The crucial role of mixed valence in the magnetoresistance properties of manganites and cobaltites

2007 ◽  
Vol 366 (1862) ◽  
pp. 83-92 ◽  
Author(s):  
B Raveau
Keyword(s):  
Colossal Magnetoresistance ◽  
Mixed Valence ◽  
Transition Element ◽  
Oxygen Stoichiometry ◽  
Orbital Ordering ◽  
Electronic Phase ◽  
Spin Glass Behaviour ◽  
Jahn Teller ◽  
Jahn Teller Effect

The mixed valence Mn 3+ /Mn 4+ and Co 3+ /Co 4+ in manganites and cobaltites with the perovskite structure is absolutely necessary for the appearance of magnetotransport properties. It is shown that in these systems the Jahn Teller effect of the transition element, the charge and orbital ordering and the oxygen stoichiometry play a key role in the appearance of large and even colossal magnetoresistance. It has been discovered that these oxides exhibit a new phenomenon, the crystallographic and electronic phase separation. It is this phenomenon that is at the origin of the competition between ferromagnetism and antiferromagnetism or spin glass behaviour and which leads to the negative magnetoresistance (MR). The doping of these materials at different sites appears then to be a means of inducing large MR effects.

Manganites

2021 ◽  
pp. 403-428
Author(s):  
Vladimir Z. Kresin ◽  
Sergei G. Ovchinnikov ◽  
Stuart A. Wolf
Keyword(s):  
Colossal Magnetoresistance ◽  
Mixed Valence ◽  
Fundamental Property ◽  
Microscopic Theory ◽  
Double Exchange ◽  
Metallic State ◽  
Electron Hole ◽  
Jahn Teller ◽  
Jahn Teller Effect ◽  
Mixed Valence Compounds

This chapter focuses on manganites. There is a large similarity between the two families of mixed-valence compounds, the cuprates and the manganites. However, manganites display colossal magnetoresistance. The most fundamental property of manganites is the strong correlation between their transport properties and their magnetic properties. This correlation is caused by the double-exchange mechanism. The Hund interaction and the Jahn–Teller effect are the key ingredients of the microscopic theory. The transition to the ferromagnetic and metallic state is of a percolative nature. The superconducting–antiferromagnetic–superconducting Josephson junction is described. One can observe giant oscillations of the Josephson current as a function of a weak external magnetic field. The main properties, including the electron–hole asymmetry can be described in the framework of a generalised two-band picture. A peculiar isotope effect can be observed.

scholarly journals ORBITAL EFFECTS IN MANGANITES

2001 ◽  
Vol 15 (19n20) ◽  
pp. 2665-2681 ◽  
Author(s):  
D. I. KHOMSKII
Keyword(s):  
Phase Diagram ◽  
Colossal Magnetoresistance ◽  
Degrees Of Freedom ◽  
Short Review ◽  
Orbital Ordering ◽  
Orbital Degeneracy ◽  
Structural Distortions ◽  
Jahn Teller ◽  
Different Parts

In this paper I give a short review of some properties of the colossal magnetoresistance manganites, connected with the orbital degrees of freedom. Ions Mn 3+, present in most of these compounds, have double orbital degeneracy and are strong Jahn–Teller ions, causing structural distortions and orbital ordering. Mechanisms leading to such ordering are shortly discussed, and the role of orbital degrees of freedom in different parts of the phase diagram of manganites is described. Special attention is paid to the properties of low-doped systems (doping 0.1≤x≤0.25), to overdoped systems (x>0.5), and to the possibility of a novel type of orbital ordering in optimally doped ferromagnetic metallic manganites.

JAHN-TELLER EFFECT OF MOLECULAR COMPLEXES IN LIQUID SOLUTIONS

1994 ◽  
Vol 08 (21n22) ◽  
pp. 1319-1334 ◽  
Author(s):  
J.W. HALLEY ◽  
X.R. WANG
Keyword(s):  
Quantum Coherence ◽  
Berry Phase ◽  
Molecular Complexes ◽  
Liquid Solution ◽  
Teller Effect ◽  
Liquid Systems ◽  
Jahn Teller ◽  
Jahn Teller Effect ◽  
Molecular Dynamics Methods

We review recent work clarifying the sense in which the Jahn-Teller effect can exist in a molecular complex in liquid solution. We review the molecular dynamics methods for modeling such liquid systems using the cupric ion in aqueous solution as an example. We review the experimental evidence for the Jahn-Teller effect in liquids, emphasizing the importance of taking the time scale of the measurement into account. Finally we discuss the role of quantum coherence and the Berry phase in the Jahn-Teller effect.

The Dielectric Loss of Single Crystal and Polycrystalline TiO2

1997 ◽  
Vol 500 ◽  
Author(s):  
Xiaoru Wang ◽  
Alan Templeton ◽  
Stuart J. Penn ◽  
Neil McN. Alford
Keyword(s):  
Dielectric Loss ◽  
Single Crystal ◽  
Mixed Valence ◽  
Dielectric Materials ◽  
Oxygen Stoichiometry ◽  
Ion Conductor ◽  
Microwave Dielectric ◽  
Valence States ◽  
Oxygen Ion Conductor

ABSTRACTThe dielectric loss of single crystal and polycrystalline TiO2 has been studied. In polycrystalline TiO2 the dielectric loss is determined by both the microstructure and by the oxygen stoichiometry. Experiments have been carried out to determine the influence of both the microstructure (particularly porosity) and the oxygen stoichiometry. The TiO2 powder has been doped with partially stabilised zirconia, an oxygen ion conductor, in order to modify the oxygen stoichiometry. Sintered discs have been examined for loss as a function zirconia doping, pore volume and as a function of temperature. The behaviour of the doped and undoped titania powders is significantly different. Since many microwave dielectric materials contain Ti eg Ba-Ti-O, Ba-Nd-Ti-O, (Ba-RE-Ti-O, RE=Rare Earth), Zr-Sn-Ti-O etc it is essential to understand the role of the titanium, particularly as it can exist in mixed valence states, and the role of oxygen and its influence on the dielectric loss.

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