Effect of Fe substitution on structure and exchange interactions within and between the sublattices of frustrated CoCr2O4.

Obtaining tunable magnetic states in geometrically frustrated multiferroic compound CoCr2O4 by tuning the sublattice magnetic coupling is indeed of high interest from the fundamental and applied points of view. In this work, Fe substitution effects in Co(Cr1-xFex)2O4 (0 ≤ x ≤0.5) are experimentally investigated through detailed measurements of the crystalline structure and magnetization. Our experiments reveal that the samples undergo a magnetic transition characterized by a sharp variation in magnetization from 94 K at x = 0 to 317 K at x = 0.5. The field-cooled process shows that a magnetization reversal phenomenon is observed under a stable positive magnetic field when the measurement undergoes the compensation temperature. A magnetic field-assisted switching effect is realized near the compensation temperature, which possesses the characteristics of high repeatability and stability. The molecular field coefficients are evaluated based on the ferrimagnetic Curie-Weiss fitting, and the exchange interactions within and between the sublattices show a relationship of |JAA| < |JAB| < |JBB|. The magnetization reversal in Co(Cr1-xFex)2O4 is considered to be attributed to the coexistence and competition of A-A, A-B and B-B magnetic interactions, as well as the weakening of spin frustration.

Vortex creep activation energies and depinning currents in CaKFe4As4 and Ba0.6K0.4Fe2As2 revealed by AC susceptibility measurements.

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe4As4 is slightly smaller than that in Ba0.6K0.4Fe2As2, which may result from its distorted FeAs4-tetrahedron in CaKFe4As4. The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values of J c0(0) ∼ 1.0 × 108 and 2.2 × 108 A cm-2 for CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors, respectively, which suggest potential applications.