Structural, transport and magnetic properties of single-crystalline samples of a praseodymium-based cubic compound PrMgNi 4 were studied. The single-crystal X-ray structural analysis revealed that Mg atoms are substituted for the Pr atoms at the 4a site by 4.5%. The χ(T ) data follow the Curie-Weiss law with an effective moment for the Pr 3+ ion. The magnetic specific heat divided by temperature, C m /T , shows a broad maximum at around 3 K, which is reproduced by a two-level model with a ground state doublet. On cooling below 1 K, C m /T approaches a constant value, which behavior is reproduced by a random two-level model. The twofold degeneracy of the ground state is lifted by symmetry lowering due to the substituted Mg atoms for the Pr atoms or strong hybridizations between the 4 f 2 electron states and conduction bands, which hinders the long-range quadrupole order.Praseodymium-based cubic compounds with a 4 f 2 configuration have attracted much attention because a variety of phenomena manifest themselves at low temperatures. Pr-filled skutterudites exhibit correlated electronic phenomena due to strong hybridization between the 4 f electron and conduction electrons (cf hybridization), e.g., heavy-fermion superconductivity, a metal-insulator transition, and higher-order multipolar ordering.[1-4] These phenomena are attributed to the interplay between the conduction electrons and multipolar degrees of freedom in quasi-degenerated ground states of the Pr ions under the crystalline electric field (CEF).In case the CEF ground state is the non-Kramers doublet of the Pr ion under cubic symmetry, the magnetic dipole is quenched but electric quadrupole and magnetic octupole become active. Thereby, the multipoles in the non-Kramers doublet could govern the low-temperature properties. A typical system is PrPb 3 , which exhibits an antiferroquadrupole (AFQ) order at T Q = 0.4 K [5][6][7]. Below T Q , the quadrupoles are modulated sinusoidally by the correlation between the quadrupoles and the conduction electrons [8,9]. Another example is PrInNi 4 , which presents a ferromagnetic transition at T C = 0.75 K.[10-12] The nonmagnetic ground state doublet could mix with the excited magnetic triplet by the ferromagnetic exchange interaction inducing a magnetic moment in the ground state which orders ferromagnetically.On the contrary, PrAg 2 In and PrMg 3 exhibit no quadrupole order. [13][14][15] The absence of the quadrupole order was discussed by considering the Kondo effect due to strong c-f hybridization, because the specific heat divided by temperature, C/T , is largely enhanced at low temperatures. However, the thermoelectric power is not enhanced and the absolute value is comparable to that of the La counterpart, indicating the conduction electrons at the Fermi level could not correlate strongly with the 4 f 2 electrons. [16,17] Moreover, in the specific heat measurements using a high-quality single crystal of PrAg 2 In with the residual resistivity ratio of RRR = 14, a cusp-type anomaly manifests itself at 0.33 K, indic...

Ytterbium-based heavy-fermion metals have recently attracted attention as magnetic refrigeration materials generating low-temperature environments below 1 K without using expensive 3He. YbCu4Ni is known to exhibit a giant value of specific heat divided by temperature C/T∼7.5J/K2mol below 0.2 K, implying high potential of magnetic refrigeration. In this paper, we report magnetic refrigeration down to 0.2 K from the initial temperatures of 1.8 K by YbCu4Ni ingots installed in a commercial 4He refrigerator. The performance is consistent with that evaluated by our DC magnetization and specific heat measurements. Our study demonstrates the high performance of YbCu4Ni without precious metals as a magnetic refrigeration material with moderately high density of Yb atoms (∼0.02Ybmol/cm3) and high thermal conductivity.

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Specific heat and electrical resistivity ρ of a cubic PrMgNi 4 were measured in magnetic fields B up to 14 T and 5 T, respectively. The magnetic specific heat C m for B = 0 exhibits a broad maximum at 5 K, which can be reproduced by a doublet-triplet two-level model, indicating the Γ 3 doublet ground state for the Pr 3+ ion under the cubic crystalline electric field (CEF). With applying B along the [100] axis, the maximum shifts to the higher temperature of 9 K at 14 T with raising the absolute value. On the other hand, in B || [110], the maximum of C m shifts to 10 K at 14 T, although the absolute value is not enhanced so much. This anisotropic behavior can be moderately reproduced by Schottky specific heat arising from splitting of the ground state doublet. Moreover, a maximum of C m at 1.5 K in B || [110] predicted from the CEF calculation does not appear, indicating possible quadrupole fluctuations in the ground state doublet due to the c-f hybridization. At around 0.7 K, both C m (T) and ρ(T) exhibit anomalies, which shift to higher temperatures with increasing B. It may arise from development of intersite correlation of the degrees of freedom in the ground state doublet.

We performed inelastic neutron scattering experiments on PrMgNi4 with the cubic MgSnCu4-type structure. The magnetic excitations were observed at 1.1, 2.5, 5.9, 10.7, and 11.7 meV. Since the energy of the excitations is constant for | Q | < 5 Å−1, they are ascribed to the crystalline electric field (CEF) excitations of the Pr3+ ions. We adopted CEF parameters of W = −3.3 K and x = 0.8 for the cubic T d point group to reproduce the strong excitations at 1.1, 10.7, and 11.7 meV. The calculated CEF level scheme reveals the Γ3 doublet ground state with the quadrupolar degrees of freedom and the excited states of Γ4 triplet (1.4 meV), Γ1 singlet (3.4 meV), and Γ5 triplet (13.5 meV). This scheme, however, does not explain the observed two excitations at 2.5 and 5.9 meV. These additional excitations may arise from splitting of the doublet by symmetry lowering due to excess Mg atoms occupying the Pr sites. This splitting is probably responsible for the absence of the long-range quadrupole order in PrMgNi4.