Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials

We report on zero-field muon spin relaxation studies of cerium based heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1 compositions muon spin relaxation functions were found to be temperature independent across T_c; no evidence for the presence of electronic magnetic moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K and superconducting below T_c=0.8 K. Muon spin realxation spectra show the gradual onset of damped coherent oscillations characteristic of magnetic order below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85% of the sample volume. No change in muon precession frequency or amplitude is detected on cooling below T_c, indicating the microscopic coexistence of magnetism and superconductivity in this material.Comment: 6 pages with 3 figures. Revision with corrected axis label (mK) in Fig.

Magnetic structure and critical behavior of GdRhIn5_{5}: resonant x-ray diffraction and renormalization group analysis

The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by resonant x-ray diffraction at the Gd LII and LIII edges, followed by a renormalization group analysis for this and other related Gd-based compounds, namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the isostructural Ce-based heavy-fermion superconductors. The ground state of GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the a-direction and antiparallel alignment along b and c. A comparison between this magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is determined by a competition between first-(J1) and second-neighbor(J2) antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an antiparallel alignment along the three directions (the so-called G-AFM structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM). In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is in a frontier between these two ground states, which may explain its non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close to the paramagnetic transition at TN = 39 K was also studied in detail. A typical second-order transition with the ordered magnetization critical parameter beta = 0.35 was experimentally found, and theoretically investigated by means of a renormalization group analysis.Comment: 22 pages, 4 figure

Effect of La doping on magnetic structure in heavy fermion CeRhIn5

The magnetic structure of Ce0.9La0.1RhIn5 is measured using neutron diffraction. It is identical to the incommensurate transverse spiral for CeRhIn5, with a magnetic wave vector q_M=(1/2,1/2,0.297), a staggered moment of 0.38(2)Bohr magneton per Ce at 1.4K and a reduced Neel temperature of 2.7 K.Comment: 5 pages, 2 figures, 1 table. Conf. SCES'200

Estudo das propriedades magnéticas de sistemas com alta correlação eletrônica

Orientador: Carlos RettoriTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Este trabalho de doutoramento apresenta, principalmente, uma sistemática de experimentos de Ressonância Paramagnética Eletrônica (RPE) e suscpetibilidade magnética para estudos das propriedades magnéticas de diversos sistemas com alta correlação eletrônica, tais como: 1) Supercondutores de alta temperatura crítica (HTS) 2) Sistemas de Férmions pesados (HFS), Materiais tipo Kondo e Compostos de Valência Intermediária (IV). 3) Perovskitas que apresentam o fenômeno de magnetoresistência colossal (CMR). Nos HFS de Eu2-xPrxCuO4 , uma sistemática de experimentos de magnetização para 0 £ x £ 1 em monocristais crescidos em diferentes cadinhos, permitiu-nos estudar o aparecimento de uma componente de ferromagnetismo fraco (WF) neste materias. Modos Raman "proibidos" foram somente observados em amostras que apresentam WF. Neste trabalho concluiu-se que para volumes de célula unitária menores que um volume crítico VC ¿ 181.1 Å 3 (próximo ao valor de V para Eu2CuO4 crescido em Al2O3/CuO) , a estrutura T¿ destes compostos apresenta distorções locais nos planos de CuO2 dando origem a componente WF e aos modos Raman "proibidos". Para o caso dos HFS RNi2B2C:Gd (R = Y, Lu), os valores obtidos para os parâmetros de troca 1/2 , extraídos dos estudos da dependência a com temperatura da largura de linha de RPE do Gd +3 e do decréscimo de Tc com o aumento da concentração de impurezas magnéticas, coincidem dentro do erro experimental. Este resultado indica que os compostos RNi2B2C (R = Y, Lu) comportam-se como um supercondutor BC convencional. Nestes compostos a interação de troca entre o Gd +3 e os elétrons de condução (c-e) mostrou-se dependende do vetor de onda q, e o parâmetro de troca J fs(0) foi encotrado positivo para (R = Y, Lu) indicando que a interação Gd +3 - c.e. é do tipo atômica. As interações elétron-elétron (relacionadas com o fator de Stoner), se mostraram significativas para as análises dos espectros de RPE. Nos estudo de RPE do Gd +3 nos compostos IV de YbInCu4 e no HFS de YbAgCu4 e seus compostos de referência (Y, Lu)(In, Ag)Cu4 , observamos um aumento do g-shift e da taxa de Korringa para os compostos de Yb em relação aos de referência. Este resultado é atribuído ao aumento da densidade de estados no nível de Fermi existente para os compostos de Yb. Para ambos os sistemas acima, a dependência da interação de troca Gd +3 e os c-e com vetor de onda q foi introduzida para analisar os dados de RPE. No entanto, somente para o composto de IV, YbInCu4 , interações elétron-elétron, se mostrou significativa para as análises dos espectros de RPE. Para o estudo dos compostos LuInNi4:Gd e Nd, uma contribuição tipo multibanda foi necessária para as análises de RPE. O valor da interação de troca entre a RE e os c-e é negativo para a banda-d (Jfd) é positivo para a banda-s (Jfs). O valor de Jfs é maior para o Nd +3 em relação ao Gd +3 para ambos os compostos, provavelmente devido ao maior contato com os c.e., resultado de um maior raio das camadas 4f para o Nd +3 . A banda eletrônica d para o caso do compostos de Ni, é provavelmente proveniente das camadas 3d incompletas do íon de Ni. No composto YBiPt, o ajuste dos dados de susceptibilidade magnética e estudos de RPE nos permitiram obter os parâmetros de campo cristalino cúbico A4 e A6 para os compostos Y0.9Nd0.1BiPt e Y0.9Yb0.1BiPt . Um limite superior para o parâmetro de campo cristalino ?b4? ~ 1 Oe, obtido da largura de linha, para o caso Gd, foi estimado. Nossos resultados sugerem que pequenos valores b4 podem ser característicos de semicondutores da gap pequeno ou semimetais com baixa densidade de portadores. Para as diferentes amostras R1-xAxMnO3 ( R = La, Pr; A = Ca, Sr) e também para os cristais CMR de La1.2Sr1.8Mn2O7 , os "loops" de histerese e os "splittings" de linha de RPE ou linhas FMR observados foram associados à inomogeneidades das amostras provenientes de uma distribuição não aleatória de vacâncias, defeitos e conteúdo de oxigênio, gerando principalmente uma distribuição de Tc nas amostras, ou para o caso das "layers" a fases extrínsecas provavelmente provenientes de fases de Lan+1MnnO3n+1 com n diferente 2Abstract: In this PhD work, we have performed systematic Electron Paramagnetic Resonance (EPR) and magnetic susceptibility studies in several Highly Correlated Electron Systems, such as: i) High- T c Superconductors (HTS), ii) Heavy Fermions (HFS), Kondo Materiais and lntermediate Valence compounds (IV) and iii) Colossal Magnetoresistance Perovskites (CMR). For lhe HFS Eu2-xPrxCuO4 (O Tc) allowed us to obtain the exchange parameter between lhe rare-earth localized magnetic moment and the conduction electrons. This parameter depends on the conductions electrons momentum transfer ? kfin - kfout ? = q, i.e. Jfs(q). The temperature dependence of lhe EPR linewidth yields a value for one of the exchange parameters, 1/2, which is in agreement with that estimated from the slope of the inicial linear decrease of T c by the Gd3+ impurities. These results indicated that the R1-xGdxNi2B2C (R = Y, Lu) compounds behave as conventional BCS superconductors. In the EPR of Gd3+ studies in IV YbInCu4 and the HFS YbAgCu4 and their reference compounds, the larger g-shift and Korringa rate were interpreted in terms of the enhancement of the density of states at lhe Ferrni leveI for the Yb-based. The extracted exchange interaction parameter was found to be q-dependent for alI compounds. From the EPR and magnetic susceptibility data the electron-electron exchange enhancement factor for the YbInCu4 was obtained. For the others compounds, including YbAgCu4, the electron- electron interactions were negligible. For the intermetallic LuInA4 (A = Cu, Ni) compounds, our results of EPR of diluted Gd3+ and Nd3+ indicate a density of states at the Fermi levei built up of a single s-band for the Cu-based compound and a multiple (s and d) bands for Ni-based system. The susceptibilty and specific heat data show negligible electron-electron exchange enhancement for both compounds. For the Cu-based system the exchange interaction between the rare-earth local moment and the conduction electrons depends on the conduction electron wave-vector. EPR and magnetic susceptibility experiments in the rare-earth ( R = Nd, Yb and Er) doped cubic semiconducting YBiPt allow us to estimate the fourth (A4) and sixth (A6) arder crystal field parameters for this compound. It is found that these parameters are ofthe same arder for alI the R studied. On the other hand, no crystal field effects were found for Gd3+ doped single crystal system. Consistent with the small gap semiconducting character of the YBiPt intermetallic compound, a Dysonian ESR lineshape with no g-shift and Korringa broadening was observed. For the CMR Rl-xAxMn03 (R = La, Pr; A = Ca, Sr) and Lal.2Srl.8Mn207, the hysteresis loops and the resonance line splittings or the FMR lines observed above T c are interpreted as inhomogeneities in the samples, probably, originated from a non-random distribution of vacancies, defects and oxygen contento The origin of the FMR lines in the layered Lal.2Srl.8Mn207 compounds was attributed to the presence of extrinsic Ruddlesden -Poper phases with n ¹ 2DoutoradoFísicaDoutor em Ciência

Magnetic, thermal and transport properties of Cd doped CeIn3_3

We have investigated the effect of Cd substitution on the archetypal heavy fermion antiferromagnet CeIn$_3$ via magnetic susceptibility, specific heat and resistivity measurements. The suppression of the Neel temperature, T$_{N}$, 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 $T_N$ 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 AFM order on its neighboring Ce moments, without affecting the valence of Ce. Moreover, the temperature dependence of the specific heat below $T_N$ is not consistent with 3D magnons in pure as well as in Cd-doped CeIn$_3$, a point that has been missed in previous investigations of CeIn$_3$ and that has bearing on the type of quantum criticality in this system