MagNetZ with Les Nagy (2021/10/20) - Beyond single grains and single domains
Channel: MagNetZ
Date: 2021-10-21

Beyond single grains and single domains – using big data to answer fundamental questions in rock magnetism

The video is also available for download at: https://earthref.org/ERDA/2486/
and can be referenced with the doi: 10.7288/V4/ERDA/2486

Title: Beyond single grains and single domains – using big data to answer fundamental questions in rock magnetism
Author: Les Nagy from Scripps Institute of Oceanography, UCSD, USA

The magnetic field is a fundamental feature of the Earth, and its structure gives us important information about the geological processes deep within our planet. Since rocks can retain records of the Earth’s field over billions of years, they provide a window into the past and can answer basic questions such as how our planet was formed. The way in which rocks acquire information and the techniques used in the laboratory to recover the ancient field is complicated by the fact that our understanding of the small magnetic particles that hold the ancient field is incomplete. Néel’s single-domain (SD) theory has shaped our understanding of magnetic recording and forms the basis of current experimental methods used to extract palaeomagnetic signals. However, despite its undeniable success, the theory is limited in the sizes and morphologies it can explain and it has long since been recognized that the majority of magnetic minerals form particles that are too large to be uniformly magnetized. These larger grains contain pseudo-single domains (PSD) and show a range of behaviors that are quite unexpected. For example, recent work has shown that so-called single-vortex (SV) domain structures are not only significantly more common than SD, since they occupy a larger range of grain sizes, but are also surprisingly thermally stable. It has also been observed that as grain sizes increase and the SD state evolves into the SV state, the transition is marked by an “unstable zone” – a region characterized by low blocking temperatures and coercivities, but an increase in the number of possible domain states that can be nucleated in the grain.
In this talk we explore how palaeomagnetic samples record information, in particular the effect of SV grains on the acquisition and recovery of ancient planetary fields. What is the current state of the art in our understanding of non-uniform recorders and what is the way towards fully characterizing the mechanisms by which rocks record information?