D. P. Rojas

Phonon softening on the specific heat of nanocrystalline metals

Specific heat enhancement in several common nanocrystalline metals is established by comparison with their bulk counterparts. Measurements were carried out in Fe, Cu, Ni and binary alloy LaAl(2). The excess specific heat is evidenced as a low temperature peak below 65 K and a high temperature slope above 150 K. The experimental data are in good agreement with a model which considers contributions from the grain boundary and core atoms in the nanoparticles. This model is supported by Raman spectroscopy measurements, showing a softening of the frequency phonon modes associated with a size reduction and increase of the atomic disorder.

Clusterization Phenomena and Magnetic Phase Transitions in Rare Earth Intermetallics

Two cases of anomalous phase transitions in Ce intermetallics are presented. In CeIn2 multiple signatures of a first order paramagnetic-ferromagnetic transition at 22 K have been experimentally detected. We discuss possible mechanisms for such behaviour and the new studies in the La1-xCexIn2 series. In the he case of CeNi1-xCux the low temperature ferromagnetic long range order appears owing to a magnetic cluster percolation process, from a cluster glass state established above the ferromagnetic state.

Antiferromagnetic behaviour of Tb2Al alloy

The structural, thermal and magnetic properties ol the Tb2Al alloy have been investigated by AC/DC magnetic susceptibility, specific heat, X-ray and neutron diffraction measurements. DC and AC-magnetic susceptibility results are consistent with an AFM order at TN = 52 K. The specific heat data show a lambda anomaly associated to the magnetic transition with a peak at 52 K (cord = 99 J/molTbK). The analysis of thermodiffractograms of neutron diffraction patterns indicates that, below the ordering temperature, the magnetic reflections can be indexed with a commensurate lattice related to the crystallographic cell (Pmna) by a propagation vector k = (1/2, 1/2, 1/2). The results are compared with those reported for other magnetic rare earth alloys of R2Al-type (with R = Nd, Gd and Dy).

Magnetization and specific heat of nanocrystalline rare-earth TbAl2, TbCu2 and GdAl2 alloys

Measurements of the magnetization and specific heat on nanocrystalline TbAl2, TbCu2 and GdAl2 with mean particle diameters below 40 nm show a different behavior from those made on bulk alloys. A reduction of the saturation magnetization quantified in TbAl2from 8.4 ?B/mol for the bulk to 5.8 ?B/mol for a 14 nm alloy is attributed to a spin disordered layer on the particle surface. In the specific heat, the lambda anomaly associated to the ferro (?c = 6.6 J/molK, 9.8 J/molK) and to the antiferromagnetic transitions (7.1 J/molK) in the bulk samples, respectively, is smeared out due to finite size effects.

Breakdown of the coherence effects and Fermi liquid behavior in YbAl 3 nanoparticles

A change in the Kondo lattice behavior of bulk YbAl3 has been observed when the alloy is shaped into nanoparticles (≈12 nm). Measurements of the electrical resistivity show inhibited coherence effects and deviation from the standard Fermi liquid behavior (T 2-dependence). These results are interpreted as being due to the effect of the disruption of the periodicity of the array of Kondo ions provoked by the size reduction process. Additionally, the ensemble of randomly placed nanoparticles also triggers an extra source of electronic scattering at very low temperatures (≈15 K) due to quantum interference effects.

Electron Spin Resonance of the Ferromagnetic YbNi2 Alloy

An electron spin resonance (ESR) signal was observed in the recently synthesized Kondo lattice with ferromagnetic (FM) correlations YbNi2 below 20 K. Significant broadening and disappearance of ESR line at temperatures above 11 K can be explained by the processes of the spin-lattice relaxation of the Yb ions via the first excited Stark doublet with the activation energy Δ ≈ 75 K. Measurements of specific heat and magnetization are in agreement with ESR results, indicating a FM behavior below TC = 10.5 K.

Heat capacity and magnetic phase diagram of the hydride CeRuSiH

The hydride CeRuSiH exhibits antiferromagnetic order with two transitions at TN1 = 7.5 and TN2 = 3.1 K. Furthermore, magnetization measurements M (H) up to H = 45 kOe shows a metamagnetic double transition at low temperatures, suggesting a complex magnetic phase diagram. Here, we present magnetization measurements and heat capacity data up to 90 kOe, which allows as to complete the magnetic phase diagram. In addition, from the analysis of the heat capacity, we propose a model for the crystal field splitting.

Pressure effects on the antiferromagnetic order of the hydride CeRuSiH

The non-magnetic heavy fermion CeRuSi upon hydrogen absorption develops an antiferromagnetic behaviour, mainly due to the pronounced expansion of the unit cell. The effects of pressure on the χ(T) and M(H) curves of the hydride CeRuSiH up to 10.3 kbar are presented. Both, the Neel temperatures and the critical fields of the metamagnetic transitions, increase with pressure, whereas the net magnetization decreases. This evolution could be understood within the framework of the Doniach diagram.

Magnetic properties of

We report measurements on magnetic properties of Ce2CuIn3Ce2CuIn3 compound. Irreversibility of the ZFC–FC curves of the DC magnetic susceptibility (M/H) at low field, and the shift of the peak with the frequency in the AC susceptibility give evidences for spin glass (SG) behavior at freezing temperature of Tf=2.1K. In addition, the relative shift per frequency decade, ΔTf/TfΔlogν=0.016, is similar to those reported for other spin glasses. We discuss the results in relation with the other isotypic, Ce2TX3Ce2TX3 (T=TransitionT=Transition metal and X=SiX=Si, Ge and In). The interplay of disorder and frustration within hexagonal CaIn2CaIn2 type of structure and competition between RKKY interaction and Kondo effect are suggested as the main causes for the arising magnetic ground state.