Low-temperature behavior of single-domain through multidomain magnetite
Low-temperature behavior of single-domain through multidomain magnetite
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DOI：
10.1016/0012-821x(94)00260-6
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发表时间：
1995-02
影响因子：
5.3
通讯作者：
S. Halgedahl;R. Jarrard
中科院分区：
文献类型：
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作者：
S. Halgedahl;R. Jarrard
我们研究了一套“生长”的合成和天然磁铁矿的低温行为，这些磁铁矿涵盖单域（SD）和多域（MD）行为。合成样品是在实验室的水介质或玻璃中生长的。天然样品包括斜长石中的 SD 磁铁矿和大八面体形式的真正 MD 磁铁矿。在所有实验中，首先对样品进行室温下的饱和剩磁测量；接下来，在 230 K 和 60 K 之间的冷却和升温过程中连续测量力矩。与其他工作人员之前报告的结果类似，SD 样品中的磁记忆很大，而真正的 MD 样品在室温和 60 K 之间循环时几乎完全消磁。伪单畴样品表现出的行为介于 SD 和真正 MD 状态之间。当这项研究的数据与 Hartstra [10] 从一定尺寸的天然磁铁矿获得的数据相结合时，发现在室温和 60 K 之间的循环中幸存下来的总剩磁百分比随着晶粒尺寸的对数线性下降，因此随着磁畴数量的增加而线性下降。这种关系表明，记忆可以对那些含磁铁矿的岩石中的晶粒尺寸提供合理的估计，而这些样本为这些岩石提供了良好的类似物。值得注意的是，一些大型天然八面体提供了 MD 对低温响应的放大视图，从而揭示了两种令人惊讶且有趣的行为类型。首先，在大约 180 K 以下，这些八面体通过一系列大的巴克豪森跳跃而消磁。其次，在 117 K 附近，这些相同的八面体表现出一个“狂野区”，其中磁矩执行大的随机偏移。我们将这两种现象解释为磁畴壁因矫顽力下降而解钉扎和不可逆位移的直接证据，并且随着温度降至各向同性点，磁畴壁可能会变宽。这种行为的一个含义是，冷却到逐渐降低的温度可以提供一种有效的方法来逐步去除MD颗粒携带的古磁性成分，即使不通过磁铁矿的各向同性点。
We have investigated the low-temperature behavior of a suite of ‘grown’ synthetic and natural magnetites that span single-domain (SD) and multidomain (MD) behavior. Synthetic samples had been grown in the laboratory either in an aqueous medium or in glass. Natural samples included SD magnetites occurring in plagioclase and truly MD magnetites in the form of large octahedra. In all experiments a sample was first given a saturation remanence at room temperature; next, moment was measured continuously during cooling and warming between 230 K and 60 K. Similar to results reported earlier by other workers, magnetic memory is large in SD samples, whereas truly MD samples are almost completely demagnetized by cycling between room temperature and 60 K. Pseudo-single-domain samples exhibit behavior that is intermediate with respect to that of the SD and truly MD states. When data from this study are combined with data obtained by Hartstra [10] from sized, natural magnetites, it is found that the percentage of total remanence that survives cycling between room temperature and 60 K decreases linearly with the logarithm of grain size and, thus, with increasing number of domains. This relation suggests that memory can provide a reasonable estimate of grain size in those magnetite-bearing rocks for which these samples provide good analogues. Remarkably, some of the large natural octahedra provide a magnified view of MD response to low temperatures and thus reveal two surprising and intriguing types of behavior. First, below approximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ‘wild zone’, where magnetic moment executes large, random excursions. We interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and, possibly, the broadening of domain walls as temperatures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by MD grains, even without passage through the isotropic point of magnetite.