Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1.
J Am Chem Soc. 2012 Jan 18;134(2):1136-46. doi: 10.1021/ja2091678. Epub 2011 Dec 14.
Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.
多铁性材料表现出自发磁矩和电偶极矩之间的耦合，在高密度存储和复杂多态存储元件设计方面具有重要的应用潜力。在这项工作中，我们通过溶剂控制法合成了 Cr(3+)掺杂的 BaTiO(3)纳米晶，并通过 X 射线衍射和拉曼光谱研究了尺寸和掺杂浓度对其结构和相变的影响。通过磁化率、磁圆二色性（MCD）和 X 射线磁圆二色性（XMCD）测量研究了这些纳米晶的磁性。我们观察到，纳米晶尺寸的减小和掺杂浓度的增加有利于稳定顺电立方相，尽管在掺杂浓度约为 5%的约 7nm 纳米晶中，铁电四方相仍部分保留。Cr(3+)掺杂被确定为四方相失稳的主要因素。磁和磁光测量的结合表明，由合成的顺磁 Cr(3+)掺杂 BaTiO(3)纳米晶制备的纳米晶薄膜表现出稳健的铁磁有序（高达约 2μB/Cr(3+)），类似于磁性掺杂透明导电氧化物。由于表面/体积比增加导致界面缺陷浓度增加，观察到的铁磁性随组成纳米晶尺寸的减小而增加。通过扫描透射 X 射线显微镜（STXM）测量的元素特异性 XMCD 光谱以高空间分辨率证实，磁有序源于 Cr(3+)掺杂剂的交换相互作用。这项工作的结果表明，通过使用优化组成和尺寸的掺杂胶体纳米晶作为功能构建块，设计和制备保留铁电相并表现出长程磁有序的多铁钙钛矿材料的方法。
