is found that the parent compound EuIn2As2 exhibits metallic behavior instead of the predicted insulating feature. To pursuit the predicted non-trivial topological state and novel feature, in this paper, we use various elements to dope the system to adjust the Fermi level. It is found that only Ca is successfully doped into the EuIn2As2 system. The systematic transport and magnetization studies are performed on the single crystal of Eu1–xCaxIn2As2. The long-range AF order is revealed to be similar to the parent compound. Above the AF transition, the magnetization violated Curie-Weiss behavior and magnetoresistance keeps negative, indicating the ferromagnetic order. With doping nearly 20% non-magnetic Ca, the magnetic properties of the system barely change, which is favorable to keeping the former predicted nontrivial topological properties in EuIn2As2. Although Ca shares the same valence with Eu, the carrier density of Eu1–xCaxIn2As2 is one order lower than that of EuIn2As2. The Ca doping brings electrons in and lifts the Fermi level. The results enrich the 2D magnetic material candidate pool and provide useful information for realizing the nontrivial topological state in the 2D AF system. Keywords:two-dimensional magnetic materials/ antiferromagnetic order/ topological insulator
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3.1.不同元素掺杂的实验结果
为了能够调节费米面, 尝试了多种元素对体系进行掺杂(如图1, 利用Ca进行掺杂改变费米能级): 比如在In的位置用Ga, Cd, Zn, Sn, Ag等元素进行掺杂; 在As位通过$ {\rm{Sb}}, {\rm{P}} $等元素进行掺杂; 在Eu位通过Sm, Ca等元素进行掺杂. 结果如表1所列, 元素掺杂的体系中掺杂元素的含量低于仪器的识别精度, 最终只有Sn, Ca元素成功地掺杂进入体系, 其中Ca元素掺杂效应较为明显. 选取Ca掺杂的$ {\rm{Eu}}_{1-x}{\rm{Ca}}_{x}{\rm{In}}_{2}{\rm{As}}_{2} $单晶进行详细研究. X射线衍射的结果表明, 制备的单晶表现出良好的结晶取向, 由于Ca原子与Eu原子半径相似, 掺杂没有引起明显的晶格变化. EDS的实验结果表明Ca成功地掺杂进了体系, 单晶为$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $. 图 1 (a) $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的晶体结构, 其中无磁的Ca元素替换掉部分磁性元素Eu; (b)体系材料能带结构示意图, 其中母体$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $处于金属态, 通过掺杂提升费米能级, 可以实现可能的拓扑态; (c)母体$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $(上图)和$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $(下图)的X射线单面晶体衍射 Figure1. (a) Crystal structure of $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $. The atoms of non-magnetic element Ca replace some of the atoms of magnetic element Eu. (b) Schematic of band structure and Fermi level of $ {\rm{Eu}}_{1-x}{\rm{Ca}}_{x}{\rm{In}}_{2}{\rm{As}}_{2} $. Fermi level can be lifted by doping. (c) X-ray diffraction pattern of single crystals of $ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $ (upper) and $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $ (lower).
表1$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $体系不同元素掺杂材料的单晶制备实验结果 Table1.Summary of results of single crystal growth of doped EuIn2As2 compounds∶ Ag, Sm, Sb
在$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $中, 长程反铁磁序的建立依赖于Eu的磁性, 而掺杂无磁Ca会在长程反铁磁体系中引入无序杂质. 理论模拟的结果表明, 建立长程反铁磁是形成非平庸拓扑态的必要条件, 过多的无序会抑制长程反铁磁序, 最终影响拓扑态的形成[26]. 因此首先研究无序杂质对于体系磁性质的影响. 图2(a)是$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $和$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的磁化数据. 如图2(a)插图所示, 可以观察到在高温下$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的磁化率随温度变化的行为满足居里-外斯定律$\chi = {C}/({T-\varTheta })$, 其中$ \chi $为磁化率, C为居里常数, Θ为体系的居里温度, 这表明在这一温度区间体系以顺磁态为主导. 随着温度降低, $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $磁化率开始偏离居里-外斯行为. 随着温度的进一步降低, 如图2(a)主图所示, 在16 K下$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $和$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的磁化率都出现了尖峰的特征, 表明体系经历了一个磁相变. 在相转变温度以下, 磁化率随温度的降低而降低, 这符合典型的反铁磁磁化行为的特征. 在母体$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $的研究中, 中子散射实验和电磁共振实验[30,31]已证实这一相变是反铁磁相变, 并会引起低温电阻率的一个尖峰行为. 对比母体, ${\rm{Eu}}_{0.81} $$ {\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2}$的磁化行为和母体非常类似, 其磁化率尖峰的转变温度也没有明显变化, 因此我们判断, 在掺杂的$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $中, 反铁磁相变依然存在. 此外观察到在磁转变温度以上, 磁化率偏离了居里-外斯定律, 这表明体系中存在磁涨落或短程磁有序. 为了研究体系中的短程磁有序, 可以根据公式$ f=\varTheta /{T}_{\rm{N}} $来计算体系的阻挫系数. 其中f为阻挫系数, $ {T}_{\rm{N}} $为反铁磁转变的奈尔温度$ ({T}_{\rm{N}}=16\;{\rm{K}}) $, Θ为体系的居里温度(由居里-外斯定律$\chi\! =\! {C}/({T\!-\!\varTheta })$拟合的高温段曲线可得${\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19} $$ {\rm{In}}_{2}{\rm{As}}_{2}$的Θ值为65.2 K, 正的Θ值表明除了长程的反铁磁序, $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $中还存在铁磁关联). 计算出$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $阻挫系数为4.3, 高于$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $的阻挫系数1.9[29]. 可见Ca的掺杂对体系中长程反铁磁序的影响不大, 但是影响了中温区的短程磁序和可能的磁涨落. 图 2 (a)温度依赖的$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $和$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的磁化率曲线$ \chi \left(T\right) $, 蓝线来自于$ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $数据, 红线来自于${\rm{Eu}}_{0.81} $$ {\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2}$数据. 测量以零场冷的方式在1000 Oe ($1~{\rm{O}}{\rm{e}}=\dfrac{{10}^{3}}{4{\text{π}}}{\rm{A}}/{\rm{m}})$下进行, 外加磁场的方向沿着ab面. 在20 K左右明显观察到一个磁化曲线的尖峰. 插图为温度依赖的$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的磁化率倒数$ 1/\chi ={{H}}/{{M}} $曲线, 其中H为外加磁场, M为样品的磁化强度, 虚线表示的是根据居里-外斯定律拟合的曲线. (b)$ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $的磁滞回线测量结果, 外加磁场分别沿着晶体的ab面和c轴. 插图为放大方框区域内的曲线, 其中黑色(右)对应磁化强度, 蓝色(左)对应磁化强度微分, 可以观察到微分曲线有一个明显的尖峰. (c)外加磁场沿晶体的ab面方向时, 样品在温度为2, 10, 15, 20和30 K时的磁化强度随磁场变化曲线. (d)外加磁场沿晶体的c方向时, 样品在温度为2, 5, 15, 20, 30, 35, 40, 60, 70和80 K时的磁化强度随磁场变化曲线 Figure2. (a) Temperature dependent χ (where χ is the magnetic susceptlilty) of $ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $ and $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $, the blue curve represents $ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $, the red curve represents $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $. The inset is the temperature dependent $ 1/\chi $ of $ {\rm{Eu}}{\rm{In}}_{2}{\rm{As}}_{2} $ and $ {\rm{Eu}}_{0.81}{\rm{Ca}}_{0.19}{\rm{In}}_{2}{\rm{As}}_{2} $ in low temperature region. (b) The magnetic hysteresis loops at 2 K with the applied field within ab plane and along c axis. The inset is the $ {\rm{d}}{{M}}/{\rm{d}}{{H}} $ curve for the applied field within ab plane. (c) The magnetic hysteresis loops at 2, 10, 15, 20 and 30 K with the applied field within ab plane. (d) The magnetic hysteresis loops at 2, 5, 15, 20, 30, 35, 40, 60, 70 and 80 K with the applied field along c axis.