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 $\rho \left(T\right)$ and the reduced magnetic entropy $S_{\text{m}}$ of $0.5R\ln 2$ at $T_{\text{N}}$. In this study, we measured $\rho \left(T\right)$, the isothermal magnetization $M\left(B\right)$, magnetic susceptibility $\chi \left(T\right)$, and specific heat $C\left(T\right)$ of ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20-x}{\mathrm{Cd}}_x$ for $x=1$ and ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20-y}{\mathrm{Ga}}_y$ for $y=1$ and 2. The sharp peak of the magnetic specific heat $C_{\text{m}}\left(T\right)$ 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 $C_{\text{m}}\left(T\right)$ 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, $C_{\text{m}}\left(T\right)$ 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.