Magnetic Ordering and Crystal Field Effects in Quasi Caged Structure Compound PrFe$_2$Al$_8$

Abstract

The compound PrFe$_2$Al$_8$ possesses a three-dimensional network structure resulting from the packing of Al polyhedra centered at the transition metal element Fe and the rare earth Pr. Along the $c$-axis, Fe and Pr form {\em chains} which are separated from each other by the Al-network. In this paper, the magnetism and crystalline electric field effects in PrFe$_2$Al$_8$ are investigated through the analysis of magnetization and specific heat data. A magnetic phase transition in the Pr lattice is identified at $T^{Pr}_{N}\approx$ 4~K in dc magnetization and ac susceptibility data. At 2~K, the magnetization isotherm presents a ferromagnetic saturation, however, failing to reach full spin-only ferromagnetic moment of Pr$^{3+}$. Metamagnetic step-like low-field features are present in the magnetization curve at 2~K which is shown to shift upon field-cooling the material. Arrott plots centered around $T^{Pr}_{N}$ display "S"-like features suggestive of an inhomogeneous magnetic state. The magnetic entropy, $S_m$, estimated from specific heat outputs a value of $R$ ln(2) at $T_{N2}$ suggesting a doublet state for Pr$^{3+}$. The magnetic specific heat is modeled by using a 9-level Schottky equation pertinent to the Pr$^{3+}$ ion with $J$ = 4. Given the crystalline electric field situation of Pr$^{3+}$, the inference of a doublet state from specific heat and consequent long-range magnetic order is an unexpected result.