E. M. Bittar

Crystal structure and physical properties of Gd3Co4Sn13 intermetallic antiferromagnet

We have synthesized single crystalline samples of Gd3Co4Sn13 intermetallic compound using a Sn-flux method. This compound crystallizes with a cubic Yb3Co4Sn13-type structure, space group Pm-3n, which has 40 atoms per unit cell. Measurements of the magnetic susceptibility, heat capacity, electrical resistivity, and electron spin resonance (ESR) revealed that Gd3Co4Sn13 is a metallic Curie-Weiss paramagnet at high temperature and presents an antiferromagnetic ordering below TN=14.5K. In the paramagnetic state, a single Gd3+ ESR line with a nearly temperature independent g∼2.005(2) is observed, and its linewidth follows a Korringa-like behavior as a function of temperature. From the Korringa rate (ΔH∕ΔT∼4Oe∕K) and g-shift (Δg∼0.012) obtained from the ESR experiments combined with the magnetic susceptibility and specific heat data for Gd3Co4Sn13, we have extracted the exchange parameters between the Gd3+ local moments and the conduction-electrons (c-e) in this compound. This exchange parameter Jfs≈10meV was f...

Electron spin resonance study of the LaIn3−xSnx superconducting system

The LaIn(3-x)Sn(x) alloy system is composed of superconducting Pauli paramagnets. For LaIn3 the superconducting critical temperature T(c) is approximately 0.7 K and it shows an oscillatory dependence as a function of Sn substitution, presenting its highest value T(c) ≈ 6.4 K for the LaSn3 end member. The superconducting state of these materials was characterized as being of the conventional type. We report our results for Gd3+ electron spin resonance measurements in the LaIn(3-x)Sn(x) compounds as a function of x. We show that the effective exchange interaction parameter J(fs) between the Gd3+ 4f local moment and the s-like conduction electrons is almost unchanged by Sn substitution and observe microscopically that LaSn3 is a conventional superconductor.

Magnetic, thermal and transport properties of Cd doped CeIn3

We have investigated the effect of Cd substitution on the archetypal heavy fermion antiferromagnet CeIn3 via magnetic susceptibility, specific-heat, and resistivity measurements. The suppression of the Neel temperature, TN, with Cd doping is more pronounced than with Sn. Nevertheless, a doping-induced quantum critical point does not appear to be achievable in this system. The magnetic entropy at TN and the temperature of the maximum in resistivity are also systematically suppressed with Cd, while the effective moment and the Curie-Weiss temperature in the paramagnetic state are not affected. These results suggest that Cd locally disrupts the antiferromagnetic order on its neighboring Ce moments, without affecting the valence of Ce. Moreover, the temperature dependence of the specific heat below TN is not consistent with three-dimensional magnons in pure as well as in Cd-doped CeIn3, a point that has been missed in previous investigations of CeIn3 and that has bearing on the type of quantum criticality in this system.

Probing the localized to itinerant behavior of the 4f electron in CeIn 3-xSn x by Gd3 + electron spin resonance

The CeIn 3-xSn x cubic heavy fermion system presents an antiferromagnetic transition at T N=10 K, for x=0, that decreases continuously down to 0 K upon Sn substitution at a critical concentration of x c0.65. In the vicinity of T N→0 the system shows non-Fermi liquid behavior due to antiferromagnetic critical fluctuations. For a high Sn content, x2.2, intermediate valence effects are present. In this work we show that Gd3 +-doped electron spin resonance (ESR) probes a change in the character of the Ce 4f electron, as a function of Sn substitution. The Gd3 + ESR results indicate a transition of the Ce 4f spin behavior from localized to itinerant. Near the quantum critical point, on the antiferromagnetic side of the magnetic phase diagram, both localized and itinerant behaviors coexist.

Heavy fermion Ce3Co4Sn13 compound under pressure

The non-magnetic heavy fermion compound Ce3Co4Sn13 was studied under pressure. We report single crystalline measurements of electrical resistivity as a function of temperature ρ(T) under pressure. Some characteristic features related to a structural transition (TS), crystalline field effects (TCEF), and a low temperature maximum (Tmax), possibly connected simultaneously to the onset of Kondo lattice coherence and short range magnetic correlations, were identified in the ρ(T) data. A pressure-temperature phase diagram with TS and Tmax was constructed by mapping these features. Like for most Ce-based heavy fermion compounds, Tmax moves to higher temperatures with pressure, indicating that it is related to the Kondo energy scale, due to the increase of hybridization induced by pressure. On the other hand, TS, associated to a superlattice distortion and probably combined with a charge density wave transition, decreases as a function of pressure. However, differently from the Sr3−xCaxIr4Sn13 system, where a su...

Competing orders in underdoped (Ba1?xKx)Fe2As2

We report 75As Nuclear Magnetic Resonance (NMR) measurements in the high-Tc superconductor (Ba1?xKx)Fe2As2 in the underdoped regime. A structural transition at Ts ? 110 K is followed by an antiferromagnetic (AFM) order at TN ? 102 K for our x = 0.16 single crystal [1]. Superconductivity (SC) also appears at Tc ? 20 K. We find that the ordered Fe moment (S) is reduced upon hole-doping. Both spectrum analysis and relaxation measurements indicate that pinned vortices are present below Tc and SC is coexisting with AFM fluctuations.

Crystal structure and low temperature physical properties of Ho2CoGa8 intermetallic antiferromagnet

We have synthesized single crystalline samples of Ho2CoGa8 intermetallic compound using a Ga-flux method. This compound crystallizes with a tetragonal structure, space group P4∕mmm, and lattice parameters a=4.219(5)A and c=10.99(2)A. This structure is a bilayer version of the HoCoGa5 (1-1-5) which hosts a series of heavy-fermion superconductors and complex antiferromagnetic intermetallic systems. Measurements of magnetic susceptibility, heat capacity, and electrical resistivity revealed that Ho2CoGa8 is a metallic Curie–Weiss paramagnet at high temperature and presents an antiferromagnetic ordering below TN∼5K. The low temperature magnetic properties of this compound show the effects of tetragonal crystalline electrical field and the Ruderman–Kittel–Kasuya–Yosid interactions and the results presented here are compared with a broader description of the evolution of the low-T magnetic properties of structurally related series of rare-earth based tetragonal 2-1-8 and 1-1-5 compounds.

Traces of superconductivity in Li-doped BaFe2 As2 single crystals

Reportamos aqui os efeitos da substituicao por Li nas propriedades fisicas de monocristais de BaFe2 As2 . Medidas de susceptibilidade magnetica revelam tracos de uma fase supercondutora (SC) com Tc maxima de 9.0K para x=0.2 em monocristais de Ba1−x Lix Fe2 As2 , com volume SC de aproximadamente 15%. De medidas de resistividade eletrica, encontramos uma clara anomalia em Tc definida por uma queda abrupta na resistividade sem atingir um estado de resistividade zero. Medidas de resistividade sobre pressao mostraram uma evolucao dessa anomalia como funcao da pressao com uma Tc,mx = 15 K em P = 16 kbar. Tambem discutimos a influencia do caminho de crescimento (fluxe de Sn ou In) sobre a qualidade e propriedades dos nossos monocristais.

La doping effects in the coupling between localized and itinerant electronic states in EuFe2As2 probed by Eu2+ ESR

In this work we present an Electron Spin Resonance (ESR) study on La-doped EuFe2As2 aiming to further understand the coupling of localized spin states and itinerant electrons in these systems. The temperature dependence of the Eu2+ ESR linewidth, g-value and intensity was investigated for both monocrystalline and powdered samples. Our results confirm the suggestion of previous ESR experiments on the pure compound [1], that the SDW transition dramatically changes the coupling between localized and itinerant states. In addition, we found a peculiar dome like shape in the Eu2+ ESR linewidth temperature dependence for temperatures between TSDW and TN.