First order reversal curve diagrams and thermal relaxation effects in magnetic particles
First order reversal curve diagrams and thermal relaxation effects in magnetic particles
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DOI：
10.1046/j.0956-540x.2001.01419.x
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发表时间：
2001-06
影响因子：
2.8
通讯作者：
C. R. Pike;A. Roberts;K. Verosub
中科院分区：
文献类型：
--
作者：
C. R. Pike;A. Roberts;K. Verosub
我们最近开发了一种表征自然粒子组合中的磁性成分的技术。该技术基于将一阶反转曲线（FORC）的磁化数据转换为二维分布函数（FORC图）的等高线图。FORC图对于获取磁粒子系统中开关场和相互作用的信息是有用的。在这里，我们检查实验数据和理论模型，以便为解释含有超顺磁粒子的样品的FORC图提供严格的框架。我们在FORC图上发现了四种不同的热松弛表现。首先，热松弛将使FORC分布向低矫顽力方向移动。其次，在中等温度下，热松弛可以在FORC图的起源处产生二次峰。这个次峰表明部分单畴粒子组合已经变成超顺磁性。在足够高的温度下，FORC分布的主峰将位于FORC图的原点附近。第三，热松弛可以产生一个小的，但系统的，向上移动的FORC分布。第四，在FORC图的下象限中，热松弛将产生靠近并平行于垂直轴的等高线。这些表现使得FORC图成为研究天然样品体中热松弛效应（超顺磁性）的有力工具，特别是当样品中含有混合磁颗粒组合时。
SUMMARY We have recently developed a technique for characterizing the magnetic components within natural particle assemblages. This technique is based on the transformation of magnetization data from first-order reversal curves (FORCs) into contour plots of a 2-D distribution function (FORC diagrams). FORC diagrams are useful for obtaining information about switching fields and interactions in magnetic particle systems. Here, we examine experimental data and a theoretical model in order to provide a rigorous framework for interpreting FORC diagrams for samples that contain superparamagnetic particles. We have found four distinct manifestations of thermal relaxation on FORC diagrams. First, thermal relaxation will shift the FORC distribution to lower coercivities. Second, at intermediate temperatures, thermal relaxation can generate a secondary peak about the origin of a FORC diagram. This secondary peak indicates that part of a singledomain particle assemblage has become superparamagnetic. At high enough temperatures, the primary peak of the FORC distribution will be located about the origin of a FORC diagram. Third, thermal relaxation can produce a small, but systematic, upward shift of a FORC distribution. Fourth, thermal relaxation will produce contours that lie near and parallel to the vertical axis in the lower quadrant of a FORC diagram. These manifestations make FORC diagrams a powerful tool for studying the effects of thermal relaxation (superparamagnetism) in bulk natural samples, particularly when the samples contain mixed magnetic particle assemblages.