  Magnetic properties of A2BB'O6 (A = rare or alkaline earth ions; B,B' =
transition metal ions) double perovskites are of great interest due to their
potential spintronic applications. Particularly fascinating is the zero field
cooled exchange bias (ZEB) effect observed for the hole doped La2-xAxCoMnO6
polycrystalline samples. In this work we synthesize La2CoMnO6,
La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 single crystals by the floating zone
method and study their magnetic behavior. The three materials are
ferromagnetic. Surprisingly, we observe no zero or even conventional exchange
bias effect for the Ca and Sr doped single crystals, in sharp contrast to
polycrystalline samples. This absence indicates that the lack of grain
boundaries and spin glass-like behavior, not observed in our samples, might be
key ingredients for the spontaneous exchange bias phenomena seen in
polycrystalline samples.
