Thermal fluctuation analysis: A new technique in rock magnetism
Thermal fluctuation analysis: A new technique in rock magnetism
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热波动分析：岩石磁性新技术
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发表时间：
1976
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通讯作者：
D. Dunlop
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作者：
D. Dunlop
热涨落分析是一种新的方法来确定“磁性晶粒尺寸”v和微观矫顽力HK的颗粒太小，直接域观察。当用于单畴颗粒时，热涨落分析是一种磁性粒度测量方法，因为v是物理颗粒体积，HK通常与颗粒形状密切相关。对于多畴粒子，v对应于畴壁的一次巴克豪森跳跃，HK描述了畴壁运动的相反方向。该技术需要三个步骤：（1）在宽温度范围内测量矫顽力Hc和饱和磁化强度，（2）以Neel（1949）理论建议的方式分离HK和“波动场”Hq对HC的贡献，以及（3）从Hq（T）和HK（T）计算v和室温HK的平均值。热波动分析是单独计算平均Vi和HK的唯一简单方法。如果热涨落是重要的，就像低场热剩磁或粘滞剩磁中涉及的细颗粒或小壁位移的情况一样，热退磁和交变场退磁不能明确地解决v和HK，因为观察到的磁化率和阻塞温度TB取决于v和HK。通过将其应用于已知晶粒尺寸的单畴材料来测试该方法。然后用涨落分析来推断500- 2200-A磁铁矿颗粒的可能的畴结构，并指出Stacey和Banerjee（1974）在检测阻碍畴壁运动的晶格缺陷的有序化过程中的缺陷。潜在的更广泛的应用包括确定岩石中磁性载体的单群体和在有利条件下的双群体的平均粒度，定义岩石和合成分散体（例如，磁记录磁带），并推导出各向异性（形状，晶体或磁弹性）或反对壁运动（应变，夹杂物或静磁）的机制。该方法的主要局限性是，如果Hc测量在TB范围内进行，则v和HK的平均值变得不稳定，并且步骤2（HK（T）和Hq（T）的分离）是困难的，除非HK主要是由于单一各向异性或壁对抗机制。
Thermal fluctuation analysis is a new method of determining ‘magnetic grain size’ v and microscopic coercive force HK in particles too small for direct domain observations. When it is used with single-domain particles, thermal fluctuation analysis is a method of magnetic granulometry, since v is the physical grain volume and HK is usually closely related to grain shape. For multidomain particles, v corresponds to one Barkhausen jump of a domain wall, and HK describes the opposition to wall motion. The technique requires three steps: (1) measurement of coercive force Hc and saturation magnetization over a broad temperature range, (2) separation of the contributions of HK and the ‘fluctuation field’ Hq to HC, in a manner suggested by the Neel (1949) theory, and (3) calculation of average values of v and room temperature HK from Hq(T) and HK(T). Thermal fluctuation analysis is the only simple means of calculating average v; and HK separately. If thermal fluctuations are important, as is always the case for the fine particles or small wall displacements involved in low-field thermoremanence or viscous remanence, thermal and alternating field demagnetization do not resolve v and HK unambiguously, because observed coercivities and blocking temperatures TB depend on both v and HK. The method is tested by applying it to single-domain materials of known grain size. Fluctuation analysis is then used to infer the possible domain structure of 500- to 2200-A magnetite particles and to point up a deficiency in the Stacey and Banerjee (1974) procedure for detecting ordering of lattice defects impeding domain wall motion. Potential broader applications include determining mean grain sizes of single, and under favorable conditions, double populations of magnetic carriers in rocks, defining average elongation for single-domain particles in rocks and synthetic dispersions (e.g., magnetic recording tapes), and deducing the mechanism of anisotropy (shape, crystalline or magnetoelastic) or of opposition to wall motion (strain, inclusion or magnetostatic). The main limitations to the method are that the averages of v and HK become nonstationary if Hc measurements are made within the TB range and that step 2 (separation of HK(T) and Hq(T)) is difficult unless HK is predominantly due to a single anisotropy or wall opposition mechanism.