The magnetocaloric effect has been studied in high quality single crystals of Fe7Se8 (3c type) grown by using Bridgman’s method. Magnetization and magnetocaloric effect measurements have been carried out in a magnetic field up to 5 T over the temperature range from 2 to 490 K. The spin reorientation transition from the easy c-axis to the easy c-plane, proceeding in an abrupt fashion, as a first-order phase transition, has been observed near the temperature TR ≈ 125 K. The magnetization curves in the vicinity of this transition were shown to have an S-shape with a clear hysteresis. The first order metamagnetic field induced transitions have been identified above and below TR. The conventional magnetocaloric effect related to the metamagnetic transitions has been found above TR, while below TR the inverse magnetocaloric effect was clearly seen. The existence of both kinds of magnetocaloric effect is important from the point of view of large rotating field entropy change in Fe7Se8 single crystals. The refrigeration capacity associated with a second order phase transition from the ferrimagnetic to the paramagnetic state at the Néel temperature TN ≈ 450 K was found to be weaker than that appearing near TR. The giant anisotropy of the magnetocaloric effect was related to the magnetic anisotropy of Fe7Se8 crystals. The one-ion model of the magnetocaloric effect has been developed and its predictions have been compared with experimental data.

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The effect of the change in size under the influence of an applied magnetic field (magnetostriction) was studied in Fe7Se8 single crystals. It was shown that a systematic change in the unit‑cell constants, induced by an external magnetic field and dependent on the direction and magnitude of this field, strongly correlates with the change in the spin reorientation temperature (TSRT) and the magnetic entropy change, which is strongly dependent on TSRT and magnetization. The obtained results suggest that the relation between magnetostriction and the change in magnetic entropy (ΔSm,), reported previously for different materials, seems to have universal character, and magnetostriction can be used as an indirect method to estimate the change in magnetic entropy in Fe7Se8 single crystals substituted with transition metals.