Combined pressure and magnetic field induced caloric effects in Fe7Se8 single crystals doped with Ni and Co ions

Abstract

The influence of Co and Ni ion doping on the crystal structure, phase transitions, and magnetic properties of the Fe7Se8 system has been investigated. The compounds studied are (Fe0.987Ni0.013)7Se8, (Fe0.955Ni0.045)7Se8, (Fe0.915Ni0.085)7Se8, (Fe0.89Ni0.11)7Se8, (Fe0.79Ni0.21)7Se8, (Fe0.975Co0.025)7Se8, (Fe0.951Co0.049)7Se8, and (Fe0.91Co0.09)7Se8 as well as the parent compound Fe7Se8. The crystals are grown using a modified Bridgman method and exist as a hexagonal NiAs-like structure with an ordered distribution of Fe vacancies that determine their magnetic properties. The substitution effect, inducing a systematic decrease in the unit-cell volume due to the ionic radii of doping ions being smaller than the radii of Fe2+ ions, is a source of chemical pressure, acting on the magnetic subsystem and determining the magnetic ordering temperature TC and the spin-reorientation temperature (TSRT). As the most important result of this paper, it was shown that hydrostatic and chemical pressures are strongly correlated in the investigated system. The character of magnetic phase transitions has been designated in the vicinity of TC and TSRT temperatures, and the magnetic entropy change ΔSm and refrigeration potential magnitudes have been determined for this temperature range. These magnetocaloric parameters were shown to depend strongly on chemical pressure. A phenomenological model developed based on the experimental measurements demonstrated that the magnetic properties of the system depend strongly on the crystal field acting on Fe2+ ions and the ratio of the numbers of Fe2+ and Fe3+ ions.