Silence of magnetic layers to magnetoresistive process and electronic separation at low temperatures in (La, Sm)Mn${}_2$Ge${}_2$

A closer look at the temperature (T) dependence of magnetoresistance (MR) of two polycrystalline magnetic compounds, LaMn${}_2$Ge${}_2$ and SmMn${}_2$Ge${}_2$, previously reported by us, is made. A common feature for both these compounds is that the low temperature MR is positive (say, below, 30 K) in spite of the fact that both are ferromagnetic at such low temperatures; in addition, MR as a function of magnetic field (H) does not track magnetization (M) in the sense that M saturates at low fields, while MR varies linearly with H. These observations suggest that the magnetic layers interestingly do not dominate low temperature magnetotransport process. Interestingly enough, as the T is increased, say around 100 K, these magnetic layers dominate MR process as evidenced by the tracking of M and MR in SmMn${}_2$Ge${}_2$. These results tempts us to propose that there is an unusual “electronic separation” for MR process as the T is lowered in this class of compounds.