Room Temperature Ferromagnetism in Cobalt-Doped LiNbO₃ Single Crystalline Films

Abstract

Cobalt-doped (5 at. %) LiNbO3 (Co:LN) single crystalline films were prepared by combinatorial laser molecular-beam epitaxy on Al2O3 (001) substrates. The oxygen atmosphere should be severely controlled to be approximately 10 Pa to obtain stoichiometric Li/Nb concentration. Determined by asymmetric X-ray diffraction and high-resolution transmission electron microscopy, the epitaxial relationship of the sample follows (003)<100>F ∥ (003)<100>S, (110)<001>F ∥ (110)<001>S, and (113)<1̅10>F ∥ (113) <1̅10>S (F and S denote the film and the substrate, respectively). The Co:LN films have a single-phase, where Co is not metallic but in the 2+ state. Co K-edge X-ray-absorption near-edge structure spectrum determines that Co2+ ions substitute for Nb5+ lattice sites, producing oxygen vacancies to compensate for the charge nonequilibrium, and shows that the Co−O bond length is greater than that of Co:LN films grown on Si (100) substrates. The Co:LN/Al2O3 films exhibit a high Curie temperature of ∼550 K and room temperature ferromagnetism of 0.58 μB/Co arising from the bound magnetic polarons mechanism based on defects. This work opens a window to a class of single-phase multiferroics by introducing magnetic dopants to ferroelectric materials and to the mechanism of dopants induced ferromagnetism in insulators.

Multiferroic response of nanocrystalline lithium niobate

“…They also found the same behavior in cobalt doped (5 at.%) LN single crystal films. [6][7][8] This magnetism has been attributed to point defects at the surface of the NPs.…”

Lithium Niobate Single Crystals and Powders Reviewed—Part II

“…However, recently some efforts have been put on as-obtained coarse LN particles that show ferromagnetism at RT. On an investigation carried out in 2018 by our group, it was found that the generation of oxygen vacancies-by thermal treatment in reducing atmospheres: thermal treatment in a reducing atmosphere (TTRA)-at the surfaces of LN:Fe powders is a mechanism that favors RT ferromagnetism at low doping concentrations of 0.44%, 0.89%, 1.47%, and 2.20% by mass of Fe2O3 (labeled as nLN:Fe-0.44-R, nLN:Fe-0.89-R, nLN:Fe-1.47-R, nLN:Fe-2.20- There is a tendency in what respects doped-LN to study single-crystal and polycrystal thin films, in comparison to polycrystalline powders; further results on the Fe impurity and reports on the doping with Co, Cu, and V-based on similar studies to those above just described-can be consulted in the literature [198,[202][203][204][205][206][207]. However, recently some efforts have been put on as-obtained coarse LN particles that show ferromagnetism at RT.…”

“…Figure 11 shows the plots of magnetic moment per transition metal-cation of LN doped with Fe (1-LN:Fe), Mn (2-LN:Mn), V (7-LN:V), as well as four different systems of LN doped with Co (labeled as 3-LN:Co, 4-LN:Co, 5-LN:Co, and 6-LN:Co). All samples were prepared as layers by different methods [190,192,198,[202][203][204][205]. As can be seen, for 2-LN:Mn and 7-LN:V, the AMM increases as the x increases until reaching a maximum; after that, the AMM begins to decrease as x increases.…”

“…As previously explained, the AMM decrease is mainly associated with the formation of secondary phases or lattice damage due to excess dopant, which destroys the magnetic coupling. There is a tendency in what respects doped-LN to study single-crystal and polycrystal thin films, in comparison to polycrystalline powders; further results on the Fe impurity and reports on the doping with Co, Cu, and V-based on similar studies to those above just described-can be consulted in the literature [198,[202][203][204][205][206][207]. However, recently some efforts have been put on as-obtained coarse LN particles that show ferromagnetism at RT.…”

Preparation and Physical Property of BTO-based Multiferroic Ceramics

“…多铁材料指材料具有两种或者两种以上的铁性 有序，铁性有序包括铁电性、铁磁性和铁弹性等。 多铁材料可以被广泛地应用在多态存储器、磁电探 测器和传感器等领域 [1][2][3][4] 。近几十年来，多铁材料被 认为是最具潜在应用价值的信息功能材料之一，受 到了科研人员的广泛关注。 ABO3 型钙钛矿氧化物材料具有独特的晶体结 构和丰富的物理性质，是当前多铁研究领域的热点 之一。其中，BiFeO3 是目前研究较多的多铁材料， 但 BiFeO3 在室温下是反铁磁性， 一定程度上限制了 其应用。 BaTiO3(BTO)是 ABO3 型钙钛矿结构的典型 铁电材料，在室温下具有强铁电性、热释电性、光 催化特性、高介电常数和高光电系数等优点 [5][6][7] 。 BTO 在室温下具有良好的铁电性， 却没有铁磁性 [8] ， 这归因于铁电性和铁磁性在钙钛矿氧化物形成机制 上表现出的互斥性。铁电性的形成要求钙钛矿氧化 物在 B 位的过渡金属离子具有空的 d 轨道 [9] ；而磁 性的形成机制是要求处于 B 位的过渡金属离子的 d 轨道被电子部分占据 [10] 。这种互斥性仅表现在单相 中，可通过在材料不同单元分别引入铁电性与铁磁 性，实现材料整体的多铁性。人们在对 BTO 多铁性 的研究上尝试了许多方法，Hiroyuki 等 [11] 通过理论 计算预测了 Mn、 Fe 等掺杂的 BTO 能够实现多铁性， Song 等 [12] 通过对 LiNbO3 进行 Co 掺杂，实现了铁 电性和铁磁性的共存，引起了人们用过渡金属离子 掺入钙钛矿氧化物的方式来诱发多铁性的兴趣。不 同的过渡金属元素 Fe、Ni 掺杂 ABO3 钙钛矿型铁电 体证明了其可以实现室温铁磁性 [13][14] 。研究表明， 少量的 Fe 或 Mn 离子的掺杂使 BTO 的晶体结构从 四方转为六方相，Dang 等 [15] 制备出 BaTi1-xMnxO3 陶瓷，发现掺杂量 x 大于 0.01 就会产生六方相，而 铁电性就会有大幅的降低。多位研究者 [16][17][18][19][20][21] [12] 。即使很少的掺杂量也会使 BTO 的晶 体结构快速从四方相完全转变为六角相，四方相是 铁电相而六角相是顺电相。 Das 等 [22] 对 BTO 采用了 Mn-Nb 共掺杂的掺杂方式，抑制了六方相的产生， 使晶体结构较为缓慢的由四方相到立方相的转变。 过渡金属离子与 Nb 共掺杂的策略可以在诱导出铁 磁性的同时也一定程度上抑制铁电性的减弱。 另外， Nb 5+ 的 d 轨道上的电子为空， 这也会增强铁电性 [9] 。 从过渡金属元素的单掺杂到过渡金属元素与 Nb 的 共掺杂这种掺杂策略调整给铁磁性的诱导和铁电性 的减弱之间增加更大的调整空间。Zheng 等 [23] 在 [25][26] 。 拉曼(Raman)散射光谱可以分析样品的晶体结 构和相变，不同掺杂陶瓷样品的拉曼光谱如图 2 所 示。 BTO 陶瓷的拉曼光谱中， 低中频区域(<500 cm -1 ) 显示有两个拉曼峰，269 cm -1 处峰的 A1(TO2)振动模 式和 306 cm -1 处峰的 B1(TO2)、E(TO2+LO2) 振动模 式；高频区域(>500 cm -1 )，A1(TO3)和 A1(LO2)振动 模式和氧八面体有关 [23,27] 。BTO 陶瓷样品的拉曼峰 窄而尖锐，表明样品的晶体结晶质量较好。在 305 cm -1 处的峰与样品的铁电性相关 [28] 。从图 2 可以看 出，三组掺杂样品的拉曼光谱在 305 cm -1 对应峰的 峰型变得平缓并且峰强减弱，说明具有铁电性的四 方相减少而顺电相的立方相增加，这是晶体结构从 四方相到立方相转变的明显标志 [29][30] 。同时 305…”