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Anomalous magnetization reversal in exchange biased thin films and nanostructures.

机译：交换偏向薄膜和纳米结构中的反常磁化反转。

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摘要

When a ferromagnet/antiferromagnet (FM/AF) bilayer is cooled below the Neel temperature TN of the AF in a magnetic field, exchange bias (EB) phenomenon arises. Although EB was discovered nearly 50 years ago, a general understanding is still lacking regarding the competing interactions and length scales involved and how they give rise to a rich variety of magnetization reversal behavior. In this thesis, I will address these questions by studying the cooling field dependence, asymmetric magnetization reversal, and spontaneous reversal in epitaxial FeF2/polycrystalline FM bilayers. Two types of transitions from negative to positive EB with increasing cooling fields were found: for in-plane twinned FeF2, a continuous transition was found, while coexistence of EB of both signs was observed for untwinned FeF2. This is attributed to the relevance between the AF "domain" size and the FM domain wall width and confirmed by micromagnetic simulation. Nanostructuring the FM significantly decreases the onset cooling field for positive EB with decreasing dot sizes when the FM dot size is comparable with the AF "domain" size (∼500nm). The high quality epitaxial exchange bias system also constitutes a model system for studying magnetization reversal asymmetry. By vector magnetometry and simulation, we found that the FM reverses through parallel domain walls, which results in highly asymmetric hysteresis loops. Also, the FM near the FM/AF interface exhibits a more asymmetric reversal than that farther away from the interface. These results unambiguously show the existence of a FM parallel domain wall and its importance in asymmetric magnetization reversal. Moreover, we found another surprising phenomenon that when positively exchange biased, the FM can spontaneously reverse its magnetization in a constant field when cooled below TN due to the strong interfacial coupling. When heating up in the same field, the FM magnetization reverses at T > TN, giving rise to a tunable thermal hysteresis. Discovery of this phenomenon suggests a revised energy competition mechanism for positive EB, which includes parallel FM and AF domain wall energy. By studying these different but strongly related magnetization reversal behaviors, we demonstrated the central role of competing length scales and interactions in heterostructured magnetic systems.
机译：当铁磁体/反磁体（FM / AF）双层在磁场中冷却到AF的尼尔温度TN以下时，会产生交换偏压（EB）现象。尽管EB是在50年前发现的，但对于竞争性相互作用和涉及的长度尺度以及它们如何引起各种各样的磁化反转行为，仍然缺乏普遍的了解。在本文中，我将通过研究外延FeF2 /多晶FM双层中的冷却场依赖性，不对称磁化反转和自发反转来解决这些问题。发现了从负EB到正EB的两种类型的跃迁，且冷却场增加了：对于平面孪晶FeF2，发现了连续跃迁，而对于未缠绕的FeF2，两种迹象的EB共存。这归因于AF“畴”尺寸与FM畴壁宽度之间的相关性，并通过微磁模拟得到了证实。当FM点尺寸与AF“域”尺寸（〜500nm）相当时，纳米结构化FM会显着减小正EB的起始冷却场，同时减小点尺寸。高质量的外延交换偏置系统也构成了研究磁化反转不对称性的模型系统。通过矢量磁力计和仿真，我们发现FM通过平行磁畴壁反转，从而导致高度非对称的磁滞回线。而且，与远离接口的FM相比，靠近FM / AF接口的FM表现出更不对称的反转。这些结果清楚地表明了FM平行畴壁的存在及其在非对称磁化反转中的重要性。此外，我们发现了另一个令人惊讶的现象，当正交换偏向时，由于强界面耦合，当冷却到TN以下时，FM可以在恒定磁场中自发地反转其磁化强度。在相同的磁场中加热时，FM磁化强度在T> TN处反转，从而产生可调节的热滞。这一现象的发现表明，正电子束的能量竞争机制得到了修正，其中包括平行的FM和AF域壁能量。通过研究这些不同但密切相关的磁化反转行为，我们证明了竞争长度尺度和相互作用在异质结构磁系统中的核心作用。

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  • 作者

    Li, Zhi-Pan.;

  • 作者单位

    University of California, San Diego.;

  • 授予单位 University of California, San Diego.;
  • 学科 Physics Condensed Matter.
  • 学位 Ph.D.
  • 年度 2006
  • 页码 187 p.
  • 总页数 187
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 O49;
  • 关键词

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