Magnetism in reduced dimensions: Nanoparticles, thin films and quantum spin systems

纳米粒子、薄膜和量子自旋系统中的纳米磁性：尺寸小于100纳米的金属基晶体，被称为纳米粒子，表现出广泛的物理性质，与它们的体相对应物截然不同。许多新的特性来自于驻留在表面的大部分原子和每个纳米粒子核心中有限数量的原子。我们的研究项目致力于回答一个基本问题：表面对纳米粒子的磁性有什么总体影响。纳米粒子表面和核心磁性二分法产生的磁挫折效应可以被“剥离”到二维表面上。作为铁磁层与反铁磁层界面的薄膜就是一个理想的例子。就像纳米粒子一样，这种薄膜表现出交换偏置效应。交换偏置是界面上被“锁定”的磁矩的结果。这种“锁定”的物理起源目前尚不清楚。通过纳米尺度结构表征的独特结合，以及从原子到体长度尺度的纳米磁性的全面描述，我们打算对“锁定”的物理学有新的见解。在某些结构中，自由度的进一步降低是可能的，其中相互作用可能只在一个晶体学方向上发生。然后创建了一维量子自旋系统，其中一个例子是霍尔丹间隙材料。这些材料表现出新的磁性。量子自旋系统提供了一种独特的理论描述，其稳健性仍需要严格的实验测试。我们的研究小组在测量和模拟静态和动态磁性方面的丰富经验将为这些材料的物理特性提供独特的见解。在上述研究领域，对知识的迫切需求是不容置疑的。首先，下一代磁性介质取决于缩小器件尺寸，而成功实现取决于对纳米磁性的理解。其次，基于纳米颗粒给药的新医疗技术需要对单个和集合纳米颗粒的磁性有全面的了解。

Nanomagnetism in nanoparticles, thin films, and quantum spin systems: Metals-based crystallites with sub-100 nm sizes, called nanoparticles, exhibit a wide range of physical properties that are strikingly different from their bulk counterparts. Many novel properties arise from the large fraction of atoms that reside on the surface and the finite number of atoms in each nanoparticle core. Our research program is dedicated to answering the fundamental question: what overall effect does the surface have on the nanoparticle magnetism. The magnetic frustration effects from the surface and core magnetism dichotomy in a nanoparticle can be "peeled" away onto a 2D surface. A thin film that is a ferromagnetic layer interfaced to an antiferromagnetic layer is an ideal example. Just like a nanoparticle, such a film exhibits the exchange bias effect. Exchange bias is the result of magnetic moments at the interface which are "locked-in". The physical origins of this "locking" are currently unknown. With a unique combination of nanoscale structural characterization, and a comprehensive description of the nanomagnetism from atomic to the bulk length scales, we intend to enable new insights into the physics of the "locking". A further reduction in the degrees of freedom is possible in certain structures where interactions are possible in only one crystallographic direction. A 1D quantum spin system is then created, an example of which are Haldane gap materials. These materials exhibit novel magnetism. Quantum spin systems present a unique theoretical description whose robustness still requires rigorous experimental tests. Our research group's significant experience measuring and modeling static and dynamic magnetism will provide unique insights into the physics of these materials. There is an unmistakable, pressing need for knowledge in the above research areas. First, the next generation of magnetic media hinges on shrinking device sizes where successful implementation depends on understanding the nanomagnetism. Second, new medical technologies based on nanoparticle drug delivery require a comprehensive understanding of the magnetism of single and collections of nanoparticles.