Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La0.5Ca0.5MnO3

Abstract

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x=0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles.

Engineering of Advanced Materials for High Magnetic Field Sensing: A Review

“…One can see that higher amount of Co increases the MR values, what could be explained by partly destroyed double exchange hopping Mn 3+ −O 2− −Mn 4+ , when cobalt ions substitute manganese in the lattice, which results in increased resistivity. It is worth noting that one must be careful when increasing the disorder in polycrystalline films—by doping with Co or other elements, or by using other means such as decreasing of grain size [ 79 ]. The increase of disorder by reducing the grain size could lead to collapse of the VRH and drive the system in the opposite direction of the Anderson localization.…”

“…The increase of disorder by reducing the grain size could lead to collapse of the VRH and drive the system in the opposite direction of the Anderson localization. As a result, the suppressing the CO/AFM state and growth of the FM contribution would lead to the decrease of resistivity [ 79 ]. Such behavior could have a great influence on magnetoresistance values and sensors operation at low temperatures.…”

Interface based field effect configuration and charge conduction mechanisms for manganite thin film heterostructures

“…These mixed valent manganites have been studied extensively in their different forms including polycrystalline bulk, nanostructure, heterostructure, device, thin film, multilayer, etc. [8][9][10][11][12][13][14] Mixed valent manganites have been investigated extensively for various possible potential applications such as capacitors, p-n junctions, field effect devices, magnetic field and temperature sensors, spin transistors, memristor devices, RS devices, nonvolatile memory devices, etc. [15][16][17][18][19][20] To fulfill the increased demand for materials with astonishing properties and characteristics for today's advanced technologies, researchers have always been active in investigating different functional materials for improving their functional properties as well as to make them more suitable for their potential practical applications including spintronic applications.…”

“…These mixed valent manganites have been studied extensively in their different forms including polycrystalline bulk, nanostructure, heterostructure, device, thin film, multilayer, etc. 8–14 Mixed valent manganites have been investigated extensively for various possible potential applications such as capacitors, p–n junctions, field effect devices, magnetic field and temperature sensors, spin transistors, memristor devices, RS devices, nonvolatile memory devices, etc. 15–20…”

New Spin on Metal-Insulator Transitions

“…A useful tool to systematically investigate and compare all these phenomena is the database for crystalline organic conductors and superconductors provided by Ganter et al [13]. The interplay of charge and spin degrees of freedom, prominently seen in manganites [14], is considered important in dimerized organic Mott systems, where dielectric anomalies have been controversially discussed [15]. In geometrically frustrated systems, the charge order can transform into a charge glass [16] and magnetic order can transform into a quantum spin liquid.…”