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Hysteresis and coercivity of hematite
Author(s) -
Özdemir Özden,
Dunlop David J.
Publication year - 2014
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2013jb010739
Subject(s) - coercivity , remanence , magnetocrystalline anisotropy , materials science , hematite , condensed matter physics , saturation (graph theory) , magnetic anisotropy , hysteresis , nucleation , magnetization , anisotropy , nuclear magnetic resonance , chemistry , metallurgy , magnetic field , physics , optics , mathematics , organic chemistry , quantum mechanics , combinatorics
In room‐temperature hysteresis, 14 submicron hematites (0.12‐0.45 µm) had large coercive forces H c (150‐350 mT), while 22 natural 1‐5.5 mm hematite crystals had H c = 0.8‐23 mT (basal‐plane measurements). Single‐domain (SD) and multidomain (MD) hematites owe their high H c mainly to magnetoelastic anisotropy, caused in fine particles by internal strains and in large crystals by defects like dislocations, with a smaller contribution by triaxial magnetocrystalline anisotropy. A strong correlation between H c and the defect moment M d measured below hematite's Morin transition also favors magnetoelastic control. Saturation remanence/saturation magnetization ratios M rs / M s and coercivity ratios H cr / H c ( H cr is remanent coercive force) are distinctive: M rs / M s = 0.5‐0.9, H cr / H c = 1.02‐1.17 for MD hematites; M rs / M s = 0.5‐0.7, H cr / H c = 1.45‐1.62 for SD hematites. In high‐temperature (20‐690°C) hysteresis, H c ( T ) ~ M s ( T ) to a power 1.8‐2.4 above 385°C. Magnetoelastic wall pinning by crystal defects is thus more likely than control by domain nucleation which depends on magnetocrystalline anisotropy. Our results compare well with existing H c vs. crystal size d data. A suggested peak in H c around 15 µm and a proposed slope change around 100 µm are both questionable. Using only near‐saturation data, H c varies continuously as d −0.61 from ≈0.1 µm to 2 mm. The SD threshold size d 0 may be >15 µm but there is no strong evidence that d 0 ≈100 µm. Direct domain observations are needed to settle the question. Augmented data sets for H c and M rs vs. d show that SD hematite is increasingly affected by thermal fluctuations below ≈0.3 µm and generally confirm a superparamagnetic threshold size d s of 0.025‐0.03 µm.