The magnetism of DySc 2 N@C 80 endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy III ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc 2 N@C 80 samples with C 60 increases the zero-field 4f electron relaxation time at 2 K to several hours. I ncorporation of magnetic ions in molecular clusters can lead to the formation of so-called single-molecule magnets (SMMs).1 These molecules are characterized by slow magnetic relaxation, making them candidates for applications in quantum computing, spintronics, and high-density storage devices. Investigations on the magnetism of single ions inside fullerenes started with Gd@C 82 , which turned out to be paramagnetic down to 3 K. 9a For Dy@C 82 , superconducting quantum interference device (SQUID) and X-ray magnetic circular dichroism (XMCD) measurements revealed paramagnetic behavior down to 1.8 K.9b−e The observed magnetic moment is reduced in this system compared to the free trivalent Dy ion, which is attributed to a quenched orbital moment due to the crystal field splitting from the carbon cage and/or electron back-donation from the cage to the Dy ion. In contrast, C 80 NCFs have a carbon cage with a closed shell, and less coupling between the moments of the metal ions and a diamagnetic cage is expected. SQUID magnetization measurements on Ln 3 N@C 80 (Ln = Tb and Ho) 10a,b are in line with a model where the LF of the N 3− ion induces an easy axis for the individual Ln III moments directed along the respective Ln−N bond. In this model the magnetic anisotropy due to the LF is strong enough that the Ln III moments do not align with the external field but instead parallel to the bond directions. ) and Er x Sc 3−x N@ C 80 10d (x = 1, 2) above 1.8 K showed paramagnetism without hysteresis.In the present case we have a single Dy III ion in a diamagnetic carbon cage (see Figure 1). Since Sc III ions are not paramagnetic, the LF due to the N 3− ion will result in magnetic anisotropy directed along the Dy−N bond. Furthermore, if the LF stabilizes a ground state with a large J z , the prerequisite for magnetic bistability is fulfilled.

Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of Tc14.6 K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal x-ray and neutron powder diffraction, superconducting quantum interference device and torque magnetometry, and muon-spin rotation (SR). Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe1.045Se0.406Te0.594. Additionally, a minor hexagonal Fe7Se8 impurity phase was identified. The magnetic penetration depth at zero temperature obtained by means of SR was found to be ab(0)=491 (8) PREPRINT (April 7, 2010) Anisotropic Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of Tc 14.6 K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe1.045Se0.406Te0.594. Additionally, a minor hexagonal Fe7Se8 impurity phase was identified. The magnetic penetration depth λ at zero temperature was found to be λ ab (0) = 491(8) nm in the ab-plane and λc(0) = 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density ρs(0) ∝ λ −2 (0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and ironpnictides. The temperature dependences of both λ ab and λc are well described by a two-gap s+s-wave model with the zero-temperature gap values of ∆S(0) = 0.51(3) meV and ∆L(0) = 2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter γ λ (T ) = λc(T )/λ ab (T ) increases with decreasing temperature, in agreement with γ λ (T ) observed in the iron-pnictide superconductors.superconducting properties of single-crystalline FeSe 0.5 Te 0.

We report on muon-spin rotation and relaxation (µSR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs0.8(FeSe0.98)2. Whereas our transport and magnetization data confirm the bulk character of the superconducting state below Tc = 29.6(2) K, the µSR data indicate that the system is magnetic below TN = 478.5(3) K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs0.8(FeSe0.98)2 a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed TN is the highest reported to date for a magnetic superconductor.

a b s t r a c tA review of our investigations on single crystals of LnFeAsO 1Àx F x (Ln = La, Pr, Nd, Sm, Gd) and Ba 1Àx -Rb x Fe 2 As 2 is presented. A high-pressure technique has been applied for the growth of LnFeAsO 1Àx F x crystals, while Ba 1Àx Rb x Fe 2 As 2 crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO 1Àx F x crystals. Resistivity measurements on LnFeAsO 1Àx F x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba 1Àx Rb x Fe 2 As 2 simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 Â 10 9 A/m 2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO 1Àx F x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba 1Àx Rb x Fe 2 As 2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO 1Àx F x . Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy ($20 meV).

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