Investigating Interfacial Exchange Interaction and Antiferromagnetic Moment Orientations in La_0.5Sr_0.5FeO₃ / La_0.7Sr_0.3MnO₃ Bilayers

Antiferromagnetic (AFM) spintronics offer several distinct advantages over their ferromagnetic (FM) counterparts, including higher storage density, reduced power consumption, and faster data processing, attributed to the terahertz-frequency spin dynamics inherent to AFM materials. A deep understanding of AFM spin transport and moment orientation is essential for developing AFM-based spintronic devices. Conventionally, AFM moment reorientation requires the application of a large magnetic field (~50 T), however, by exploiting perpendicular interfacial coupling between AFM and FM moments, called spin-flop coupling, AFM moments can be reoriented within the interface plane using magnetic fields as low as tenths of a tesla. In this case, upon application of a magnetic field, the FM moments reorient and the interfacial AFM moments rotate to maintain the perpendicular coupling. AFM La_1-xSr_xFeO₃ / FM La_0.7Sr_0.3MnO₃ (LSMO) heterostructures are one of the few systems which exhibit spin-flop coupling. However, this effect vanishes at an AFM thickness of 18 unit cells (u.c.) in (001)-oriented La_0.7Sr_0.3FeO₃ / LSMO superlattices [1]. To increase this critical thickness, this study investigated Sr doping, x = 0.5 in La_1-xSr_xFeO₃ due to the reduced magnetocrystalline anisotropy. The LSMO layer thickness was held constant at 85 u.c. while the LSFO layer thickness was systematically varied between 10 and 85 u.c. X-ray magnetic linear dichroism measurements were used to reveal the extent of the reorientation of the AFM moments with increasing La_0.5Sr_0.5FeO₃ (LSFO) thickness. While perpendicular moment coupling persists at the interface of all bilayers, spin-flop coupling at the AFM layer surface vanishes at an LSFO thickness of 60 u.c., suggesting the emergence of an easy AFM plane. At this thickness, AFM magnetocrystalline anisotropy dominates and prevents the formation of a monodomain AFM state through spin-flop coupling. Finally, a pixel-by-pixel analysis of the AFM domain images captured using photoemission electron microscopy enabled precise identification of distinct AFM domains and determination of their moment orientations. This detailed understanding of spin-flop coupling in LSFO / LSMO bilayers will lay the foundation of complex oxides for AFM spintronics.
References:
[1] Y. Takamura et al., Phys. Rev. B 80, 180417 (2009).