Tunable exchange bias effect in Sr-doped double perovskite La2NiMnO6

Structural and magnetic properties of polycrystalline La2-xSrxNiMnO6 (0.1 <= x <= 0.4) compounds are investigated by x-ray diffraction, x-ray absorption near-edge spectroscopy, Raman spectroscopy and magnetometry. All the samples have a rhombohedral structure and show a slight decrease in the unit cell with Sr doping. X-ray absorption spectra demonstrate that as the Sr-doping level increases, the Mn ions remain in the tetravalent state while the Ni ions partially change from the bivalence to the trivalence state. Magnetic measurements reveal that the Sr dopant has little influence on the ferromagnetic transition temperature but leads to significant reduction in the saturated magnetic moments. The latter suggests that the Sr dopant enlarges the Ni/Mn antisite disorders in these double perovskite oxides, which is confirmed by the Raman spectra where both the antisymmetric stretching and symmetric stretching modes at around 540 cm(-1) and 680 cm(-1), respectively, exhibit an increase in the linewidth and decrease in the intensity with Sr doping. The field-cooled magnetic hysteresis loops at 10K show obvious shift both in the field and magnetization axes for all these samples, indicating the presence of exchange bias effect. This effect is attributed to the exchange coupling between Ni/Mn ordered ferromagnetic phases with antiferromagnetic antiphase boundaries caused by the Ni/Mn antisite disorders. Moreover, it is found that the exchange bias field significantly increases with Sr doping. The hysteresis loops measured under different cooling fields for the compound with x = 0.4 further reveal that the exchange bias field does not depend monotonically on the cooling field but shows a maximum at about 5 kOe. Our results suggest that the exchange bias effect in the Sr-doped La2NiMnO6 can be effectively tuned by the Sr-doping level and the cooling field, which is of special interest for applications.