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Recent progress of a new type diluted magnetic semiconductors with independent charge and spin doping

Deng Zheng Zhao Guo-Qiang Jin Chang-Qing

Deng Zheng, Zhao Guo-Qiang, Jin Chang-Qing. Recent progress of a new type diluted magnetic semiconductors with independent charge and spin doping. Acta Phys. Sin., 2019, 68(16): 167502. doi: 10.7498/aps.68.20191114
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Recent progress of a new type diluted magnetic semiconductors with independent charge and spin doping

Deng Zheng, Zhao Guo-Qiang, Jin Chang-Qing
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  • Due to the potential applications for spintronics devices, diluted ferromagnetic semiconductors (DMS) have received extensive attention for decades. However, in classical Ⅲ–Ⅴ based DMS material, such as (Ga,Mn)As, heterovalent (Ga3+, Mn2+) doping results in lack of individual control of carrier and spin doping, and seriously limited chemical solubility. The two difficulties prevent furtherincrease of the Curie temperature of the Ⅲ–Ⅴ based DMS. To overcome these difficulties, a series of new types of DMS with independent spin and charge doping have been synthesized, such as Ⅰ–Ⅱ–Ⅴ based LiZnAs and Ⅱ–Ⅱ–Ⅴ based (Ba,K)(Zn,Mn)2As2. In these new materials, isovalent (Zn,Mn) substitution is only spin doping, while charge is independently doped by heterovalentsubstitution of non-magnetic elements. As a result (Ba,K)(Zn,Mn)2As2 obtains the reliable record of Curie temperature (230 K) among DMS in which ferromagnetic ordering is mediated by itinerate carriers. In this review, we summarize the recent development of the new DMS materials with following aspects: 1) the discovery and synthesis of several typical new DMS materials; 2) physical properties studies with muon spin relaxation and in-situ high pressure techniques; 3) single crystal growth, Andreev reflection junction based on single crystal and measurements of spin polarization.
      Corresponding author: Jin Chang-Qing, jin@iphy.ac.cn
    • Funds: Project supported by the National Basic Research Program of China (Grant Nos. 2017YFB0405703, 2018YFA03057001) and the National Natural Science Foundation of China (Grant No. 11534016).

    现代信息社会进入物联网和大数据时代, 对信息载体提出了运算更快、容量更大、能耗更低的要求, 然而基于传统半导体材料的信息技术正面临摩尔定律已接近极限的挑战. 从新材料探索这一源头出发, 探索颠覆性信息技术成为破解后摩尔时代难题的共识. 在诸多候选材料中, 兼具半导体材料和磁性材料双重特性的稀磁半导体, 因其与现有半导体产业的兼容性而被人们寄予厚望. 稀磁半导体早在1991年就受到美国国家科学研究委员会的关注, 在其发布的咨文中指出, 稀磁半导体在信息通讯、处理和存储等方面有着广泛的应用前景[1]. 如果稀磁半导体在操控电荷的同时亦能调控其自旋自由度, 那将获得集运算、通讯、存储于一体的单一器件. 例如, 稀磁半导体将能用于半导体自旋场效应晶体管(spin-FET)、非易失性存储器(NVM)、自旋发光二极管(spin-LED)和光隔离器等[24]. 这些器件会实现新一代低功耗、超高速的信息处理技术, 让未来的生活发生翻天覆地的变化[5].

    最初的稀磁半导体材料可以追溯到20世纪60年代的EuSe等化合物, 但是这些材料的磁性难以调控, 居里温度过低(150 K以下), 再加上其单晶难以生长, 使研究逐渐淡出人们的视线. 之后虽然也陆续出现的Ⅱ-Ⅳ、Ⅱ-Ⅵ等稀磁半导体, 但使稀磁半导体受到人们的广泛关注, 还要归功于以(Ga,Mn)As为代表的Ⅲ-Ⅴ体系[6,7]. 它能够与已经成熟的Ⅲ-Ⅴ族半导体产业集成, 与Ⅲ-Ⅴ稀磁半导体构成的器件可能拥有丰富的磁、光、电耦合现象. 稀磁半导体的实际应用既需要异质结又需要高于室温的居里温度(TC), 而“能否得到室温下工作的磁性半导体”也是《Science》创刊125周年之际发布的125个重大科学问题之一[8]. 中国科学院半导体研究所的科研团队在这个领域做出了突出的贡献, 他们将(Ga,Mn)As的最高居里温度提高到200 K[9,10].

    然而在(Ga,Mn)As等Ⅲ-Ⅴ体系中, 也存在一些难以克服的瓶颈: (Ga3+,Mn2+)异价掺杂使Mn的含量难以有效提高, 并且Mn离子极易进入间隙位. 这既阻碍了材料居里温度的提升, 又使得材料性能对生长工艺极为敏感. 另一方面, (Ga3+,Mn2+)异价掺杂同时引入自旋和电荷(图1(a)), 这种捆绑使得材料的载流子浓度和类型难以单独调控, 导致得理论模型构建困难, 难以得到一个普适性的物理图像[11]. 这些难题成为制约(Ga,Mn)As等Ⅲ-Ⅴ体系进一步走向实用化的主要瓶颈.

    图 1  (a) (Ga,Mn)As的晶体结构, 其中Mn同时引入电荷和自旋; (b) Li(Zn,Mn)As的晶体结构, 其中Mn仅引入自旋, 电荷来自Li含量的变化[12]
    Fig. 1.  (a) Crystal structure of (Ga,Mn)As, in which dependent charge and spin doping by Mn2+ dopant; (b) crystal structure of Li(Zn,Mn)As, in which spin is doped by isovalent (Zn,Mn) substitution while charge is doped by controlling Li concentration[12].

    为了破解上述难题, 我们设计了通过自旋、电荷掺杂机制分离研制新型稀磁半导体(简称新型稀磁半导体)材料的方案: 通过等价掺杂磁性离子引入自旋, 在不同阳离子位进行异价非磁性离子掺杂引入电荷[1216]. 例如在新型稀磁半导体(Ba,K)(Zn,Mn)2As2 (简称BZA)中, 通过等价的(Zn2+,Mn2+)掺杂引入自旋(局域磁矩), 另一方面通过(Ba2+,K+)掺杂引入电荷(载流子), 从而成功实现了稀磁半导体中电荷、自旋掺杂的分离, 克服了(Ga,Mn)As的主要瓶颈[17,18]. BZA中得到了最高230 K的居里温度, 刷新了可控型稀磁半导体居里温度的纪录[11,19]. 本文依照新材料研制、物性研究、简单原型器件构建这一“全链条”研究模式, 介绍新型稀磁半导体的发展[14,15,2037].

    Li(Zn,Mn)As是首个发现的电荷与自旋掺杂分离的新型稀磁半导体材料, 它与铁基超导LiFeAs化学组分接近. Li(Zn,Mn)As和(Ga,Mn)As有着相近的晶体结构, 同属立方晶系, 空间群均为F-43m. [ZnAs4]四面体构成了Li(Zn,Mn)As晶格的“骨架”, 而Li+离子穿插于Zn2+离子之间(图1(b)). GaAs和LiZnAs均为直接带隙半导体, 有着相近的能带结构和能隙宽度(LiZnAs为1.61 eV, GaAs为1.52 eV)[12]. Li(Zn,Mn)As中通过(Zn2+,Mn2+)等价磁性元素替代引入自旋、非磁性元素Li的过量掺杂引入电荷, 从而实现了电荷与自旋掺杂机制的分离. 通过调控Li的含量引入电荷, 可以使电导行为从半导体性转变为金属性, 同时载流子浓度也急剧增加, 例如母体LiZnAs中载流子浓度为np ~ 1017 cm–3(空穴型), 而Li1.1ZnA的载流子浓度为np ~ 1020 cm–3(同样为空穴型)[12]. 同时掺杂电荷和局域自旋(即Mn离子)的样品呈现铁磁性, 在固定Li的含量时, 样品的居里温度随Mn浓度的增加而上升. 如图2(a)所示, Li(Zn,Mn)As系列在配比为Li1.1(Zn0.9Mn0.1)As的样品中得到了50 K的TC. 图2(b)是样品磁滞回线M(H)的测量结果, Li(ZnMn)As的矫顽力仅为30 Oe左右, 这为瞄准应用的自旋低场调控提供了可能.

    图 2  (a)不同Mn含量Li1.1(Zn,Mn)As的M(T)曲线; (b)不同Mn含量Li1.1(Zn,Mn)As的M(H)曲线, 其中灰色的磁滞回线对应了上方的横坐标; (c)样品Li1.1(Zn0.95Mn0.05)As在低温下的霍尔电阻, 15 K以下表现出了明显的反常霍尔效应[12]
    Fig. 2.  (a) Temperature dependence of magnetization of Li1.1(Zn1xMnx)As; (b) field dependence of magnetization of Li1.1(Zn1xMnx)As at various tempeartures; (c) Hall resistance of Li1.1(Zn0.95Mn0.05)As and the anomalous Hall effect below 15 K[12].

    传统稀磁半导体中, 由于天然的“低固溶度”的限制, 一些情况下会出现磁性团簇, 而这些磁性团簇将干扰人们对材料本征铁磁的研究. 为了排除Li(Zn,Mn)As的铁磁性来源与团簇的可能性, 我们进行了反常霍尔效应的观测. 反常霍尔效应(anomalous Hall effect, AHE)源于磁性材料内的自旋轨道耦合, 是铁磁半导体的重要表现, 是载流子与局域磁矩耦合的重要证据. 如图3(c)所示, 在居里温度以下Li(Zn,Mn)As呈现出显著的反常霍尔效应, 证实了铁磁序是Li(ZnMn)As的本征属性[12].

    图 3  (a) BZA的晶体结构中, Mn引入自旋, K引入电荷; (b) Li(Zn,Mn)As中[Zn/MnAs4]四面体的间距; (c) BZA中[Zn/MnAs4]四面体的间距[17]
    Fig. 3.  (a) Crystal structure of BZA, in which spin is doped by isovalent (Zn,Mn) substitution while charge is doped by (Ba,K)substitution; (b) distance of nearest [Zn/MnAs4] tetrahedra (4.20 Å) in Li(Zn,Mn)As; (c) distance of nearest [Zn/MnAs4] tetrahedra (2.91 Å) in BZA[17].

    稀磁半导体中磁性离子相距较远, 在维持一定浓度的前提下, 寻找更为理想的晶体结构以缩小磁性离子的平均间距, 这可以理解为变相引入“化学压力”, 将极有可能得到更高的居里温度. 在这一材料设计思路的指导下, 我们合成了(Ba,K)(Zn,Mn)2As2, 属于四方ThCr2Si2结构, 空间群为I4/mmm(图3(a)). 其中同价态(Zn2+,Mn2+)掺杂提供自旋, 异价(Ba2+,K1+)替代引入电荷, 同样使得电荷与自旋掺杂机制分离[15]. 相比于Li(Zn,Mn)As, 最近邻Mn离子的距离从4.20 Å压缩到2.91 Å(图3(b)图3(c)), 最高居里温度从而由Li(Zn,Mn)As中的50 K大幅增加至BZA中的230 K[18]. 此结构的另一大优势在于, 掺杂电荷载流子(K+)的Ba2+层与掺杂磁性离子(Mn2+)的ZnAs层彼此分离, 在空间上避免了电荷对自旋的干扰. 这一点对针对BZA理论模型的构建尤为重要[38].

    图4(a)所示, (Ba,K)(Zn,Mn)2As2的电阻测量结果同样表明K的掺杂量对材料的导电性起着至关重要的作用, 仅5%的K就将材料从半导体行为转变为金属行为. 对于结构相同仅掺杂量略微变化的一系列样品, 可以认为载流子的迁移率几乎相同, 那么电阻率的减小就意味着载流子浓度的增加. (Ba,K)(Zn,Mn)2As2的磁性同时受载流子浓度(K含量)和局域磁矩浓度(Mn含量)的影响, 通过成分优化, 我们在配比为Ba0.7K0.3(Zn0.85Mn0.15)2As2的样品中获得了230 K的居里温度(图4(b))[18,39]. 居里温度以下, 样品中同样观察到了AHE效应(图4(c)), 证实了铁磁序是(Ba,K)(Zn,Mn)2As2的本征属性.

    图 4  (a)不同K, Mn含量(Ba,K)(Zn,Mn)2As2ρ(T)曲线; (b)样品(Ba0.7K0.3)(Zn0.85Mn0.15)2As2M(T)和M(H), M(T)中上升的拐点即为230 K的居里温度点; (c)样品(Ba0.85K0.15)(Zn0.9Mn0.1)2As2在低温下的霍尔电阻, 50 K以下表现出了明显的反常霍尔效应[17]
    Fig. 4.  (a) Temperature dependence of resistivity of (Ba,K)(Zn,Mn)2As2; (b) temperature dependence of magnetization and field dependence of magnetization (inset) of (Ba0.7K0.3)(Zn0.85Mn0.15)2As2, the upturn point, namely Curie temperature is 230 K; (c) Hall resistance of (Ba0.85K0.15)(Zn0.9Mn0.1)2As2 and the anomalous Hall effect below 50 K[17].

    Li(Zn,Mn)P, Li(Cd,Mn)P是与Li(Zn,Mn)As同构的稀磁半导体材料, 其显著的特点是: Li(Zn,Mn)P在载流子浓度比Li(Zn,Mn)As低2个数量级的条件下仍然达到34 K的居里温度, 未来极有希望通过引入额外的载流子实现高居里温度[15,40]; 与Li(Zn,Mn)P相比, 4d元素Cd的加入使Li(Cd,Mn)P的载流子浓度大幅提升, 并在后者中发现了80%以上的巨大负磁阻, 这拓展了新型稀磁半导体的应用范围[36]. (Ba,K)F(Zn,Mn)As是首个发现的氟-砷基的新型稀磁半导体, 其晶体结构与“1111”型铁基超导体相同, 为ZrCuSiAs型的四方结构. 其中[ZnAs4]四面体仍然构成结构的主体. 与“122”型BZA类似, 通过在Ba2+位掺杂K+控制载流子, 在Zn2+位掺杂Mn2+引入自旋. 由于加入了负离子性极强的F-离子, 相比于BZA, (Ba,K)F(Zn,Mn)As的半导体性得到了极大的增强, 这对材料将来的应用是非常有利的[29,41,42].

    以(Ga,Mn)As为代表的Ⅲ-Ⅴ体系中, Mn2+捆绑式掺杂自旋和电荷也给机理研究带来了诸多不便, 以至于至今仍未形成完全统一的磁性机理[11]. 新型稀磁半导体中电荷掺杂与磁性离子掺杂完全分离, 以BZA为例, 电荷掺杂发生在Ba位, 在晶体结构和电子结构上与磁性层(Zn,Mn)As层隔离, 这将极大地简化铁磁模型的构建[38]. 同时BZA能够以多晶和单晶的形式制备, 极大地丰富了材料研究手段: μ子自旋弛豫、中子非弹性散射(INS)等对磁性材料至关重要的表征技术可以应用在BZA上[33]. 由于电荷与自旋掺杂的分离, 新型稀磁半导体被认为非常适合理论研究, 并能为阐明稀磁半导体中磁相互作用起源以及磁有序的微观机制提供重要线索.

    μ子自旋弛豫(muon spin relaxation, μSR)利用μ子磁矩在样品内部局域磁场中的拉莫进动来探测样品的磁性. 与中子技术相比, μSR可以探测的磁信号提高了10倍以上, 因此非常适合用于研究稀磁半导体的磁有序以及磁动力学性质. 我们以Li1.1(Zn0.95Mn0.05)As(TC = 30 K)为例, 介绍μSR实验对于新型稀磁材料所能够提供的信息. 一般而言, 样品都会在零场(zero field, ZF)模式与弱垂直场(weak transverse field, WTF)模式下进行测试, 为了简单起见, 我们仅介绍ZF模式的结果. 根据理论模型, 假设样品中存在铁磁相和顺磁相, ZF谱上铁磁相和顺磁相的响应各不相同, 铁磁谱表现为A的快速衰减, 顺磁谱则反之. 因此ZF谱由铁磁谱与顺磁谱叠加构成, ZF谱可以写为(1)式的形式[12]:

    ${\rm{Asymmetry}} = {A_{{\rm{mag}}}}{\rm{G}}_Z^L\left( t \right) + {A_{{\rm{para}}}}\exp \big( { - {{\left( {\lambda t} \right)}^\beta }} \big),$

    (1)

    其中Amag代表铁磁相比例, Apara代表顺磁相比例. 如图5(a)所示, 随着温度的下降, 快速衰减的成分出现, 并且其所占比例逐渐增多, 说明样品中铁磁体积分数迅速上升. 通过拟合(1)式, 可以分别获得铁磁相和顺磁相的体积分数. 拟合结果汇总在图5(b)中, 图中清晰地显示TC以下铁磁相含量迅速升高, 直到达到100%. 并且ZF与WTF得到的体积分数非常吻合, 十分有力地表征了铁磁相变. 这个结果表明进入铁磁态后Li(Zn,Mn)As中的所有的局域自旋长程有序排列, 也就证明铁磁性是Li(Zn,Mn)As的本征属性. 同样, 我们在BZA和(Ba,K)F(Zn,Mn)As等新型稀磁半导体上也获得了类似的结果(图5(c)和5(d)), 均证明了铁磁性是材料的本征属性[17].

    图 5  μSR测试结果汇总 (a) ZF模式下Li1.1(Zn0.95Mn0.05)As的时间谱; (b) ZF模式与WTF模式下Li1.1(Zn0.95Mn0.05)As铁磁含量的拟合结果, 两者互相吻合[12]; (c) ZF模式下(Ba0.80K0.20)(Zn0.9Mn0.1)2As2的时间谱; (d) ZF模式与WTF模式下(Ba0.80K0.20)(Zn0.9Mn0.1)2As2铁磁含量的拟合结果, 铁磁体积含量与SQUID的测量结果吻合[17]
    Fig. 5.  Results of muon spin relaxation measurements: (a) Time spectra of Li1.1(Zn0.95Mn0.05)As in ZF process; (b) the volume fraction of the magnetically ordered region in Li1.1(Zn0.95Mn0.05)As, derived from ZF and WTF spectra[12]; (c) time spectra of (Ba0.80K0.20)(Zn0.9Mn0.1)2As2 in ZF process; (d) the volume fraction of the magnetically ordered region in (Ba0.80K0.20)(Zn0.9Mn0.1)2As2, derived from ZF and WTF spectra[17].

    BZA的TC已经达到230 K, 距室温仅有一步之遥, 而物理压力是调节稀磁半导体磁性的有效手段[43]. BZA等新型稀磁半导体材料具有较小的体弹模量, 易于压缩, 在较低的压力范围内就可以有效地调控材料的物理性能. 在得到居里温度的最优化压力区间后, 可以设计相应的等价态化学替代, 产生相应化学内压从而将物理压力的效果固化, 这将开辟常规条件下提升BZA的居里温度的新途 径[32,37]. 之前报道稀磁半导体压力调节实验中, 普遍选用活塞圆筒装置结合电输运测量的手段进行压力研究, 这种方式存在测量方式不直接, 压力范围有限(< 3 GPa)等缺点[43,44]. 由于BZA能够以体材料的形式制备, 因此可以结合金刚石压砧技术与同步辐射磁元二色(XMCD)这种直接观测铁磁序的手段, 研究其铁磁序在高压下的演化.

    图6(a)是样品(Ba0.75K0.25)(Zn0.95Mn0.05)2As2(TC = 150 K)常压下As的K边X射线吸收谱(XAS)以及XMCD谱. 由于BZA中载流子大多集中在As的p轨道, 因此As的K边上观察到的XMCD信号表明p轨道电子出现了很大程度的极化, 这是BZA内Mn-As之间产生p-d电子杂化的直接证据, 也证明了BZA中铁磁性来源于巡游电子的诱导[26]. 图6(b) 2 K时高压原位的As-K边XMCD谱, 其强度随着压力的变化而下降, 说明压力对材料磁性的有效调控. 由于XMCD谱的强度与TC成正相关, 因此可以推测样品的TC随压力增加而持续下降[26].

    图 6  (Ba0.75K0.25)(Zn0.95Mn0.05)2As2的高压表征 (a)常压下As的K边XAS和XMCD谱; (b) 2 K时高压原位的As-K边XMCD谱, 插图为XMCD谱的最高值随压力的变化; (c) 不同压力的电阻-温度曲线; (d)常温下XES谱随压力的演化, 插图为ΔE'随压力变化[26,34]
    Fig. 6.  In-situ high pressure properties of (Ba0.75K0.25)(Zn0.95Mn0.05)2As2: (a) As K-edge XAS near edge structure (black curve) and XMCD (blue curve) data takenat T = 2 K and ambient pressure; (b) pressure-dependent As K-edge XMCD signal, the inset is XMCD peak intensity normalized to unity at ambient pressure, dreen data points are compression data, while the red data point was obtained on decompression; (c) temperature-dependent resistance plots at various pressures; (d) X-ray emission spectra at high pressures and room temperature, the spectra were shifted in the vertical for clarity, red solid lines are fits using three Gaussian functions; left corner inset: energy difference ΔE'[26,34].

    稀磁半导体的铁磁序来源于载流子诱导的长程有序局域磁矩. 因此为了研究TC下降的原因, 需要研究载流子和局域磁矩分别随压力的演化. 通过高压原位电输运测量, 我们发现BZA在压力作用下, 其导电性发生半导体行为到金属行为的转变(图6(c)), 说明压力使载流子巡游性增加, 这与TC下降的结果并不吻合[26]. 为了研究Mn离子局域磁矩随压力的演化, 我们进行了原位X射线发射谱(XES)研究. 这里重点关注Mn的Kβ谱, 它由主峰Kβ1,3和卫星峰Kβ' 组成, Kβ1,3与Kβ'的能量差($ \Delta E'$)与Mn离子上的局域磁矩成正比. 我们发现 $\Delta E' $随压力变化并不大, 尤其是0 GPa和10 GPa下的$\Delta E'$几乎完全相同(图6(d)), 这说明Mn离子上的局域磁矩并不随压力发生明显变化[34].

    我们进一步通过高压同步辐射XRD研究材料晶体结构随压力的演化, 图7所示BZA的晶格在高压下发生各向异性畸变, 具体表现在: 1) ZnAs层间As-As压缩率远高于层内压缩率, 导致层间As-As发生强烈杂化, 进而导致层内载流子向层间转移, 使得Mn之间铁磁耦合减弱[45]; 2) ZnAs层内的[MnAs4]四面体产生畸变, 偏离理想的四面体构型, As-Zn-As夹角α逐渐偏离理想四面体构型的夹角(~109.4°), 这使得Mn的d电子与As的p电子交叠减少, 进而导致p-d杂化减弱[34].

    图 7  (Ba0.80K0.20)(Zn0.95Mn0.05)2As2的晶格常数随压力的变化, 左下角插图显示了ZnAs层间As-As距离, 右上角插图显示了MnAs4四面体内的As-Zn-As夹角α[34]
    Fig. 7.  Lattice parameters of (Ba0.80K0.20)(Zn0.95Mn0.05)2As2 as a function of pressure. Data were normalized to unity at ambient pressure. Left corner inset: crystal structure of (Ba,K)(Zn,Mn)2As2, and the pressure dependence of interlayer As-As distance d. Upper right corner inset: MnAs4 tetrahedron geometry, and the pressure dependence of As-Mn-As bond angle α in the MnAs4 tetrahedron[34].

    总而言之, 压力使晶格发生畸变, 从而使p-d交换作用减弱, 进而削弱了铁磁关联并降低居里温度. 尽管如此, 未来如果能通过不同尺寸离子的化学掺杂抵消压力下结构的畸变, 将很有可能得到更高的居里温度. 另一方面, 可以通过薄膜应力模拟定向外压力, 比如仅在Zn/MnAs层内产生压力, 从而避免物理压力带来的晶格畸变, 则极有可能实现居里温度的提升.

    无论是为了机理研究还是未来的自旋器件应用, 自旋极化率对稀磁半导体材料而言都是一个关键参数. 前人已经利用安德烈夫反射结研究了经典稀磁体系的自旋极化率, 证明了这是目前测量自旋极化率最为有效的方法之一[46]. 我们以BZA单晶为基础, 使用材料外延生长技术, 成功构建了Pb-BZA安德烈夫反射结, 并测量到了较高的自旋极化率. 关于安德烈夫反射结的构建将在下一节进行详细讨论.

    我们通过测量安德烈夫反射结的I-V曲线可以得到归一化的安德烈夫反射谱G/G0(V), 这里微分电导G(V) = dI/dV, 而G0是0.25 T磁场下, 将铅的超导完全压制后得到的微分电导. 图8(a)所示的是不同温度下的反射谱, 我们利用修正的Blonder-Tinkham-Klapwijk (BTK)理论对测量安德烈夫反射谱进行分析, 拟合结果中的两个关键参数Pb的超导能隙Δ = 1.3 meV, 界面势垒Z = 0.38, 均在合理范围, 说明拟合过程是正确的[47]. 最终得到的自旋极化率为P = 66%, 这个数值高于(Ga,Mn)Sb中得到的57%, 以及第一种被预言半金属NiMnSb的50%. 紧接着我们在散铁磁性(asperomagnetic ordering)的(BaNa)(ZnMn)2As2(简称BNZA)单晶上也构建了同样的AR结, 尽管(BaNa)(ZnMn)2As2并不具有长程铁磁序, 但是如图8(b)所示仍在其中观测到了约为50%的自旋极化率[48].

    图 8  (a)不同温度下Pb-BZA结的安德烈夫反谱以及拟合结果[47]; (b)低温下Pb-BNZA结的安德烈夫反谱以及拟合结果[48]
    Fig. 8.  (a) The Andreev reflection spectra of Pb-BZA heterjunction and the best BTK fit[47]; (b) Andreev reflection spectra of Pb-BNZA and the best BTK fit[48].

    探索基于新型稀磁半导体的简单异质结器件, 是将其向应用推广的关键一步. 结合新型稀磁半导体的特征, 我们设计并构建了基于BZA单晶的安德烈夫反射结. 首先, 我们成功生长了接近厘米尺寸的BZA单晶样品. 如图9(a)所示, 2θ模式下的XRD谱中仅得到(00K)的衍射峰, 说明BZA单晶是沿着ab面生长的, 这与其层状的晶体结构是对应的. 如图9(b)所示, BZA单晶表现出了显著的磁各向异性, 沿c方向的磁矩比ab面内的磁矩大一个数量级, 表明其易磁化轴是沿着c方向的. 无论c方向或者ab面内的磁矩, 均在50 K附近出现了磁化率的突然上升, 表明其居里温度为50 K[47]. 利用环氧树脂作为包覆层, 我们采用点接触的方式在BZA单晶上外延了铅(Pb)薄膜, 构成安德烈夫反射结的核心部分(Pb-BZA结), 反射结的整体结构如图9(c)所示. 利用此反射结, 我们成功测量了BZA的自旋极化率.

    图 9  (a) 2θ模式下BZA单晶的XRD谱, 插图为单晶照片以及晶体结构示意图; (b)沿不同方向测量的BZA单晶的M(T)曲线; (c)以BZA单晶为基础构造的安德烈夫反射结示意图[47]
    Fig. 9.  (a) XRD pattern of BZA single crystal with 2θ process, the insets are photographic of the single crystal and crystal structure of BZA; (b) temperature dependence of magnetization of the single crystal along ab-plane and c-axis; (c) sketch of the Andreev reflection junction based on BZA single crystal[47].

    基于BZA单晶的安德烈夫反射结的成功构建, 为下一步探索基于新型稀磁半导体的多组合异质结奠定了技术基础. 源于新型稀磁半导体结构的特殊性, 它拥有多种结构相同的功能材料. 新型稀磁半导体发现之初, 人们就注意到Li(Zn,Mn)As与“111”型铁基超导体LiFeAs和反铁磁体LiMnAs在合适的晶面上拥有匹配的晶格, 将可以互相组合, 形成界面完美的异质结. 这个特点在BZA上表现得更加显著, 如图10所示, “122”型铁基超导体(Ba,K)Fe2As2(超导转变温度38 K)和反铁磁体BaMn2As2(奈尔温度625 K)与BZA同属与四方ThCr2Si2结构, 它们在ab面内的晶格失配度小于5%. 如果利用外延生长技术生长这些材料的异质结, 可以预期它们将拥有近乎完美的界面. 这个结构上的优势是(Ga,Mn)As等传统稀磁半导体材料所不具备的.

    图 10  三类不同功能材料的结构示意图 (a)超导体(Ba,K)Fe2As2; (b)铁磁性稀磁半导体BZA; (c)反铁磁体BaMn2As2[17]
    Fig. 10.  Crystal structure and lattice parameters of (a) superconductor (Ba,K)Fe2As2; (b) ferromagnetic DMS BZA; (c) antiferromagnetic BaMn2As2[17].

    我们将能设计铁磁体、反铁磁体以及超导体等多种功能材料构成的多组合异质结, 为探索新的物理效应和新的应用提供重要基础. 例如BZA与铁基超导体(Ba,K)Fe2As2构成的安德烈夫反射结, 将能用于研究自旋轨道耦合、超导配对机制等凝聚态物理的关键问题. 这一先进的设计理念与诸多国际同行不谋而合, 国际电气与电子工程师学会在近期发布的关于自旋电子学演生材料的路线图文章中, 将发展基于BZA的同结构多组合异质结(iostructural DMS junction with multiple order parameters)选为未来稀磁半导体的主要研究方向之一[11].

    目前新型稀磁半导体材料的最高TC已经达到230 K, 更加接近距离室温铁磁[11]. 事实上我们已经发现BZA中, 最近邻Mn离子间在室温以上仍然存在铁磁关联[24]. 如果能通过组分优化、压力调控等手段增强这一铁磁交换作用, 则极有希望将BZA的居里温度提升至室温以上.

    新型稀磁半导体实现了电荷与自旋掺杂机制的分离, 能够以单晶的形式制备, 这为稀磁半导体的实验表征与理论研究提供了理想对象. 稀磁半导体理论学家认为BZA上得到的物理图像将具有普适性, 这为探索稀磁半导体中磁相互作用起源以及磁有序的微观机制提供了一个机会[19,38].

    得益于BZA上已经达到的高居里温度以及可能实现的同结构多组合异质结, 国际电气与电子工程师学会将BZA选为稀磁半导体未来15年发展的重点材料之一. 如图11所示, 他们在近期发布的关于自旋电子学衍生材料的路线图文章中, 规划了基于BZA的两个主要研究方向, 即1)在BZA基础上寻找居里温度高于室温的稀磁半导体材料; 2)发展基于BZA的同结构多组合异质结[11]. 总而言之, BZA等自旋、电荷掺杂机制分离新型稀磁半导体的出现, 为人们呈现了稀磁半导体领域发展的光明前景.

    图 11  稀磁半导体发展路线图[11]
    Fig. 11.  Roadmap on DMS[11].

    感谢与Y. J. Uemura, 宁凡龙, A. Fujimori, 顾波, S. J. L. Billinge, 赵建华, S. Maekawa, 李永庆等的卓有成效的合作与讨论.

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    其他类型引用(3)

  • 图 1  (a) (Ga,Mn)As的晶体结构, 其中Mn同时引入电荷和自旋; (b) Li(Zn,Mn)As的晶体结构, 其中Mn仅引入自旋, 电荷来自Li含量的变化[12]

    Figure 1.  (a) Crystal structure of (Ga,Mn)As, in which dependent charge and spin doping by Mn2+ dopant; (b) crystal structure of Li(Zn,Mn)As, in which spin is doped by isovalent (Zn,Mn) substitution while charge is doped by controlling Li concentration[12].

    图 2  (a)不同Mn含量Li1.1(Zn,Mn)As的M(T)曲线; (b)不同Mn含量Li1.1(Zn,Mn)As的M(H)曲线, 其中灰色的磁滞回线对应了上方的横坐标; (c)样品Li1.1(Zn0.95Mn0.05)As在低温下的霍尔电阻, 15 K以下表现出了明显的反常霍尔效应[12]

    Figure 2.  (a) Temperature dependence of magnetization of Li1.1(Zn1xMnx)As; (b) field dependence of magnetization of Li1.1(Zn1xMnx)As at various tempeartures; (c) Hall resistance of Li1.1(Zn0.95Mn0.05)As and the anomalous Hall effect below 15 K[12].

    图 3  (a) BZA的晶体结构中, Mn引入自旋, K引入电荷; (b) Li(Zn,Mn)As中[Zn/MnAs4]四面体的间距; (c) BZA中[Zn/MnAs4]四面体的间距[17]

    Figure 3.  (a) Crystal structure of BZA, in which spin is doped by isovalent (Zn,Mn) substitution while charge is doped by (Ba,K)substitution; (b) distance of nearest [Zn/MnAs4] tetrahedra (4.20 Å) in Li(Zn,Mn)As; (c) distance of nearest [Zn/MnAs4] tetrahedra (2.91 Å) in BZA[17].

    图 4  (a)不同K, Mn含量(Ba,K)(Zn,Mn)2As2ρ(T)曲线; (b)样品(Ba0.7K0.3)(Zn0.85Mn0.15)2As2M(T)和M(H), M(T)中上升的拐点即为230 K的居里温度点; (c)样品(Ba0.85K0.15)(Zn0.9Mn0.1)2As2在低温下的霍尔电阻, 50 K以下表现出了明显的反常霍尔效应[17]

    Figure 4.  (a) Temperature dependence of resistivity of (Ba,K)(Zn,Mn)2As2; (b) temperature dependence of magnetization and field dependence of magnetization (inset) of (Ba0.7K0.3)(Zn0.85Mn0.15)2As2, the upturn point, namely Curie temperature is 230 K; (c) Hall resistance of (Ba0.85K0.15)(Zn0.9Mn0.1)2As2 and the anomalous Hall effect below 50 K[17].

    图 5  μSR测试结果汇总 (a) ZF模式下Li1.1(Zn0.95Mn0.05)As的时间谱; (b) ZF模式与WTF模式下Li1.1(Zn0.95Mn0.05)As铁磁含量的拟合结果, 两者互相吻合[12]; (c) ZF模式下(Ba0.80K0.20)(Zn0.9Mn0.1)2As2的时间谱; (d) ZF模式与WTF模式下(Ba0.80K0.20)(Zn0.9Mn0.1)2As2铁磁含量的拟合结果, 铁磁体积含量与SQUID的测量结果吻合[17]

    Figure 5.  Results of muon spin relaxation measurements: (a) Time spectra of Li1.1(Zn0.95Mn0.05)As in ZF process; (b) the volume fraction of the magnetically ordered region in Li1.1(Zn0.95Mn0.05)As, derived from ZF and WTF spectra[12]; (c) time spectra of (Ba0.80K0.20)(Zn0.9Mn0.1)2As2 in ZF process; (d) the volume fraction of the magnetically ordered region in (Ba0.80K0.20)(Zn0.9Mn0.1)2As2, derived from ZF and WTF spectra[17].

    图 6  (Ba0.75K0.25)(Zn0.95Mn0.05)2As2的高压表征 (a)常压下As的K边XAS和XMCD谱; (b) 2 K时高压原位的As-K边XMCD谱, 插图为XMCD谱的最高值随压力的变化; (c) 不同压力的电阻-温度曲线; (d)常温下XES谱随压力的演化, 插图为ΔE'随压力变化[26,34]

    Figure 6.  In-situ high pressure properties of (Ba0.75K0.25)(Zn0.95Mn0.05)2As2: (a) As K-edge XAS near edge structure (black curve) and XMCD (blue curve) data takenat T = 2 K and ambient pressure; (b) pressure-dependent As K-edge XMCD signal, the inset is XMCD peak intensity normalized to unity at ambient pressure, dreen data points are compression data, while the red data point was obtained on decompression; (c) temperature-dependent resistance plots at various pressures; (d) X-ray emission spectra at high pressures and room temperature, the spectra were shifted in the vertical for clarity, red solid lines are fits using three Gaussian functions; left corner inset: energy difference ΔE'[26,34].

    图 7  (Ba0.80K0.20)(Zn0.95Mn0.05)2As2的晶格常数随压力的变化, 左下角插图显示了ZnAs层间As-As距离, 右上角插图显示了MnAs4四面体内的As-Zn-As夹角α[34]

    Figure 7.  Lattice parameters of (Ba0.80K0.20)(Zn0.95Mn0.05)2As2 as a function of pressure. Data were normalized to unity at ambient pressure. Left corner inset: crystal structure of (Ba,K)(Zn,Mn)2As2, and the pressure dependence of interlayer As-As distance d. Upper right corner inset: MnAs4 tetrahedron geometry, and the pressure dependence of As-Mn-As bond angle α in the MnAs4 tetrahedron[34].

    图 8  (a)不同温度下Pb-BZA结的安德烈夫反谱以及拟合结果[47]; (b)低温下Pb-BNZA结的安德烈夫反谱以及拟合结果[48]

    Figure 8.  (a) The Andreev reflection spectra of Pb-BZA heterjunction and the best BTK fit[47]; (b) Andreev reflection spectra of Pb-BNZA and the best BTK fit[48].

    图 9  (a) 2θ模式下BZA单晶的XRD谱, 插图为单晶照片以及晶体结构示意图; (b)沿不同方向测量的BZA单晶的M(T)曲线; (c)以BZA单晶为基础构造的安德烈夫反射结示意图[47]

    Figure 9.  (a) XRD pattern of BZA single crystal with 2θ process, the insets are photographic of the single crystal and crystal structure of BZA; (b) temperature dependence of magnetization of the single crystal along ab-plane and c-axis; (c) sketch of the Andreev reflection junction based on BZA single crystal[47].

    图 10  三类不同功能材料的结构示意图 (a)超导体(Ba,K)Fe2As2; (b)铁磁性稀磁半导体BZA; (c)反铁磁体BaMn2As2[17]

    Figure 10.  Crystal structure and lattice parameters of (a) superconductor (Ba,K)Fe2As2; (b) ferromagnetic DMS BZA; (c) antiferromagnetic BaMn2As2[17].

    图 11  稀磁半导体发展路线图[11]

    Figure 11.  Roadmap on DMS[11].

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Metrics
  • Abstract views:  10969
  • PDF Downloads:  184
  • Cited By: 11
Publishing process
  • Received Date:  19 July 2019
  • Accepted Date:  15 August 2019
  • Available Online:  19 August 2019
  • Published Online:  20 August 2019

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