Magnetic and transport characteristics in Gd 0.925 La 0.075 Mn 2 Ge 2 with two-dimensional alignment of Mn atoms

We present our recent experimental results on magnetic and transport properties in intermetallic compound ${\mathrm{Gd}}_{0.925}{\mathrm{La}}_{0.075}{\mathrm{Mn}}_{2}{\mathrm{Ge}}_{2}$ with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type layered structure. The results obtained indicate that ${\mathrm{Gd}}_{0.925}{\mathrm{La}}_{0.075}{\mathrm{Mn}}_{2}{\mathrm{Ge}}_{2}$ reveals two kinds of first order transitions below the ferrimagnetic Curie temperature ${T}_{t1},$ from canted ferrimagnetic to noncollinear antiferromagnetic states and from noncollinear antiferromagnetic to reentrant canted ferrimagnetic states at ${T}_{t2}$ and ${T}_{t3}$ with decreasing temperature. Between ${T}_{t2}$ and ${T}_{t3},$ a field induced metamagnetism from noncollinear antiferromagnetism to canted ferrimagnetism appears at relatively lower fields, accompanied with fractal-like multistep transitions, the so called ``devil's stair case.'' Furthermore, a negative giant magnetoresistance effect $(\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}\ensuremath{\sim}15%)$ is also observed at the field induced metamagnetic transition. The mechanism of this negative GMR is discussed on the basis of the results of comprehensive measurements of the resistivities using single-crystalline ${\mathrm{Gd}}_{0.925}{\mathrm{La}}_{0.075}{\mathrm{Mn}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{TbMn}}_{2}{\mathrm{Ge}}_{2}.$ These unusual features might be characteristic of layered structure, two-dimensional alignment of Mn atoms.