Article

# Mesoscopic magnetic states in metallic alloys with strong electronic correlations: a percolative scenario for CeNi 1-x Cux.

Departamento CITIMAC, Universidad de Cantabria, 39005 Santander, Spain.

Physical Review Letters (Impact Factor: 7.94). 04/2007; 98(16):166406. DOI:10.1103/PhysRevLett.98.166406 Source: PubMed

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**ABSTRACT:**The interplay between disorder and strong correlations has been observed experimentally in disordered cerium alloys such as Ce(Ni, Cu) or Ce(Pd, Rh). In the case of Ce(Ni, Cu) alloys with a Cu concentration x between 0.6 and 0.3, the first studies have shown a smooth transition with decreasing temperature from a spin glass phase to ferromagnetism; for x smaller than 0.2, a Kondo phase has been observed. The situation is more complicated now due to the recent observation of magnetic clusters. The competition between the Kondo effect, the spin glass (SG) and the ferromagnetic (FE) ordering has been extensively studied theoretically. The Kondo effect is described by the usual mean-field approximation; we have treated the SG behavior successively by the Sherrington-Kirkpatrick model, then by the Mattis model and finally by the van Hemmen model, which takes both a ferromagnetic part and a site-disorder random part for the intersite exchange interaction. We present here the results obtained by the van Hemmen-Kondo model: for a large Kondo exchange J(K), a Kondo phase is obtained while, for smaller J(K), the succession of an SG phase, a mixed SG-FE one and finally an FE one has been obtained with decreasing temperature. This model improves the theoretical description of disordered Kondo systems by providing a simpler approach for further calculations of magnetic clusters and can, therefore, account for recent experimental data on disordered cerium systems.Journal of Physics Condensed Matter 02/2011; 23(9):094207. · 2.36 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**The magnetic and transport properties of PrIr(2)B(2) and PrIr(2)B(2)C have been investigated by dc and ac magnetic susceptibility, specific heat, electrical resistivity and magnetoresistance measurements. PrIr(2)B(2) forms in CaRh(2)B(2)-type orthorhombic crystal structure (space group Fddd). At low fields the dc magnetic susceptibility of PrIr(2)B(2) exhibits a sharp anomaly near 46 K which is followed by an abrupt increase below 10 K with a peak at 6 K, and split-up in ZFC and FC data below 46 K. In contrast, the specific heat exhibits only a broad Schottky type hump near 9 K which indicates that there is no long range magnetic order in this compound. The thermo-remanent magnetization is found to decay very slowly with a mean relaxation time τ = 3917 s. An ac magnetic susceptibility measurement also observes two sharp anomalies; the peak positions strongly depend on the frequency and shift towards high temperature with an increase in frequency, obeying the Vogel-Fulcher law as expected for a canonical spin-glass system. The two spin-glass transitions occur at freezing temperatures T(f1) = 36 K and T(f2) = 3.5 K with shifts in the freezing temperatures per decade of frequency δT(f1) = 0.044 and δT(f2) = 0.09. An analysis of the frequency dependence of the transition temperature with critical slowing down, τ(max)/τ(0) = [(T(f)-T(SG))/T(SG)](-zν), gives τ(0) = 10(-7) s and exponent zν = 8, and the Vogel-Fulcher law gives an activation energy of 84 K for T(f1) and 27.5 K for T(f2). While zν = 8 is typical for spin-glass system, the characteristic relaxation time τ(0) = 10(-7) s is very large and comparable to that of superspin-glass systems. An addition of C in PrIr(2)B(2) leads to PrIr(2)B(2)C which forms in LuNi(2)B(2)C-type tetragonal structure (space group I4/mmm) and remains paramagnetic down to 2 K. The specific heat data show a broad Schottky type anomaly, which could be fairly reproduced with CEF analysis which suggests that the ground state is a CEF-split singlet and the first excited state singlet is situated 15 K above the ground state. The Sommerfeld coefficient γ∼300 mJ mol(-1) K(-2) of PrIr(2)B(2)C is very high and reflects a heavy fermion behaviour in this compound. We believe that the heavy fermion state in PrIr(2)B(2)C has its origin in low lying crystal field excitations as has been observed in PrRh(2)B(2)C.Journal of Physics Condensed Matter 09/2011; 23(37):376001. · 2.36 Impact Factor - [show abstract] [hide abstract]

**ABSTRACT:**The Sherrington-Kirkpatrick model, in the presence of a Gaussian random field, is studied through the replica formalism within a one-step replica-symmetry-breaking procedure. This model treated in the replica-symmetry approximation does not exhibit a spin-glass phase, since the corresponding order parameter becomes trivially induced by the random field. However, such a phase appears naturally through the present approach, being associated with the onset of replica-symmetry breaking. It is shown that the low-temperature negative-entropy problem, characteristic of the replica-symmetry approximation, is practically resolved within this improved approach. Phase diagrams, the corresponding order parameters, and thermodynamic properties are computed; the present results are expected to be very close to the correct ones, i.e., those that could be obtained from the full replica-symmetry-breaking procedure.Journal of Statistical Mechanics Theory and Experiment 01/2011; 7(07). · 1.87 Impact Factor

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