Abstract
AbstractThe term “pseudo‐single domain” (PSD) has been used to describe the transitional state in rock magnetism that spans the particle size range between the single domain (SD) and multidomain (MD) states. The particle size range for the stable SD state in the most commonly occurring terrestrial magnetic mineral, magnetite, is so narrow (~20–75 nm) that it is widely considered that much of the paleomagnetic record of interest is carried by PSD rather than stable SD particles. The PSD concept has, thus, become the dominant explanation for the magnetization associated with a major fraction of particles that record paleomagnetic signals throughout geological time. In this paper, we argue that in contrast to the SD and MD states, the term PSD does not describe the relevant physical processes, which have been documented extensively using three‐dimensional micromagnetic modeling and by parallel research in material science and solid‐state physics. We also argue that features attributed to PSD behavior can be explained by nucleation of a single magnetic vortex immediately above the maximum stable SD transition size. With increasing particle size, multiple vortices, antivortices, and domain walls can nucleate, which produce variable cancellation of magnetic moments and a gradual transition into the MD state. Thus, while the term PSD describes a well‐known transitional state, it fails to describe adequately the physics of the relevant processes. We recommend that use of this term should be discontinued in favor of “vortex state,” which spans a range of behaviors associated with magnetic vortices.
Highlights
More than 55 years have passed since Stacey [1961, 1962] coined the term ‘pseudo-single domain’ (PSD) to describe a transitional magnetic state between the stable single domain (SD) and multi-domain (MD) states
The observation that the vertical separation between the upper and lower peaks of the first-order reversal curve (FORC) distribution depends on the magnitude of vortex nucleation/annihilation fields raises questions about whether a broader distribution of single vortex nucleation/annihilation fields associated with broader particle size distributions could give rise to the FORC diagrams observed for geological samples with ‘PSD’ behavior (e.g., Figure 3f)
We present evidence from FORC diagrams, micromagnetic simulations, and electron holography to make the case that the ‘PSD’ state is better described by the vortex state
Summary
More than 55 years have passed since Stacey [1961, 1962] coined the term ‘pseudo-single domain’ (PSD) to describe a transitional magnetic state between the stable single domain (SD) and multi-domain (MD) states. The observation that the vertical separation between the upper and lower peaks of the FORC distribution depends on the magnitude of vortex nucleation/annihilation fields raises questions about whether a broader distribution of single vortex nucleation/annihilation fields associated with broader particle size distributions could give rise to the FORC diagrams observed for geological samples with ‘PSD’ behavior (e.g., Figure 3f) The main peaks in the FORC diagrams indicate that both the coercivity and strength of vortex nucleation/annihilation fields vary with applied field angle with respect to the easy axis of magnetization These micromagnetic results, provide a direct link between the physics of vortex nucleation/annihilation and expected FORC responses [Pike and Fernandez, 1999]. While many new things will be learned with application of such new techniques, the observations of Donnelly et al [2017] strengthen our case for referring to the ‘PSD’ state as the vortex state because the complex magnetization structures that they documented are associated with magnetic vortices and their interactions within a magnetic material
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