Unconventional magnetic and transport properties in Gd1-xLaxMn2Ge2 with two-dimensional arrangement of Mn atoms

In this paper, we present our recent experimental results of magnetic and t ransport properties of Gd1-xLaxMn2Ge2 intermetallic compounds with the ThCr 2Si2-type layered structure. The results obtained indicate that, in GdMn2Ge 2, a first-order transition from a collinear antiferromagnetic to a colline ar ferrimagnetic state appears with decreasing temperature at T-t3, below t he Neel temperature T-N. In Gd1-xLaxMn2Ge2 compounds with x = 0.05 and 0.07 5, after ordering ferrimagnetically at T-t1, two kinds of first-order trans itions from a canted ferrimagnetic to a non-collinear antiferromagnetic sta te and from a non-collinear antiferromagnetic to a reentrant canted ferrima gnetic state occur at T-t2 and T-t3. In Gd0.925La0.075Mn2Ge2, a field-induc ed metamagnetic transition from non-collinear antiferromagnetism. to canted ferrimagnetism occurs at relatively low fields, accompanied by fractal lik e multi-step transitions, the so called "devil's stair-case". Furthermore, a negative giant magnetoresistance (GMR) effect (Delta rho/rho similar to 1 5%) was observed at the field-induced metamagnetic transition. The mechanis m of this negative GMR was clarified by comprehensive measurements of the r esistivity on single crystals Gd0.925La0.075Mn2Ge2 and TbMn2Ge2. With furth er increasing x, only canted ferrimagnetism appears with a compensation tem perature for 0.10<x<0.40, whereas no compensation behavior appears for x>0. 50. The phase diagram obtained indicates that the overall magnetism is cont rolled by the Mn-Mn intralayer distance in the tetragonal c-plane, reflecti ng the two-dimensional arrangement of Mn atoms. (C) 2001 Elsevier Science B .V. All rights reserved.