Magnetic ordering and valence instability in Kondo system CeMn$_{2-x}$Cu$_{x}$Si$_{2}$

The transition from a 3d-antiferromagnetically ordered mixed valence system to a Kondo lattice system has been studied for the CeMn$_{2-x}$Cu$_{x}$Si$_{2 }$(0 $\le \quad x \quad \le $ 1) series. The Ce L$_{3}$-edge x-ray absorption result shows that the series evolves from a Ce mixed valence system at $x$ = 0 to a nearly trivalent system at $x$ = 2. The resistivity results show that in the low Ce valence region ($x$ greater than 1.0), the system exhibits a crystalline-field modified Kondo lattice behavior. In the high valence region, however, the system displays a behavior of the prototype mixed valence compound CePd$_{3}$. Magnetization (M) data were taken in both zero-field-cooled (ZFC) and field cooled (FC) processes. It is found that for 0 $\le \quad x \quad <$ 0.4, Neel temperature decreases rapidly with the increase of the Cu concentration $x$. In the range of $x$ from 0.4 $\le $ x $<$ 0.8, ferromagnetic phase is observed below 150 K, and both the ordering temperature $T_{c}$ and Qurie-Weiss temperature $\theta $ decrease with the increase of $x$. For $x \quad \ge $ 0.8, the $M$ (T) curves are reversible and display paramagnetic behavior. Thus, the system is non-magnetic as it approaches heavy-fermion compound CeCu$_{2}$Si$_{2}$. Finally, a magnetic phase diagram is proposed for this compound series.