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We present a new generation of a scanning MicroSQUID microscope operating in an inverted dilution refrigerator. The MicroSQUIDs have a size of 1.21$ \ \mu$m\textsuperscript{2} and a magnetic flux sensitivity of 120 $\mu\Phi_{0} / \sqrt{\textrm{Hz}}$ and thus a field sensitivity of %$550^{-6} \ \Phi_{0} / \sqrt{\textrm{Hz}}$ 550$ \ \mu \textrm{G}/ \sqrt{\textrm{Hz}}$. The scan range at low temperatures is about 80 $\mu$m and a coarse displacement of 5 mm in x and y direction has been implemented. The MicroSQUID-to-sample distance is regulated using a tuning fork based force detection. A MicroSQUID-to-sample distance of 420 nm has been obtained. The reliable knowledge of this distance is necessary to obtain a trustworthy estimate of the absolute value of the superconducting penetration depth. An outlook will be given on the ongoing direction of development.

International audience; The effect of selenium substitution by sulphur on the structural and physical properties of antiferromagnetic TlFe1.6+δSe2 has been investigated via neutron, x-ray and electron diffraction, and transport measurements. The 5 sqrt(a)× 5sqrt(a)×c super-cell related to the iron vacancy ordering found in the pure TlFe1.6Se2 selenide is also present in the S-doped TlFe1.6+δ(Se1−xSx)2 compounds. Neutron scattering experiments show the occurrence of the same long range magnetic ordering in the whole series i.e. the 'block checkerboard' antiferromagnetic structure. In particular, this is the first detailed study where the crystal structure and the 5 a× 5 a antiferromagnetic structure is characterized by neutron powder diffraction for the pure TlFe1.6+δS2 sulphide over a large temperature range. We demonstrate the strong correlation between occupancies of the crystallographic iron sites, the level of iron vacancy ordering and the occurrence of block antiferromagnetism in the sulphur series. Introducing S into the Se sites also increases the Fe content in TlFe1.6+δ(Se1−xSx)2 which in turn leads to the disappearance of the Fe vacancy ordering at x = 0.5 ± 0.15. However, by reducing the nominal Fe content, the same 5 a× 5 a×c vacancy ordering and antiferromagnetic order can be recovered also in the pure TlFe1.6+δS2 sulphide with a simultaneous reduction in the Néel temperature from 435 K in the selenide TlFe1.75Se2 to 330 K in the sulphide TlFe1.5S2. The magnetic moment remains high at low temperature throughout the full substitution range, which contributes to the absence of superconductivity in these compounds.

We determine the pressure phase diagram of the 1111 compounds CaFeAsF and SrFeAsF, up to 20 GPa and down to 4 K by electrical resistivity measurements and the change of structure up to 40 GPa at room temperature. The antiferromagnetic transition temperature, as determined by the derivative peak, shows a minimum at ~5 GPa (10 GPa) for the Ca (Sr) compound. For CaFeAsF, superconductivity appears at this minimum, coincident with the development of a previously reported monoclinic phase. For SrFeAsF, where the orthorhombic and the monoclinic phase were reported to coexist, superconductivity exists above P≥1 GPa. Both phase diagrams can be scaled by a shift of ~10 GPa pressure at which the volume of SrFeAsF and that of CaFeAsF at ambient pressure coincide. The difference of our phase diagram with that of electron-doped 1111 samples is accounted for by hole doping under pressure, which we verified through electron band structure calculations. Fil: Freitas, Daniele C.. Centre National de la Recherche Scientifique; Francia. Universite Joseph Fourier; Francia Fil: Garbarino, Gaston Leonel. European Synchrotron Radiation Facility; Francia Fil: Weht, Ruben Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Sow, Amadou. Universite Joseph Fourier; Francia. Centre National de la Recherche Scientifique; Francia Fil: Zhu, Xiyu. Chinese Academy of Sciences; República de China Fil: Han, Fei. Chinese Academy of Sciences; República de China Fil: Cheng, Peng. Chinese Academy of Sciences; República de China Fil: Ju, Jing. Chinese Academy of Sciences; República de China Fil: Wen, Hai Hu. Chinese Academy of Sciences; República de China Fil: Nunez Regueiro, Manuel Daniel. Universite Joseph Fourier; Francia. Centre National de la Recherche Scientifique; Francia