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

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 (Fe_0.987Ni_0.013)₇Se₈, (Fe_0.955Ni_0.045)₇Se₈, (Fe_0.915Ni_0.085)₇Se₈, (Fe_0.89Ni_0.11)₇Se₈, (Fe_0.79Ni_0.21)₇Se₈, (Fe_0.975Co_0.025)₇Se₈, (Fe_0.951Co_0.049)₇Se₈, and (Fe_0.91Co_0.09)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 T_C and the spin-reorientation temperature (T_SRT). 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 T_C and T_SRT temperatures, and the magnetic entropy change ΔS_m 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. [ABSTRACT FROM AUTHOR]