Abstract:
Interfaces between LaMnO 3 and SrMnO 3 provide a unique environment where the spin, charge and orbital degrees of freedom of each of the constituents may reconstruct to give rise to states that do not occur in the bulk phase diagram of La 1-x Sr x MnO 3 . We have synthesized ordered analogues of La 1-x Sr x MnO 3 for x =0.33 and x =0.67, in superlattices of (LaMnO 3 ) 2n (SrMnO 3 ) n for n = 1 – 5, and (LaMnO 3 ) m (SrMnO 3 ) 2m for m = 1 - 3 respectively, using ozone-assisted oxide MBE. Here, n and m represent integer layers of the constituents. In the (LaMnO 3 ) 2n (SrMnO 3 ) n superlattices, a metal-insulator transition occurs upon increasing n , being metallic for n≤ 2 and insulating for n ≥ 3. Using polarized neutron reflectometry, we show that the insulating state at low temperatures for n = 5 and n = 3 is associated with a strongly modulated ferromagnetic order parameter, where the ferromagnetism has a maximum in the LaMnO 3 , and is strongly suppressed in the SrMnO 3 . For the n = 1 metallic superlattice, the transport and magnetic properties are very similar to those of the random alloy La 2/3 Sr 1/3 MnO 3 , suggesting a homogeneous distribution of charge and magnetization. This is interpreted in terms of a Mott-like transition from a 3-D uniform ferromagnetic metal at low n to a modulated ferromagnetic insulator at high n upon increasing the distance between the LaMnO 3 /SrMnO 3 interfaces. Furthermore, the insulating state that emerges at high n obeys variable-range hopping, which implies a finite density of states at the Fermi level. This is unlike LaMnO 3 or SrMnO 3 , which are both gapped insulators. We suggest that these high n insulators are interfacial in nature, where the states at the Fermi level arise out of charge transfer at the LaMnO 3 /SrMnO 3 interface, which are then localized by canting/frustration of spins due to coexisting FM/AF interactions within the superlattices. Using resonant Bragg scattering at the O- K edge, we see evidence for this electronic reconstruction at LaMnO 3 /SrMnO 3 interfaces while cooling through the ferromagnetic transition. In the (LaMnO 3 ) m (SrMnO 3 ) 2m system ( x =0.67), which is expected to order antiferromagnetically, we have observed signatures of an increase in the magnetic ordering temperature in the superlattices, above the known bulk Néel temperature for randomly alloyed La 1/3 Sr 2/3 MnO 3 .