C. B. R. Jesus

Temperature dependence of coercive field of ZnFe2O4 nanoparticles

Structural and magnetic measurements on ZnFe2O4 nanoparticles obtained through co-precipitation chemical method are reported. The Rietveld analysis of X-ray patterns reveal that (i) our samples are single phase, and (ii) the average particle size increases with synthesis temperature. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the average blocking temperature increases for increasing mean particle size. Besides, one can observe via magnetization measurements that our particle size distribution also increases as a function of synthesis temperature. Finally, we have observed that the coercive field does not decay with the square root of temperature following the Neel relaxation and the Bean-Livingston approaches. In order to fit our experimental data, we have used a generalized model that proposes a temperature dependence of blocking temperature due to the coexistence of blocked and unblocked particles. This proposed generalized model shows good agreement with our ...

The role of chelating agents on the structural and magnetic properties of α-Fe2O3 nanoparticles

In this work we have studied the role of the addition of chelating agents on the structural and magnetic properties of α-Fe2O3 nanoparticles obtained by the co-precipitation method. The precursors were prepared for the addition of different concentrations of the chelating agents: sucrose and glycerine. To obtain the nanoparticles, these precursors were heated in the temperature range between 200 and 400 °C. The samples have been characterized via x-ray diffraction (XRD), scanning electron microscopy (SEM), and magnetization measurements. The XRD data confirm that the crystalline phase is already formed at temperatures around 200 °C and there is a preferential growth to the (110) crystallographic plane to the sample at 0.01 mol/l of sucrose. Besides, a more careful analysis performed in the XRD, SEM, and zero field cooling and field cooling magnetization data clearly show the dependence of the size, shape, and size distribution of the samples as function of the chelating agent concentration.

Magnetic properties of nearly stoichiometric CeAuBi2 heavy fermion compound

Motivated by the interesting magnetic anisotropy found in the heavy fermion family CeTX2 (T = transition metal and X = pnictogen), here, we study the novel parent compound CeAu1−xBi2−y by combining magnetization, pressure dependent electrical resistivity, and heat-capacity measurements. The magnetic properties of our nearly stoichiometric single crystal sample of CeAu1−xBi2−y (x = 0.92 and y = 1.6) revealed an antiferromagnetic ordering at TN = 12 K with an easy axis along the c-direction. The field dependent magnetization data at low temperatures reveal the existence of a spin-flop transition when the field is applied along the c-axis (Hc ∼ 7.5 T and T = 5 K). The heat capacity and pressure dependent resistivity data suggest that CeAu0.92Bi1.6 exhibits a weak heavy fermion behavior with strongly localized Ce3+ 4f electrons. Furthermore, the systematic analysis using a mean field model including anisotropic nearest-neighbors interactions and the tetragonal crystalline electric field (CEF) Hamiltonian allo...

Evolution of the magnetic properties along the RCuBi2 (R = Ce, Pr, Nd, Gd, Sm) series of intermetallic compounds

In this paper, the evolution of the magnetic properties along the series of intermetallic compounds RCuBi2 (R = Ce, Pr, Nd, Gd, Sm) is discussed. These compounds crystallize in a tetragonal ZrCuSi2 (P4/nmm) structure, and our single crystals of RCuBi2 grown from Bi-flux show no evidence for Cu-deficiency [Ye et al., Acta Crystallogr. C 52, 1325 (1996)] as previously reported for R = Ce. For R = Ce, Pr, Gd, and Sm, we found an antiferromagnetic ordering at TN ∼ 16 K, 4.2 K, 13.6 K, and 4.9 K, respectively. For R = Nd, we saw no evidence for a magnetic phase transition down to T = 2 K. These values of TN clearly show a dramatic breakdown of the De Gennes factor in this series. We discuss our data taken into account the tetragonal crystalline electrical field and the anisotropic Ruderman-Kittel-Kasuya-Yoshida magnetic interaction between the R-ions in this family of compounds.

Electron spin resonance of Gd3+ in the intermetallic Gd1–xYxNi3Ga9 (0 ≤ x ≤ 0.90) compounds

In this work, experiments of X-ray diffraction, magnetic susceptibility, heat capacitance, and Electron Spin Resonance (ESR) carried out in the Gd1–xYxNi3Ga9 (0 ≤ x ≤ 0.90) compounds grown through a Ga self flux method are reported. The X-ray diffraction data indicate that these compounds crystallize in a trigonal crystal structure with a space group R32. This crystal structure is unaffected by Y-substitution, which produces a monotonic decrease of the lattice parameters. For the x = 0 compound, an antiferromagnetic phase transition is observed at TN = 19.2 K, which is continuously suppressed as a function of the Y-doping and extrapolates to zero at x ≈ 0.85. The ESR data, taken in the temperature range 15 ≤ T ≤ 300 K, show a single Dysonian Gd3+ line with nearly temperature independent g-values. The linewidth follows a Korringa-like behavior as a function of temperature for all samples. The Korringa rates (b =  ΔH/ΔT) are Y-concentration-dependent indicating a “bottleneck” regime. For the most diluted sa...

Tuning the structural transition by chemical substitutions in Eu3Ir4Sn13 intermetallic compound

Resumo The intermetallic compound Eu3Ir4Sn13 is known to present a peculiar structural distortion at TS~60K and an antiferromagnetic transition at TN~11K.[1]. In this work, Eu3Ir4Sn13 single crystals were grown by Sn self-flux technique and a small pecentage of Ga was added. We will show how Ga-substitution affects the transitions and, compare with previous hidrostatic pressure results, we discuss the relation between the structural transition and the magnetic and eletronic properties of the compound.

Chemical substitution effects on YMn2 intermetallic antiferromagnet

In this study we present the results of synthesis and characterization of single crystals of the antiferromagnetic intermetallic compound YMn2, unalloyed and with chemical substitutions, using a Sn-flux technique. In particular, we inspect the effects of Cu chemical substitutions in the Mn site of YMn2, and investigate the Cu-concentration dependence of the magnetic properties of this compound. Comparing our results to previous ones, we discuss the interplay between its doping induced properties.

The role of Ni vacancies on the physical properties of CeNixBi2-y single crystals

The search for new superconductors (SC) represents an exciting field in condensed matter physics. Recently, CeNi0.8Bi2 has been reported as a new bulk SC (Tc ~ 4.2 K) in which Ni vacancies play an essential role for the emergence of superconductivity. Here we report experiments of magnetic susceptibility and heat capacity combined with energy dispersive X- ray spectroscopy analysis on CeNixBi2-y (x = 0.62, 0.70, 0.74, 0.78) single crystals. Our data show a systematic enhancement of the antiferromagnetic transition temperature (TN) with x, as well as the low temperature Ce3+ magnetic anisotropy. In particular, we find that the crystal- field ground state doublet becomes more isolated from the excited states as one approaches the Ni-rich end. As a consequence of such subtle crystal-field evolution, our analyses suggest that both the magnetic frustration and the hybridization between Ce3+ 4f and conduction electrons are decreasing along the series. In addition, SC transitions are observed in both La and Ce members with no bulk signature, strongly indicating the presence of impurity SC phases.

Electron Spin resonance of Gd3+ in three dimensional topological insulator Bi2Se3

Bi2Se3 has been claimed to be a three dimensional topological insulator (TI) with topologically protected metallic surface states with exotic properties. We have performed electron spin resonance (ESR) measurements on Gd3+ doped (x ≈ 0.01) Bi2Se3 single crystal grown from stoichiometric melt. For the studied crystals, our preliminary results revealed a partly resolved Gd3+ fine structure spectrum with Dysonian (metallic character) lines. At room temperature, the central line has a g ≈ 1.98, a linewidth ΔH ≈ 95 G and the spectra have a overall splitting of roughly 1300 Oe. As the temperature is decreased, the Gd3+ ESR ΔH of the central line presents a very small Korringa-like behavior b =ΔH/ΔT ≈ 0.013 Oe/K and nearly T-independent g-value. However, for T ≲ 40 K, ΔH shows a stronger narrowing effect evolving to Korringa-like behavior (b ≈ 0.15 Oe/K) for T ≲ 30 K. Concomitantly with the change in ΔH behavior, the Gd3+ central line g value starts to decrease reaching a value of 1.976 at T =4.2 K. The ESR results are discussed in terms of possible effects of protected topological surface states enlightened by complementary data from macroscopic measurements.