Enhanced metal–insulator transition and magnetoresistance in melt-processed La0.67Ca0.33MnO3 and Ho-doped manganites

Abstract

We report significant enhancement of magnetoresistance (MR) in melt-processed La0.67Ca0.33MnO3 (LCM) and in Ho-doped LCM samples. The LCM system exhibits surprising enhancement of metal–insulator transition (TIM) that coincides with the Curie transition temperature (Tc), TIM≈Tc, illustrating the enhanced percolative transport and spin-polarization through grain boundaries. Surprisingly, 1 mol % of addition of Ho enhances Tc of ∼70 K with remarkable colossal MR. The nanoscale Ho distribution is consistent with the magnetic inhomogeneity-induced MR due to phase segregation in Ho-doped sample whereas charge-segregation picture is valid for undoped LCM.

The effect of Ho-doping and Ho-adding on electronic transport and magnetic properties of La0.67Ca0.33MnO3

“…3, in which it displays a remarkable MR at room temperature. The low temperature MR is associated with grain boundaries and is caused by spindependent tunneling between grains, which indicates the FM and CO state remains coexistent almost all over the measured temperature region exhibiting the percolative conduction through metallic regions embedded in the insulating matric [21]. These results have been verified by the STM image shown in Ref.…”

“…The CMR effect that occurs near T c is associated with bulk and it occurs as the spins align and so the mobility of carriers increases. For the LCMO sample, the MR collapses rapidly below T c indicating that the sample has a sharp phase boundary between ferromagnetic metallic (FMM) and charge ordering (CO) state [21]. However, a substantial MR remains at the low temperature for Ho-LCM.…”

“…We observed that T c was nearly 70 K higher in Ho-LCM than that in Ho-free LCMO, but was nearly 35 K lower in Ho-doping LCM sample. Pradhan et al [21] attributed this phenomenon to the opening of new conduction channels blocked at GB. In this article, a reasonable explanation is given based on the changes of structural parameters gained from Rietveld refinement of X-ray powder diffraction.…”

“…However, for bulk ceramic material, nearly 100% spin polarization remains an intriguing problem both for technological and scientific point of view. Therefore, the enhanced spin polarization is significant to investigate the structural MR at T c ET IM (metal-insulator transition) in charge transport across the interface of the GB [21].…”

Enhancement of ordering temperatures of La0.67Ca0.33MnO3 thin films by adding Ho atoms

“…Furthermore, the shifted XRD peaks for the sample with 0.6 mol % Ho implied that above a certain level of addition, Ho was also going to lattice sites along with LCMO matrix. Based on these results, the enhancement in the ordering temperature is attributed to presence of nano-size Ho in the LCMO matrix, which will act as conduction channels between grain boundaries [7,8]. Above 0.40, the decrease in T MI is due to the substitutional effects.…”