Transdomain thermoremanent magnetization

Abstract

Transdomain thermoremanent magnetization (TRM) is produced when thermally activated transitions between different domain structures become blocked during cooling. This paper investigates transdomain remanence in the pseudo‐single‐domain size range (<1 μm) in magnetite. We use a one‐dimensional micromagnetic model of single‐domain (SD), two‐domain (2D) and three‐domain (3D) structures to determine the energies of these states, the energy barriers between them, blocking temperatures for SD ↔ 2D and 2D ↔ 3D transitions, and relaxation times for the resulting TRMs. Energy barriers are very high at 20°C, from 2000kT to >6000kT for SD ↔ 2D transitions. Transdomain viscous remanent magnetization (VRM) will not occur, even on geological timescales, by a one‐dimensional excitation such as edge nucleation of a domain wall. Transdomain blocking temperatures, at which energy barriers fall to 25kT–60kT, are ≥553°C for SD ↔ 2D and ≥574°C for 2D ↔ 3D transitions. There are two separate blocking temperatures, e.g. TB1 for SD ↔ 2D and TB2 for 2D ↔ SD transitions. Usually, only the higher of the two has practical significance because the favored (lower energy) state is already 100% populated at this temperature. Our theory is the first to make quantitative predictions of transition paths, relaxation times, and blocking temperatures for transdomain TRM. It is also quite robust. Relaxing the one‐dimensional constraint and introducing crystal defects would make it easier to nucleate domains, but energy barriers and blocking temperatures would not be reduced greatly. Our principal conclusion, that only the lowest energy state at blocking is significantly populated, is a fundamental consequence of the Boltzmann statistics of equilibrium states and is unaffected by the details of transitions between states. Grain interactions may be responsible for the multiplicity of states observed in large titanomagnetite grains following replicate TRM experiments.