Observation of Kosterlitz-Thouless spin correlations in the colossally magnetoresistive layered manganite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$
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The spin correlations of the bilayer manganite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$ have been studied using neutron scattering. On cooling within the paramagnetic state, we observe purely two-dimensional behavior with a crossover to three-dimensional scaling close to the ferromagnetic transition. Below $T_C$, an effective finite size behavior is observed. The quantitative agreement of these observations with the conventional quasi two-dimensional Kosterlitz-Thouless model indicates that the phase transition is driven by the growth of magnetic correlations, which are only weakly coupled to polarons above $T_C$.Keywords:
Manganite
Colossal Magnetoresistance
This paper discusses the preparation of the manganite polycrystalline samples La0.7-xYxCa0.3MnO3 by the solid-state reaction technique. The effects that the different substitution of Y for La affects the crystal structure, electromagnetic properties and magnetoresistance effects are also studied. No peak is observed on the resistance-temperature curves(R-T curves) of the sample La0.6Y0.1Ca0.3MnO3 with the decrease of the tempeature, whereas onepeak is observed for the sample La0.5Y0.2Ca0.3MnO3, and the complex transport properties are shown. The magnetoresistance(Rm) of the two samples increases with the decrease of the temperature, but no peak is observed. The sample La0.5Y0.2Ca0.3MnO3 shows large magnetoresistance over a wide temperature range, moreover, the magnetoresistance effects strengthen with the increase of Y substituting for La from x=0.1 to x=0.2.
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The authors present results for the magnetoresistance in a number of single crystals of YBCO. They all have Tc in the vicinity of 93K with transitions widths of around 0.5K. Between 93K and 100K the positive magnetoresistance due to superconducting fluctuations is present and the authors are able to fit the data to theory yielding values of the coherence length in the {open_quote}c{close_quote} axis of around 0.2nm. However, above 100K a negative contribution to the resistivity appears. Above about 115K the magnetoresistance is dominated by this negative contribution. The negative contribution is proportional to the field and its magnitude is consistent with a spin fluctuation contribution to the magnetoresistance. The magnitude of the effect increases as the temperature is reduced until it peaks at around 100K.
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Magnetoresistance is an effect in which the electrical resistance of a material changes in a magnetic field. This phenomenon is of great importance in magnetic data recording, where a magnetoresistive sensor detects the patterns of bits on magnetic storage media. In his Perspective, Goldman discusses results reported in the same issue by Kimura et al . ( p. 1698 ) that reveal how current is carried in an important group of materials, the layered manganites.
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Colossal Magnetoresistance
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Manganite
Colossal Magnetoresistance
Atmospheric temperature range
Magnetic refrigeration
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Solid-state physics
Thermal Hall effect
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An intrinsic characteristic of the “colossal” magnetoresistance manganite compounds is that the resistance and the magnetoresistance vary strongly with temperature over the small temperature regime in which the magnetoresistance is exceptionally large. We propose a heterostructure constructed of layers of varying composition manganites which extends the regime of large magnetoresistance and greatly broadens the sharp peak in resistance. Data from a prototype heterostructure are presented that demonstrate the effectiveness of this method.
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We report on a series of magnetoresistance measurements of in situ grown granular Sn wires with widths from 1100--2000 \AA{}. The magnetoresistance is measured within the superconducting resistive transition of wires with different normal state resistances and the results are compared to two-dimensional (2D) granular Sn films. Both the wires and the 2D films exhibit two distinct magnetic-field regimes: a low-field weak positive magnetoresistance regime and a high-field strong positive magnetoresistance regime. In addition, the wires exhibit strong reproducible magnetoresistance oscillations within the low-field regime near the superconductor-insulator transition, which are not observed in the 2D films. We attribute these magnetoresistance oscillations to the effects of screening currents circulating around phase coherent loops of weakly linked superconducting grains. The observed magnetoresistance at different field strengths in the wires and films allows for a clear and coherent interpretation of the mechanisms causing the magnetoresistance behavior, and the magnetic-field tuned superconductor-insulator transition.
Colossal Magnetoresistance
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Significance Magnetoresistance is the change of resistance in the presence of an external magnetic field. In rare-earth manganite compounds, this change is orders of magnitude stronger than usual and it is promising for developing new spintronic and electronic devices. The colossal magnetoresistance (CMR) effect has been observed only in chemically doped manganite compounds. We report the realization of CMR in a compressed single-valent LaMnO 3 manganite compound. Pressure generates an inhomogeneous phase constituted by two components: a nonconductive one with a unique structural distortion and a metallic one without distortion. The CMR takes place when the competition between the two phases is at a maximum. We identify phase separation as the driving force for generating CMR in LaMnO 3 .
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Colossal enhancement of magnetoresistance has been achieved over a broad temperature range which extends upto the room temperature, in ferromagnetic metal-charge ordered insulator manganite multi-layers. The artificially created phase coexistence in the multilayers reproduce the characteristic signatures of metastability in the magnetotransport properties commonly observed in electronically phase-separated manganites.
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Colossal Magnetoresistance
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Manganite
Lanthanum manganite
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