The perovskite manganites with generic formula RE1?xAExMnO3 (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator?metal/paramagnetic?ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g.?structure, phase diagram, double-exchange mechanism, Jahn?Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g.?pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

https://iopscience.iop.org/article/10.1088/0953-8984/20/27/273201/pdf

Low field magnetotransport in manganites

The perovskite manganites of general formula RE_1-xAe_xMnO_3 (RE= rare earth,AE=Ca, Sr, Ba and Pb)have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). They exhibit extraordinary large magnetoresistance pronounced as CMR in the vicinity of insulator-metal/paramagnetic-ferromagnetic transition at a relatively large applied magnetic fields. However, for applied aspectes, occurence of significant CMR at low applied magnetic fields would be required. This review consists of of two sections: In the first section we have extensively reviewed the salient features e.g. structure, phase diagram, double exchange mechansim, Jahn Teller effect, different types of ordering and phase separation of CMR mangnaites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for doped manganites such as polycrystalline La_0.67Ca_0.33MnO_3 films, Ag admixed La_0.67Ca_0.33MnO_3 films, polycrystalline (La_0.7Ca_0.2Ba_0.1MnO_3)and epitaxial (La_0.67Ca_0.33MnO_3) films on different substrates, nanophasic La_0.7Ca_0.3MnO_3, mangnaite-polymer composites (La_0.7Ba_0.2Sr_0.1MnO_3-PMMA and La_0.67Ca_0.33MnO_3-PMMA)and double layered polycrystalline (La_1.4Ca_1.6-xBa_xMn_2O_7) and films (La_1.4Ca_1.6Mn_2O_7). Some other potential magnetoresistive materials e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc have aslo been briefly dealt with. The review concludes with the summary of results for low field magnetotransport behaviour and prospectes for applications.

Low Field Magnetotransport in Manganites

We have synthesized La0.67Ca0.33MnO3 (LCMO):xZnO () composites through a citrate gel route and have characterized them for magnetic and magnetotransport properties. In lower concentrations (), ZnO mostly goes into the perovskite lattice substituting Mn in LCMO and segregates less in the grain boundary region, but at higher concentration (x>0.13) it segregates mostly at the grain boundaries of LCMO and influences the transport properties significantly. A model is proposed which describes the overall resistivity of the system as a parallel combination of a low resistive intragrain conducting path and a high resistive intergrain insulating path. Using this approach, the grain and grain boundary contributions to the overall resistivity are separated for all the composites. The field dependent resistivity shows that all the composites have higher values of MR at the transition temperatures (TMI) compared to that in pure LCMO (x = 0). The highest value of MR is obtained for x = 0.10 and is 76.6% at 80 kOe field near TMI.

Magnetic and electrical transport properties of La0.67Ca0.33MnO3 (LCMO):xZnO composites

Magnetic and electrical transport properties of La~0~.~6~7Ca~0~.~3~3MnO~3 (LCMO):xZnO composites

Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The grain-boundary structure and the temperature dependence of resistivity were investigated for (1-x)LCMO + xYSZ, where LCMO and YSZ represent La2/3Ca1/3MnO3 and yttria-stabilized zirconia, respectively. It is shown that the YSZ doped samples for x 2%. The metal-insulator transition temperature (Tp) decreases for x 2% as x is increased. LFMR increases with x for x 2%. The experimental observations are discussed on the basis of scanning electron microscopy (SEM) analysis, which reveals YSZ appearing at the grain boundaries of LCMO for x 2%.

Y P Yang

Papers

Grain boundaries and low-field transport properties in colossal magnetoresistance materials