Colossal magnetoresistance

About: Colossal magnetoresistance is a research topic. Over the lifetime, 3658 publications have been published within this topic receiving 130104 citations.

Enhanced magnetoresistance in sintered granular manganite/insulator systems

Abstract: We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.

Trapping, self-trapping and the polaron family

Abstract: The earliest ideas of the polaron recognized that the coupling of an electron to ionic vibrations would affect its apparent mass and could effectively immobilize the carrier (self-trapping). We discuss how these basic ideas have been generalized to recognize new materials and new phenomena. First, there is an interplay between self-trapping and trapping associated with defects or with fluctuations in an amorphous solid. In high dielectric constant oxides, like HfO2, this leads to oxygen vacancies having as many as five charge states. In colossal magnetoresistance manganites, this interplay makes possible the scanning tunnelling microscopy ( STM) observation of polarons. Second, excitons can self-trap and, by doing so, localize energy in ways that can modify the material properties. Third, new materials introduce new features, with polaron-related ideas emerging for uranium dioxide, gate dielectric oxides, Jahn-Teller systems, semiconducting polymers and biological systems. The phonon modes that initiate self-trapping can be quite different from the longitudinal optic modes usually assumed to dominate. Fourth, there are new phenomena, like possible magnetism in simple oxides, or with the evolution of short-lived polarons, like muons or excitons. The central idea remains that of a particle whose properties are modified by polarizing or deforming its host solid, sometimes profoundly. However, some of the simpler standard assumptions can give a limited, indeed misleading, description of real systems, with qualitative inconsistencies. We discuss representative cases for which theory and experiment can be compared in detail.

Carrier Density Collapse and Colossal Magnetoresistance in Doped Manganites

Abstract: A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped charge-transfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pair formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse describes the resistivity peak and the colossal magnetoresistance of doped manganites.

Atomic-scale images of charge ordering in a mixed-valence manganite

Abstract: Transition-metal perovskite oxides exhibit a wide range of extraordinary but imperfectly understood phenomena. The best known examples are high-temperature superconductivity in copper oxides1, and colossal magnetoresistance in manganese oxides (‘manganites’)2,3. All of these materials undergo a range of order–disorder transitions associated with changes in charge, spin, orbital and lattice degrees of freedom. Measurements of such order are usually made by diffraction techniques, which detect the ionic cores and the spins of the conduction electrons. Unfortunately, because such techniques are only weakly sensitive to valence electrons and yield superpositions of signals from distinct submicrometre-scale phases, they cannot directly image phase coexistence and charge ordering, two key features of the manganites. Here we present scanning tunnelling microscope measurements of the manganite Bi1-xCaxMnO3. We show that charge ordering and phase separation can be resolved in real space with atomic-scale resolution. By taking together images and current–voltage spectroscopy data we find that charge order correlates with both structural order and the local conductive state (either metallic or insulating). These experiments provide an atomic-scale basis for descriptions4 of manganites as mixtures of electronically and structurally distinct phases.

Magnetoresistance from quantum interference effects in ferromagnets

Abstract: The desire to maximize the sensitivity of read/write heads (and thus the information density) of magnetic storage devices has stimulated interest in the discovery and design of new magnetic materials exhibiting magnetoresistance Recent discoveries include the 'colossal' magnetoresistance in the manganites and the enhanced magnetoresistance in low-carrier-density ferromagnets An important feature of these systems is that the electrons involved in electrical conduction are different from those responsible for the magnetism The latter are localized and act as scattering sites for the mobile electrons, and it is the field tuning of the scattering strength that ultimately gives rise to the observed magnetoresistance Here we argue that magnetoresistance can arise by a different mechanism in certain ferromagnets--quantum interference effects rather than simple scattering The ferromagnets in question are disordered, low-carrier-density magnets where the same electrons are responsible for both the magnetic properties and electrical conduction The resulting magnetoresistance is positive (that is, the resistance increases in response to an applied magnetic field) and only weakly temperature-dependent below the Curie point