Potential giant magnetoresistance perovskites: The manganites La1−xAx+MnO3 (A+=K+, Na+, Rb+, Ag+, Tl+, Cu+, and Li+) (abstract)

Abstract

Rare-earth alkaline earth substitute manganites perovskites, Ln1−xAx2+MnO3 (A=Ca, Sr, Ba, Pb), have retained considerable attention in regard to their colossal magnetoresistance coefficient for compositions in the range 0.1⩽x⩽0.4. Compounds where A2+ cations replaced monovalent A+ cations such as K+ Ag+, etc., have been known for a long time1 and have even been tested as oxidation catalysts for exhaust gazes of internal combustion engines. Surprisingly enough, however, they have given rise to very little investigation as potential giant MR candidates. Preliminary structural features (cell distortions), magnetic and electric properties versus temperature are presented for the series of compounds of general composition La1−xAx+Mn1−2x3+Mn2x4+O3, where A+=Rb, K, Na, Ag, Tl, Cu, and Li. Lanthanum can be partly or totally replaced by Pr, Nd or other rare-earth cations or yttrium. Most of the phases of the lanthanum subseries are rhombohedral (R-3c) and ferromagnetic. Curie temperatures are strongly dependent on x, with a maximum around 330 K for Na, K, or Ag. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, uncharacteristic of a conventional Brillouin-type curve. This may be related to a first-order crystallographic transition accompanying the isolant–metal transition. The average magnetic moment of Mn is about 3.55 μB at 20 K for the composition La0.8K0.2MnO3. Preliminary magnetoresistance measurements on bulk ceramic samples display a MR effect of the same amplitude as the case of the alkaline earth substituted La manganites.