Separating multidomain and single‐domain‐like remanences in pseudo‐single‐domain magnetites (215–540 nm) by low‐temperature demagnetization

It has long been hypothesized that the pseudo‐single‐domain properties of submicron magnetites are a result of coexisting multidomain and single‐domain‐like remanences. We have separated these two types of remanence by low‐temperature demagnetization (LTD), in which samples are cooled through the low‐temperature magnetite isotropic point and rewarmed to room temperature, all in zero field. At the isotropic point, the first crystalline anisotropy constant vanishes, and multidomain remanence due to displaced domain walls will be unpinned. We measured stepwise alternating field (AF) and thermal demagnetization curves of weak‐field thermoremanent magnetization (TRM) and anhysteretic remanent magnetization and of saturation isothermal remanent magnetization (IRMs), both with and without prior LTD, for magnetites with mean particle sizes of 215, 390, and 540 nm. The memory fraction of remanence recovered after LTD, which represents the single‐domain‐like component, decreased from 44% for 215‐nm grains to 23% for 540‐nm grains in the case of IRMs. Memory of TRM was approximately twice as large. AF decay curves were soft and multidomain‐like before LTD but became harder and single‐domain‐like in shape after LTD. LTD removed most low unblocking temperature TRM and IRMs. Distributed low unblocking temperatures are therefore a multidomain phenomenon, probably resulting from successive adjustments of domain walls to changing internal fields and pinning strengths during heating.