Theory of Stable Multidomain Thermoviscous Remanence Based on Repeated Domain Wall Jumps

Abstract

We developed a theory of multidomain (MD) thermoviscous remanence that can be solved analytically to yield an expression relating the blocking temperature of a MD grain to its relaxation time. This expression is analogous to Néel's widely used theory of single‐domain (SD) thermoviscous remanence but yields a different time‐temperature relationship. The theory is based on a two‐domain model with a domain wall (DW) that can jump between different pinning sites. In contrast to previous theories of this kind, our theory considers repeated DW jumps rather than a single jump in isolation. It is shown that while SD remanence behavior is fully described by the two quantities (V,HK0), that is, volume and microscopic coercivity, MD particle remanence depends on three quantities: volume, Barkhausen volume, and DW pinning field, denoted (V,VBark,HK0). “Pullaiah” nomograms of this new theory show that while small MD grains behave almost identically to SD grains, larger grains show different slopes depending on the above quantities. The theory predicts that MD grains can be highly stable remanence carriers, in particular showing a high thermal stability. Grains with weak pinning fields, however, while thermally stable, are highly unstable under alternating field (AF) demagnetization, being demagnetized under fields of a few millitesla. Our theory also explains (1) why samples dominated by MD grains show curved vector demagnetization plots for both thermal and AF demagnetization, as well as (2) why MD grains affect thermal demagnetization plots even at high temperatures, while their remanence is completely removed within the first few AF demagnetization steps.

Remagnetization Under Hydrothermal Alteration of South Tibetan Paleocene Lavas: Maghemitization, Hematization, and Grain Size Reduction of (Titano)magnetite

“…We suggest that it represents the TVRM2 acquired while E 1 d was cooling after peak heating to 130–145°C during hydrothermal event II (Huang et al., 2022). This hypothesis is supported by the consistence of the temperature range (<300°C) over which the LTC is isolated, and with the MD theory‐predicted unblocking temperature range (<300–350°C) for the TVRM2 (Berndt & Chang, 2019).…”

“…Hydrothermal event I, induced by extensive intrusion of dikes in the E 1 d and E 2 n and massive ignimbrite eruption in E 2 p, heated the E 1 d strata up to 300°C for 1 million years (Myr) (Huang et al., 2022). Considering that the Curie temperatures for titanomagnetite in the E 1 d samples are 549–568°C (Figure 3), the MD theory of titanomagnetite (Berndt & Chang, 2019) predicts that such heating could reset the primary remanence for all grains whose unblocking temperature is below 450°C in the laboratory; this would induce the acquisition of a thermoviscous remanent magnetization (TVRM1). Similarly, MD theory also predicts that hydrothermal event II, during which the E 1 d lavas were heated to 130–145°C for 1 Myr (Huang et al., 2022), could result in replacement of the portion of TVRM1 that unblocks below 300–350°C by a new TVRM (TVRM2).…”

Two-step movement of tsunami boulders unveiled by modified viscous remanent magnetization and radiocarbon dating

“…In contrast to the general random walk model, the magnetic particle must wait for a certain time at the local energy minima before each jump can occur (Supplementary Text) 30 . There are waiting times before the jumps of particles in local energy minima can occur in the rugged energy landscape 21 , which is similar to the energy landscape of MD particles 15 . However, in the coarse-grained landscape of energies, the organized sets of energy barriers create deep valleys and the saddle point corresponds to overturning magnetic spins of large clusters (Fig.…”

“…Although Walton 26 developed a theory regarding VRM acquisition for assemblies of SD particles with different volumes and coercivity, it has been pointed out that the physical meaning of the equations is the replacement of a previous VRM acquired at a temperature over a certain time by a new VRM with equal intensity obtained at a higher temperature over a shorter time period 10 , 29 . On the other hand, Berndt and Chang 15 advanced previous MD theories by considering the effect of repeated domain wall jumps over many pinning sites, which implies that the VRM stability of MD remanent magnetization. Nagy et al 16 numerically derived Pullaiah curves for vortex-state structures.…”

“…The remanent magnetization of rocks is not only attributed to SD particles but also to non-SD particles (i.e., vortex-state and multi-domain, MD, particles); moreover, the non-SD effects are considered to be the cause of the older ages. The temporal stability curves of uniform vortex-state domain and MD particles are much shallower and yield higher unblocking temperatures than the Pullaiah nomogram 15 , 16 . Hence Sato et al 17 considered the behavior of an assembly of magnetic particles with various domain states and reported that the relaxation of geologic samples is often described in terms of the stretched exponential or Kohlrausch–Williams–Watts function 18 , 19 .…”