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We report selected experimental results on the spin dynamics and spin freezing at ferromagnetic quantum phase transitions to illustrate some of the most prominent escape routes by which ferromagnetic quantum criticality is avoided in real materials. In the transition metal Heusler compound Fe2TiSn we observe evidence for incipient ferromagnetic quantum criticality. High pressure studies in MnSi reveal empirical evidence for a topological non-Fermi liquid state without quantum criticality. Single crystals of the hexagonal Laves phase compound Nb1−yFe2+yprovide evidence of a ferromagnetic to spin density wave transition as a function of slight compositional changes. Last but not least, neutron depolarisation imaging in CePd1−xRhxunderscore evidence taken from the bulk properties of the formation of a Kondo cluster glass.

We report selected experimental results on the spin dynamics and spin freezing at ferromagnetic quantum phase transitions to illustrate some of the most prominent escape routes by which ferromagnetic quantum criticality is avoided in real materials. In the transition metal Heusler compound Fe2TiSn we observe evidence for incipient ferromagnetic quantum criticality. High pressure studies in MnSi reveal empirical evidence for a topological non-Fermi liquid state without quantum criticality. Single crystals of the hexagonal Laves phase compound Nb1−yFe2+yprovide evidence of a ferromagnetic to spin density wave transition as a function of slight compositional changes. Last but not least, neutron depolarisation imaging in CePd1−xRhxunderscore evidence taken from the bulk properties of the formation of a Kondo cluster glass.

Die Restaustenit-Analysemittels Röntgen-Beugungsverfahren wird häufig nach standardisierten Verfahren wie ASTM E975-03 [1] oder SAE-SP-452 [2] durchgeführt. Die Verfahren basieren auf der Analyse der integralen Intensitätsverhältnisse von wenigen Beugungsreflexen für die einzelnen Phasen. Die Anpassung erfolgt modellfrei bezüglich der unterliegenden strukturellen Phasen. Dabei ist die genaue Kenntnis der Mikrostruktureigenschaften (Texturierungen, Eigenspannungszustände), die sich empfindlich auf die Intensitäten der Beugungsreflexe auswirken können, und ein geringer Anteil an weiteren Phasen, beispielsweise Carbiden, eine notwendige Voraussetzung. Dies trifft für technologisch relevante Proben häufig nicht zu. Als Folge sind daher oftmals große Streubreiten der Restaustenitgehalte bei den standardisierten Analysen zu beobachten, siehe z. B. [3]. Im vorliegenden Artikel wird ein Vergleich zwischen dem standardisierten Verfahren und einer Analyse unter Verwendung einer durch Strukturmodelle gestützten Gesamtprofilanpassung für exemplarische Proben mit leichten Vorzugsorientierungen und für den Einsatz von 1-D- und 2-D-Detektoren vorgestellt.

The magnetic excitation spectrums of charge stripe ordered La2−xSrxNiO4x= 0.45 and x= 0.4 were studied by inelastic neutron scattering. We found the magnetic excitation spectrum of x= 0.45 from the ordered Ni2+S = 1 spins to match that of checkerboard charge ordered La1.5Sr0.5NiO4. The distinctive asymmetry in the magnetic excitations above 40meV was observed for both doping levels, but an additional ferromagnetic mode was observed in x= 0.45 and not in the x= 0.4. We discuss the origin of crossover in the excitation spectrum between x= 0.45 and x= 0.4 with respect to discommensurations in the charge stripe structure.

Disorder in decagonal quasicrystals is studied by “contrasting methods”, i.e. by joint analysis of X-ray and neutron data and by use of the anomalous dispersion method, to learn about the relative influence of the transition metals. First experimental results with decagonal Al–Ni–Co samples of different origin are discussed qualitatively. A comparative study of Patterson maps from neutron and X-ray Bragg data of d-Al72.5Ni16.5Co11gives direct evidence for particular sites occupied by Co.

Synchrotron measurements at DESY and ESRF of decagonal quasicrystals with the nominal composition Al70Co15Ni15and Al72Co16Ni12, show that Bragg reflections with high |Hi∗|values are surrounded by up to four weak subsidiary peaks. The positions of these additional peaks depend solely on the position of the Bragg reflection in the reciprocal perpendicular subspace and can be explained by a linear phason strain model. This model sheds new light on the current superorder models [K. Hradil et al. Phil. Mag. A 79 (1999) 1963] and is a further step towards an understanding of the real structures of decagonal quasicrystals.

Abstract:: The dynamics of the decagonal AlNiCo phase has been investigated on a single-grain quasicrystalline sample using inelastic neutron scattering. The decagonal structure can be viewed as a periodic stacking of quasiperiodic planes. The anisotropy between the modes propagating in the periodic and quasiperiodic directions is found to be much weaker than theoretically predicted. A strong resonance splitting is observed at an energy transfer of 15 meV for transverse modes polarized in the quasiperiodic plane.

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Diffuse scattering of the thermodynamically stable decagonal quasicrystal (QC) Al70Ni15Co15 has been measured at a synchrotron radiation source. Perpendicular to the unique 'periodic' direction there is an alternating sequence of 'Bragg' and (modulated) diffuse layers. Within the zeroth Bragg layer, strong diffuse streaks and diffuse satellite reflections, as well as very weak and slowly varying diffuse scattering, were measured up to high values of Q. Satellites and diffuse streaks, already known from precession photographs, were explored in the q-scan mode yith a step width of 0.01 in h and k (pseudo-monoclinic setting with a* = 0.081 08 Å-1, b* = 0.081 09 Å-1 in the QC plane, c* = 0.245 16 Å-1 in the periodic direction, y = 72°). The positions, but not the intensities of all registered satellite reflections obey an overall tenfold symqetry around the Bragg reflection. The satellite vector magnitude -qs- = 3.3 × 10 -2 Å-1 corresponds to a translational superperiod of the QC substructure with a 30 Å period. First- and second-order satellites were always observed only in pairs around the Bragg reflections, superimposed on the diffuse streaks through the Bragg reflection and the satellites. These observations may be interpreted by lamellar domains extended parallel to the unique axis. The full widths at half-maximum of the satellites are between 1.4 × 10-3 and 2.0 × 10-3 Å-1, corresponding to correlation lengths between 700 and 500 Å of the domain ordering. The diffuse streaks indicate faults in the regular sequence of the lamellar domains. An additional uniform diffuse background is observed, reflecting a high degree of disorder in the (projected) QC arrangement of the basic columnar-like structural elements.

Commensurate satellite reflections in diffraction patterns of decagonal Al-Ni-Co and Al-Cu-Co phases indicate a periodic superorder on the quasiperiodic substructure. From the intensity characteristics of these satellites a box-like modulation function is derived which may be visualized in different ways including approximant phases. An interpretation by fine-scale periodic lamella microdomains matches closely high-resolution electron microscopy observations and characteristic structural features of the average structure.

The title compounds have an isotypic averaged structure but differ in significant detail with respect to disorder. Disorder diffuse scattering has been studied in situ up to 1150 K with a heating camera and X-ray films and an image-plate system at the synchrotron source Deutsches Elektronensynchrotron/HASYLAB. With the oscillation technique (primary beam perpendicular to the 'periodic' direction) the area between the zeroth and the fourth Bragg layer was explored. There are modulated diffuse layers and diffuse phenomena, in particular diffuse streaks, within the Bragg layers. The diffuse layers are related to a superperiod along the 'periodic' direction and the streaks arise from disorder in the quasicrystalline packing. A different temperature behaviour of the diffuse layers compared with the diffuse streaks was observed. The former decrease in intensity above 770 K and vanish completely at about 1070 K, whereas the intensity of the diffuse streaks decreases between 1120 and 1150K. Both processes are time dependent but fully reversible. This decrease in disorder in the quasicrystalline packing occurs in the same temperature regime where an extension of the stability field of the decagonal Al70Ni15Co15 phase was reported earlier.