Numerical simulations of magnetic systems with competing interactions

磁性现象可以发生在各种各样的材料中。随着计算能力的进步，我们能够更多地了解磁矩在日益复杂的系统中是如何排列的。这些力矩可以与晶体中的单个原子相关联，也可以作为薄膜中较大纳米簇的一部分。在每种情况下，系统的几何形状在决定磁矩方向的排列方面起着重要作用，这可能相当复杂。利用数值方法研究了磁矩的取向随温度、压力和外加磁场的变化情况。微观水平上的力矩方向细节决定了材料的大样本在实验室和实际设备应用中的表现。我们的部分工作将集中在几何上受挫的反铁磁体上，其中晶体中相邻原子之间的相互作用不能全部满足原子磁矩反平行的偏好。受挫的反铁磁性材料的新例子经常被发现，例如那些还表现出技术上重要的磁电效应的材料，其中施加的磁场会产生可检测的电压。我们的数值技术将应用于进一步了解这一令人兴奋和潜在的重要新现象。我们工作的一个相关焦点是数据存储行业对各种目的感兴趣的薄铁磁薄膜。这包括在硬盘驱动器中使用的材料。随着数据密度的增加，磁性粒度必须减小，直到达到超顺磁极限，此时热运动导致自发矩反转，数据丢失。我们的工作将有助于揭示薄膜需要什么样的特性来防止这种情况的发生。我们进行的理论和数值研究的一个显著好处是与Memorial和其他地方的实验组密切合作。这对实习研究人员来说尤其有益。

Magnetic phenomena can occur in a huge variety of materials.  With advances in computing power, we are able to learn more about how magnetic moments are arranged in increasingly complex systems.  These moments can be associated with individual atoms in a crystal or as a part of larger nanometer clusters in thin films.  In each case, the geometry of the system plays a major role in determining the arrangement of the magnetic-moment directions, which can be quite complex. Numerical techniques have been developed to study how the orientation of the magnetic moments change with changing temperature, pressure, and applied magnetic field.  The details of the moment orientations on a microscopic level determine how larger samples of the materials will behave in the laboratory and in real device applications.     Part of our work will focus on geometrically frustrated antiferromagnets, where the interactions between neighboring atoms in a crystal cannot all satisfy the preference of the atomic magnetic moments to be anti-parallel. New examples of frustrated antiferromagnetic materials are being discovered regularly, such as those which also exhibit the technologically important magnetoelectric effect where an applied magnetic field induces a detectable voltage. Our numerical techniques will be applied to further understand this exciting and potentially significant new phenomenon.    A related focus of our work is on thin ferromagnetic films of interest to the data storage industry for a variety of purposes.  This includes materials used in hard drives.  As the density of data increases, the magnetic grain size must decrease until the superparamagnetic limit is reached, where thermal motion causes spontaneous moment reversal and data is lost.  Our work will aid in revealing what properties of the thin films are needed to prevent this from occurring.      A significant benefit of the theoretical and numerical studies we perform is the close collaboration with experimental groups at Memorial and elsewhere.  This can be particularly rewarding for trainee researchers.