Chasing Tails: Insights From Micromagnetic Modeling for Thermomagnetic Recording in Non‐Uniform Magnetic Structures

Abstract

The structural, chemical, and thermal evolution of the deep Earth has modified both the power and shape of the geomagnetic field over time, and has left its paleomagnetic trace in rocks on the surface of our planet. This vast record holds the story of some of the most dramatic geological changes in the young and adolescent Earth (Landeau et al., 2022;Tarduno et al., 2020). However, reliable paleomagnetic records of these events have proven difficult to obtain and paleointensity studies are often plagued by large uncertainties and high failure rates, and frequently, the origin of the failure of recording fidelity remains poorly understood.Experimental protocols for determining the ancient field intensity rely on Néel's thermal activation theory (Néel, 1955(Néel, , 1988. Powerful as it is, it only applies to rocks containing uniformly magnetized, single-domain (SD) particles, with sizes smaller than ∼80 nm of equivalent spherical volume diameter (ESVD). Néel theory predicts that a thermoremanent magnetization (TRM) is acquired (blocked) at a temperature T b and is proportional to the geomagnetic field strength. Furthermore, this magnetization should be completely demagnetized by zero-field re-heating to an unblocking temperature T u , where T u = T b . When this is not the case, the remaining magnetization is referred to as a magnetic "tail." This "reciprocity" of blocking and unblocking (Thellier & Thellier, 1959) implies that the ancient field can be determined by replacing the original magnetization, across any temperature interval, with one generated in the laboratory. A related concept is that of "additivity" whereby the magnetization blocked when cooled from the Curie temperature T C , is the same as that acquired through heating and cooling steps at sequential temperature intervals up to T C .

Magnetic Recording Stability of Taenite‐Containing Meteorites

“…Assessing these effects over the overall recorded magnetic signal in meteoritic plessite, despite their relevance on meteoritic paleomagnetism, is beyond the scope of this work. Our results provides another numerical evidence that non‐SD carriers (vastly observed in both terrestrial and extraterrestrial samples) can record stable magnetization states over billion‐years timescales (Mansbach et al., 2022; Nagy et al., 2017, 2022; Shah et al., 2018; Valdez‐Grijalva et al., 2018), which reinforce the potential of taenite‐containing (stony‐)iron meteorites in providing direct evidences of ancient thermal history of their parent bodies.…”

“…We can expect, therefore, that non‐SD taenite grains are somehow common (or, at least, occur in a significant number) and, hence, are likely to carry a significant proportion of the total magnetic signal recorded by these microstructures. We reinforce, however, that understanding how non‐SD states records magnetizations is a matter of intense research (Nagy et al., 2022). There is still the possibility of taenite and tetrataenite inclusions to coexist in the same microstructure (e.g., see Mansbach et al.…”

“…We can expect, therefore, that non-SD taenite grains are somehow common (or, at least, occur in a significant number) and, hence, are likely to carry a significant proportion of the total magnetic signal recorded by these microstructures. We reinforce, however, that understanding how non-SD states records magnetizations is a matter of intense research (Nagy et al, 2022). There is still the possibility of taenite and tetrataenite inclusions to coexist in the same microstructure (e.g., see Mansbach et al (2022)), which would indicate that multiple records of ancient magentic activity can be recorded in the same sample: one event recorded by non-SD taenite at temperatures >400° to 320°C, and the other one recorded by (SD or MD, Mansbach et al (2022)) at ≲320°C.…”

Using Preisach Theory to Evaluate Chemical Remanent Magnetization and Its Behavior During Thellier‐Thellier‐Coe Paleointensity Experiments

“…The recording mechanisms of these various remamances differs. Whilst much attention has historically been given to TRM acquisition (e.g., Nagy et al., 2022; Néel, 1949), CRM acquisition is less understood (e.g., Haigh, 1958; Shcherbakov et al., 1996).…”

Thermal Resolution of Unblocking Temperatures (TROUT): A Method for “Unmixing” Multi‐Component Magnetizations

“…Louis Néel proposed a theory (Néel, 1949) that these particles can be magnetized in one of several “easy” directions, and some consistent amount of energy is required to rotate their magnetic moment from one “easy” direction to another. While the energy needed is not always constant (see e.g., Nagy et al., 2022), the concept of energy barriers and remagnetization holds true. Partial demagnetization of a specimen is achieved by randomizing the moments of particles with energy barriers that can be overcome by the thermal or AF treatment step.…”