Micromagnetic Experiments on Small Single Particles: Defects, Nucleation, and Wall-Pinning

小磁性颗粒中伪单畴（PSD）行为的物理起源一直是激烈争论的主题，因为PSD颗粒被认为携带了岩石中大部分的稳定残留物。两个因素对PSD行为的贡献最大：磁畴状态的转变和钉住畴壁的缺陷。尽管缺陷的重要作用已经被认识了几十年，但最近的岩石磁学理论和实验强调，我们对它们在关键问题上的影响知之甚少，例如畴壁成核、壁钉、剩磁对交变场（AF）退磁的稳定性、热剩磁（TRM）的获取，甚至从某些磁性矿物的PSD颗粒中提取古强度的潜力。精心选择的磁性矿物的单个颗粒可以提供成核事件和壁缺陷相互作用的“放大”视图。赤铁矿退磁磁场弱，磁畴少，是这样一个放大镜。这种性质的结合为研究缺陷对磁性行为的内在影响打开了一扇窗。我们将使用大的单片赤铁矿来更好地了解缺陷是如何控制成核和畴壁钉住的。本文提出了六组核心实验。这将决定：(1)在室温下诱导的类sd（或接近类sd）状态的热稳定性；(2)弱场TRM的域态；(3)成核场的温度依赖性；(4)打开成熟畴壁的关键场的温度依赖性；(5)携带强场残余和弱场非磁滞磁化的真正多畴赤铁矿的AF稳定性；(6)临界脱钉场的频率分布。

HalgedahlEAR-0125993The physical origins of pseudosingle-domain (PSD) behavior in small magnetic particles have been subjects of lively debate, because PSD grains are thought to carry much of the stable remanence in rocks. Two ingredients contribute most to PSD behavior: transitions in magnetic domain state and defects that pin domain walls. Despite the important role of defects having been recognized for decades, recent theories and experiments in rock magnetism underscore how little we understand about their impact on key problems such as domain wall nucleation, wall-pinning, the stabilities of remanences to alternating field (AF) demagnetization, acquisition of thermal remanent magnetization (TRM), and even the potential for extracting paleointensities from PSD grains of certain magnetic minerals. Single particles of judiciously-chosen magnetic minerals can provide "magnified" views of both nucleation events and wall-defect interactions. Owing mainly to its weak demagnetizing field and few domains, hematite is such a magnifier. This combination of properties opens up a window for studying the intrinsic impact of defects on magnetic behavior. We will use large, single platelets of hematite to better understand how defects control nucleation and domain wall pinning. Six sets of core experiments are proposed here. These would determine: (1) the thermal stability of SD-like (or near SD-like) states induced at room temperature; (2) domain states for weak-field TRM; (3) temperature dependences of nucleation fields; (4) temperature dependences of critical fields that unpin mature domain walls; (5) AF stabilities of truly multidomain hematite carrying strong-field remanence and weak-field anhysteretic magnetization, and (6) the frequency distribution of critical unpinning fields.