Colossal magnetoresistance
Encyclopedia
Colossal magnetoresistance (CMR) is a property of some materials, mostly manganese
-based perovskite
oxide
s, that enables them to dramatically change their electrical resistance
in the presence of a magnetic field
. The magnetoresistance
of conventional materials enables changes in resistance of up to 5%, but materials featuring CMR may demonstrate resistance changes by orders of magnitude.
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
was given early on. In this model, the spin orientation of adjacent Mn-moments is associated with kinetic exchange of eg-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 Helmholt 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 theroetical 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. With SP calculations on covalent ferromagnets, the exchange-correlation integral in the LSDA-DFT 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 Tl2Mn2O7. Microstructural effects have also been investigated for polycrystalline samples and it has been found that the magnetoresistance is often dominated by the tunneling of spin polarized electrons between grains, giving rise to an intrinsic grain-size dependence to the magnetoresistance.
Hitherto, however, a fully quantitative understanding of the CMR effect has been elusive and it is still the subject of current research activities. Early prospects of great opportunities for the development of new technologies have not yet come to fruition.
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
-based perovskite
Perovskite
A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s, that enables them to dramatically change their electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
in the presence of a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. The magnetoresistance
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
of conventional materials enables changes in resistance of up to 5%, but materials featuring CMR may demonstrate resistance changes by orders of magnitude.
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
Double-exchange mechanism
The double-exchange mechanism is a type of a magnetic exchange that may arise between ions in different oxidation state. First proposed by Clarence Zener, this theory that predicts the relative ease with which an electron may be exchanged between two species, and has important implications for...
was given early on. In this model, the spin orientation of adjacent Mn-moments is associated with kinetic exchange of eg-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 Helmholt 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 theroetical 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)
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
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. With SP calculations on covalent ferromagnets, the exchange-correlation integral in the LSDA-DFT 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 Tl2Mn2O7. Microstructural effects have also been investigated for polycrystalline samples and it has been found that the magnetoresistance is often dominated by the tunneling of spin polarized electrons between grains, giving rise to an intrinsic grain-size dependence to the magnetoresistance.
Hitherto, however, a fully quantitative understanding of the CMR effect has been elusive and it is still the subject of current research activities. Early prospects of great opportunities for the development of new technologies have not yet come to fruition.