S. B. Oseroff

Thermally activated exchange narrowing of the Gd3+ ESR fine structure in a single crystal of Ce1-xGdxFe4P12 (x≈0.001) skutterudite

We report electron spin resonance (ESR) measurements in the Gd3+ doped semiconducting filled skutterudite compound Ce1-xGd xFe4P12 (x-0.001). As the temperature T varies from T- 150 K to T- 165 K, the Gd3+ ESR fine and hyperfine structures coalesce into a broad inhomogeneous single resonance. At T- 200 K the line narrows and as T increases further, the resonance becomes homogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest that the origin of these features may be associated with a subtle interdependence of thermally activated mechanisms that combine: (i) an increase with T of the density of activated conduction carriers across the T-dependent semiconducting pseudogap; (ii) the Gd3+ Korringa relaxation process due to an exchange interaction JfdS.s between the Gd3+ localized magnetic moments and the thermally activated conduction carriers; and (iii) a relatively weak confining potential of the rare earth ions inside the oversized (Fe 2P3)4 cage, which allows the rare earths to become rattler Einstein oscillators above T- 148 K. We argue that the rattling of the Gd3+ ions, via a motional narrowing mechanism, also contributes to the coalescence of the ESR fine and hyperfine structure.

Electron spin resonance of Gd3+ and Nd3+ in LuInA(4) (A = Cu,Ni)

Low-temperature (1.6 K{approx_lt}T{approx_lt}60 K) data of electron spin resonance for Gd{sup 3+} and Nd{sup 3+} diluted in LuInA{sub 4} (A=Cu,Ni) compounds are presented. The results are interpreted in terms of a density of states at the Fermi level built up of a single {ital s} band for the Cu-based system and a multiple (s and d) bands for the Ni-based system. The susceptibility 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 (Gd{sup 3+} and Nd{sup 3+}) local moment and the conduction electrons depends on the conduction-electron wave vector. {copyright} {ital 1999} {ital The American Physical Society}

Gradual transition from insulator to semimetal of Ca1-x Eux B6 with increasing Eu concentration

The local environment of Eu2+ (4 f7, S=7 2) in Ca1-x Eux B6 (0.003≤x≤1.00) is investigated by means of electron spin resonance (ESR). For x 0.003 the spectra show resolved fine and hyperfine structures due to the cubic crystal electric field and nuclear hyperfine field, respectively. The resonances have Lorentzian line shape, indicating an insulating environment for the Eu2+ ions. For 0.003 x 0.07, as x increases, the ESR lines broaden due to local distortions caused by the Eu Ca ions substitution. For 0.07 x 0.30, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both insulating and metallic environments for the Eu2+ ions. In contrast to Ca1-x Gdx B6, the fine structure is still observable up to x≈0.15. For x 0.30 the fine and hyperfine structures are no longer observed, the line width increases, and the line shape is purely Dysonian, anticipating the semimetallic character of EuB6. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu2+ 4f electrons. High-field ESR measurements for x 0.15 reveal smaller and anisotropic linewidths, which are attributed to magnetic polarons and Fermi surface effects, respectively.

ESR of Gd{sup 3+} and Er{sup 3+} in Pr{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4}

Electron spin resonance (ESR) of Gd{sup 3+} and Er{sup 3+} in single crystals of Pr{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4} (0{le}{ital x}{le}0.15) at liquid-helium temperature shows crystal field (CF) effects corresponding to a {ital C}{sub 4{ital v}} point symmetry. Upon doping with Ce{sup 4+}, a reduction of about 23% in the second-order CF parameter {vert_bar}{ital b}{sub 20}{vert_bar} is found with no significant change for the other CF spin-Hamiltonian parameters. The resonance lines broaden and present a Dysonian line shape at higher Ce{sup 4+} concentration, which is consistent with an increase in the CF inhomogeneity and the metallic character of the compound. Magnetic-susceptibility measurements present a small increase in the low-temperature anisotropy upon doping, consistent with a smaller {l_angle}{vert_bar}{ital B}{sub 0}{sup 2}{vert_bar}{r_angle} crystal-field parameter for Pr{sup 3+} in Pr{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4}. The reduction in the CF parameters is tentatively attributed to charge transfer from Ce atoms to the CuO{sub 2} planes. The exchange parameters {ital j}{sub Gd-Pr} and {ital j}{sub Pr-Pr} are estimated from the ESR and susceptibility measurements.

ESR of Gd3+ and Er3+ in Pr2-xCexCuO4.

Electron spin resonance (ESR) of Gd{sup 3+} and Er{sup 3+} in single crystals of Pr{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4} (0{le}{ital x}{le}0.15) at liquid-helium temperature shows crystal field (CF) effects corresponding to a {ital C}{sub 4{ital v}} point symmetry. Upon doping with Ce{sup 4+}, a reduction of about 23% in the second-order CF parameter {vert_bar}{ital b}{sub 20}{vert_bar} is found with no significant change for the other CF spin-Hamiltonian parameters. The resonance lines broaden and present a Dysonian line shape at higher Ce{sup 4+} concentration, which is consistent with an increase in the CF inhomogeneity and the metallic character of the compound. Magnetic-susceptibility measurements present a small increase in the low-temperature anisotropy upon doping, consistent with a smaller {l_angle}{vert_bar}{ital B}{sub 0}{sup 2}{vert_bar}{r_angle} crystal-field parameter for Pr{sup 3+} in Pr{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4}. The reduction in the CF parameters is tentatively attributed to charge transfer from Ce atoms to the CuO{sub 2} planes. The exchange parameters {ital j}{sub Gd-Pr} and {ital j}{sub Pr-Pr} are estimated from the ESR and susceptibility measurements.

Eu2+ spin dynamics in the filled skutterudites EuM4Sb12 (M = Fe, Ru, Os)

We report evidence for a close relation between the thermal activation of the rattling motion of the filler guest atoms, and inhomogeneous spin dynamics of the Eu2+ spins. The spin dynamics is probed directly by means of Eu2+ electron spin resonance (ESR), performed in both X-band (9.4 GHz) and Q-band (34 GHz) frequencies in the temperature interval 4.2 < T < 300 K. A comparative study with ESR measurements on the Beta-Eu8Ga16Ge30 clathrate compound is presented. Our results point to a correlation between the rattling motion and the spin dynamics which may be relevant for the general understanding of the dynamics of cage systems.

Finite Size Effect on the Resonant Microwave Absorption of Er^{3+} Doped Ag Nanoparticles

The in∞uence of flnite sample size efiects and structural defects on the ground state of Er 3+ in diluted Ag:Er alloys is studied. The choused metallic systems were, nanoparticles (5{ 10nm) of the alloy Ag1ixErx (x = 0:04) and the bulk alloy of Ag1ixErx (x = 0:001) which was used as a reference. The Ag:Er alloy nanoparticles were prepared by chemical synthesis that gives an excellent morphological and crystalline homogeneity and the bulk alloy of Ag:Er by a conventional arc-melting. The nanoparticles resonant microwave absorption (Electron Spin Resonance) at X (9.5GHz)-band of Er 3+ obtained at low-T (4{20K) show a T-independent g-value of 6.74(4) and linewidth of 50(5)Oe. However, above T … 20K an exponential T- dependence of the linewidth is observed. A preliminary interpretation of these results suggests that: i) the exchange interaction, JfsS.s, between the Er 3+ localized magnetic moment and the hosts conduction-electrons has been quenched; and ii) at high-T the spin-lattice relaxation is mainly due to the spin-orbit coupling via lattice phonons involving Er 3+ crystal fleld exited levels.

ESR ofGd3+in magnetically orderedEu2CuO4

Electron spin resonance (ESR) experiments of Gd{sup 3+} in the antiferromagnetic (AF) ordered phase ({ital T}{lt}{ital T}{sub {ital N}}) of Eu{sub 2}CuO{sub 4} can be interpreted in terms of four magnetically nonequivalent rare-earth sites with local internal fields {ital H}{sub {ital i}}={plus_minus}310(30) Oe along the [100] and [010] directions. The internal field is well described by a dipolar magnetic field of a noncollinear AF array of 0.35(4) {mu}{sub {ital B}} per Cu moment aligned along the [100] and [010] directions. This is consistent with recent results of magnetic-field-dependent neutron-diffraction experiments. From the ESR and magnetic susceptibility data, the crystal field parameters for Gd{sup 3+} and Eu{sup 3+} in Eu{sub 2}CuO{sub 4} are determined. The exchange parameters between the rare earths are also estimated. {copyright} {ital 1996 The American Physical Society.}

ESR of Gd{sup 3+} in magnetically ordered Eu{sub 2}CuO{sub 4}

Electron spin resonance (ESR) experiments of Gd{sup 3+} in the antiferromagnetic (AF) ordered phase ({ital T}{lt}{ital T}{sub {ital N}}) of Eu{sub 2}CuO{sub 4} can be interpreted in terms of four magnetically nonequivalent rare-earth sites with local internal fields {ital H}{sub {ital i}}={plus_minus}310(30) Oe along the [100] and [010] directions. The internal field is well described by a dipolar magnetic field of a noncollinear AF array of 0.35(4) {mu}{sub {ital B}} per Cu moment aligned along the [100] and [010] directions. This is consistent with recent results of magnetic-field-dependent neutron-diffraction experiments. From the ESR and magnetic susceptibility data, the crystal field parameters for Gd{sup 3+} and Eu{sup 3+} in Eu{sub 2}CuO{sub 4} are determined. The exchange parameters between the rare earths are also estimated. {copyright} {ital 1996 The American Physical Society.}