Magnetic properties of uniform spherical magnetite particles prepared from ferrous hydroxide gels

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.

Section: Sample Preparation and Characterizationmentioning

confidence: 62%

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

Dunlop¹,

Argyle²

1991

“…Comparison of calculated M rs / M s for randomly oriented assemblages (solid circles) and experimental data on sized synthetic magnetites (open symbols) [ Levi and Merrill , 1978; Dunlop , 1986; Amin et al , 1987; Argyle and Dunlop , 1990]. The expected value of M rs / M s (0.866) for a single‐domain magnetite governed by magnetocrystalline anisotropy is shown by a horizontal line.…”

Section: Resultsmentioning

confidence: 99%

Three‐dimensional micromagnetic modeling of randomly oriented magnetite grains (0.03–0.3 μm)

Fukuma

Dunlop

2006

[1] We report three-dimensional micromagnetic structures obtained for magnetite cubes with h100i edges in the size range 0.03-0.3 mm around the single-domain threshold size. We employed the Metropolis algorithm to find equilibrium micromagnetic structures starting from uniformly magnetized initial states. Directions of initial magnetizations were randomly set so as to mimic a randomly oriented assemblage. Small grains (<0.07 mm) exhibit single-domain equilibrium structures parallel to a magnetic easy axis h111i irrespective of initial magnetization directions. Vortex structures emerge for small pseudosingle-domain grains above 0.07 mm. Axes of the vortices are parallel to h100i in the threshold size range but become randomly oriented with increasing grain size. Neither lamellar domain structures nor multiple vortex structures were found for grains smaller than 0.3 mm. Reduced saturation remanent magnetization decreased down to $0.1 with increasing grain size over a relatively narrow grain size range (0.07-0.15 mm). The grain size variation of reduced magnetization depends on microscopic magnetization structures inside a single grain and the degree of randomness of grain magnetization directions in an assemblage.

“…Reduced saturation remanences Mrs/M s measured for our magnetites and in previous studies [Levi and Merrill, 1978; Ozdemir and Banerjee, 1982;Dunlop, 1986a;Amin et al, 1987] are compared in Figure 15 with grain-size dependences predicted by various domain structure models. Lamellar two-domain (2D) structures in cubes [Argyle and Dunlop, 1984] and spheres [Veitch 1984] owe their remanences to the moment of the domain wall.…”

Section: Size Dependence Of Saturation Remanencementioning

confidence: 99%

Low‐temperature and high‐temperature hysteresis of small multidomain magnetites (215–540 nm)

Argyle¹,

Dunlop²

1990

Approximately equidimensional magnetite crystals, with mean sizes of 215, 390, and 540 nm, respectively, have been produced by reducing hematite crystals. Isothermal magnetic hysteresis properties show a clear progression toward multidomain‐like behavior as the mean grain size increases. Saturation remanences Mrs are only 5–10% of saturation magnetization Ms, coercive forces HC are low (5.5–8 mT), and both Mrs and HC have grain‐size dependences compatible with those previously established for smaller and larger hydrothermally produced magnetites. Coercivities during remanence acquisition are greater than those measured during demagnetization. The difference between acquisition and destructive fields increases in the larger grains as a result of the increasing importance of the internal demagnetizing field. The low‐temperature transition is well expressed in the Mrs and HC data of the 540‐nm sample but is more subdued for smaller grains. Magnetostrictive, magnetocrystalline, and magnetostatic mechanisms in turn govern coercivity as the temperature rises. Remanence and coercivity ratios, Mrs/Ms and HR/HC, are almost temperature independent up to 500°C, indicating that domain wall configurations resulting from saturating fields are about the same at any temperature. A thermal fluctuation analysis of high‐temperature coercive force data suggests that regions 200–250 nm in size are thermally activated as a unit in grains of all sizes; these are likely domain walls. Apparent demagnetizing factors calculated from both low‐ and high‐temperature data are consistent with a mixture of two‐domain (2D) and three‐domain (3D) grains in all samples. However, theories of remanence in conventional 2D and 3D grains or in mixtures of 2D, 3D, and metastable single‐domain grains do not explain the data in a satisfying way.