The 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.

The study of the paleomagnetic signal recorded by rocks allows scientists to understand Earth’s past magnetic field and the formation of the geodynamo. The magnetic recording fidelity of this signal is dependent on the magnetic domain state it adopts. The most prevalent example found in nature is the pseudo–single-domain (PSD) structure, yet its recording fidelity is poorly understood. Here, the thermoremanent behavior of PSD magnetite (Fe₃O₄) particles, which dominate the magnetic signatures of many rock lithologies, is investigated using electron holography. This study provides spatially resolved magnetic information from individual Fe₃O₄ grains as a function of temperature, which has been previously inaccessible. A small exemplar Fe₃O₄ grain (~150 nm) exhibits dynamic movement of its magnetic vortex structure above 400°C, recovering its original state upon cooling, whereas a larger exemplar Fe₃O₄ grain (~250 nm) is shown to retain its vortex state on heating to 550°C, close to the Curie temperature of 580°C. Hence, we demonstrate that Fe₃O₄ grains containing vortex structures are indeed reliable recorders of paleodirectional and paleointensity information, and the presence of PSD magnetic signals does not preclude the successful recovery of paleomagnetic signals.

Equi-atomic FeRh is a very interesting material as it undergoes a magnetostructural transition from an antiferromagnetic (AF) to a ferromagnetic (FM) phase between 75–105 °C. Its ability to present phase co-existence separated by domain walls (DWs) above room temperature provides immense potential for exploitation of their DW motion in spintronic devices. To be able to effectively control the DWs associated with AF/FM coexistence in FeRh thin films we must fully understand the magnetostructural transition and thermomagnetic behaviour of DWs at a localised scale. Here we present a transmission electron microscopy investigation of the transition in planar FeRh thin-film samples by combining differential phase contrast (DPC) magnetic imaging with in situ heating. We perform quantitative measurements from individual DWs as a function of temperature, showing that FeRh on NiAl exhibits thermomagnetic behaviour consistent with the transition from AF to FM. DPC imaging of an FeRh sample with HF-etched substrate reveals a state of AF/FM co-existence and shows the transition from AF to FM regions proceeds via nucleation of small vortex structures, which then grow by combining with newly nucleated vortex states into larger complex magnetic domains, until it is in a fully-FM state.

The thermal demagnetization of pseudo‐single‐domain (PSD) magnetite (Fe₃O₄) particles, which govern the magnetic signal in many igneous rocks, is examined using off‐axis electron holography. Visualization of a vortex structure held by an individual Fe₃O₄ particle (~250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic‐induction maps. Stepwise demagnetization of the remanence‐induced Fe₃O₄ particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed that its vortex state remains stable under heating close to its unblocking temperature and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe₃O₄ particle exhibits thermomagnetic behavior comparable to a single‐domain carrier, and thus, vortex states are considered reliable magnetic recorders for paleomagnetic investigations.

The mapping of electrostatic potentials and magnetic fields in liquids using electron holography has been considered to be unrealistic. Here, we show that hydrated cells ofMagnetospirillum magneticumstrain AMB-1 and assemblies of magnetic nanoparticles can be studied using off-axis electron holography in a fluid cell specimen holder within the transmission electron microscope. Considering that the holographic object and reference wave both pass through liquid, the recorded electron holograms show sufficient interference fringe contrast to permit reconstruction of the phase shift of the electron wave and mapping of the magnetic induction from bacterial magnetite nanocrystals. We assess the challenges of performingin situmagnetization reversal experiments using a fluid cell specimen holder, discuss approaches for improving spatial resolution and specimen stability, and outline future perspectives for studying scientific phenomena, ranging from interparticle interactions in liquids and electrical double layers at solid–liquid interfaces to biomineralization and the mapping of electrostatic potentials associated with protein aggregation and folding.