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Topological effects in nanomagnetism: from superparamagnetism to chiral quantum solitons
Advances in Physics ( IF 23.750 ) Pub Date : 2012-02-01 , DOI: 10.1080/00018732.2012.663070
Hans-Benjamin Braun

Micromagnetics has been the method of choice to interpret experimental data in the area of microscopic magnetism for several decades. In this article, we show how progress has been made to extend this formalism to include thermal and quantum fluctuations in order to describe recent experimental developments in nanoscale magnetism. For experimental systems with constrained dimensions such as nanodots, atomic chains, nanowires, and thin films, topological defects such as solitons, vortices, skyrmions, and monopoles start to play an increasingly important role, all forming novel types of quasiparticles in patterned low-dimensional magnetic systems. We discuss in detail how soliton–antisoliton pairs of opposite chirality form non-uniform energy barriers against thermal fluctuations in nanowires or pillars. As a consequence of their low barrier energy compared to uniform reversal, they limit the thermal stability of perpendicular recording media. For sufficiently short samples, the non-uniform energy barrier continuously merges into the conventional uniform Néel–Brown barrier. Partial formation of chiral domain walls also determines the magnetic properties of granular nanostructured magnets and exchange spring systems. For a long time, the reconciliation between micromagnetics and quantum mechanics has remained an unresolved challenge. Here it is demonstrated how inclusion of Berry's phase in a micromagnetic action allows for a semiclassical quantization of spin systems, a method that is demonstrated by the simple example of an easy-plane spin. This powerful method allows for a description of quantum dynamics of solitons and breathers which in the latter case agrees with the anisotropic spin-½ XYZ-model. The domain wall or soliton chirality plays an important role as it is coupled to the wavevector of the quasiparticle dispersion. We show how this quantum soliton chirality is detected by polarized neutron scattering in one-dimensional quantum antiferromagnets.

中文翻译:

纳米磁性中的拓扑效应:从超顺磁性到手性量子孤子

几十年来,微磁学一直是解释微观磁学领域实验数据的首选方法。在本文中,我们展示了如何将这种形式主义扩展到包括热和量子涨落,以描述纳米级磁性的最新实验发展。对于诸如纳米点、原子链、纳米线和薄膜等尺寸受限的实验系统,孤子、涡旋、斯格明子和单极子等拓扑缺陷开始发挥越来越重要的作用,所有这些都在图案化的低维中形成新型准粒子磁系统。我们详细讨论了相反手性的孤子-反孤子对如何形成非均匀能垒来对抗纳米线或支柱中的热波动。由于与均匀反转相比,它们的势垒能量较低,因此限制了垂直记录介质的热稳定性。对于足够短的样品,非均匀能量势垒不断融合到传统的均匀 Néel-Brown 势垒中。手性畴壁的部分形成也决定了粒状纳米结构磁体和交换弹簧系统的磁性。长期以来,微磁学和量子力学之间的协调一直是一个悬而未决的挑战。这里展示了如何在微磁作用中包含 Berry 相位允许自旋系统的半经典量化,这种方法由简单平面自旋的简单示例演示。这种强大的方法允许描述孤子和呼吸器的量子动力学,在后一种情况下,这与各向异性自旋 1/2 XYZ 模型一致。畴壁或孤子手性起着重要的作用,因为它与准粒子色散的波矢耦合。我们展示了如何通过一维量子反铁磁体中的极化中子散射来检测这种量子孤子手性。
更新日期:2012-02-01
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