Enhanced ordering temperatures in antiferromagnetic manganite superlattices

The disorder inherent to doping by cation substitution in the complex oxides can have profound effects on collective-ordered states. Here, we demonstrate that cation-site ordering achieved through digital-synthesis techniques can dramatically enhance the antiferromagnetic ordering temperatures of manganite films. Cation-ordered (LaMnO 3 ) m /(SrMnO 3 ) 2 m superlattices show Néel temperatures ( T N ) that are the highest of any La 1− x Sr x MnO 3 compound, ∼70 K greater than compositionally equivalent randomly doped La 1/3 Sr 2/3 MnO 3 . The antiferromagnetic order is A-type, consisting of in-plane double-exchange-mediated ferromagnetic sheets coupled antiferromagnetically along the out-of-plane direction. Through synchrotron X-ray scattering, we have discovered an in-plane structural modulation that reduces the charge itinerancy and hence the ordering temperature within the ferromagnetic sheets, thereby limiting T N . This modulation is mitigated and driven to long wavelengths by cation ordering, enabling the higher T N values of the superlattices. These results provide insight into how cation-site ordering can enhance cooperative behaviour in oxides through subtle structural phenomena. One of the attractions in studying oxide heterostructures is the unusual physical phenomena that they enable. It is now demonstrated that the enforced cation ordering in thin oxide superlattices leads to significantly enhanced magnetic ordering temperatures.