Synthesis and study of transport and magnetic properties of magnesium cage compounds $R$Ni$_{2}$Mg$_{20}$ ($R$ = Pr and Nd)

TL;DR

This study synthesizes magnesium cage compounds RNi2Mg20 with R = Pr and Nd, investigating their electrical, magnetic, and thermal properties, revealing magnetic ordering and low-temperature excitations related to their electronic states.

Contribution

It provides new insights into the magnetic and transport behaviors of RNi2Mg20 compounds, especially regarding their low-temperature specific heat and magnetic ordering phenomena.

Findings

Pr compound shows a doublet-triplet energy gap of 14 K.

Nd compound exhibits ferromagnetic order below 1.5 K.

Specific heat peaks correspond to crystal field excitations and magnetic ordering.

Abstract

We have synthesized magnesium cage compounds $R$Ni$_{2}$Mg$_{20}$ ($R$ = Pr and Nd). We report the measurements of electrical resistivity $\rho$, magnetic susceptibility $\chi$, isothermal magnetization $M$, and specific heat $C$. Polycrystalline samples with $R$ = Pr and Nd were obtained by annealing, while the counterparts for $R$ = La, Ce, and Y were found to be absent. The ${\rho}(T)$ data for $R$ = Pr and Nd monotonically decrease on cooling from 300 K to 40 K and exhibit shoulders at around 13 K and 15 K, respectively. The Curie-Weiss behaviors of the ${\chi}(T)$ data indicate the trivalent states of both the Pr and Nd ions. For $R$ = Pr, the maximum in $C(T)$ at around 7 K is reproduced by a doublet-triplet two-level model with an energy gap of 14 K. The peak in $C(T)$ at 0.7 K is attributed to a short-range order of quadrupolar degrees of freedom in the non-Kramers ground doublet. On the other hand, for $R$ = Nd, a maximum in $C(T)$ at around 9 K is explained by thermal excitation from a ground state doublet to an excited quartet separated by 23 K. Upon further cooling, $C(T)$ shows a lambda-shaped peak at 1.5 K. By applying magnetic fields up to 2 T, the peak becomes broad and shifts to higher temperatures, which is a characteristic of a ferromagnetic order.