Physical properties of the magnetically frustrated very-heavy-fermion compound YbCu4Ni

Abstract

The physical properties of the very heavy fermion YbCu4Ni were characterized through structural, magnetic, thermal and transport studies along nearly four decades of temperature ranging between 50 milikelvin and 300 K. At high temperature, the crystal electric field levels splitting was determined with ∆1(Γ6) = 85 K and ∆2(Γ8) ≈ 200 K, the latter being a quartet in this cubic symmetry. An effective magnetic moment µ ef f ≈ 3µB is evaluated for the Γ7 ground state, while at high temperature the value for a Yb 3+ ion is observed. At low temperature this compounds shows the typical behavior of a magnetically frustrated system undergoing a change of regime at a characteristic temperature T * ≈ 200 mK into a sort of Fermi-liquid type 'plateau'of the specific heat: Cm/T |T →0 = const. The change in the temperature dependence of the specific heat coincides with a maximum and a discontinuity in respective inductive and dissipative components of the ac-susceptibility. More details from the nature of this ground state are revealed by the specific heat behavior under applied magnetic field.

Thermomagnetic properties of very heavy fermions suitable for adiabatic demagnetisation refrigeration at low temperature

“…1a belong to groups (II) and (III) respectively and better apply for CTD requirements because of their moderate slope allows a more convenient heat absorption more homogeneously distributed in temperature. Within group (II) one can mention YbCu 4 Ni [8] which represents the family of compounds showing a C m /T | T →0 ≈ 7.5±0.5 J/mol K 2 'plateau' followed by a common power law above a change of regime at T = T * [11]. The compounds included in group (III) are characterized by showing NFL behavior, like (Yb,Sc)Co 2 Zn 20 [7] and CeNi 9−x T x (where T = Co [14] with x = 0.1, and T = Cu with x = 0.4 [15] respectively).…”

“…Another characteristic is related with the size of the cooling window as a function of the applied field, as a Comparison of two characteristic parameters between CMN [3], YbPd2in [6],YbCu4.6Au0.4 [8] and Ce4Pt12Sn25 [17]. as a function of applied field: a) Field dependence of the entropy Sm(B) at 0.4K and b) cooling window ∆T at different fields in the adiabatic process.…”

“…5 in a logarithmic T dependence. There one can see how the density of excitations is distributed in energy in NFL systems (with a C m (T )/T ∝ − ln(T /T 0 ) dependence) respect to those with a power law (C f it (T )/T ∝ 1/T 1.24 [8]) like YbCu 4 Ni at T > T * also included into the figures.…”

“…Since C m /T is proportional to the density of excitations, the rich family of intermetallic heavy Fermions (HF) provides asignificant number of candidates for ADR applications. Among them are the recently synthesized Yb-intermetallic compounds [5][6][7][8][9][10], some of which have been tested as proper candidates for the proposed improvements because of their very large C m /T | T →0 values. These compounds were labeled as very HF (VHF) [11] because they largely exceed the values of classical HF [12].…”

“…All these paramagnetic materials do not order magnetically down to about 200 mK despite of their robust magnetic moments, providing large ∂M/∂T coefficients. As mentioned before, the entropy computed as: S m (T ) = C m /T dT is the basic parameter for the study of ARD processes because its thermal trajectory characterizes the efficiency and applicability of [6][7][8]. CMN salt (upper T scale) is included for comparison [3].…”