The 12442-type Fe-based superconductor is the only system that possesses two FeAs layers between neighboring insulating layers, which is worth the in-depth investigations. In this work, millimeter-sized single crystals of KCa2Fe4As4F2 were grown using a self-flux method. The chemical compositions and crystal structure were characterized carefully. Superconductivity with the critical transition T c = 33.5 K was confirmed by both the resistivity and magnetic susceptibility measurements. Moreover, the upper critical field H c2 was studied by the resistivity measurements under different magnetic fields, where an anisotropy of 8 was revealed near the superconducting transition. Importantly, a rather steep increase for the in-plane H c2 ab with cooling, dμ0 H c2 ab /dT| T c = −50.9 T/K, was observed. This value is several times higher than that of other systems of Fe-based superconductor and indicates an extremely high upper critical field. Possible origins for this behavior were discussed. The finding in our work is a great promotion both for understanding the physical properties and for the high-field applications of 12442-type Fe-based superconductors.

In solid state science, multifunctional materials and especially multiferroics have attracted a great deal of attention, as they open the possibility for next generation spintronic and data storage devices. Interestingly, while many of them host coexisting 3d and 4f elements, the role of the coupling between these two magnetic entities has remained elusive. By means of single crystal neutron diffraction and inelastic neutron scattering experiments we shed light on this issue in the particular case of the multiferroic oxide DyMn2O5. This compound undergoes a first order magnetic transition from a high temperature incommensurate phase to a low temperature commensurate one. Our investigation reveals that although these two phases have very different magnetic structures, the spin excitations are quite similar indicating a fragile low temperature ground state with respect to the high temperature one. Such a rare scenario is argued to be a manifestation of the competition between the exchange interaction and 4f magnetic anisotropy present in the system. It is concluded that the magnetic structure, hence the ferroelectricity, can be finely tuned depending on the anisotropy of the rare earth.