Decay‐rate dependence of anhysteretic remanence: Fundamental origin and paleomagnetic applications

Abstract

[1] We have measured the intensity of anhysteretic remanent magnetization (ARM) as a function of alternating field (AF) decay rate. For synthetic and natural single-domain (SD) and pseudo-single-domain (PSD) magnetites, ARM intensity increases as decay rate decreases. Multidomain (MD) magnetites have the opposite response, ARM increasing as the decay rate increases. These are identical to the SD/PSD and MD dependences of thermoremanent magnetization on cooling rate. For all grain sizes and domain structures, ARM intensity increases as the AF decay rate used to achieve an initial demagnetized state decreases. Decay-rate differences in ARM intensity are a property of low-and medium-coercivity grains, as shown by annealing and by stepwise AF demagnetizing samples. We interpret the SD results to mean that increased AF exposure time permits a closer approach to equilibrium magnetization. An approximate thermal activation theory based on Néel [1949] and an exact theory by Egli and Lowrie [2002] predict 6-11% increases in ARM for an order of magnitude decrease in decay rate, in reasonable accord with the observed 12% increase for 0.065 mm SD grains. For MD grains, we hypothesize that increased exposure time (slower decay) permits more efficient self-demagnetization, reducing ARM. Low-coercivity grains experience the largest self-demagnetizing fields and therefore have the largest decay-rate response. Initial-state decay-rate response is attributed to longer exposure times leaving domain walls more strongly pinned in deeper potential wells (the net self-demagnetizing field is zero in the demagnetized state). Acquisition decay-rate, annealing, and initial-state responses of PSD grains are a blend of SD and MD responses. Because ARM is the most frequently used normalizer in relative paleointensity determination, it is important either to use a standard decay rate or else to remove the decay-rate dependence by demagnetizing the ARM to $30% of its initial value (ARM 0.3 ). A standard demagnetization level for the normalizing ARM is particularly important when comparing paleointensity records from different laboratories.

Environmental magnetism and paleoclimatic interpretation of the Sanmenxia loess-paleosol sequence in the southeastern extremity of the Chinese Loess Plateau

“…In a range of SD and larger grain sizes, ARM is more sensitive to finer grains than χ and SIRM, and therefore is widely used to estimate the concentration of SD-PSD ferrimagnetic grains [9,11,24,25] . Zhou et al [1] argued that the magnetic enhancement in Fig.…”

Theoretical considerations on the anhysteretic remanent magnetization of interacting particles with uniaxial anisotropy

“…Examples are given by the AF decay rate dependence [Egli and Lowrie, 2002;Yu and Dunlop, 2003], and anisotropy due to a preferred orientation of the easy axes [Potter, 2004]. The chain rule of derivatives…”

“…On the other hand, Xu and Dunlop [1995] developed a model for the ARM acquisition in multidomain grains. Other properties such as the additivity and reciprocity of partial ARM [Yu et al, 2002a[Yu et al, , 2002b, and the dependence of the ARM on the decay rate of the alternating field [Yu and Dunlop, 2003], have been investigated.…”

Magnetic characteristics of topsoil from Xinjiang, Northwestern China, and their implications

“…The content of SSD and PSD ferrimagnetic grains can be estimated by χ ARM . Ratios like χ ARM / SIRM and SIRM/χ can be used to indicate grain size of ferrimagnetic mineral (Liu et al, 2001;Egli and Lowrie, 2002;Yu and Dunlop, 2003;Liu et al, 2004;Liu et al, 2007). The results of the χ fd % indicate the SP component in both South Xinjiang and North Xinjiang is low, and the SP component in Kumutage is also low, but it is much higher in Yili Valley.…”

“…The thermomagnetic curves and hysteresis loops can be used to determine magnetic mineral type and grain size (Heller and Liu, 1984;Thompson and Oldfield, 1986;Evans and Heller, 2003;Yu and Dunlop, 2003). Figures 3(a) The magnetic parameters can be used to indicate the type, grain size and the concentration of magnetic minerals in top-soil samples.…”