Abstract To characterize the magnetic stability of paleomagnetically important single vortex (SV) particles, we systematically studied three synthetic magnetite samples with mean sizes of 79, 299, and 685 nm all within the SV size range. We examined the rock magnetic properties of these samples during low‐temperature oxidation both from experimental observations and numerical micromagnetic modeling. It is found that: (a) Low‐temperature oxidation shifts the particle size range of magnetically unstable (MU) zones, characterized by suppressed coercivity Bc and remanence ratio Mrs/Ms. (b) A distinct MU zone emerges near the SV/multi‐vortex boundary in larger particles (685 nm), where domain states transition to hard‐axis‐aligned vortices, reducing stability. (c) Multiple MU zones are likely to be common in most rock samples, contributing to their non‐ideal paleomagnetic behavior. Our work provides a framework for identifying MU zones and refining SV particle applications in paleomagnetic studies.

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