Magnetoresistance

Measurements of magnetization versus temperature and field have been conducted on La1-xCaxMnO3 compounds which for some values of x exhibit colossal magnetoresistence (CMR). Recent theories propose that polaron forming double exchange interactions and Jahn-Teller distortions create a strong interaction amongst charge, spin and lattice degrees of freedom which is responsible for the unusually large magnotoresistence. In this work the enhancement of paramagnetic susceptibility is studied in order to investigate the possible existence of magnetic polarons. Enhancements of the effective moments are found in the range 0.15 < x < 0.94, which are qualitatively consistent with the existence of magnetic polarons. The findings are discussed within the framework of recent experimental and theoretical findings.

Magnetic and electrical properties for La(1-x)Gd(x)0.7Ca(0.3)MnO3 were studied at ambient and high pressure. For x > 0.1 the ferromagnetic saturation moment M is smaller than that expected for complete ferromagnetic alignment of the Mn moments. Gd substitution lowers both the paramagnetic-ferromagnetic transition temperature T(C), the metal-insulator transition temperature T(MI) and dramatically increases the electrical resistivity rho. T(MI) is observed to be significantly lower than T(C) for x > 0.1. The disagreement of T(C) and T(MI) is a result of the A-site ion size difference which causes the coexistence of ferromagnetic La-Ca-Mn-O and antiferromagnetic Gd-Ca-Mn-O phases and leads to the anomalous electrical transport properties. Samples with the largest average A-site ionic radius <rA> show the highest T(C) or T(MI) and are least sensitive to pressure.