Hysteresis and coercivity of hematite

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.