Competition between ferromagnetic metallic and paramagnetic insulating phases in manganites

Abstract

La 0.67 Ca 0.33 Mn 1Ϫx Cu x O 3 (xϭ0 and 0.15͒ epitaxial thin films were grown on the ͑100͒ LaAlO 3 substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic ͑FM͒ and paramagnetic insulating ͑PI͒ phases plays an important role in the observed colossal magnetoresistance ͑CMR͒ effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we calculated the resistivity as functions of temperature and magnetic field. The model not only qualitatively accounts for some main features related to the CMR effect, but also quantitatively agrees with the experimental observations.

Influence of Ca-deficiency on the magneto-transport properties in La0.8Ca0.2MnO3 perovskite and estimation of magnetic entropy change

“…In this expression, T mod C is PI-FM transition temperature used in this model and near/ equal to T C , U 0 is taken as the energy difference for temperature well below T mod C . 12 So, the total resistivity in whole temperatures ranges can be written as…”

“…Then the electrical resistivity of the system at any temperature is determined by the change of the FM volume fractions in both regions. According to Li et al, 12 the resistivity is formulated as…”

“…(1) and (2)) cannot describe the electric behavior near the metallic-insulator temperature T q . In order to understand the transport mechanism in the entire temperature range, Li et al 12 have developed a new phenomenological model based on the phase segregation mechanism (percolation approach).…”

“…In this state, the electrical conduction is governed by small polaron hopping mechanism. [9][10][11] In order to understand the electrical behavior in the whole temperature range, Li et al 12 have developed a new model based on the phase segregation mechanism. 13 This model supposes that the materials are composed of paramagnetic and ferromagnetic regions.…”

Теплофизические свойства монокристалла La-=SUB=-0.825-=/SUB=-Sr-=SUB=-0.175-=/SUB=-MnO-=SUB=-3-=/SUB=-

“…2 (справа) приведена температурная зависимость электросопротивления для образца La 0.825 Sr 0.175 MnO 3 . Отметим, что температурный ход имеет характерный для большинства легированных манганитов вид и демонстрирует переход металл-диэлектрик при T MI = 307 K. В данной работе мы не будем рассматривать подробный анализ электросопро-тивления, так как в литературе имеется огромное количество работ подробного анализа электросопро-тивления манганитов с аналогичной температурной зависимостью [19][20][21][22]. Кратко отметим, что в высокотем-пературной парамагнитной области зависимость ρ(T ) носит полупроводниковый характер и может быть интерпретирована на основе концепции поляронов малого радиуса, представляющих собой электрон или дырку, движущиеся вместе с вызванной им деформацией кристаллической решетки.…”