Colossal magnetoresistance (CMR) is a property of some materials, mostly
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
-based
perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known a ...
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s, that enables them to dramatically change their
electrical resistance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
in the presence of a
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. The
magnetoresistance
Magnetoresistance is the tendency of a material (often ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance. Some occur in b ...
of conventional materials enables changes in resistance of up to 5%, but materials featuring CMR may demonstrate resistance changes by orders of magnitude.
This technology may find uses in
disk read-and-write head
A disk read-and-write head is the small part of a disk drive which moves above the disk platter and transforms the platter's magnetic field into electrical current (reads the disk) or, vice versa, transforms electrical current into magnetic fi ...
s, allowing for increases in
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
data density. However, so far it has not led to practical applications because it requires low temperatures and bulky equipment.
History
Initially discovered in mixed-valence perovskite manganites in the 1950s by G. H. Jonker and J. H. van Santen, a first theoretical description in terms of the
double-exchange mechanism The double-exchange mechanism is a type of a magnetic exchange that may arise between ions in different oxidation states. First proposed by Clarence Zener, this theory predicts the relative ease with which an electron may be exchanged between two s ...
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, Wollan and Koehler, and later on by Jirak et al. and Pollert et al.
However, the double exchange model did not adequately explain the high insulating-like resistivity above the transition temperature. In the 1990s, work by R. von Helmolt et al. and Jin et al. 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.
Theory
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
The Stoner criterion is a condition to be fulfilled for the ferromagnetic order to arise in a simplified model of a solid. It is named after Edmund Clifton Stoner.
Stoner model of ferromagnetism
Ferromagnetism ultimately stems from Pauli exc ...
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. 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
Pyrochlore () is a mineral group of the niobium end member of the pyrochlore supergroup.
The general formula, (where A and B are metals), represent a family of phases isostructural to the mineral pyrochlore.
Pyrochlores are an important class of ...
. 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.
[For a review see:]
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.
See also
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Giant magnetoresistance
Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter G ...
References
External links
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{{DEFAULTSORT:Colossal Magnetoresistance
Magnetoresistance
Quantum electronics
Spintronics