Based on low-field magnetization measurements on a series of single crystals, we present a scheme of the Eu 2+ spin alignment in EuFe 2 (As 1−x P x ) 2 . We explain observations of the Eu 2+ ordering previously reported, reconciling different existing phase diagrams. The magnetic moments of the Eu 2+ ions are slightly canted, yielding a ferromagnetic contribution along the c direction that becomes stronger with pressure, until superconductivity sets in. The spin-density wave as well as the superconducting phase coexist with an antiferromagnetic interlayer coupling of the canted spins. Reducing the interlayer distance finally leads to a ferromagnetic Eu 2+ interlayer coupling and to the suppression of superconductivity.

By systematic investigations of the magnetic, transport, and thermodynamic properties of single crystals of EuFe(2)(As(1-x)P(x))(2) (0≤x≤1), we explore the complex interplay of superconductivity and Eu(2+) magnetism. Below 30 K, two magnetic transitions are observed for all P substituted crystals, suggesting a revision of the phase diagram. In addition to the canted A-type antiferromagnetic order of Eu(2+) at ∼20  K, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of EuFe(2)(As(1-x)P(x))(2) coexists with superconductivity around x≈0.2.

Despite decades of intense research, the origin of high-temperature superconductivity in cuprates and iron-based compounds is still a mystery. Magnetism and superconductivity are traditionally antagonistic phenomena; nevertheless, there is basically no doubt left that unconventional superconductivity is closely linked to magnetism. But this is not the whole story; recently, also structural effects related to the so-called nematic phase gained considerable attention. In order to obtain more information about this peculiar interplay, systematic material research is one of the most important attempts, revealing from time to time unexpected effects. Europium-based iron pnictides are the latest example of such a completely paradigmatic material, as they display not only spin-density-wave and superconducting ground states, but also local Eu magnetism at a similar temperature scale. Here we review recent experimental progress in determining the complex phase diagrams of europium-based iron pnictides. The conclusions drawn from the observations reach far beyond these model systems. Thus, although europium-based iron pnictides are very peculiar, they provide a unique platform to study the common interplay of structural-nematic, magnetic and electronic effects in high-temperature superconductors.

Our comprehensive study on EuFe_{2}As_{2} reveals a dramatic reduction of magnetic detwinning fields compared to other AFe_{2}As_{2} (A=Ba, Sr, Ca) iron pnictides by indirect magnetoelastic coupling of the Eu^{2+} ions. We find that only ∼0.1  T are sufficient for persistent detwinning below the local Eu^{2+} ordering; above T_{Eu}=19  K, higher fields are necessary. Even after the field is switched off, a significant imbalance of twin domains remains constant up to the structural and electronic phase transition (190 K). This persistent detwinning provides the unique possibility to study the low temperature electronic in-plane anisotropy of iron pnictides without applying any symmetry-breaking external force.

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A new approach to MRI thermometry using encapsulated hyperpolarized xenon is demonstrated. The method is based on the temperature dependent chemical shift of hyperpolarized xenon in a cryptophane-A cage. This shift is linear with a slope of 0.29 ppm °C(-1) which is perceptibly higher than the shift of the proton resonance frequency of water (ca. 0.01 ppm °C(-1)) that is currently used for MRI thermometry. Using spectroscopic imaging techniques, we collected temperature maps of a phantom sample that could discriminate by direct NMR detection between temperature differences of 0.1 °C at a sensor concentration of 150 μM. Alternatively, the xenon-in-cage chemical shift was determined by indirect detection using saturation transfer techniques (Hyper-CEST) that allow detection of nanomolar agent concentrations. Thermometry based on hyperpolarized xenon sensors improves the accuracy of currently available MRI thermometry methods, potentially giving rise to biomedical applications of biosensors functionalized for binding to specific target molecules.

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