Editing Iron (section)

===Magnetic properties===
[[File:Magnetization curves.svg|thumb|upright=1.15|left|Magnetization curves of 9 ferromagnetic materials, showing saturation. 1.{{nbsp}}Sheet steel, 2.{{nbsp}}Silicon steel, 3.{{nbsp}}Cast steel, 4.{{nbsp}}Tungsten steel, 5.{{nbsp}}Magnet steel, 6.{{nbsp}}Cast iron, 7.{{nbsp}}Nickel, 8.{{nbsp}}Cobalt, 9.{{nbsp}}Magnetite<ref>{{cite book|first=Charles |last=Steinmetz |year=1917 |title=Theory and Calculation of Electric Circuits |url=https://archive.org/details/in.ernet.dli.2015.162619 |publisher=McGraw-Hill|section=fig. 42}}</ref>]]
Below its [[Curie point]] of {{Convert|770|C|F K|abbr=on}}, α-iron changes from [[paramagnetic]] to [[ferromagnetic]]: the [[Spin (physics)|spins]] of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall [[magnetic field]].<ref name="cullity">{{cite book |last=Cullity |author2=C. D. Graham |title=Introduction to Magnetic Materials, 2nd|publisher=Wiley–IEEE|year=2008 |location=New York |page=116 |url=https://books.google.com/books?id=ixAe4qIGEmwC&pg=PA116 |isbn=978-0-471-47741-9}}</ref> This happens because the orbitals of those two electrons (d<sub>''z''<sup>2</sup></sub> and d<sub>''x''<sup>2</sup> − ''y''<sup>2</sup></sub>) do not point toward neighboring atoms in the lattice, and therefore are not involved in metallic bonding.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}}

In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into [[magnetic domain]]s, about 10 micrometers across,<ref name="Metallo">{{Cite book| chapter-url={{Google books|hoM8VJHTt24C|page=PA24|keywords=|text=|plainurl=yes}}|pages=24–28|title =Metallographer's guide: practice and procedures for irons and steels|first1 = B.L.|last1 = Bramfitt|first2= Arlan O.|last2 = Benscoter|chapter = The Iron Carbon Phase Diagram|publisher = ASM International| date = 2002| isbn = 978-0-87170-748-2}}</ref><!--https://books.google.com/books?id=brpx-LtdCLYC--> such that the atoms in each domain have parallel spins, but some domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.

Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field. This effect is exploited in devices that need to channel magnetic fields to fulfill design function, such as [[electrical transformer]]s, [[magnetic recording]] heads, and [[electric motor]]s. Impurities, [[lattice defect]]s, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists even after the external field is removed – thus turning the iron object into a (permanent) [[magnet]].<ref name="cullity" />

Similar behavior is exhibited by some iron compounds, such as the [[Ferrite (magnet)|ferrites]] including the mineral [[magnetite]], a crystalline form of the mixed iron(II,III) oxide {{chem2|Fe3O4}} (although the atomic-scale mechanism, [[ferrimagnetism]], is somewhat different). Pieces of magnetite with natural permanent magnetization ([[lodestone]]s) provided the earliest [[compass]]es for navigation. Particles of magnetite were extensively used in magnetic recording media such as [[computer memory|core memories]], [[magnetic tape]]s, [[floppy disk|floppies]], and [[hard disk drive|disk]]s, until they were replaced by [[cobalt]]-based materials.