Exchange and crystal field effects in the ESR spectra of Eu(2+) in LaB(6)

Electron spin resonance of Eu(2+) (4f(7), S=7/2) in a La hexaboride (LaB(6)) single crystal shows a single anisotropic Dysonian resonance. From the observed negative g shift of the resonance, it is inferred that the Eu(2+) ions are covalent exchange coupled to the B 2p-like host conduction electrons. From the anisotropy of the spectra (linewidth and field for resonance), we found that the S ground state of Eu(2+) ions experience a cubic crystal field of a negative fourth order crystal field parameter (CFP), b(4)=-11.5(2.0) Oe, in agreement with the negative fourth order CFP, A(4), found for the non-S ground state R hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole R hexaboride family.761

Magnetic structure and enhanced T-N of the rare-earth intermetallic compound TbRhIn5: Experiments and mean-field model

In this work the physical properties of the intermetallic compound TbRhIn5 were investigated by means of temperature-dependent magnetic susceptibility, electrical resistivity, heat-capacity, and resonant x-ray magnetic diffraction experiments. TbRhIn5 is an intermetallic compound that orders antiferromagnetically at T-N=45.5 K, the highest ordering temperature among the existing RRhIn5 (1-1-5, R=rare earth) materials, which in contrast to what is expected from a de Gennes scaling along the RRhIn5 series. The x-ray resonant diffraction data have allowed us to solve the magnetic structure of TbRhIn5. Below T-N, we found a commensurate antiferromagnetic structure with a propagation vector (1/2,0,1/2) and the Tb moments oriented along the c axis. Strong (over two orders of magnitude) dipolar enhancements of the magnetic Bragg peaks were observed at both Tb absorption edges L-II and L-III, indicating a fairly high polarization of the Tb 5d levels. Using a mean-field model including an isotropic first-neighbor exchange interaction (J(R-R)) and the tetragonal crystalline electrical field (CEF), we evaluate the influence of the CEF effects in the physical properties of TbRhIn5. The results reported here seem to corroborate a general trend of CEF-driven effects on T-N along the RRhIn5 series.742

Evolution of the magnetic properties and magnetic structures along the RmMIn3m+2 (R = Ce, Nd, Gd, Tb; M = Rh, Ir; and m=1,2) series of intermetallic compounds

We discuss the evolution of the magnetic properties and magnetic structures along the series of intermetallic compounds RmMIn3m+2 (R=Ce, Nd, Gd, Tb; M=Rh, Ir; and m=1,2). The m=1,2 are, respectively, the single layer and bilayer tetragonal derivatives of their cubic RIn3 relatives. Using a mean field model including an isotropic first-neighbors Ruderman-Kittel-Kasuya-Yoshida interaction (K) and the tetragonal crystalline electrical field (CEF), we demonstrated that, for realistic values of K and CEF parameters, one can qualitatively describe the direction of the ordered moments and the behavior of the ordering temperature for these series. The particular case, where the rare-earth ordered moments lie in the ab plane or are tilted from the c axis and T-N can be reduced by tuning the CEF parameters, revealed an interesting kind of frustration that may be relevant to the physical properties of complex classes of materials such as the RmMIn3m+2 (M=Rh, Ir, and Co; m=1,2) heavy-fermion superconductors. (C) 2006 American Institute of Physics.99

Magnetic field dependence and bottlenecklike behavior of the ESR spectra in YbRh2Si2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Electron spin resonance (ESR) experiments at different fields or frequencies (4.1 <=nu <= 34.4 GHz) in the Kondo lattice (T-K similar or equal to 25 K) YbRh2Si2 single-crystal compounds confirmed the observation of a single anisotropic Dysonian resonance with g(perpendicular to c)congruent to 3.55 and no hyperfine components for 4.2 less than or similar to T less than or similar to 20 K. However, our studies differently reveal that (i) the ESR spectra for H-perpendicular to c show strong-field-dependent spin-lattice relaxation, (ii) a weak-field and temperature-dependent effective g value, (iii) a dramatic suppression of the ESR intensity beyond 15% of Lu doping, and (iv) a strong sample and Lu-doping (<= 15%) dependence of the ESR data. These results suggest a different scenario where the ESR signal may be associated to a coupled Yb3+-conduction electron resonant collective mode with a strong bottleneck and dynamiclike behavior.793Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)NSF (USA)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Synthesis and characterization of TM-doped CuO (TM = Fe, Ni)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Polycrystalline Cu1-xTMxO samples (x = 0 and 0.06; TM = Ni2+ and Fe3+) were grown using a co-precipitation method. The structural and magnetic properties were investigated by means of temperature dependent magnetic susceptibility and room temperature X-ray powder diffraction (XRPD). The XRPD analyses of the samples reveal the formation of single phase with structure isomorphous to the CuO. Interestingly, T-dependent magnetization shows the reduction of Neel temperature, T-N, from 213 K in the copper oxide to 70 K in the Fe-doped sample (x = 0.06). Because in the Ni-doped samples TN seems to be unaffected, this decrease in TN is believed to be due to the different electronic structure of the dopant. The ferromagnetic behavior observed at room temperature in all samples can be related to both the level of oxygen (excess or vacancy) of our samples and to the difference in the magnetic structure of the dopant. (c) 2008 Elsevier B.V. All rights reserved.35442-4448304832Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Synthesis and characterization of NiO and NiFe2O4 nanoparticles obtained by a sucrose-based route

Crystalline oxide powders were synthesized in nanoscale dimensions by a simple and novel chemical route, which is based on the use of sucrose as a chelating agent. The starting solutions were evaporated at 60 degrees C and the resulting gel was heated up to 300, 600 or 750 degrees C. The process was able to produce nickel oxide and nickel ferrite, characterized by structural and microscopic techniques. The average size of the particle was estimated by both Scherrer's equation and electron microscopy, and the results indicated that particles with a high crystallinity and a mean size in the range of 11-36nm were obtained. This synthesis route was able to produce NiFe2O4 and NiO nanoparticles at temperatures as low as 300 and 350 degrees C, respectively. (c) 2007 Elsevier Ltd. All rights reserved.68459459

Asymmetric impedance in field-annealed Co-based amorphous wires and its bias field dependence

Giant magnetoimpedance (GMI) was measured in Joule heated Co68.18Fe4.32Si12.5B15 amorphous wires. Samples heat treated in open air showed an asymmetric giant magnetoimpedance (AGMI) behaviour. The AGM I profile, measured at selected frequency values and bias circumferential fields, can be related to the exchange bias interaction between the soft magnetic amorphous material and a harder magnetic crystalline phase formed on the surface of the wire. The present data show that AGMI behaviour take place owing to a soft magnetic layer With unidirectional anisotropy that develops in the surface of the wire. (c) 2005 Elsevier B.V. All rights reserved.299241942

Structural and magnetic properties of nanocrystalline particles in an amorphous Fe73.5Nb3CuSi13.5B9 matrix

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We report structural and magnetic properties of fine particles embedded in an amorphous magnetic matrix. As-quenched amorphous Fe73.5Nb3CuSi13.5B9 ribbons (FINEMET) were submitted to the thermal treatments of several times (1 <= t <= 240 min) at 570 degrees C using a conventional furnace. The analyses of the X-ray diffraction patterns at room temperature reveal that our samples consist of single phase Fe3Si nanocrystals embedded in a residual amorphous phase. Magnetic measurements show that the saturation moment at T = 450 degrees C increases as a function of annealing time. This behavior is attributed to an increase of the fraction of nanocrystallites in the residual amorphous phase. (C) 2008 Elsevier B.V. All rights reserved.35442-4448714873Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP