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A Strain-Induced Charge Carrier-Type Crossover in a Complex Oxide

Epitaxial thin films of lanthanum cobaltite (LaCoO3-d) under tensile strain—the elongation of a material via application of a tensile force—are ferromagnetic, whereas the bulk material is not. No clear understanding of this surprising phenomenon has yet emerged. Also, while the magnetism of this material has been heavily studied, relatively little attention has been paid to electronic transport, due to the insulating nature of the strain-stabilized ferromagnetic state. Here, researchers grew epitaxial LaCoO3-δ films on a variety of substrates that induced lattice mismatches ranging from 1.4% of compression to 2.5% of tensile strain. They then studied the structure, magnetism, and electronic transport of the films using synchrotron x-ray diffraction (SXRD) at the U.S. Department of Energy’s Advanced Photon Source (APS) as well as an array of other techniques; the results were published in Physical Review Materials. Ferromagnetism was observed under tension, polarized neutron reflectometry indicating a relatively uniform magnetization depth profile. Electrically, the films were found to have similar semiconducting properties to bulk LaCoO3-δ. Hall effect measurements, however, revealed a striking and unanticipated inversion of the majority carrier type, from p-type under compression to n-type under tension. This can be understood based on strain-induced orbital population changes, exposing a previously undetected link between magnetism and electronic transport in thin films of this material.

Epitaxial strain in thin films generally arises due to lattice mismatch between a film and its substrate and can occur either during film growth or due to thermal expansion mismatch. Tuning of this strain can engineer novel magnetic behavior in thin films, particularly oxides. Growth of fully strained perovskite films on substrates with tensile or compressive lattice mismatch of up to several percent has yielded many advances, including strain stabilization of ferroelectricity in quantum paraelectric SrTiO3 and multiferroicity in strained EuTiO3.

The discovery nearly 15 years ago of ferromagnetism in tensile-strained films of LaCoO3-δ provided a particularly intriguing example of a strain-induced state radically different from that of the bulk material, as the bulk material is diamagnetic in its ground state. Warming bulk LaCoO3-δ to as little as 30 K generates nonzero spin states, and its famous spin-state transition or spin-state crossover. This 30 K–80 K thermally excited spin-state transition has been extensively studied in bulk LaCoO3-δ, both experimentally and theoretically, although the exact nature of the excited spin-state remains controversial. At higher temperatures, around 500 K, a broad insulator-metal transition occurs in bulk LaCoO3-δ, accompanied by a second change in spin state. The nature of this transition is also debated.

While the spin-state transition of Co3+ ions in bulk LaCoO3-δ is well established, reports of weak ferromagnetism at low temperatures are common. Defect-driven magnetism is thought to occur, potentially including surface ferromagnetism, and weak ferromagnetism associated with magnetic excitons/spin-state polarons forming around oxygen vacancies. The spin state and magnetic order in LaCoO3-δ are thus extremely sensitive to perturbations such as line defects, point defects, and strain. LaCoO3-δ has therefore emerged as an important system in which to study the interplay among magnetism, structure, disorder, and defects, with epitaxial films being of particular interest.

While magnetism has been detected under both tensile and compressive heteroepitaxial strain, tensile strain strongly favors ferromagnetism. Magnetic force microscopy confirms long-range ferromagnetism under tension, while only short-range ferromagnetism is seen under compression. As expected from the observation of ferromagnetism, suppression of the transition to the low spin state at low temperatures has been reported based on x-ray absorption spectroscopy. The intermediate spin state, high spin state, and various ordered and disordered combinations have been reported using several techniques.

Using the extensive array of research techniques including scanning probe and transmission electron microscopy, magnetometry, polarized neutron reflectometry, resistivity, and Hall effect measurements, the researchers, from the University of Minnesota, Augsburg University, the National Institute of Standards and Technology (NIST), and Argonne National Laboratory systematically characterized the structure, morphology, magnetism and electronic transport in 10- to 22-nm-thick LaCoO3-δ films grown on SrTiO3, LSAT, LaAlO3, and SrLaAlO4 substrates. These substrates generated LaCoO3-δ lattice mismatches from 1.4% compression to 2.5% tensile strain.

The researchers obtained high-quality, smooth films exhibiting superstructures associated with both oxygen vacancy ordering and periodic in-plane ferroelastic domains, as measured by SXRD at the X-ray Science Division Surface Scattering & Microdiffraction Group‘s beamline 33-ID at the APS (Fig. 1). As expected, ferromagnetism was found to occur only under tensile strain. Polarized neutron reflectometry measurements carried out at the Polarized Beam Reflectometer at the NIST Center for Neutron Research confirmed that this ferromagnetism is a bulk property, although unit-cell-level interfacial dead layers do form. Semiconducting transport occurred regardless of the sign of the strain, similar to bulk, albeit with lower resistivity and activation energy. Most importantly, Hall effect measurements revealed a striking and unanticipated sign reversal, from p-type in the bulk and under compression, to n-type under tension. The inversion was accompanied by a significant increase in electron mobility for tensile versus compressive films. These results were interpreted, with the aid of density-functional-theory based calculations, in terms of a tensile-strain-induced redistribution of orbital occupancies, essentially associated with specific d orbitals of Co being strongly stabilized under tensile strain. This results in lowering of the electron effective mass, an increase in the hole effective mass, and thus a natural explanation for the observed phenomenon.

These results provide deepened understanding of the ferromagnetic state in strained epitaxial LaCoO3-δ, including exposing a previously undetected link between ferromagnetism and electronic transport. — Chris Palmer

See: Vipul Chaturvedi1, Jeff Walter1, 2, Arpita Paul1, Alexander Grutter3, Brian Kirby3, Jong Seok Jeong1, Hua Zhou4, Zhan Zhang4, Biqiong Yu1, Martin Greven1, K. Andre Mkhoyan1, Turan Birol1 , and Chris Leighton1*, “Strain-induced majority carrier inversion in ferromagnetic epitaxial LaCoO3-δ thin films,” Phys. Rev. Mater. 4, 034403 (2020). DOI:10.1103/PhysRevMaterials.4.034403

Author affiliations: 1University of Minnesota, 2Augsburg University, 3NIST, 4Argonne National Laboratory

Correspondence: *

This work was supported primarily by the U.S. Department of Energy (DOE) through the University of Minnesota (UMN) Center for Quantum Materials under Grant No. DESC- 0016371. Computational work by A.P. was supported by the National Science Foundation (NSF) through the UMN MRSEC under Award No. DMR-1420013. Parts of this work were performed in the Characterization Facility, UMN, which receives partial support from the NSF through the MSREC. This research used resources of the APS, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.

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