Colossal magnetoresistance

Initially discovered in mixed-valence perovskite manganites in the 1950s by G. H. Jonker and J. H. van Santen,{{Cite journal | doi = 10.1016/0031-8914(50)90033-4| title = Ferromagnetic compounds of manganese with perovskite structure| journal = Physica| volume = 16| issue = 3| pages = 337| year = 1950| last1 = Jonker | first1 = G. H. | last2 = Van Santen | first2 = J. H. |bibcode = 1950Phy....16..337J }}{{Cite journal |last1=Van Santen |first1=J. H. |last2=Jonker |first2=G. H. |date=1950 |title=Electrical conductivity of ferromagnetic compounds of manganese with perovskite structure |url= |journal=Physica |volume=16 |issue=7 |pages=599–600 |doi=10.1016/0031-8914(50)90104-2|bibcode=1950Phy....16..599V }} a first theoretical description in terms of the double-exchange mechanism was given early on. In this model, the spin orientation of adjacent Mn moments is associated with kinetic exchange of e_g-electrons. Consequently, alignment of the Mn spins by an external magnetic field causes higher conductivity. Relevant experimental work was done by Volger,{{Cite journal | doi = 10.1016/S0031-8914(54)80015-2| title = Further experimental investigations on some ferromagnetic oxidic compounds of manganese with perovskite structure| journal = Physica| volume = 20| issue = 1| pages = 49–66| year = 1954| last1 = Volger | first1 = J.|bibcode = 1954Phy....20...49V }} Wollan and Koehler,{{Cite journal | doi = 10.1103/PhysRev.100.545| title = Neutron Diffraction Study of the Magnetic Properties of the Series of Perovskite-Type Compounds [(1-x)La, x Ca]MnO_{3}| journal = Physical Review| volume = 100| issue = 2| pages = 545| year = 1955| last1 = Wollan | first1 = E. O.| last2 = Koehler | first2 = W. C.|bibcode = 1955PhRv..100..545W }} and later on by Jirak et al.{{Cite journal | doi = 10.1016/0304-8853(85)90144-1| title = Neutron diffraction study of Pr1 − xCaxMnO3 perovskites| journal = Journal of Magnetism and Magnetic Materials| volume = 53| issue = 1–2| pages = 153| year = 1985| last1 = Jirák | first1 = Z.| last2 = Krupička | first2 = S.| last3 = Šimša | first3 = Z.| last4 = Dlouhá | first4 = M.| last5 = Vratislav | first5 = S.|bibcode = 1985JMMM...53..153J }} and Pollert et al.{{Cite journal | doi = 10.1016/0022-3697(82)90142-1| title = Structural study of Pr1−xCaxMnO3 and Y1−xCaxMnO3 perovskites| journal = Journal of Physics and Chemistry of Solids| volume = 43| issue = 12| pages = 1137| year = 1982| last1 = Pollert | first1 = E.| last2 = Krupička | first2 = S.| last3 = Kuzmičová | first3 = E.|bibcode = 1982JPCS...43.1137P }}

However, the double exchange model did not adequately explain the high insulating-like resistivity above the transition temperature.{{cite book |first1=J.N. |last1=Lalena |first2=D.A. |last2=Cleary |title=Principles of Inorganic Materials Design |edition=2nd |publisher=Wiley |page=361 |year=2010 |isbn=9780470567531}} In the 1990s, work by R. von Helmolt et al.{{Cite journal | doi = 10.1103/PhysRevLett.71.2331| pmid = 10054646| title = Giant negative magnetoresistance in perovskitelike La2/3Ba1/3Mn Ox ferromagnetic films| journal = Physical Review Letters| volume = 71| issue = 14| pages = 2331–2333| year = 1993| last1 = von Helmolt | first1 = R.| last2 = Wecker | first2 = J.| last3 = Holzapfel | first3 = B.| last4 = Schultz | first4 = L.| last5 = Samwer | first5 = K.|bibcode = 1993PhRvL..71.2331V }} and Jin et al.{{Cite journal | doi = 10.1126/science.264.5157.413| title = Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films| journal = Science| volume = 264| issue = 5157| pages = 413–5| year = 1994| last1 = Jin | first1 = S.| last2 = Tiefel | first2 = T. H.| last3 = McCormack | first3 = M.| last4 = Fastnacht | first4 = R. A.| last5 = Ramesh | first5 = R.| last6 = Chen | first6 = L. H.|bibcode = 1994Sci...264..413J | pmid=17836905| s2cid = 39802144}} initiated a large number of further studies. Although there is still no complete understanding of the phenomenon, there is a variety of theoretical and experimental work providing a deeper understanding of the relevant effects.

One prominent model is the so-called half-metallic ferromagnetic model, which is based on spin-polarized (SP) band structure calculations using the local spin-density approximation (LSDA) of the density functional theory (DFT) where separate calculations are carried out for spin-up and spin-down electrons. The half-metallic state is concurrent with the existence of a metallic majority spin band and a nonmetallic minority spin band in the ferromagnetic phase.

This model is not the same as the Stoner Model of itinerant ferromagnetism. In the Stoner model, a high density of states at the Fermi level makes the nonmagnetic state unstable. In SP calculations of covalent ferromagnets using DFT-LSDA functionals, the exchange-correlation integral takes the place of the Stoner parameter. The density of states at the Fermi level does not play a special role.{{cite book |first=R. |last=Zeller |title=Computational Nanoscience: Do It Yourself |publisher=John von Neumann Institute for Computing |location=Jũlich |year=419–445 |isbn=3-00-017350-1 |pages=2006 |url=|editor-first=J. |editor-last=Grotendorst |editor2-first=S. |editor2-last=Blũgel |editor3-first=D. |editor3-last=Marx |series=NIC Series |volume=31 }} A significant advantage of the half-metallic model is that it does not rely on the presence of mixed valency as does the double exchange mechanism and it can therefore explain the observation of CMR in stoichiometric phases like the pyrochlore {{chem|Tl|2|Mn|2|O|7}}. Microstructural effects in polycrystalline samples have also been investigated and it has been found that the magnetoresistance is often dominated by the tunneling of spin-polarized electrons between grains, resulting in the magnetoresistance having an intrinsic dependence on grain size.{{harvnb|Lalena|Cleary|2010|pp=361–2}}For a review see:{{cite book

|last=Dagotto |first=E. |title=Nanoscale Phase Separation and Colossal Magnetoresistance |publisher=Springer

|url=https://www.springer.com/materials/book/978-3-540-43245-6

|isbn=978-3-662-05244-0 |year=2003 |series=Springer Series in Solid-State Sciences }}

A fully quantitative understanding of the CMR effect remains elusive and it is still the subject of much current research. Early promises of the development of new CMR-based technologies have not yet come to fruition.

• Giant magnetoresistance

{{reflist|2}}

• {{cite web |url=http://www.physorg.com/news106576042.html |title=New Clues to Mechanism for Colossal Magnetoresistance |year=2007 |publisher=phys.org}}
• {{cite web |url=http://www.ornl.gov/sci/cmsd/theory/cmr/index.html |title=Materials Science and Technology Division |publisher=Physical Sciences Directorate, Oak Ridge National Laboratory}}
• {{cite journal |first=Josep |last=Fontcuberta |year=1999 |journal=Phys. World |title=Colossal magnetoresistance |volume=12 |issue=2 |page=33 |doi=10.1088/2058-7058/12/2/29}}
• {{cite web |date=February 1999 |title=Colossal magnetoresistance |website=Physicsweb |url=http://physicsweb.org/articles/world/12/2/12 |archive-url=https://web.archive.org/web/20041213043205/http://physicsweb.org/articles/world/12/2/12 |archive-date=2004-12-13 }}

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