Effect of copper on the magnetism of half doped bilayer manganite

• All samples x≤0.2, are biphasic consisting of an AFM and a FM phase. Their transition temperatures decrease with increasing copper.
• Room temperature resistivity (ρ280K) maximizes at x~0.1 due the short-circuiting of more insulating AFM phase by the less insulating FM phase.
• Magnetization is irreversible below TN.
• At x≥0.2 a spin glass transition appears below ~30 K only in the FM phase where the interlayer interaction becomes heterogeneous.

We have studied the effect of substituting copper at the manganese site on the structure, resistivity and magnetism of the bilayer manganite LaSr2Mn2−xCuxO7 (0≤x≤0.3). All samples form in tetragonal structure with I4/mmm space group. Copper was established to be present in the +2 state by the ESCA spectra. Although no structural transition was observed with this substitution, the electrical transport and magnetic properties change significantly. The parent composition consists of an antiferromagnetic phase ordering at 210 K (=TN(x)) and a ferromagnetic phase with its spins ordering at 135 K (=TC(x)). Charge ordering is not observed. These transition temperatures agree with those of the majority La0.96Sr2.04Mn2O7 and minority La1.2Sr1.8Mn2O7 phases observed by Battle et al. The transport data is also more consistent with two manganites conducting in parallel. A freezing transition appears just above the TN(x) of each composition x≤0.15. The transition temperatures of both phases decrease with increasing copper concentration due to the replacement of the strong intersheet antiferromagnetic (AFM) superexchange Mnn+–Mnn+(n=3,4) with a much weaker Cu2+–Mn3+interaction. But in the ferromagnetic (FM) composition this substitution replaces the FM intersheet interaction with an AFM interaction and thereby produces heterogeneity in the intersheet interactions leading to a canonical spin glass transition in the minority phase of the x=0.2 composition.