Editing Hematite (section)

==Magnetism==
Hematite shows only a very feeble response to a [[magnetic field]]. Unlike magnetite, it is not noticeably attracted to an ordinary magnet. Hematite is an [[antiferromagnetic]] material below the [[Morin transition]] at {{cvt|250|K|C}}, and a [[spin canting|canted]] antiferromagnet or weakly [[ferromagnetic]] above the Morin transition and below its [[Néel temperature]] at {{cvt|948|K|C}}, above which it is [[paramagnetic]].

The magnetic structure of α-hematite was the subject of considerable discussion and debate during the 1950s, as it appeared to be ferromagnetic with a Curie temperature of approximately {{cvt|1000|K|C}}, but with an extremely small [[magnetic moment]] (0.002&nbsp;[[Bohr magneton]]s). Adding to the surprise was a transition with a decrease in temperature at around {{cvt|260|K|C}} to a phase with no net magnetic moment. It was shown that the system is essentially antiferromagnetic, but that the low symmetry of the [[Ion#Anions and cations|cation]] sites allows [[spin–orbit coupling]] to cause [[spin canting|canting of the moments]] when they are in the plane perpendicular to the ''c'' axis. The disappearance of the moment with a decrease in temperature at {{cvt|260|K|C}} is caused by a change in the [[anisotropy]] which causes the moments to align along the ''c'' axis. In this configuration, spin canting does not reduce the energy.<ref>{{cite journal |last=Dzyaloshinsky |first=I. E. | title=A thermodynamic theory of "weak" ferromagnetism of antiferromagnetics |journal=Journal of Physics and Chemistry of Solids |volume=4 |pages=241–255 |year=1958 |doi=10.1016/0022-3697(58)90076-3 |issue=4 |bibcode=1958JPCS....4..241D}}</ref><ref>{{cite journal |last=Moriya |first=Tōru |title=Anisotropic Superexchange Interaction and Weak Ferromagnetism |journal= Physical Review|volume=120 |page=91 |issue=1 |year=1960 |doi=10.1103/PhysRev.120.91 |bibcode=1960PhRv..120...91M|url=https://hal-cea.archives-ouvertes.fr/cea-01550620/file/PhysRevB.93.214414.pdf }}</ref> The magnetic properties of bulk hematite differ from their nanoscale counterparts. For example, the Morin transition temperature of hematite decreases with a decrease in the particle size. The suppression of this transition has been observed in hematite [[nanoparticles]] and is attributed to the presence of impurities, water molecules and defects in the crystals lattice. Hematite is part of a complex solid solution oxyhydroxide system having various contents of H2O (water), hydroxyl groups and vacancy substitutions that affect the mineral's magnetic and crystal chemical properties.<ref>{{cite journal |last1=Dang |first1=M.-Z. |last2=Rancourt |first2=D. G. |last3=Dutrizac |first3=J. E. |last4=Lamarche |first4=G. |last5=Provencher |first5=R. |title=Interplay of surface conditions, particle size, stoichiometry, cell parameters, and magnetism in synthetic hematite-like materials |journal=Hyperfine Interactions |volume=117 |issue=1–4 |year=1998 |pages=271–319 |doi=10.1023/A:1012655729417 |bibcode=1998HyInt.117..271D|s2cid=94031594 }}</ref> Two other end-members are referred to as protohematite and hydrohematite.

Enhanced [[Magnetic coercivity|magnetic coercivities]] for hematite have been achieved by dry-heating a two-line ferrihydrite precursor prepared from solution. Hematite exhibited temperature-dependent magnetic coercivity values ranging from {{convert|289|to(-)|5,027|Oe|0|lk=in}}. The origin of these high coercivity values has been interpreted as a consequence of the subparticle structure induced by the different particle and [[crystallite]] size growth rates at increasing annealing temperature. These differences in the growth rates are translated into a progressive development of a subparticle structure at the nanoscale (super small). At lower temperatures (350–600&nbsp;°C), single particles crystallize. However, at higher temperatures (600–1000&nbsp;°C), the growth of crystalline aggregates, and a subparticle structure is favored.<ref>{{cite journal|last1=Vallina|first1=B.|last2=Rodriguez-Blanco|first2=J. D.|last3=Brown|first3=A. P.|last4=Benning|first4=L. G.|last5=Blanco|first5=J. A.|year=2014|title=Enhanced magnetic coercivity of α-Fe<sub>2</sub>O<sub>3</sub> obtained from carbonated 2-line ferrihydrite|journal=Journal of Nanoparticle Research|volume=16|issue=3|pages=2322|bibcode=2014JNR....16.2322V|doi=10.1007/s11051-014-2322-5|s2cid=137598876|url=http://eprints.whiterose.ac.uk/78765/19/Vallina%20et%20al%20JNR%20-%20final%20version%202014_with_coversheet.pdf}}</ref>

<gallery widths="200px" heights="200px" class="center">
File:Hematite - Titanomagnitite.jpg|A microscopic picture of hematite
File:Hematite structure.jpg|Crystal structure of hematite
</gallery>