Manganites

Manganites are substances based on manganese , representatives of the class of transition metal oxides. Of greatest interest are compounds of the type $La_{1-x}A_{x}MnO_{3}$ ${\ displaystyle La_ {1-x} A_ {x} MnO_ {3}}$ $La _ {{1-x}} A_ {x} MnO_ {3}$ where A is a divalent element (Ca, Ba, Sr, ...). The concentration x of element A can vary widely $o\leq x\leq 1$ ${\ displaystyle o \ leq x \ leq 1}$ $o \ leq x \ leq 1$ , while the physical properties of manganites change dramatically. The system goes through a chain of phase transitions with various types of ordering: magnetic, structural, electronic.

Manganites have been studied for over 50 years and are of great interest due to the discovery of colossal magnetoresistance relatively recently (1994) ^[1] . This effect can serve as the basis for technical applications, it is observed in the concentration range x, where there is a ferromagnetic metal phase and consists in the fact that the electrical resistance $\rho$ ${\ displaystyle \ rho}$ $\ rho$ decreases when a magnetic field is applied. Magnitude of effect $\delta \rho /\rho$ ${\ displaystyle \ delta \ rho / \ rho}$ $\ delta \ rho / \ rho$ in fields of the order of 1 T, it can reach tens of percent. The maximum effect occurs in the vicinity of the Curie temperature .

For example, connection $LaMnO_{3}$ ${\ displaystyle LaMnO_ {3}}$ $LaMnO_ {3}$ is an antiferromagnetic dielectric with a magnetic structure of type A, when the lanthanum is replaced by calcium, the system becomes a ferromagnetic metal, and when $x>0,5$ ${\ displaystyle x> 0.5}$ $x> 0.5$ - again an antiferromagnetic dielectric with a magnetic structure of type G in the final state $CaMnO_{3}$ ${\ displaystyle CaMnO_ {3}}$ $CaMnO_ {3}$ and type C in the intermediate concentration range ^[2] . With increasing temperature, the ferromagnetic phase changes paramagnetic with a sharp drop in conductivity. The behavior of electrical resistance on temperature strongly depends on the concentration of the doped element (on the degree of doping of the initial compound with a divalent element). The appearance of a metallic state upon passing through the Curie point and strong magnetoresistance - phenomena that are closely related to each other, are a typical property of manganites.

The appearance of the metallic ferromagnetic phase in manganites was explained back in 1951 by Zener on the basis of the assumption of a strong intraatomic exchange between the localized spin and the delocalized electron . Due to this connection, the electron spin is aligned parallel to the ion spin. And the electron, thus, is able to move from the node to the node of the lattice, lowering the total energy of the system. In this case, the ferromagnetic state arises not because of the exchange interaction of ions, but because of the kinetic effect. This mechanism was called double exchange:

Mn → O → Mn (double transition of an electron through an intermediate oxygen ion).

Due to this effect, manganites can be included in the so-called class of highly correlated electronic systems.

Application

The use of manganites as substances with colossal magnetoresistance can be in a new developing branch of electronics - spintronics .

Notes

↑ Coey J. MD, Viret M., von Molnar S., Adv. Phys. 48167 (1999)
↑ Jin S. et al. Science 264 413 (1994)

Literature

Izyumov Yu. A., Scriabin Yu. N. Double exchange model and unique properties of manganites // Uspekhi Fizicheskikh Nauk , 2001, No. 2, p. 121-148;
Kagan M. Yu., Kugel K. I. Inhomogeneous charge states and phase separation in manganites // Uspekhi Fizicheskikh Nauk , 2001, No. 6, p. 577-596;
Voronov V.K., Podoplelov A.V. Modern Physics: Textbook. - M .: KomKniga, 2005 .-- 512 p. ISBN 5-484-00058-0 , chap. 3 Condensed matter, p. 3.7 Physical properties of manganites, p. 133-138.

[1] Coey J. MD, Viret M., von Molnar S., Adv. Phys. 48167 (1999)

[2] Jin S. et al. Science 264 413 (1994)

Manganites

Application

Notes

Literature

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