Magnetism is a ubiquitous phenomenon in nature and plays an important role in device applications, e.g., hard disk. Manganese-based perovskites (manganites), an important class of magnetic materials, exhibit a variety of rich physics and hold great promise for application such as magnetic data storage. The widespread interests in manganites also stems from the strong correlation between electrons, which could be easily altered by structural distortions, electronic doping, resulting in dramatic change of electronic and magnetic properties. It is therefore imperative to understand the delicate interplay between these degrees of freedom and develop synergistic approach for accurate control of physical properties.
Recently, a group of researchers led by University of Science and Technology of China (USTC), Argonne National Lab and Southeast University (China), used BM 33 at Advanced Photon Source to probe the correlation between the structural distortion and ferromagnetism at the interface between LaMnO3 and SrTiO3. Their work was published in July 2014 in Nature Communications. This international collaboration also includes researchers from Beijing Synchrotron Radiation Facility and University of Illinois Urbana-Champaign.
The research team performed a systematic investigation of magnetic properties in (LaMnO3)M/(SrTiO3)N superlattices and found a strong correlation between magnetism and structural distortion at the interface, as revealed by high-resolution synchrotron X-ray diffraction. Due to various complexities of interfacial structural distortion, the researchers utilized a novel method to determine the oxygen octahedral rotation (OOR) by measuring the half-order bragg peaks, a method that was originally proposed and developed a few years ago in Argonne (S. May et al., Phys. Rev. B 82, 014110 (2010)). They found that the low periodicity superlattice (M=N=2) exhibits a strong oxygen octahedral rotation along the c axis, which quickly diminishes as the superlattice periodicity increases to M=N=4. Accordingly, the ferromagnetic (FM) order is strongly suppressed for low periodicity superlattice. First-principles calculation directly confirms that the change in OOR plays an important role in the reduction of FM order. While suppression of FM at manganite interface is well-known and its origin is still under debate, the current observation of an enhanced c-axis OOR brings new insight into this well-known issue, the so-called “magnetic dead layer”.
The researchers also found that the FM order is strongest when the superlattice periodicity is close to N=M=6. This asymptotic behavior implies a delicate balance of electronic interactions, which depend sensitively on the film thickness, in addition to well-known factors such as interfacial structure, charge doping, etc. They found that N=M=6 superlattice with the strongest FM order features a OOR pattern with a-a-c0 with α=β~4°. This novel phase of LaMnO3+δ, achieved so far only in thin films, suggests a new direction of magnetic control at manganite interface.
The current work therefore provides direct evidence that the interfacial OOR in manganites is intimately linked to their magnetic properties. Recently, there have been considerable research interest in modifying/creating novel interfacial functional properties by OOR and many exciting applications have been theoretically predicted. With the rapid progress of advanced synchrotron techniques, one would expect more excitement in the near future.
Author affiliations: 1University of Science and Technology of China, 2Argonne National Lab, 3Southeast University, 4Beijing Synchrotron Radiation Facility, 5University of Illinois Urbana-Champaign.
This work was supported by NBRPC (2012CB922000, 2014CB921102), NSFC (Grant Nos. 11104258, 11034006, 11374279, 11274060, and 51322206), CAS (XDB01020000, KJCX2-EW-J02), SRFDP (20113402110046), and FRFCU (WK2340000035). A.B. acknowledges support of the Materials Science and Engineering Division, U.S. Department of Energy, Office of Science, Basic Energy Sciences. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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