Pr-magnetism in the quasi-skutterudite compound PrFe₂Al₈

Abstract

The intermetallic compound PrFe2Al8 that possesses a three-dimensional network structure of Al polyhedra centered at the transition metal element Fe and the rare earth Pr is investigated through neutron powder diffraction and inelastic neutron scattering in order to elucidate the magnetic ground state of Pr and Fe and the crystal field effects of Pr. Our neutron diffraction study confirms long-range magnetic order of Pr below TN = 4.5 K in this compound. Subsequent magnetic structure estimation reveals a magnetic propagation vector k = ( 1 2 0 1 2 ) with a magnetic moment value of 2.5 µB/Pr along the orthorhombic c-axis and evidence the lack of ordering in the Fe sublattice. The inelastic neutron scattering study reveals one crystalline electric field excitation near 19 meV at 5 K in PrFe2Al8. The energy-integrated intensity of the 19 meV excitation as a function of |Q|(Å −1 ) follows the square of the magnetic form factor of Pr 3+ thereby confirming that the inelastic excitation belongs to the Pr sublattice. The second sum rule applied to the dynamic structure factor indicates only 1.6(2) µB evolving at the 19 meV peak compared to the 3.58 µB for free Pr 3+ , indicating that the crystal field ground state is magnetic and the missing moment is associated with the resolution limited quasi-elastic line. The magnetic order occurring in Pr in PrFe2Al8 is counter-intuitive to the symmetry-allowed crystal field level scheme, hence, is suggestive of exchange-mediated mechanisms of ordering stemming from the magnetic ground state of the crystal field levels.

Electronic and magnetic properties of quasi-skutterudite PrCo2Ga8 compound

“…Such observations are largely attributed to induced magnetism which may occur through the formation of a pseudo-doublet ground state arising from the admixture of the ground state singlet and the first CEF level when the magnetic exchange energy is comparable to or higher than the first CEF excitation. In iso-structural compound PrFe 2 Al 8 , both macroscopic and microscopic measurements have confirmed the setting in of an AFM ordering below T N = 4 K [2,17]. Below about 0.8 K, C p (T )/T shows an upturn that develops into a logarithmic-like divergence.…”

“…In previous studies, similar magnetic ground state has been observed for systems predicted to have a CEF singlet ground state. Typical examples are the iso-structural compounds PrCo 2 Al 8 [1] and PrFe 2 Al 8 [2,17]. Other relevant examples are PrCoAl 4 [29], PrSi [30] and PrTl 3 [31].…”

Absence of a long-range ordered magnetic ground state in Pr3Rh4Sn13studied through specific heat and inelastic neutron scattering

“…This is due mainly to the fact that the crystal field level schemes for Pr in many of these high symmetry caged environments dictate a singlet ground state. In spite of which, several instances are found in the literature of Pr-based intermetallics (not only R 3 T 4 X 13 ) where long-range magnetic order or a spin glass state is established than the predicted non-magn etic state [21][22][23][24][25][26]. A primary reason for this observation is the admixture of the ground state energy with higher energy levels which could permit magnetic ordering.…”

Superzone Gap Formation and Possible Kondo-like Features in the Heavy Fermion PrFe2Ga8 Compound

“…The local symmetry of the rare-earth ion in 1:2:8 compounds is m (C s in Schoenflies notation) in the space group Pbam (where R occupies the 4g Wyckoff position) which predicts 9 singlets for the Pr 3+ ion with J = 4. However, clear signatures of magnetic ordering have been experimentally observed in Pr-based 1:2:8 compounds that our group investigated recently [3][4][5]. A long-range magnetic order can be established in a Pr compound due to the admixture of the ground state singlet with higher lying crystalline electric field (CEF) energy levels accompanied by the condition that the magnetic exchange energy is comparable to or higher than the first CEF excitation [6].…”