Magnetic domain patterns observed on synthetic Ti‐rich titanomagnetite as a function of temperature and in states of thermoremanent magnetization

Abstract

The fundamental temperature dependence of magnetic domain structure has been recognized for several decades, but its relationship to thermoremanent magnetization (TRM) has been unexplored. As one step toward investigating this possible link, we have studied Bitter patterns on synthetic Ti‐rich titanomagnetite (Al0.1Mg0.1Ti0.6Fe2.2O4; Curie point of approximately 75°C) as a function of temperature in a weak field (0.42 Oe), in states of weak field TRM, and after alternating field demagnetization. Two different types of patterns were studied with the following goals: (1) simple Kittel‐like patterns of straight walls were studied in order to gain insight into the nature of TRM domain structures and the various possible mechanisms by which particles acquire weak field TRM; (2) mazelike patterns, which occupy regions of high surface stress, were studied in order to estimate the temperature dependence of magnetostriction constant in this material. Results of experiments to study Kittel‐like patterns suggest that two different mechanisms, and two resulting species of domain structure, could account for fundamental differences between multidomain and pseudosingle‐domain thermoremanence in titanomagnetite. In accordance with traditional models, typical multidomain TRM results from wall pinning in a particle that contains several domains whose volumes are approximately equal to each other. In contrast, pseudosingle‐domain TRM is acquired when denucleation of domains and domain walls yields a domain structure with a large associated “intrinsic” moment. The following observations support these conclusions. In a particle with a Kittel‐like pattern, the number of domains varies from one TRM experiment to the next. This variation indicates that a grain can acquire TRM in a range of local energy minimum states, rather than in one particular domain configuration of absolute minimum energy. Many such TRM patterns represent the near‐ideal local energy minimum domain states, consisting of several domains of approximately equal widths, usually envisioned in models of multidomain thermoremanence. Observations suggest that a particle can maintain such states during cooling if weakly pinned walls adjust readily to demagnetizing fields generated during the denucleation process. In this case, TRM results from small displacements of pinned walls with respect to a state of minimum magnetostatic energy; several observations suggest that blocking of walls at pinning centers in the classical manner occurs mainly after the final nucleation event as the particle cools toward room temperature from the Curie point. In contrast, we have observed examples in which denucleation leaves behind an anomalously large domain where several approximately uniform domains existed prior to denucleation; several smaller domains survive elsewhere in the particle, virtually unaffected by the transition. Evidently, strong wall‐pinning forces sometimes prevent surviving walls from responding to the change of internal field which must accompany the denucleation process. It is observed that denucleation can yield single‐domain‐like states during TRM acquisition. Thus denucleation, rather than complete failure to nucleate during TRM acquisition, is one mechanism for producing single‐domain‐like states in particles which ordinarily favor walls. Possibly, such states exist by virtue of strong pinning forces at surface defects, into which denucleating domains and domain walls collapse. Particles with an odd number of approximately uniform domains also are observed in TRM states, and such particles also may carry large intrinsic moments owing to domain imbalance; possibly, such domain configurations result from the final denucleation episode during cooling. Thus these observations indicate a strong link between TRM acquisition mechanisms, magnetic domain transitions, and the accessibility of domain states. We hypothesize that denucleation could account for the most desirable properties of TRM associated with pseudosingle‐domain titanomagnetite particles, namely, high intensity and high thermal stability. The second class of patterns, mazelike patterns, was studied to estimate the temperature dependence of domain wall energy and magnetostriction constant from the observed temperature dependence of domain width. The temperature dependence of magnetostriction constant obtained in this manner for AMTM60 is similar to that obtained for magnetite in earlier studies.