Chemical Strain Engineering of Magnetism in Oxide Thin Films

Arxiv preprint arXiv: …, 2010

Epitaxial strain is a proven route to enhancing the properties of complex oxides, however, the details of how the atomic structure accommodates strain are poorly understood due to the difficulty of measuring the oxygen positions in thin films. We present a general methodology for determining the atomic structure of strained oxide films via x-ray diffraction, which we demonstrate using LaNiO 3 films. The oxygen octahedral rotations and distortions have been quantified by comparing the intensities of half-order Bragg peaks, arising from the two unit cell periodicity of the octahedral rotations, with the calculated structure factor. Combining ab initio density functional calculations with these experimental results, we determine systematically how strain modifies the atomic structure of this functional oxide.

Physical Review B, 2017

Strain-phonon coupling, in terms of the shift in phonon frequencies under biaxial strain, is studied by density functional theory calculations for twenty perovskite oxides strained in their (111)-and (001)-planes. While the strain-phonon coupling under (001)-strain follows the established, intuitive trends, the response to (111)-strain is more complex. Here we show that strain-phonon coupling under (111)-strain can be rationalized in terms of the Goldschmidt tolerance factor and the formal cation oxidation states. The established trends for coupling between (111)-strain and in-phase and out-of-phase octahedral rotational modes as well as polar modes provide guidelines for rational design of (111)-oriented perovskite thin films.

Scientific Reports, 2015

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from first-principles calculations. The proposed mechanism is completely general, however for illustrative purposes, we demonstrate the concept on vanadium based perovskites where we reveal an unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks to the intimate coupling between Jahn-Teller distortions and electronic degrees of freedom, the electric field control of Jahn-Teller distortions is of general relevance and may find broad interest in various functional devices.

ACS Applied Materials & Interfaces, 2020

We have probed the structural and magnetic properties of PrVO 3 (PVO) thin lms grown on the (001)-, (110)-and (111)-oriented SrTiO 3 (STO) substrates. By changing the substrate orientation, (1) the out-of-plane orientation of lm can be tuned to [110], [100] / [010], and [011] / [311], (2) the number of crystal variants in the lm can be varied, for ex. we observe single domain lm on (110)oriented STO, whereas two domains in the lm grown on the (111)-oriented STO substrate. The lattice strain induced by using dierent oriented substrates has direct inuence on the magnetic properties of PVO lms. The magnetic moment of PVO lms radically enhances from 0.4 µ B /f.u. for STO (001) to 2.3 µ B /f.u. for STO (111). While, lms on (001)-oriented STO substrate display out-of-plane anisotropy, an in-plane anisotropy is observed for lms grown on the (110)-and (111)-oriented STO substrates. In addition, a strong uniaxial magnetic anisotropy is also extracted for a partially relaxed lm on the (110)-oriented STO substrate. Such ndings can help oxide community in the better understanding of magnetic anisotropy in vanadate thin lms, a subject that still lacks scientic investigations.

Physical Review B, 2008

We study how strain affects orbital ordering and magnetism at the interface between SrMnO3 and LaMnO3 from density-functional calculations and interpret the basic results in terms of a threesite Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t2g core spins and the ferromagnetic double exchange of the itinerant eg electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the MnO6 octahedron, splitting the Mn-eg states into x 2-y 2 and 3z 2-1 states, with the former being lower in energy, if the strain is tensile in the plane, and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the x 2-y 2 orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction of the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the 3z 2-1 orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the (LaMnO3)1/(SrMnO3)1 and (LaMnO3)1/(SrMnO3)3 superlattices for various strain conditions.

We show that a ferromagnetic (FM) order in the orthorhombic CaRuO3, which is a non-magnetic and iso-structural analog of FM system SrRuO3, can be established and stabilized by the means of tensile epitaxial strain. Investigations on the structural and magnetic property correlations in the CaRuO3 films with different degrees of strain reveal that the FM moment increases with increasing the tensile strain. This is an experimental verification to the theoretical predictions of scaling of the tensile epitaxial strain and the magnetic order in this system. Our studies further establish that the tensile strain is more efficient than the chemical route to induce the FM order in CaRuO3 as the magnetic moment in these strained films is larger than that in chemically modified CaRu0.9Cr0.1O3 films.

EPL, 2006

Epitaxial thin films of multiferroic perovskite BiMnO3 were synthesized on SrTiO3 substrates, and orbital ordering and magnetic properties of the thin films were investigated. The ordering of the Mn 3+ eg orbitals at a wave vector ( 1 4 1 4 1 4 ) was detected by Mn K-edge resonant x-ray scattering. This peculiar orbital order inherently contains magnetic frustration. While bulk BiMnO3 is known to exhibit simple ferromagnetism, the frustration enhanced by in-plane compressive strains in the films brings about cluster-glass-like properties.

physica status solidi (b), 2015

Advanced Materials Interfaces, 2019

Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is a controversial subject that is critical for developing efficient energy storage and conversion materials. In this work, high‐quality epitaxial thin films of the model perovskite La0.5Sr0.5Mn0.5Co0.5O3−δ (LSMC), under compressive or tensile strain, are characterized with a combination of in situ and ex situ bulk and surface‐sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is observed that the effect of strain on reducibility is dependent on the “effective strain” induced on the chemical bonds. In‐plain strain, and in particular the relative BO length bond, is the key factor controlling which of the B‐site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it...

Physical Review B, 2012

We have examined the atomic and electronic structures of perovskite lanthanum cobaltite (LaCoO3) thin films using Co K-edge x-ray absorption fine structure (XAFS) spectroscopy. Extended XAFS (EXAFS) demonstrates that a large difference between in-plane and out-of-plane CoO bond lengths results from tetragonal distortion in highly strained films. The structural distortions are strongly coupled to the hybridization between atomic orbitals of the Co and O atoms, as shown by x-ray absorption near edge spectroscopy (XANES). Our results indicate that increased hybridization is not the cause of ferromagnetism in strained LaCoO3 films. Instead, we suggest that the strain-induced distortions of the oxygen octahedra increase the population of eg electrons and concurrently depopulate t2g electrons beyond a stabilization threshold for ferromagnetic order. Discoveries of novel properties in transition metal (TM) oxides, such as superconductivity in cuprates 1 and colossal magnetoresistance in manganites, 2 have prompted a flurry of research on these materials and proposals for numerous oxide thin film based electronic devices. 3,4 Fabrication of complex oxides in the form of thin films is crucial to the development of such devices. However, when synthesized as a thin film, the properties of a material can differ substantially from those of their bulk counterpart as a consequence of epitaxial strain. A striking example is the appearance of ferromagnetism in strained LaCoO 3 (LCO), 5-12 a perovskite oxide that lacks long-range magnetic order in bulk form. Below 100 K, LCO exists in a nonmagnetic insulating state, where the six valence electrons of Co 3+ occupy the t 2g levels of the crystal field split 3d states. The t 6 2g e 0 g configuration of Co 3+ is commonly referred to as the low spin (LS) state. Above 100 K, LCO becomes paramagnetic and semiconducting. Upon heating to 500 K, a transition to a paramagnetic and metallic phase occurs. These phase transitions are believed to coincide with spin state transitions of Co 3+ ions to t 5 2g e 1 g intermediate spin (IS) or t 4 2g e 2 g high spin (HS) states. However, the exact nature of the spin states present in LCO is not fully understood. 13 The spin states of the perovskite cobaltites are strongly influenced by competition between the crystal field splitting (∆ CF) of the Co(3d) states into e g and t 2g levels, which favors a LS configuration, and Hund exchange, which favors a HS configuration. The effects of chemical and hydrostatic pressure on the nonmagnetic to paramagnetic transition temperature illustrate the sensitivity of the spin state to the e g-t 2g gap (∆), where ∆ = ∆ CF − W/2, and W is the overlap between Co(3d) derived e g and O(2p) orbitals. 14,15 The dependence of the orbital overlap and crystal field splitting on the CoO distance (r Co−O) and the CoO Co angle (θ) are approximated by the expressions W ∝ r −3.5 Co−O sin(θ/2) and ∆ CF ∝ r −5 Co−O. 6,14 Therefore, ∆ can be reduced by an increase in r Co−O or θ. In the case of chemical pressure, replacing lanthanum with a rare earth ion having a smaller ionic radius reduces θ. This increases ∆ and thereby raises the nonmagnetic to paramagnetic transition temperature. 13-15 On the other hand, application of external pressure can induce a transition to the LS state by reducing r Co−O and therefore increasing ∆. 14 Numerous studies have demonstrated that epitaxial LCO thin films have a ferromagnetic ground state with a Curie temperature near 85 K. 5-12 These works indicate that tetragonal distortion of LCO is critical to the appearance of ferromagnetism, a conclusion supported by theoretical studies. 16 However, the local atomic structure resulting from tetragonal distortion is unclear. Local structure is strongly correlated with magnetic properties, and therefore without knowledge of the local structure the nature of ferromagnetism cannot be well understood. Previous extended x-ray absorption fine structure (EXAFS) studies of LCO thin films deposited on (LaAlO 3) 0.3 (Sr 2 AlTaO 6) 0.7 indicate that all six CoO bonds are of equal length. 17 This result supports the conclusion that ferromagnetic order in LCO is due to strain induced suppression of the Jahn-Teller distortion that has been suggested to occur in bulk LCO. 5 However, existence of a Jahn-Teller distortion in bulk LCO has been called into question by recent studies. 18,19 Furthermore, in other TM perovskite oxide thin films, epitaxial strain causes significant distortion of oxygen octahedra and can also alter octahedral rotations. 20-23 In addition to affecting the spin state as discussed above, distortion and rotation of oxygen octahedra can alter magnetic properties by changing the magnetic exchange energy, which increases with additional overlap between Co(3d) derived e g and O(2p) orbitals, W. In this way, strain induced changes in CoO hybridization can modify the magnetic properties of ferromagnetic perovskite oxides. 20,23 Increased CoO hybridization has also been posited as a requirement for ferromagnetic ordering in LCO. 6 In order to provide insight into the nature of ferromagnetism in epitaxial LCO, we have undertaken a detailed investigation of the local and electronic structures of LCO thin films. Bulk LCO has a rhombohedral structure belonging to space group R3c with a pseudocubic lattice parameter (a 0) of 3.830Å. 24 The present study examines ∼ 20 nm thick LCO films pulsed-laser deposited on SrTiO 3 (STO), a 0 = 3.905Å, and LaAlO 3 (LAO), a 0 = 3.791Å, substrates for direct comparison. 7 The films are capped with two unit cells of STO. Further details of the deposition and properties of these films can be found in Refs. 7 and 8. STO and LAO substrates have respective lattice misfits of 2.0 % and −1.0 % with LCO. However, while the LCO film deposited on STO is coherently strained, having an in-plane lattice parameter (a f) equal to that of the substrate, the film deposited on LAO is not, having a f equal to 3.842Å, presumably due to the increased misfit between LCO and LAO at the growth temperature. 8,25 Thermal stress that occurs upon cooling from the growth temperature has a substantial impact on lattice parameters of LCO thin films. 6 The out-of-plane lattice parameters (c f) of LCO films deposited on STO and LAO substrates are 3.781Å and 3.864Å, respectively. 7,8 Lattice parameters were measured by x-ray diffraction at beamlines 6-ID-C and 33-BM-C of the Advanced Photon Source (APS). The different strain states that result from the two substrates have been shown to strongly affect the magnetic properties of LCO, with films deposited on STO being ferromagnetic and films deposited on LAO showing magnetic behavior consistent with that of a spin glass. 7 Thus, differences in the local and electronic structures of these two films reveal aspects of the material that likely determine the magnetic state of LCO. Co K-edge x-ray absorption fine structure (XAFS) spectroscopy was performed at the National Institute of Standards and Technology (NIST) beamline X23A2 of the National Synchrotron Light Source (NSLS) in order to examine the local atomic and electronic structures of the LCO films. A four-element silicon drift detector was used for collection of EXAFS data, while the near edge portion of the XAFS spectrum was measured using a single-element