W. Felsch

The magnetic behavior of CeB6: Comparison between elastic and inelastic neutron scattering, initial susceptibility and high-field magnetization

We report measurements of the elastic and inelastic neutron scattering, initial susceptibility and high-field magnetization on thoroughly prepared poly- and single crystalline samples of CeB6. Part of these experiments have been performed at temperatures down to 60 mK and magnetic fields up to 70 kØe. Our neutron-diffraction data provide the first proof that CeB6 is an antiferromagnet belowTN≃2K as has been suggested by previous bulk experiments. The reduced value of the low-temperature magnetic moment both below and aboveTN points to the existence of a Kondo effect of theΓ7 crystal-field (CF) ground state of Ce3+. From the low-temperature width of the quasielastic neutron line, the Kondo temperature is inferred to beTK≃3 K. The thermal variation of the initial susceptibility (forT>20K) is semiquantitatively explained invoking, besides the Kondo effect, aΓ7-Γ8 CF splitting of ≃70 K and magnetic interactions, which are about 10 times stronger betweenΓ8 states than those betweenΓ7 states. This largeΓ8-Γ8 exchange interaction is also assumed to account for the most striking result of this work, i.e. the lack of any CF-transition peak up to 44 meV in our inelastic neutron-scattering spectra.

Magnetic correlations in Eu x Sr1?x S and the ferromagnet-spin glass transition

Neutron (Bragg and small angle) scattering and susceptibility measurements are used to study magnetic ordering in EuxSr1−xS with ferromagnetic nearest neighbor exchangeJ1 and antiferromagnetic next-nearest neighbor exchangeJ2. We present data for 0.50≦x≦0.70 which cannot be analyzed within the merely geometrical treatments of percolation theory. Breakdown of ferromagnetism occurs atxc=0.51, far above the percolation thresholdxp=0.136, and a spin-glass phase is observed in the intermediate concentration regime. Close toxc, the ferromagnetic state is also displaced by the spinglass phase at lower temperatures. Both properties are a general characteristic of diluted systems with competing interactions. An effective decoupling of finite magnetic clusters from the ferromagnetic net arises from frustration, which enhances the ground-state entropy. Anomalous properties below the Curie temperatureTc as well as atTc support this microscopic picture.

Structural and electronic properties of epitaxial V2O3 thin films

Thin films of V2O3 with thickness 4?300?nm were grown on -oriented sapphire substrates by reactive dc magnetron sputtering. X-ray diffraction, pole figure measurements and scanning tunnelling microscopy show high crystallinity and epitaxy to the substrate with a faceted surface structure, and the absence of strain. Measurements of the electrical resistivity, scanning tunnelling and x-ray absorption spectroscopy show a metal?insulator transition near 150?K that is connected with the opening of an energy gap and a characteristic modification of the absorption spectrum at the vanadium-2p and oxygen-1s edges. These observations reveal that the films have bulk-like properties.

Delocalization of the Ce 4f shell in amorphous Ce75.5Co24.5

The amorphous alloy Ce75.5Co24.5 prepared by melt spinning has been studied through measurements of the magnetic susceptibility, magnetization, electrical resistivity, thermoelectric power and specific heat. The results are interpreted in terms of a homogeneous intermediate valence state of the Ce ions. This is inferred from a temperature-independent magnetic susceptibility at low temperature and the absence of magnetic ordering, a large linear term in the specific heat, and aT2 dependence of the electrical resistivity at low temperature followed by a steep increase with temperature up to 50 K. At this temperature, the thermoelectric power displays a maximum. The intrinsic properties are partially obscured at low temperatures by a contribution from roughly a few percent of magnetic ‘impurities’, presumably Ce3+ ions. They manifest themselves by an increase of the susceptibility towards low temperatures and by a broad Schottky-like contribution to the specific heat resulting from the excitation of magnetic clusters.

Ion-beam induced magnetic anisotropies in iron films

Abstract 100–300 nm thin Fe layers evaporated onto crystalline and amorphous Si or SiO2 substrates were irradiated, at 77 K, with 1014–1016 Xe+-ions/cm2 at 450 keV beam energy. The magnetizations in the films were measured by means of Perturbed Angular Correlation (PAC) spectroscopy with implanted 111In tracer ions, or the Magneto-Optic Kerr Effect (MOKE). Upon ion implantation, dramatic changes of the magnetic anisotropy were observed which are attributed to ion-beam enhanced lateral grain growth. Very little influence of the deposition parameters (type and cristallinity of substrate, evaporation rate) on the anisotropic magnetization was found.

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureTf(ν) is determined for the dilute spin glass systems (La, Gd)B6 and (Y, Gd)Al2 in the frequency range 10–1,000 Hz. While for (La, Gd)B6,Tf(ν) is found to be weak, for (Y, Gd)Al2Tf(ν) is even stronger than for the previously studied system (La, Gd)Al2. Both, measurements of the temperature dependence of the susceptibility nearTf and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B6 and ferromagnetic for (Y, Gd)Al2 as well as (La, Gd)Al2.

Cerium-derived anomalies in the magnetic and transport properties of amorphous Ce-Cu alloys

Abstract Measurements of the bulk magnetic properties and transport coefficients of the amorphous alloys Ce 26 Cu 74 and Ce 72 Cu 28 were used to investigate the influence of structural disorder and alloy composition on the Kondo effect in concentrated cerium systems. Incoherent Kondo scattering from the Ce 4f states split by the local “crystal” fields is manifested for both alloys by pronounced anomalies in the thermoelectric power and electrical resistivity at temperatures above 20K. The data indicate a narrow distribution of the overall crystal field splittings and hence a rather uniform structural short range order. Differences in the transport properties below 20K are interpreted by a different strength of the sd-f exchange interaction in the two alloys. A maximum in the resistivity and a minimum in the magnetoresistivity of a-Ce 26 Cu 74 , features reflecting coherence effects in the crystalline Kondo lattice compound CeCu 6 , are explained in terms of magnetic interactions between the Ce ions which reduce the Kondo effect.

Ion-beam-induced magnetic texturing of thin nickel films

Abstract The texturing of thin metallic and ceramic films caused by heavy-ion implantation is a common observation, but still lacks quantitative explanations. Magnetic texturing effects of polycrystalline Fe-films irradiated with Xe ions have recently been reported. In this paper, we present results of similar experiments for 18–400 nm thick polycrystalline Ni-films on Si (1 0 0) substrates after irradiating them with 2×1014–1×1016 Xe+-ions/cm2 at 77 K and at ion energies of 6–450 keV, in the presence of a small magnetic field of the order of 1 G. The magnetization was measured by means of the magneto-optical Kerr effect (MOKE), as function of several parameters such as fluence, energy and spot size of the ion beam, thickness and shape of the Ni film and the thermal annealing procedures after irradiation.

Magnetic and structural properties of cerium/iron multilayers

Abstract Ce/Fe multilayers with modulation lengths between 18 and 200 A were prepared by ion-beam sputtering in an UHV system and structurally characterized by X-ray diffraction at small angles and 57 Fe conversion-electron Mossbauer spectroscopy. Good periodicity and sharp concentration profiles at the interfaces are found. The magnetic properties are unusual. For Fe-layer thicknesses below 25 A, relatively independent of the Ce-layer thickness, the Curie temperature is reduced to values below 180 K. This is accompanied by a distinct decrease of the spontaneous magnetization and the appearance of hysteresis in the low-field susceptibility at low temperatures. Isothermal magnetization curves point to noncollinear magnetic order. Evidently, these phenomena are closely related to the transition in the Fe layers from the bcc crystalline to an amorphous structure occurring near 25 A according to X-ray diffraction at large angles and RHEED diagrams.

Interfacial magnetism of Ce/Fe and CeH∼2/Fe multilayers studied by 57Fe Mössbauer spectroscopy

Abstract The distribution of the magnetic hyperfine fields and the magnetic anisotropy of Ce/Fe and CeH ∼2 /Fe multilayers have been studied by Mossbauer spectroscopy between 4.2 and 300 K. The extension of the Ce-Fe interaction defining a ‘magnetic interface’ exceeds the structural extension of the interface and is more than two times larger at the Ce/Fe than at the CeH ∼2 /Fe interface. This is proposed to be a consequence of a strong hybridization between the Ce-5d and Fe-3d states that is suppressed by hydrogenation. From the relative intensity of the Mossbauer lines we show that for the CeH ∼2 /Fe system with sufficiently thin Fe layers the orientation of the spontaneous magnetization is perpendicular to the film plane at low temperatures and switches to a parallel alignment at higher temperatures in a sharp transition. In contrast, the Ce/Fe system is spontaneously magnetized in the film plane.