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 transport properties of Gd1-xLaxMn2Ge2 intermetallic compounds with the ThCr2Si2-type layered structure. The results obtained indicate that, in GdMn2Ge2, a first-order transition from a collinear antiferromagnetic to a collinear ferrimagnetic state appears with decreasing temperature at T-t3, below the Neel temperature T-N. In Gd1-xLaxMn2Ge2 compounds with x = 0.05 and 0.075, after ordering ferrimagnetically at T-t1, two kinds of first-order transitions from a canted ferrimagnetic to a non-collinear antiferromagnetic state and from a non-collinear antiferromagnetic to a reentrant canted ferrimagnetic state occur at T-t2 and T-t3. In Gd0.925La0.075Mn2Ge2, a field-induced metamagnetic transition from non-collinear antiferromagnetism. to canted ferrimagnetism occurs at relatively low fields, accompanied by fractal like multi-step transitions, the so called "devil's stair-case". Furthermore, a negative giant magnetoresistance (GMR) effect (Delta rho/rho similar to 15%) was observed at the field-induced metamagnetic transition. The mechanism of this negative GMR was clarified by comprehensive measurements of the resistivity on single crystals Gd0.925La0.075Mn2Ge2 and TbMn2Ge2. With further increasing x, only canted ferrimagnetism appears with a compensation temperature 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 controlled by the Mn-Mn intralayer distance in the tetragonal c-plane, reflecting the two-dimensional arrangement of Mn atoms. (C) 2001 Elsevier Science B.V. All rights reserved.