Single‐domain critical sizes for coercivity and remanence

Abstract

, so in this size range the SD state is less stable. To calculate the critical sizes, we use rigorous nucleation theory and obtain analytical expressions. The analytical form allows us to explore the effect of grain shape, stress, crystallographic orientation and titanium content in titanomagnetites. We adapt the theory to cubic anisotropy with K1 < 0, which allows us to apply the expressions to titanomagnetites. We find that the size range for SD coercivity is always small. The size range for SD remanence can vary enormously depending on the anisotropy. If the easy axes are oriented favorably, the SD state can occur in large x = 0.6 titanomagnetite grains. Ensembles of magnetite grains with aspect ratios greater than 5 have SD-like remanence but low coercivity. However, most synthetic magnetite grains are nearly equant, and the predicted size range for SD remanence is small to nonexistent. This, rather than grain interactions, may be the reason they have properties such as saturation remanence that do not agree well with standard SD theory.

Growth, structure, morphology, and magnetic properties of Ni ferrite films

“…When grain size is smaller than the single-domain critical size, M reversal mechanism can be described as coherent rotation. Due to this mechanism, H c increases with increasing grain size [16]. When the grain size is much bigger than single-domain critical size, M reversal mechanism turns into a domain wall motion; therefore, H c decreases as grain size increases [12].…”

Characterizing the superparamagnetic grain distribution f(V, Hk) by thermal fluctuation tomography

“…For MD grains there is no reason to expect the behavior to obey equations (4) - (8), and although we have not yet run the experiment on such a sample, we expect that the proportional misfit would be much higher than 10%. Single-domain grains larger than a threshold size near 50-70 nm are likely to reverse incoherently [Newell and Merrill, 1999], and may Figure 16. Low-T behavior of a composite sample containing a mixture of two stratigraphic levels in the Tiva Canyon Tuff: Remanent magnetization as a function of (a) applied DC backfield and (b) derivative curves, for temperatures from 10 K to 300 K (DT = 10), measured in steps of 5 mT.…”

“…[59] There are two major limiting factors: (1) the restricted range of ''viewing angles'' available for input to the tomographic algorithm, a limitation imposed by the physics of magnetic blocking; and (2) the string of critical assumptions involved, including: uniform magnetization of the grains; coherent reversal [Newell and Merrill, 1999]; angular dependence of switching field H SW (8) according to Stoner-Wohlfarth theory [Stoner and Wohlfarth, 1948;see also Stephenson and Shao, 1994;Dunlop and Ö zdemir, 1997;Madsen, 2002]; dominant shape anisotropy, so that H K (T) is proportional to M S (T); lack of magnetostatic interactions; and the grain size and temperature dependence of the thermal fluctuation field as developed from Néel theory by Egli and Lowrie [2002].…”

Anhysteretic remanent magnetization of fine magnetic particles

“…In general, a ≫ 1 when Δ is of the same order of magnitude as Δ 0 : a ≈ 14 with μ 0 H K = 60 mT and m = 5 × 10 −17 A m 2 . For SD magnetite grains that switch by coherent rotation, μ 0 H K < 300 mT and m < 10 −16 A m 2 [ Newell and Merrill , 1999], and consequently, using equation (25), a > 8. For reasons explained later, the estimation of f ± is important only for values of Δ around Δ 0 , and a ≫ 1 can be assumed in equation (17).…”