Effect of Ga and Cd substitutions on the first-order antiferromagnetic transition in NdCo2Zn20

The cubic neodymium-based compound ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20}$ exhibits a first-order antiferromagnetic (AFM) transition at ${T}_{\text{N}}=0.53$ K. Strong magnetic fluctuations at temperatures up to 5 K were suggested by the downward curvature of the electrical resistivity $\ensuremath{\rho}(T)$ and the reduced magnetic entropy ${S}_{\text{m}}$ of $0.5R\mathrm{ln}2$ at ${T}_{\text{N}}$. In this study, we measured $\ensuremath{\rho}(T)$, the isothermal magnetization $M(B)$, magnetic susceptibility $\ensuremath{\chi}(T)$, and specific heat $C(T)$ of ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20\ensuremath{-}x}{\mathrm{Cd}}_{x}$ for $x=1$ and ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20\ensuremath{-}y}{\mathrm{Ga}}_{y}$ for $y=1$ and 2. The sharp peak of the magnetic specific heat ${C}_{\text{m}}(T)$ at ${T}_{\text{N}}$ for $x$ = 0 is changed to a weak and broad maximum at 0.55 K for $x$ = 1. This drastic change in ${C}_{\text{m}}(T)$ suggests that the isovalent Cd substitution for Zn disorders the exchange interactions between the Nd moments to hinder the first-order AFM transition. On the other hand, ${C}_{\text{m}}(T)$ for $y=1$ and 2 exhibits a lambda-shaped anomaly, which is a characteristic of a second-order AFM transition, at elevated temperatures of 0.78 and 1.5 K, respectively. The stabilization of the AFM order by the Ga substitution indicates that $4p$ electron doping strengthens the AFM interaction. We therefore propose that the first-order transition in ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20}$ is maintained by competitive magnetic interactions inherent in the Nd diamond sublattice.