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

纳米颗粒、薄膜和量子自旋系统中的纳米磁性：尺寸小于100 nm的金属基微晶，称为纳米颗粒，表现出与其块体对应物截然不同的各种物理性质。许多新的性质产生于驻留在表面的大部分原子和每个纳米颗粒核心中的有限数量的原子。我们的研究计划致力于回答一个基本问题：表面对纳米颗粒磁性有什么总体影响。纳米颗粒中的表面和核心磁性二分法产生的磁阻效应可以被“剥离”到2D表面上。作为与反铁磁层接口的铁磁层的薄膜是理想的例子。就像纳米粒子一样，这种薄膜表现出交换偏置效应。交换偏置是界面处的磁矩“锁定”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组合，以及对从原子到块体长度尺度的纳米磁性的全面描述，我们打算使人们能够对“锁定”的物理学有新的见解。在某些结构中，自由度的进一步减少是可能的，在这些结构中，相互作用只可能在一个晶体方向上发生。然后创建了一个一维量子自旋系统，其中一个例子是霍尔丹带隙材料。这些材料显示出新奇的磁性。量子自旋系统提出了一种独特的理论描述，其稳健性仍然需要严格的实验测试。我们研究小组在静态和动态磁性测量和建模方面的重要经验将为这些材料的物理提供独特的见解。毫无疑问，对上述研究领域的知识有着迫切的需求。首先，下一代磁性介质取决于缩小的设备尺寸，而成功的实施取决于对纳米磁性的理解。其次，基于纳米药物输送的新医疗技术需要全面了解单个和多个纳米颗粒的磁性。

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.