Structural, magnetic, and thermal measurements performed on CeCo 1−x Fe x Si alloys are reported. Three regions can be recognized: (i) Co rich (x 0.20) with a decreasing long-range antiferromagnetic order which vanishes at finite temperatures, (ii) an intermediate region (0.20 < x 0.30) showing a broad magnetic anomaly (C A ) in specific heat, and (iii) the nonmagnetic region progressively changing from a non-Fermi-liquid-type behavior towards a Fermi-liquid one as Fe concentration increases. The C A anomaly emerges as an incipient contribution above T N already at x = 0.10, which indicates that this contribution is related to short-range correlations likely of quasi-two-dimensional type. Both T N transition and C A anomaly are practically unaffected by an applied magnetic field up to B ≈ 10 T.

We report a detailed study of the magnetic properties of CeCo0.85Fe0.15Si under high magnetic fields (up to 16 Tesla) measuring different physical properties such as specific heat, magnetization, electrical resistivity, thermal expansion and magnetostriction. CeCo0.85Fe0.15Si becomes antiferromagnetic at [Formula: see text] K. However, a broad tail (onset at [Formula: see text] K) in the specific heat precedes that second order transition. This tail is also observed in the temperature derivative of the resistivity. However, it is particularly noticeable in the thermal expansion coefficient where it takes the form of a large bump centered at T X . A high magnetic field practically washes out that tail in the resistivity. But surprisingly, the bump in the thermal expansion coefficient becomes a well pronounced peak fully split from the magnetic transition at T N . Concurrently, the magnetoresistance also switches from negative to positive above T N . The magnetostriction is considerable and irreversible at low temperature ([Formula: see text] at 2 K) when the magnetic interactions dominate. A broad jump in the field dependence of the magnetostriction observed at low T may be the signature of a weak ongoing metamagnetic transition. Taking altogether the results indicate the importance of the lattice effects on the development of the magnetic order in these alloys.

The non-magnetic heavy fermion behavior of CeRuGe is destroyed by hydrogen insertion. The resulting hydride CeRuGeH, investigated by magnetization, thermoelectric, electrical resistivity and specific heat measurements, exhibits an antiferromagnetic ordering below T(N) = 4.0(2) K weakly influenced by the Kondo effect. Below T(N), a metamagnetic double transition induced by an applied magnetic field was evidenced for CeRuGeH. This hydride presents a simple field-temperature phase diagram in comparison to that determined for the equivalent compound CeRuSiH.

We have tuned the paramagnetic heavy Fermion (HF) system CeTiGe to a more magnetic state and toward quantum criticality (QC) by partially substituting La for Ce. The induced volume expansion leads to a pronounced decrease of the Kondo temperature (T K ), from %50 K in pure CeTiGe to less than 4 K at large La content. Since the former value is of the order of the crystal field splitting, while the latter is well below, the systems evolves from a large degeneracy N ¼ 6 Kondo system at x ¼ 0 to a conventional N ¼ 2 Kondo system at x ! 0:8. There, the first excited crystal field doublets become apparent in the T dependence of the specific heat C P (T) at an excitation energy D CF % 30 K. The reduction of the degeneracy is expected to enhance the decrease of T K , explaining the strong reduction by more than one order of magnitude observed in this system. The decrease of T K drives the system toward QC as evidenced by a divergence of C P (T)/T and x(T) toward low temperatures in the x ¼ 0:8 sample.

A very strong magnetoelastic effect in the CeCo1−xFexSi alloys is reported. The strength of the magnetostrictive effect can be tuned upon changing x. The moderate low-temperature linear magnetostriction observed at low Fe concentrations becomes very large ( ∆L L (16T, 2K) = 3×10 −3 ) around the critical concentration (xc ≈ 0.23) at which the long-range antiferromagnetic order vanishes. Upon increasing doping through the non-magnetic region (x > xc), the magnetostriction strength gradually weakens again. Remarkably the low-temperature magnetostriction at the critical concentration shows a pronounced S-like shape (centered at Bm ∼ 6 T) resembling other well-known Ce-based metamagnetic systems like CeRu2Si2 and CeTiGe. Unlike what is observed in these compounds, however, the field dependence of the magnetization shows only a minor upturn around Bm vaguely resembling a metamagnetic behavior. The subtle interplay between magnetic order and the Kondo screening seems to originate an enhanced valence susceptibility slightly changing the Ce ions valence, ultimately triggering the large magnetostriction observed around the critical concentration.