Magnetization of La_2−xSr_xNiO_4+δ (0⩽x⩽0.5): Spin-glass and memory effects

We have studied the magnetization of a series of spin-charge-ordered La_2−xSr_xNiO_4+δ single crystals with 0⩽x⩽0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature T_F1∼50 K and a smaller irreversibility that persists up to near the charge-ordering temperature. We observed memory effects in the thermoremnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed and that there is a pronounced increase in the thermoremnant magnetization when the system is warmed through a spin reorientation transition.