Low-field magnetic-properties of ErCo₂ intermetallic compound

The magnetic behavior of ErCo₂ at low fields has been investigated over the temperature range extending from 4 to 300 K. Magnetic hysteresis loops, at 4.2 K between the fields of ∓ 1.5 kOe, have been recorded for both zero field-cooled (ZFC) and field-cooled (FC) cases. Strong thermo-magnetic hysteresis effects were observed in the dc temperature dependence of magnetization measurements at temperatures below T = 35 K. A sharp drop occurs in both the ac susceptibility and the resistivity at the same temperature. All these behaviors indicate an apparent first-order magnetic transition. The same experiments were repeated on the same sample, but for two different thermal histories (annealed at 670 ° C for 2 months, then slowly cooled down to room temperature and annealed at 900 ° C for 2 days, then quenched to room temperature). Neither the transition temperature nor the magnetic hysteresis has been affected considerably by these thermal treatments. From magnetization measurements for different magnetic histories such as ZFC and FC cases and also thermal cycling of the remnants, it is concluded that the cobalt moments are created at T_c accompanied by a lattice expansion with a small distortion along the 〈 1 1 1 〉 direction (easy axes). Each induced cobalt moment has an orbital component coupled to the 〈 1 1 1 〉 direction. Close to its critical temperature, the slightest applied field, even earth's field, will have an effect first on the erbium sites whose magnetic moments are oriented along the field direction and then will trigger the cobalt moment formation process in the opposite direction. The existence of the cobalt orbital moment can account consistently for many aspects of low-field M versus H curves below the Curie temperature for both ZFC and FC cases. Furthermore, we have observed that the magnetization, at temperatures below T_c, depends on the history. The reduction in the magnetization after each M versus H cycle (the reduction in the magnetization occurs substantially after the first cycle of the applied field and followed by the relaxation to the equilibrium state where the rate of reduction decreases for the following cycles). This behavior may be attributed to the spin melting of some cobalt atoms, initially quenched along the direction of erbium magnetic moments. We also discuss the Dzyaloshinsky-Moriya (DM) anisotropic interactions as one of the possible mechanisms to explain the origin of the large coercivity observed at low fields.