Magnetic fatigue of magnetite: effect of cyclic loading on the magnetic and microstructural behavior at elevated temperatures

岩石的磁性受到地壳内部变形过程的影响，从板块构造的缓慢过程，到火山爆发到高动态地震事件。到目前为止，磁性能的修改主要是研究静态变形。然而，人们普遍认为，构造应力首先不均匀地积累，然后迅速增加。如果构造载荷在高温下表现出循环或疲劳特性，则这种快速应力增加可能特别占主导地位。这一假设提出了一个问题，即岩石的磁性是否对高温下的循环变形敏感。令人惊讶的是，这些信息在地球科学文献中是不可用的。然而，我们的初步调查清楚地表明，循环变形是导致磁性的退化或磁疲劳的磁铁矿轴承矿石。在500°C下的初步实验揭示了增加的加载频率与Verwey转变温度、剩磁和磁化率的变化之间的非线性关系，以及磁化率及其各向异性随着循环加载持续时间的增加而急剧非线性下降。我们的项目旨在测试磁疲劳假设，特别考虑不可逆（微裂纹，空位形成，孪生）和可逆效应（磁致伸缩，弹性应变）。为此，将在各种条件下对从磁铁矿矿石、磁铁矿单晶和含磁铁矿花岗岩制备的样品进行循环加载，随后通过不同的磁性和显微结构方法进行研究。在此基础上，建立了与磁铁矿磁性有关的疲劳变形图。我们期望从这个项目的磁场应力敏感性，这是目前约束不佳，但可能是有用的变形岩石的磁信号的复杂解释，因为它们发生在地壳中无处不在的基本知识。

Magnetic properties of rocks are influenced by deformation processes within the Earth crust, ranging from slow processes in plate tectonics, through volcanic eruptions to the high dynamic seismic events. Up to now, modifications of magnetic properties are mainly investigated in relation to static deformation. However, it is generally accepted that tectonic stresses first accumulate non-uniformly, and then rapidly increase. Such a rapid stress increase may be especially dominant if the tectonic loading exhibits a cyclic or a fatigue character at elevated temperatures. This assumption raises the question whether magnetic properties of rocks are sensitive to cyclic deformation at elevated temperatures. Surprisingly, such information is unavailable in the geoscience literature. Our preliminary investigations, however, show clearly that cyclic deformation is causing a degradation of magnetic properties or a magnetic fatigue of a magnetite-bearing ore. Preliminary experiments at 500°C reveal a non-linear relationship between the increasing loading frequency and the variations in the Verwey transition temperature, magnetic remanence and coercively as well as a drastic non-linear drop of magnetic susceptibility and its anisotropy with increasing cyclic loading duration. Our project aims to test the magnetic fatigue hypothesis with a special consideration of irreversible (microcracks, vacancy formation, twinning) and reversible effects (magnetostriction, elastic strains). For this purpose, samples prepared from a magnetite ore, single crystals of magnetite and magnetite-bearing granite will be cyclically loaded under various conditions and subsequently studied by different magnetic and microstructural methods. On the basis of the obtained results, fatigue deformation maps related to the magnetic behaviour for magnetite will be constructed. We expect from this project basic knowledge on the magnetic field stress sensitivity, which is currently poorly constrained but might be useful for a sophisticated interpretation of magnetic signals from deformed rocks as they occur ubiquitous in the Earth crust.