We report critical current density (Jc) in tetragonal FeS single crystals, similar to iron based superconductors with much higher superconducting critical temperatures (Tc's). The Jc is enhanced 3 times by 6% Se doping. We observe scaling of the normalized vortex pinning force as a function of reduced field at all temperatures. Vortex pinning in FeS and FeS0.94Se0.06 shows contribution of core-normal surface-like pinning. Reduced temperature dependence of Jc indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path (δl), in contrast to KxFe2−ySe2 where spatial variations in Tc (δTc) prevails.

NiFe2O4 and ZnFe2O4 ferrites have been prepared by soft mechanochemical synthesis. The formation of spinel phase and crystal structure of sintered powders were analyzed by X-ray diffraction, Raman spectroscopy, and transmission microscopy. In order to confirm phase formation and cation arrangement, Mössbauer measurements were done. Investigation of the magnetization as a function of magnetic field confirms an expected change of the degree of inversion in the spinel structure with the sintering. The electrical DC/resistivity/conductivity was measured in the temperature range of 298-423 K. Impedance spectroscopy was performed in the wide frequency range (100 Hz-10 MHz) at different temperatures.

We have successfully synthesized a new layered iron oxychalcogenide BaFe2OSe2 single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the c-axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the b-axis and are bridged by oxygen along the a-axis. Physical property measurements indicate that BaFe2OSe2 is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long range antiferromagnetic (AFM) transition at 240 K, corresponding to the peak in specific heat measurement and two glassy transitions at 115 K and 43 K. The magnetic entropy up to 300 K is much smaller than the expected value for Fe 2+ in tetrahedral crystal fields and Mösbauer spectrum indicates that long range magnetic order is unlikely at 294 K. Both results suggest that a short range magnetic correlations exist above the room temperature.

No abstract

The critical properties of the single-crystalline van der Waals bonded ferromagnet Fe3−xGeTe2 were investigated by bulk dc magnetization around the paramagnetic (PM) to ferromagnetic (FM) phase transition. The Fe3−xGeTe2 single crystals grown by self-flux method with Fe deficiency x ≈ 0.36 exhibit bulk FM ordering below Tc = 152 K. The Mössbauer spectroscopy was used to provide information on defects and local atomic environment in such crystals. Critical exponents β = 0.372(4) with a critical temperature Tc = 151.25(5) K and γ = 1.265(15) with Tc = 151.17(12) K are obtained by the Kouvel-Fisher method whereas δ = 4.50(1) is obtained by a critical isotherm analysis at Tc = 151 K. These critical exponents obey the Widom scaling relation δ = 1 + γ/β, indicating self-consistency of the obtained values. With these critical exponents the isotherm M (H) curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation m = f±(h), where m and h are renormalized magnetization and field, respectively. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg (d = 3, n = 3) spins coupled with the attractive long-range interactions between spins that decay as J(r) ≈ r −(3+σ) with σ = 1.89.