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

Abstract

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.

Rock magnetic response to climatic changes in west Philippine Sea for the last 780 ka: Discussion based on relative paleointensity assisted chronology

“…So ARM/k and ARM/SIRM can be valid indicators for magnetic size (Argyle and Dunlop, 1990;Stoner et al, 2003). k ARM (anhysteresis susceptibility, it is the ARM divided by biasing direct field) and k of two cores are projected to the model of Stoner et al (2003) and we obtained a size estimated to be smaller than 2 mm (Fig.…”

Resolving the Origin of Pseudo‐Single Domain Magnetic Behavior

“…Illustration of the gradual rather than sharp change in magnetic properties for magnetite with increasing grain size, which has been attributed to “pseudo‐single domain” behavior. (a) Plot of M rs /M s versus grain size (after Dunlop (), who synthesized data from Argyle & Dunlop (); Day et al (), Heider et al (), Özdemir & Dunlop (), and Worm & Markert ()) and (b) B c versus grain size (after Dunlop & Özdemir (), who synthesized data from Amin et al (), Argyle & Dunlop (), Day et al (), Dunlop (), Heider et al (), Levi & Merrill (), Özdemir & Banerjee (), Özdemir & Dunlop (), Schmidbauer & Schembera (), and Worm & Markert ()). The “harder” magnetic properties in crushed magnetite are attributed to the higher state of internal stress compared to the hydrothermally grown or glass ceramic crystals.…”

Nanofabrication of two‐dimensional arrays of magnetite particles for fundamental rock magnetic studies

“…This improves resist adhesion and makes smaller particle sizes achievable. Day et al, 1977] for nanofabricated samples from this study (solid triangles) compared with published data for previously nanofabricated magnetites , for magnetites precipitated from aqueous solution [Dunlop, 1986], for magnetites prepared by reducing hematite [Argyle and Dunlop, 1990], and for magnetites synthesized chemically by decomposition of oxalates followed by reduction [Schmidbauer and Schembera, 1987;Schmidbauer and Keller, 1996]. Grain sizes are shown in nm next to each data point.…”