The reversal of the spontaneous exchange bias effect and zero-field-cooling magnetization in La1.5Sr0.5Co1−xFexMnO6: the effect of Fe doping

The crystal structure, electronic structure and magnetic properties were systematically studied in a series of Fe-doped La1.5Sr0.5CoMnO6 double perovskites. The X-ray diffraction patterns of the samples are all refined with a rhombohedral (Rc) structure. The parameters a and c continuously increase with increasing Fe doping concentration x. X-ray photoelectron spectroscopy (XPS) spectra of the Mn, Co, and Fe 2p core levels, consistent with the soft X-ray absorption spectroscopy (XAS) spectra of Mn, Co, and Fe L2,3 edges, indicate that their valence states are Mn3+ and Mn4+, Co2+ and Co3+, and Fe3+, respectively. However, relative to samples with x ≤ 0.1, there is an abrupt change of photon energy in the Co- and Fe-2p XAS spectra for x ≥ 0.2, implying the spin state transition is from high to low. In addition, this is further confirmed by a comparison between the calculated effective spin moment from the paramagnetic data and the theoretical value. Interestingly, we demonstrate the reversal of both zero-field-cooling magnetization and the sign switching of the spontaneous exchange bias (SEB) with the doping concentration from magnetic measurements. The magnetization reverses from positive to negative with the temperature decreasing across the compensation temperature at the critical concentration x = 0.2. Meanwhile, the exchange bias field of the SEB reverses from large negative values to positive ones. Our findings allow us to propose that the spin state transition caused by inhomogeneity is considered to play an important role in the reversal of the magnetization and the SEB effect.