J.B. Marimon da Cunha

First-order Kondo-frustated antiferromagnetic ordering in the heavy electron compound YbAs

Abstract170Yb Mössbauer measurements show that the heavy electron compound YbAs undergoes a first order paramagnetic-antiferromagnetic transition centered at TN=0.58K. The saturated 4f-shell magnetic moments are reduced by 47% relative to the value predicted by a crystal field model. A study of the line shapes below TN in the presence of an external field shows that the RKKY exchange energy is about 20 times bigger than kB TN. These results show that YbAs presents a magnetic order frustrated by the Kondo effect.

170Yb Mössbauer study of the heavy-electron pnictides YbP, YbAs and YbSb

Abstract 170Yb Mossbauer measurements on the heavy-electron compounds YbP, YbAs, and YbSb have probed the competition between RKKY exchange interactions and Kondo hybridisation. Magnetic ordering occurs below TN ∼ 0.5 K with reduced magnetic moments; the RKKY exchange interaction was measured and found to be much greater than kBTN.

Some structure and magnetic effects of Ga incorporation on α-FeOOH

The effects of Ga incorporation on goethite were investigated by using X-ray diffractometry (XRD), particle-induced X-ray emission (PIXE), susceptibility measurements and Mossbauer spectroscopy (MS). Samples GaxFe1−xOOH were prepared with x=0.00, 0.01, 0.03, 0.05, 0.10, 0.35 and 0.70. All the samples are single-phase and an accurate observation of the structure refinement procedure makes clear the sample-dependent structural behaviour. A model is proposed in which the main effect of Ga incorporation is to reduce the crystallite dimensions.

Magnetic behaviour of the antiferromagnet

The magnetic behaviour of the antiferromagnet was investigated through heat capacity, magnetic susceptibility and magnetization measurements. The broad magnetic susceptibility curve (peaked at 15 K) presents an inflection typical of two-dimensional systems. This inflection is observed near 10 K, the temperature at which early reported Mossbauer measurements indicate the onset of magnetic order. The heat capacity measurements have shown a clear transition at 8.7 K, where Mossbauer measurements show complete ordering. Magnetization curves, for temperatures between 1.4 K and 5 K, display a noticeable change in slope, suggesting a spin-flop transition induced by a field higher than 10 T. It is argued that this transition, which disappears for , is evidence of two-dimensional behaviour.

Ferromagnetic Laves phase intermetallics doped with sp impurities: A Mössbauer study of Zr(Fe1−xSix)2

The effect of Si on the hyperfine parameters of the pseudobinary compounds Zr(Fe1−xSix)2 is studied by Mössbauer spectroscopy in the C15 phase (x≦0.17). In these pseudobinaries, two magnetic sites of the pure compound coexist with a third site associated with the impurities. The local effects in Zr(Fe,Si)2 are qualitatively discussed, together with the observed trends of the hyperfine parameters. A comparison with Zr(Fe,Al)2 compounds is also presented.

Structure and magnetic properties of layered (FexCo1 -x)Ta2O6 compounds

Abstract Samples of (FexCo1 − x)Ta2O6 were prepared from pure FeTa2O6 and CoTa2O6. From X-ray diffraction (XRD) measurements followed by Rietveld refinement, it is demonstrated that the solid solution obeys the Vegards law. Mossbauer spectroscopy (MS) measurements at 4.2 and 300 K show that for x ⩾ 0.8 the Fe → Co substitution has no apparent effect on the hyperfine parameters.

161Dy Mössbauer measurements in DyBa2Cu3O7−x

Superconducting and non-superconducting samples of DyBa2Cu3O7−x have been examined by Mössbauer spectroscopy down to 0.05 K. The saturated Dy3+ sublattice 4f shell magnetic moments and quadrupole interactions show fairly wide distributions in both samples. These are attributed to crystal field inhomogeneities. Neither the Dy3+ paramagnetic relaxation rates nor the isomer shifts show any evidence of coupling between Dy3+ and a possible conduction band.

Europium charge states in sputtered amorphous EuPdx (x = 2.4 and 3.0)

Abstract 151 Eu Mossbauer spectroscopy measurements on two amorphous samples of EuPd x ( x = 2.4 and 3.0) prepared by sputtering, show the presence of inhomogeneous mixed valent (Eu 2+ and Eu 3+ ) states with sample averaged valencies 2.25 and 2.85 respectively. Magnetic ordering and hyperfine fields are visible on the Eu 2+ components. The two charge states have the same effective Debye temperature. The amorphous and recrystallised samples are compared.

Mössbauer absorption in some high Tc superconductors:170Yb in YbBa2Cu3O7−x,166Er in ErBa2Cu3O7−x, and57Fe in YBa2Cu3O7−x

For the high Tc compound YbBa2Cu3O7−x, Mossbauer absorption measurements show that magnetic ordering occurs within the Yb3+ sublattice at 0.35K (μsat=1.7μB). For ErBa2Cu3O7−x the Er3+ sublattice orders at 0.7K (μsat=4μB). Measurements on57Fe diluted into YBa2Cu3O7−x shows that no ordered magnetic moments exist within the Cu sublattices down to at least 4.2K.