The dependence of magnetic domain structure upon magnetization state with emphasis upon nucleation as a mechanism for pseudo‐single‐domain behavior

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Abstract
The stable carriers of the paleomagnetic record in rocks are commonly fine pseudo‐single‐domain (PSD) particles between about 1 and 20 μm in diameter. However, the underlying mechanism which determines PSD behavior has previously remained in debate. To study this problem, magnetic domain patterns have been observed with the Bitter method on particles of natural pyrrhotite and intermediate titanomagnetite in various magnetization states, including natural remanent magnetization (NRM), after alternating field demagnetization of the NRM and during high‐field hysteresis. From these observations it is evident that two primary mechanisms govern high‐field hysteresis behavior: (1) ‘bulk’ pinning of fully developed domain walls by intervening energy barriers, and (2) nucleation, the creation and expansion of walls into the particle volume to produce a fully developed domain structure. These observations demonstrate that nucleation can explain several important aspects of PSD behavior. Typically, multidomain particles are entirely governed by bulk wall pinning, since they readily develop walls whose dimensions are comparable to the particle diameter prior to or upon removal of a strong field. For these grains the average domain wall spacing is relatively insensitive to the particular magnetization state, being approximately proportional to (r being the grain diameter), in accordance with equilibrium calculations. In contrast, particles that are at least partially controlled by nucleation fail to develop a discernable domain wall structure at saturation remanence. However, these same grains can easily accommodate walls in other magnetization states. It is found that nucleation can involve two physically distinct but sometimes experimentally inseparable processes: (1) initial creation of walls at surface imperfections, and (2) unpinning of minute wall fragments from strong potential energy ‘traps’ near the grain surface. When either process fails, a grain remains locked in a metastable, single‐domain‐like state at saturation remanence and requires application of a reverse field Hn, called the nucleation field, before walls appear. In pyrrhotite, the probability for nucleation failure rises with decreasing grain size and has been determined through observation to be given by f(w = 0) = 1.2 exp (−0.46 )r being in micrometers). Such single‐domain‐like particles can thus contribute substantially to the saturation remanence in the PSD range. The nucleation field determined experimentally for pyrrhotite is given by Hn ≃ 1200/ Oe. In many fine pyrrhotites, Hn is sufficiently strong to reverse completely the magnetization through a single Barkhausen jump of the nucleated wall. These observations thereby demonstrate that nucleation becomes increasingly more dominant as the particles become smaller, a manifestation of the random distribution of active nucleation sites. Nucleation may therefore account for much of the magnitude and grain size dependence of hysteresis parameters in the PSD range as well as resulting in a gradual transition between multidomain and PSD behavior. Fine particles completely controlled by nucleation during hysteresis behave in a strikingly parallel manner to classical single domains and are therefore quite appropriately described as being pseudo‐single‐domain.