Effect of Ni substitution on the fragile magnetic system ${\text{La}}_5{\text{Co}}_2{\text{Ge}}_3$

${\text{La}}_5{\text{Co}}_2{\text{Ge}}_3$ is an itinerant ferromagnet with a Curie temperature, $T_C$, of $\sim$ 3.8 K and a remarkably small saturated moment of 0.1 ${\mu }_B\mathrm{/}\text{Co}$. Here we present the growth and characterization of single crystals of the ${\text{La}}_5{\text{(Co}}_{1-x}{\text{Ni}}_x{)}_2{\text{Ge}}_3$ series for 0.00 $\le x\le$ 0.186. We measured powder X-ray diffraction, composition as well as anisotropic temperature dependent resistivity, temperature and field dependent magnetization along with heat capacity on these single crystals. We also measured muon-spin rotation/relaxation ($\mu \text{SR}$) for some Ni substitutions ($x$ = 0.027, 0.036, 0.074) to study the evolution of internal field with Ni substitution. Using the measured data we infer a low temperature, transition temperature-composition phase diagram for ${\text{La}}_5{\text{(Co}}_{1-x}{\text{Ni}}_x{)}_2{\text{Ge}}_3$. We find that $T_C$ is suppressed for low dopings, $x\le 0.014$; whereas for $0.036\le x\le 0.186$, the samples are antiferromagnetic with a Neel temperature, $T_N$, that goes through a weak and shallow maximum ($T_N\sim$ 3.4 K for $x\sim$ 0.07) and then gradually decreases to 2.4 K by $x$ = 0.186. For intermediate Ni substitutions, $0.016\le x\le 0.027$, two transition temperatures are inferred with $T_N>T_C$. Whereas the $T-x$ phase diagram for ${\text{La}}_5{\text{(Co}}_{1-x}{\text{Ni}}_x{)}_2{\text{Ge}}_3$ and the $T-p$ phase diagram determined for the parent ${\text{La}}_5{\text{Co}}_2{\text{Ge}}_3$ under hydrostatic pressure are grossly similar, changing from a low doping or low pressure ferromagnetic (FM) ground state to a high doped or pressure antiferromagnetic (AFM) state, perturbation by Ni substitution enabled us to identify an intermediate doping regime where both FM and AFM transitions occur.