Absence of spontaneous exchange bias effect on La2-xAxCoMnO6 (A = Ca, Sr) single crystals grown by the floating zone technique

The double perovskite material La2-xAxCoMnO6 with Sr and Ca doping (for x = 0.5) showed by various experiments to present the conventional (CEB) and spontaneous exchange bias (SEB) effect [1]. In contrast to polycrystalline samples, this work finds the absence of zero-field-cooled exchange bias effect (ZEB) on single crystals based on magnetic characterization of the same compounds. This result corroborates that the SEB/ZEB effect is deeply correlated to the role played by the spin glass-like phase as a critical ingredient, not observed in our AC susceptibility measurements. The single crystals were grown by the floating zone technique and oriented by X-ray Laue backscattering on the [001] axis. The powder x-ray diffraction pattern shows the La2CoMnO6 (LCMO) and Ca-doped (LCCMO) on an orthorhombic Pnma crystal structure and rhombohedral R-3c for Sr-doped (LSCMO). In addition, our experiments indicate structural changes by doping, which increases the bond angles Co/Mn-O-Co/Mn to closer to 180o, gradually shifting the ferromagnetic transition to higher temperatures.