Micromagnetic constraints on Pullaiah curves for magnetite

Fifty years ago, Pullaiah et. al (1975) derived temperature-dependent relaxation curves for single-domain (SD) magnetite, which have since been widely applied to a range of paleomagnetic problems. However, ideal SD behaviour is restricted to a narrow particle-size range. Most of the stable geological remanence carried by magnetite is instead held by vortex-state particles, for which SD theory fails to provide an adequate description. This study presents new numerical results for micromagnetically determined, temperature-dependent relaxation curves for submicron oblate, prolate and equidimensional cuboctahedral magnetite particles from 45-200 nm, with varying elongations. MERRILL was used to compute local energy-minimum (LEM) states over the full temperature range from 20 to 579 °C, and the nudged elastic band (NEB) method was employed to obtain energy barriers for use in Néel–Arrhenius estimates of relaxation times. The resulting relaxation curves are analysed and compared with the classical Pullaiah curves, highlighting the implications for interpreting paleomagnetic records carried by vortex-state particles.