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

Abstract

${\text{La}}_5{\text{Co}}_2{\text{Ge}}_3$ is an itinerant ferromagnet with a Curie temperature, $T_C$, of $\sim$ 3.8 K and a remarkably small saturated moment of 0.1 ${\mu }_B/\text{Co}$. 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.

arXiv comment24 pages, 40 figures

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