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Carreras Oropesa, W. G., S. Encina, P. Pedrazzini, V. F. Correa, J. G. Sereni, V. Vildosola, García D.J., and P. S. Cornaglia. "Minimal model for the magnetic phase diagram of CeTi1-xScxGe, GdFe1-xCoxSi, and related materials." Journal of Magnetism and Magnetic Materials 503 (2020): 166614.
Abstract: We present a theoretical analysis of the magnetic phase diagram of CeTi1-xScxGe and GdFe1-xCoxSi as a function
of the temperature and the Sc and Co concentration x, respectively. CeScGe and GdCoSi, as many other RTX
(R=rare earth, T=transition metal, X=p-block element) compounds, present a tetragonal crystal structure
where bilayers of R are separated by layers of T and X. While GdFeSi and CeTi0.75Sc0.25Ge are ferromagnetic,
CeScGe and GdCoSi order antiferromagnetically with the R 4f magnetic moments on the same bilayer aligned
ferromagnetically and magnetic moments in nearest neighbouring bilayers aligned antiferromagnetically. The
antiferromagnetic transition temperature TN decreases with decreasing concentration x in both compounds and
for low enough values of x the compounds show a ferromagnetic behavior. Based on these observations we
construct a simplified model Hamiltonian that we solve numerically for the specific heat and the magnetization.
We find a good qualitative agreement between the model and the experimental data. Our results show that the
main magnetic effect of the Sc→Ti and Co→Fe substitution in these compounds is consistent with a change in
the sign of the exchange coupling between magnetic moments in neighbouring bilayers. We expect a similar
phenomenology for other magnetic RTX compounds with the same type of crystal structure.
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Correa, V. F., A. G. Villagrán Asiares, D. Betancourth, S. Encina, P. Pedrazzini, P. S. Cornaglia, García D.J., J. G. Sereni, B. Maiorov, N. Caroca-Canales et al. "Strong magnetoelastic effect in CeCo1−xFexSi as Néel order is suppressed." Physical Review B 100 (2019): 184409.
Abstract: 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 nonmagnetic region
(x > xc), the magnetostriction strength gradually weakens again. The interplay between magnetic order and the
Kondo screening appears to cause an enhanced valence susceptibility slightly changing the Ce ions valence,
ultimately triggering the large magnetostriction observed around the critical concentration. Previous studies
of the evolution of the lattice parameters with x as well as magnetization and x-ray absorption spectroscopy
measurements support this hypothesis.
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Encina, S., and P. Pedrazzini. "Low Temperature Thermoelectric Power of Ce(Pd{1-x}Cux)2Si2." Journal of Low Temperature Physics 179, no. 1-2 (2015): 21–27.
Abstract: We present the thermoelectric power S(T) of the Ce(Pd 1−x Cu x ) 2 Si 2 alloy for temperatures \(1.5\,\mathrm{K} K. We observe three characteristic features across the \(0 substitution range: two positive maxima and a negative minimum, that are typical for Ce compounds that display, or lie close to, magnetism. Our analysis of the data shows that the high- T maximum is related to the Kondo effect on excited crystal-field levels, but that the low- T one cannot be simply associated with the Kondo scale, TK . We speculate that disorder induced by alloying can be at the origin of this discrepancy and can also be responsible for the low S(T) measured at low temperatures in the \(0.2 concentration range. We have extended electrical resistivity measurements on Ce(PdCu)Si 2 ( x=0.5 ) down to T∼40 mK in applied fields as high as 16 T.
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Encina, S., and P. Pedrazzini. "Poder termoeléctrico del Ce(Pd1-xCux)2Si2." In Anales de la Asociacion Fisica Argentina, 43–46. Vol. 24., 2013.
Abstract: Se presenta un estudio del poder termoeléctrico, S(T), a bajas temperaturas de la aleación Ce(Pd1-xCux)2Si2. Las curvas deS(T) tienen una dependencia similar en todo el rango de sustitución, con tres anomalÃas: un máximo positivo de altatemperatura, un mÃnimo negativo y un máximo de bajas temperaturas. Se discute brevemente el origen de las anomalÃas,para lo cual se compara S(T) con datos de resistividad eléctrica provenientes de la literatura. Para realizar las medicionesde poder termoeléctrico se diseñó y construyó un dispositivo que puede ser montado en diversos crióstatos.
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Encina, S., P. Pedrazzini, J. G. Sereni, and C. Geibel. "Low temperature thermopower and magnetoresistance of Sc-rich CeSc1-xTixGe." Physica B: Condensed Matter 536 (2018): 133–136.
Abstract: In CeSc1-xTixGe, Ti-alloying reduces the record-high antiferromagnetic (AFM) ordering temperature found in CeScGe at TN=46K and induces ferromagnetism for x≥0.5. In this work we focus on the AFM side, i.e. Sc-rich samples, and study their thermopower S(T) and magnetoresistance Ï(H,T). The measured S(T) is small in comparison with the thermopower of other Ce-systems and shows some features that are compatible with a weak hybridization between the 4f and band states. This is a further hint pointing to the local character of magnetism in this alloy. Magnetic fields up to 16T have a minor effect on the electrical resistivity of stoichiometric CeScGe. On the other hand, for x=0.65, we find that fields above 4T suppress the hump in Ï(T). Furthermore, the 4.2K magnetoresistance displays a strong decrease in the same field range, also in coincidence with magnetization results from the literature. Our results indicate that Ï(T,H) is a proper tool to assess the H−T phase diagram of this system.
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Sereni, J. G., P. Pedrazzini, M. G. Berisso, A. Chacoma, S. Encina, T. Gruner, N. Caroca-Canales, and C. Geibel. "Local character of the highest antiferromagnetic temperature of Ce systems in Sc-rich CeTi 1− x Sc x Ge." Physical Review B 91, no. 17 (2015): 174408.
Abstract: The highest antiferromagnetic (AFM) temperature in Ce based compounds has been reported for CeScGe with TN=47K, but its local or itinerant nature has not been deeply investigated yet. In order to shed more light into this unusually high ordering temperature we have investigated structural, magnetic, transport, and thermal properties of CeTi1−xScxGe alloys within the range of stability of the CeScSi-type structure: 0.25≤x≤1. Along this concentration range, this strongly anisotropic system presents a complex magnetic phase diagram with a continuous modification of its magnetic behavior, from ferromagnetism for 0.25≤x≤0.50 (with 7K≤TC≤16K) to AFM for 0.60≤x≤1 (with 19K≤TN≤47K). The onset of the AFM phase is associated to a metamagnetic transition with a critical field increasing from Hcr=0 at x≈0.55 to ≈6T at x=1, coincident with an increasing contribution of the first excited crystal electric field doublet. At a critical point xcr≈0.65 a second transition appears at TL≤TN. In contrast to observations in itinerant systems like CeRh2Si2 or CeRh3B2, no evidences for significant hybridization of the 4f electrons at large Sc contents were found. Therefore, the exceptionally large TN of CeScGe can be attributed to an increasing Ruderman-Kittel-Kasuya-Yosida interaction between Ce double layers as Sc content grows.
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Sereni, J. G., P. Pedrazzini, M. Gómez Berisso, A. Chacoma, S. Encina, T. Gruner, N. Caroca-Canales, and C. Geibel. "Exploring high temperature magnetic order in CeTi 1- x Sc x Ge." In Journal of Physics: Conference Series, 012005. Vol. 592., 2015.
Abstract: We studied the magnetic, transport, and thermodynamic properties of the alloy CeTi 1- x Sc x Ge in order to shed some light into the origin of the exceptionally large antiferromagnetic (AFM) ordering temperature T N = 47 K in pure CeScGe. We observed a complex magnetic phase diagram, which present an interesting dichotomy: Despite strong changes in the nature of the ordered state, from ferromagnetic (FM) for x ≤ 0.55 to AFM for x > 0.55, the ordering temperature increases smoothly and continuously from T C = 7 K at x = 0.25 to T N = 47 K at x = 1. Within the AFM regime we observe a metamagnetic transition at a critical field increasing from H = 0 at x ≈ 0.55 to μ 0 * H ≈ 6 Tesla at x = 1. Furthermore a second transition appears at T L ≤ T N for x ≥ 0.65. In contrast to observations in CeRh 2 Si 2 or CeRh 3 B 2 , we found no evidence for a strong hybridization of the 4f electrons at large Sc contents. Therefore the exceptionally large T N of CeScGe could be attributed to the unusually strong RKKY interaction in this type of compounds.
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