arXiv subjects

J. G. Sereni

Publications and source records attributed to J. G. Sereni.

30 records · Page 2Linked to original sources

arXiv2013

From magnetic to Fermi Liquid behavior in CeCo{1-x}Fe{x}Si alloys

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 temperature, ii) an intermediate region (0.20 < x < 0.30) showing a broad magnetic anomaly (C_A) in specific heat and iii) the non-magnetic 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 not affected by applied magnetic field up to B ~ 10 Tesla.

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arXiv2011

Tri-critical point and suppression of the Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn by Ni doping

Structural, magnetization and heat capacity measurements were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 \leq x \leq 0.25$) alloys, covering the full range of the Mo$_2$FeB$_2$ structure stability. In this system, the two transitions observed in Ce$_2$Pd$_2$Sn (at $T_N=4.8$\,K and $T_C=2.1$\,K respectively) converge into a tri-critical point at $T_{cr}\approx 3.4$\,K for $x\approx 0.3$, where the intermediate antiferromagnetic AF phase is suppressed. The $T_N(x)$ phase boundary decrease is due to an incipient Kondo screening of the Ce-4f moments and local atomic disorder in the alloy. Both mechanisms affect the formation of Ce-magnetic dimers on which the Shastry-Sutherland lattice (SSL) builds up. On the contrary, the $T_C(x)$ transition to the ferromagnetic ground state increases as a consequence of the weakening of the AF-SSL phase. Applied magnetic field also suppresses the AF phase like in the stoichiometric compound.

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arXiv2011

Approaching Quantum Criticality in ferromagnetic Ce_2 (Pd_1-x Rh_x)_2 In alloys

Low temperature magnetic and thermal (C_m) properties of the ferromagnetic (FM) alloys Ce_2.15 (Pd_1-x Rh_x)_1.95 In_0.9 were investigated in order to explore the possibility for tuning a quantum critical point (QCP) by doping Pd with Rh. As expected, the magnetic transition observed at T = 4.1K in the parent alloy decreases with increasing Rh concentration. Nevertheless it splits into two transitions, the upper being antiferromagnetic (AF) whereas the lower FM. The AF phase boundary extrapolates to T_N = 0 for x_cr ~ 0.65 whereas the first order FM transition vanishes at x ~ 0.3. The QC character of the T_N => 0 point arises from the divergent T dependence of the tail of C_m/T observed in the x = 0.5 and 0.55 alloys, and the tendency to saturation of the maximum of C_m(T_N)/T as observed in exemplary Ce compounds for T_N => 0. Beyond the critical concentration the unit cell volume deviates from the Vegard's law in coincidence with a strong increase of the Kondo temperature.

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arXiv2011

Structural, magnetic and thermal properties of the substitution series Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn

Structural, magnetization and heat capacity studies were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 \leq x \leq 1$) alloys. The substitution of Pd atoms by isoelectronic Ni leads to a change in the crystallographic structure from tetragonal (for $x \leq 0.3$) to centered orthorhombic lattice (for $x \geq 0.4$). The volume contraction thorough the series is comparable to the expected from the atomic size ratio between transition metal components. The consequent weak increase of the Kondo temperature drives the two transitions observed in Ce$_2$Pd$_2$Sn to merge at $x = 0.25$. After about a 1% of volume collapse at the structural modification, the system behaves as a weakly magnetic heavy fermion with an enhanced degenerate ground state. Notably, an incipient magnetic transition arises on the Ni-rich size. This unexpected behavior is discussed in terms of an enhancement of the density of states driven by the increase of the $4f$-conduction band hybridization and the incipient contribution of the first excited crystal field doublet on the ground state properties.

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arXiv2010

Electron concentration effects on the Shastry-Sutherland phase stability in Ce_{2-x}Pd_{2+y}In_{1-z} solid solutions

The stability of a Shastry-Sutherland ShSu phase as a function of electron concentration is investigated through the field dependence of thermal and magnetic properties of the solid solution Ce_{2-x}Pd_{2+y}In_{1-z} in the antiferromagnetic branch. In these alloys the electronic (holes) variation is realized by increasing $Pd$ concentration. The AF transition T_M decreases from 3.5K to 2.8K as $Pd$ concentration increases from y=0.2 to y=0.4. By applying magnetic field, the ShSu phase is suppressed once the field induced ferromagnetic polarization takes over at a critical field B_{cr} which increases with $Pd$ content. A detailed analysis around the critical point reveals a structure in the maximum of the dM/dB derivative, which is related with incipient steps in the magnetization M(B) as predicted by the theory for the ShSu lattice. The crossing of M(B) isotherms, observed in ShSu prototype compounds, is also analyzed. The effect of $In$ substitution by $Pd$ is interpreted as an increase of the number of 'holes' in the conduction band and results in a unique parameter able to describe the variation of the magnetic properties along the studied range of concentration.

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arXiv2010

Suppression of the Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn at a field induced critical point

The magnetic phase diagram of Ce$_2$Pd$_2$Sn is investigated through the field dependence of thermal, transport and magnetic measurements performed at low temperature. The upper transition, $T_M=4.8$ K is practically not affected by magnetic field up to B=1 T, whereas the lower one $T_C(B)$ rapidly increases from 2.1 K joining $T_M$ in a critical point at $T_{cr}=(4.2\pm 0.3$)K for $B_{cr}=(0.12\pm 0.03)$ T. At that point the intermediate phase, previously described as an unstable Shastry-Sutherland phase, is suppressed. A detailed analysis around the critical point reveals a structure in the maximum of the $\partial M/\partial B(B)$ derivative which could be related to the formation of a novel phase in that critical region.

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arXiv2009

Unstable Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn

Thermal ($C_P$), magnetic ($M$ and $χ_{ac}$) and transport ($ρ$) measurements on Ce$_{2}$Pd$_{2}$Sn are reported. High temperature properties are well described by the presence of two excited crystal field levels at $(65\pm 5)$K and $(230\pm 20)$K, with negligible hybridization (Kondo) effects. According to literature, two transitions were observed at $T_M=4.8$K and $T_C =2.1$K respectively. The upper transition cannot be considered as a standard anti-ferromagnetic because of frustration effects in a triangular network of Ce-atoms and the positive sign of the paramagnetic temperature $θ_P^{LT} = 4.4$K. The nature of the this intermediated phase is described accounting for the formation of ferromagnetic (F) Ce-dimers disposed in a quasi-2D square lattice, resembling a Shastry-Sutherland pattern. According to hysteretic features in $ρ(T)$ and $χ_{ac}(T)$, the lower F-transition is of first order, with $C_P(T<T_C)$ revealing a gap of anisotropy $E_g\approx 7$K.

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arXiv2006

Quantum criticality in the cubic heavy-fermion system CeIn_{3-x}Sn_x

We report a comprehensive study of CeIn$_{3-x}$Sn$_x$ $(0.55 \leq x \leq 0.8)$ single crystals close to the antiferromagnetic (AF) quantum critical point (QCP) at $x_c\approx 0.67$ by means of the low-temperature thermal expansion and Grüneisen parameter. This system represents the first example for a {\it cubic} heavy fermion (HF) in which $T_{\rm N}$ can be suppressed {\it continuously} down to T=0. A characteristic sign change of the Grüneisen parameter between the AF and paramagnetic state indicates the accumulation of entropy close to the QCP. The observed quantum critical behavior is compatible with the predictions of the itinerant theory for three-dimensional critical spinfluctuations. This has important implications for the role of the dimensionality in HF QCPs.

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arXiv2006

Quantum Criticality in doped CePd_1-xRh_x Ferromagnet

CePd_1-xRh_x alloys exhibit a continuous evolution from ferromagnetism (T_C= 6.5 K) at x = 0 to a mixed valence (MV) state at x = 1. We have performed a detailed investigation on the suppression of the ferromagnetic (F) phase in this alloy using dc-(χ_dc) and ac-susceptibility (χ_ac), specific heat (C_m), resistivity (ρ) and thermal expansion (β) techniques. Our results show a continuous decrease of T_C (x) with negative curvature down to T_C = 3K at x*= 0.65, where a positive curvature takes over. Beyond x*, a cusp in cac is traced down to T_C* = 25 mK at x = 0.87, locating the critical concentration between x = 0.87 and 0.90. The quantum criticality of this region is recognized by the -log(T/T_0) dependence of C_m/T, which transforms into a T^-q (~0.5) one at x = 0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegard's law (at x_V~0.75) and increasing hybridization effects on high temperature χ_dc and ρ. Coincidentally, a Fermi liquid contribution to the specific heat arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C_m/T flattening is observed for x > x_cr rather the mentioned power law divergence, which coincides with a change of sign of β. The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.

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arXiv2004

Low temperature magnetic phase diagram of the cubic non-Fermi liquid system CeIn_(3-x)Sn_x

In this paper we report a comprehensive study of the magnetic susceptibility (χ), resistivity (ρ), and specific heat (C_P), down to 0.5 K of the cubic CeIn_(3-x)Sn_x alloy. The ground state of this system evolves from antiferromagnetic (AF) in CeIn_3(T_N=10.2 K) to intermediate-valent in CeSn_3, and represents the first example of a Ce-lattice cubic non-Fermi liquid (NFL) system where T_N(x) can be traced down to T=0 over more than a decade of temperature. Our results indicate that the disappearance of the AF state occurs near x_c ~ 0.7, although already at x ~ 0.4 significant modifications of the magnetic ground state are observed. Between these concentrations, clear NFL signatures are observed, such as ρ(T)\approx ρ_0 + A T^n (with n<1.5) and C_P(T)\propto -T ln(T) dependencies. Within the ordered phase a first order phase transition occurs for 0.25 < x < 0.5. With larger Sn doping, different weak ρ(T) dependencies are observed at low temperatures between x=1 and x=3 while C_P/T shows only a weak temperature dependence.

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arXiv2004

Complex magnetic phase diagram of CePd_{2-x}Ni_xAl_3 around its critical concentration

Thermal, magnetic and transport measurements are reported on CePd_{2-x}Ni_xAl_3 in the 0<= x <= 1 range, including the effect of pressure (p) and magnetic field on some selected samples. The low temperature results indicate that long range antiferromagnetic order is stable up to x=0.2, while between 0.25 and 0.5 magnetic disorder gives rise to non-Fermi-liquid (NFL) behavior. In this region, the low temperature specific heat can be described by the sum of two components, the major one showing a C_P/T=gamma_0 -gamma_1 T^(1/2) dependence, while the minor one includes a decreasing number of degrees of freedom related to a residue of short range order. The latter extrapolates to zero between 0.45<x_{cr}<0.5. Electrical resistivity (rho) studies performed under pressure for x=0.5 allow to investigate the evolution of the NFL state right above the critical point, where the exponent of rho ~ T^n is found to increase from n=1 (for p=0) up to n=2 (for p=12 kbar). The latter is observed for x=1.0 already at normal pressure, indicating the onset of the Fermi Liquid behavior. Doping and pressure effects are compared by fitting high temperature resistivity data employing a unique function which allows to describe the evolution of the characteristic energy of this series through a large range of concentration and pressure.

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arXiv2001

Study of the Ce(Rh$_{1-x}$Pd$_x$)$_2$Si$_2$ alloy: evidence for itinerant character of the magnetic order in CeRh$_2$Si$_2$

We present electrical resistivity and specific heat measurements of alloys on the Rh rich side of the phase diagram of the Ce(Rh$_{1-x}$Pd$_x$)$_2$Si$_2$ system and compare the results with those obtained at intermediate and low Rh concentrations. The analysis of the x-evolution of the entropy and the scaling behaviour of $C_{el}(T)$ and $ρ(T)$ clearly confirm a separation of the magnetic phase diagram into two regions. The region $x\le0.3$, showing a concentration independent characteristic temperature $T_o\approx45 {\rm K}$, while for $x>0.3$ $T_o$ decreases down to $T_o(x=1)\approx15 {\rm K}$. This characteristic temperature is obtained by scaling the $C_{el}(T,x>0.3)$ results as a function of the reduced temperature $T/T_o$. At low Pd-content, $T_N$ decreases very rapidly from $T_N = 36 {\rm K}$ in pure CeRh$_2$Si$_2$ to $T_N=18 {\rm K}$ at x=0.1. With higher Pd concentration it stabilized at $T_N\approx15 {\rm K}$ whereas the magnitude of the anomalies in $C_{el}(T)$ and in the susceptibility around $T_N$ are further reduced and disappear at $x\approx0.3$. This is in contrast to the behavior found on the Pd-rich side, where $T_N$ decreases continuously to zero with increasing Rh content. The pronounced differences observed between both phase boundaries and the drastic effect of doping on the Rh rich side suggest an itinerant character in CeRh$_2$ Si$_2$, in contrast to the localized character of CePd$_2$Si$_2$. A further evidence for the itinerant character in CeRh$_2$Si$_2$ is given by the $ρ(T)$ dependence observed for $x\le0.3$, which scales with $ρ(T)$ of the prototype itinerant compound YCo$_2$.

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