Structural, magnetic, and magnetocaloric properties of Fe7Se8 single crystals.

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 T_R ≈ 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 T_R. The conventional magnetocaloric effect related to the metamagnetic transitions has been found above T_R, while below T_R 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 T_N ≈ 450 K was found to be weaker than that appearing near T_R. 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. [ABSTRACT FROM AUTHOR]