Micromagnetic determination of the FORC response of paleomagnetically significant magnetite assemblages

Micromagnetic modelling allows the systematic study of the effects of particle size and shape on the first-order reversal curve (FORC) magnetic hysteresis response for magnetite particles in the single-domain (SD) and pseudo-single domain (PSD) particle size range. The interpretation of FORCs, though widely used, has been highly subjective. Here, we use micromagnetics to model randomly oriented distributions of particles to allow more physically meaningful interpretations. We show that one commonly found type of PSD particle - namely single vortex (SV) particles - has far more complex signals than SD particles, with multiple peaks and troughs in the FORC distribution, where the peaks have higher switching fields for larger SV particles. Particles in the SD to SV transition zone have the lowest switching fields. Symmetrical and prolate particles display similar behavior, with distinctive peaks forming near the vertical axis of the FORC diagram. In contrast, highly oblate particles produce ‘butterfly’ structures, suggesting that these are potentially diagnostic of particle morphology. We also consider FORC diagrams for distributions of particle sizes and shapes and produce an online application that users can use to build their own FORC distributions. There is good agreement between the model predictions for distributions of particle sizes and shapes, and the published experimental literature.

微磁模拟能够系统地研究粒径和形状对单畴（SD）和准单畴（PSD）粒径范围内磁铁矿颗粒的一阶反转曲线（FORC）磁滞响应的影响。FORC的解释尽管被广泛应用，但具有很强的主观性。在此，我们利用微磁学对随机取向的颗粒分布进行建模，以便做出更具物理意义的解释。我们表明，一种常见的PSD颗粒类型——即单涡旋（SV）颗粒——比SD颗粒具有复杂得多的信号，在FORC分布中有多个峰和谷，其中较大的SV颗粒的峰具有更高的转换场。从SD到SV过渡区的颗粒具有最低的转换场。对称和长球形颗粒表现出相似的行为，在FORC图的纵轴附近形成独特的峰。相比之下，高度扁球形颗粒产生“蝴蝶”结构，这表明这些结构可能对颗粒形态具有诊断性。我们还考虑了粒径和形状分布的FORC图，并制作了一个在线应用程序，用户可借此构建自己的FORC分布。粒径和形状分布的模型预测与已发表的实验文献之间具有良好的一致性。