Session A9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Cobaltite And Ferrite Thin Films

Ferromagnetic insulating state in tensile-strained LaCoO$_3$ thin films

With local density approximation + Hubbard $U$ (LDA+$U$) calculations, we show that the ferromagnetic (FM) insulating state observed in tensile-strained LaCoO$_3$ epitaxial thin films is most likely a mixture of low-spin (LS) and high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM states, including the intermediate-spin (IS) state (\textit{metallic} within LDA+$U$), which consists of IS Co only, and the insulating IS/LS mixture state, the HS/LS state is the most favorable one. The FM order in HS/LS state is stabilized via the superexchange interactions between adjacent LS and HS Co. We also show that Co spin state can be identified by measuring the electric field gradient (EFG) at Co nucleus via nuclear magnetic resonance (NMR) spectroscopy.   

Aberration-corrected STEM-EELS studies of epitaxial La0.5Sr0.5CoO3 thin films

Cobaltite thin films provide a unique opportunity to study magneto-electronic phase separation, which can be strong in this reduced dimensionality environment. Here we present an investigation of epitaxial La0.5Sr0.5CoO3 thin films on SrTiO3 and LaAlO3 substrates by scanning transmission electron microscopy and electron energy loss spectroscopy. The different degrees of strain and also different orientations of the substrates (such as (001) vs. (110)) induce major changes of the crystal structure and the depth profile of the chemical composition, observed both in the La/Sr and O sub-lattices. These effects can lead to lower effective doping level at the interface, favoring interfacial magneto-electronic phase separation. Research Council Starting Investigator Award (JS, NB) and the U.S. Dept. of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Div. (MV, SJP). Work at UMN supported by NSF and DOE (scattering).   

Atomic-Scale Imaging and Control of Interface Magnetic States in Vacancy Ordered Cobaltite Thin Films

Magnetic properties of complex oxide thin films are strongly affected by strain, chemical composition, and octahedral tilt of the substrate. Here, we study lanthanum/strontium cobaltite (La$_{0.5}$Sr$_{0.5}$CoO$_{3-x,}$ LSCO) thin films via quantitative aberration-corrected scanning transmission electron microscopy and Electron Energy Loss Spectroscopy (EELS) to explore the coupling between magnetic properties, ionic behavior, and oxygen octahedral tilts. LSCO films were grown by PLD in identical conditions on two different substrates, LSAT (cubic) and NGO (orthorhombic). These substrates have nearly identical lattice parameters, but different octahedral tilts. The film on NGO appears to be La$_{0.5}$Sr$_{0.5}$CoO$_{2.5}$, while the film on LSAT is less oxygen deficient. Comparison of measured lattice parameters with the first-principles calculations allows us to determine oxygen content in the film. In La$_{0.5}$Sr$_{0.5}$CoO$_{2.5}$/NGO films, EELS reveals different valence states of Co at the interface depending on termination, resulting in different magnetic states. Therefore changes in octahedral tilts can induce changes in oxygen stoichiometry and interface magnetic states of the vacancy ordered structures.   

Stress and magnetism in LaCoO$_{3}$ films

Cobaltates exhibit a wide variety of exciting electronic properties resulting from strong electron correlations; these include superconductivity, giant magnetoresistance, metal-insulator transition, and strong thermoelectric effects. This makes them an excellent platform to study correlated electron physics, as well as being useful for various applications in electronics and sensors. In the ground state in the bulk, the prototypical complex cobalt oxide LaCoO$_{3}$ is in a spin-compensated low-spin state (t$_{2g}^{6})$, which results in the ground state being nonmagnetic. In a recent experiment, Fuchs \textit{et al.} (\textit{Phys. Rev. B} \textbf{75}, 144402 (2007)) have demonstrated that a ferromagnetic ground state could be stabilized by epitaxial tensile strain resulting in a Curie temperature ($T_{C})$ of $\sim $90 K when LaCoO$_{3}$ (LCO) is grown on SrTiO$_{3}$ (STO) using pulsed laser deposition. In this talk I will discuss our recent successful attempt to integrate a LCO/STO heterostructure with Si (001) using molecular beam epitaxy. We have grown strained, epitaxial LaCoO$_{3}$ on (100)-oriented silicon using a single crystal STO buffer (Appl.Phys. Lett. \textbf{98}, 053104 (2011)). SQUID magnetization measurements confirm that the ground state of the strained LaCoO$_{3}$ is ferromagnetic with a $T_{C}$ of 85 K. Our first-principles calculations of strained LaCoO$_{3}$ using the LSDA+$U$ method show that beyond biaxial tensile strain of 2.5{\%} local magnetic moments, originating from the high spin state of Co$^{3+}$, emerge in a low spin Co$^{3+}$ matrix. Ferromagnetism found in tensile-strained LaCoO$_{3}$ is tightly coupled to the material's orbital and structural response to applied strain. Theoretical calculations show how LaCoO$_{3}$ accommodates tensile strain \textit{via} spin state disproportionation, resulting in an unusual sublattice structure.   

Ordering of defects induced by epitaxy in LaCoO$_{3}$ films

In the bulk, LaCoO3 (LCO) undergoes a spin state transition from a diamagnet to a paramagnet with increasing temperature. Recent studies of epitaxial LCO thin films have resulted in the stabilization of a higher spin state and ferromagnetic ordering at low temperatures. Here, we explore the effects of epitaxy on the electronic structure of LCO films with X-ray absorption spectroscopy (XAS) and scanning transmission electron microscopy (STEM). We find differences in XAS spectra in coherently strained thinner films compared to the thicker partially relaxed films which may be due to differences in Co valence and bonding. STEM and electron energy loss spectroscopy of thinner LCO films reveal ordered defect planes that appear to be associated with a change in the O and Co bonding environments. In films on LaAlO3 strained in compression periodic planes occur parallel to the substrate-film interface, while films on SrTiO3 strained in tension have perpendicular defect planes. Correlation with magnetic data suggests that defect rich regions may exhibit greater ferromagnetism.   

Spin state disproportionation and ferromagnetism in strained LaCoO3: Ab-initio study

Strain engineering of artificial oxide heterostructures opens up routes for the creation of novel electronic phases that do not exist in the bulk. To fully exploit the functionalities of the oxide, understanding its electronic and structural response to epitaxial strain is crucial. One example is the recent demonstration of biaxial tensile strain stabilizing an insulating ferromagnetic ground state in normally non-magnetic LaCoO3. However, theoretical understanding is incomplete. In this talk, using the LSDA+U method we discuss the origin of strain induced transition to insulating ferromagnetic ground state in LaCoO3. We show that beyond biaxial tensile strain of 2.5{\%} local magnetic moments, originating from high spin state of Co3+, emerge in low spin Co3+ matrix. We further show that these local moments are ferromagnetically coupled via superexchange interaction. In contrast, we find that compressive strain by itself is not able to stabilize a magnetic state, that agrees with recent experiment. Ferromagnetism found in tensile-strained LaCoO3 is tightly coupled to the material's orbital and structural response to applied strain. We discuss how LaCoO3 accommodates tensile strain via spin state disproportionation, resulting in an unusual sublattice structure.   

Percolation-Type Ferromagnetic Order in Epitaxial Strained LaCoO$_3$ Thin Films

Support for percolation-type ferromagnetic order in LaCoO$_3$ thin films is provided by x-ray diffraction and Co $K$-edge x-ray absorption fine structure (XAFS) spectroscopy. X-ray diffraction shows considerable changes in structure with respect to bulk LaCoO$_3$, and extended XAFS (EXAFS) demonstrates a large Jahn-Teller like distortion of the oxygen octahedra in highly strained films. Structural distortions of the oxygen octahedra are strongly coupled to the hybridization between orbitals of Co and O atoms, as shown by x-ray absorption near edge spectroscopy (XANES). Our results indicate that increased Co-O hybridization, and therefore increased magnetic exchange energy, does not cause ferromagnetism to occur in LaCoO$_3$ thin films. Instead, we suggest that the strain-induced distortions of the oxygen octahedra increase the population of $e_g$ electrons and concurrently depopulate $t_{2g}$ electrons beyond a stabilization threshold for ferromagnetic order.   

Imaging Local Magnetic Domain Rearrangement in Strained LaCoO$_3$ Thin Films Using Magnetic Force Microscopy

Previous studies we have conducted on thin films of lanthanum cobaltate (LCO) under tensile strain have revealed a tendency toward local magnetic domain rearrangement into streak-like configurations near the ferromagnetic to paramagnetic phase transition. Moreover, the persistence of these streak-like characteristics to lower temperatures after field-cooling appears to be linked to the strength of the applied magnetic field in which these films are field-cooled. This tendency has not yet been verified for thin films of LCO under compressive strain which could indicate whether this magnetic domain rearrangement is intrinsic to thin film samples of LCO or is merely an effect of tensile strain. Using magnetic force microscopy, we investigate the microscale magnetic properties of a thin film of LCO under compressive strain, prepared by molecular beam epitaxy and deposited on a lanthanum aluminate substrate. We observe these properties across a wide temperature range and compare our results to global magnetic characteristics of this film as measured by a SQUID magnetometer.   

Origin of ferromagnetic ordering in LaCoO$_{3}$ epitaxial thin films

LaCoO$_{3}$ (LCO) film has received attention due to its unexpected ferromagnetic (FM) ordering, which is distinctly different from the bulk counterpart. Although the exact origin has not been understood, previous studies have suggested that the epitaxial strain should play an important role. In this work, we show that the FM ordering could be related to a locally-ordered microstructure. We used PLD to deposit epitaxial LCO thin films on various substrates in order to impose different degree of strain. XRD and XAS studies showed that the films are of high quality, without any secondary phases or changes in the Co valence. In addition, all the films were coherently-strained. From the STEM investigation, however, we noticed that some of the films had an unexpected stripe-like superstructure along the $<$100$>$ direction. While the microstructure resembles that of oxygen-vacancy or charge ordering, typically found in doped transition metal oxides, we could rule out such possibilities and interpret it as a nanoscale twin boundary. The strain induced structural change seems to originate the FM ordering.   

Magnetic properties of LuFeO$_3$ thin films

In order to extract their intrinsic magnetic properties, we have grown LuFeO$_3$ thin films epitaxially on Al$_2$O$_3$ (0001), a substrate with minimum magnetic impurities, using pulsed laser deposition. The magnetization measurements reveal strong anisotropy between in-plane and out-of-plane, not only in terms of coercivity and remanence, but also obvious in the zero field cool and field cool splitting. Further experiment using X-ray magnetic linear dichroism suggest magnetic ordering higher than 290 K and a spin reorientation at lower temperature. Over all, the films appear weak-ferromagnetic.   

Fabrication and properties of LuFeO3 thin film

We have succeeded in growing the hexagonal LuFeO3 single crystalline thin films on Al2O3(0001) substrates using Pulsed Laser Deposition (PLD). The structures, epitaxial relation between film and substrate, ferroelectric and magnetic properties of the samples were characterized by RHEED, LEED, XRD, AFM, TEM, PFM and SQUID magnetometry. The structure of our hexagonal LuFeO3 films is consistent with that of YMnO3, and the samples exhibit a piezoelectric effect at room temperature. RHEED data are consistent with a structural change from a polar P63cm (185) to non-polar P63/mmc (194) at 1050 K. SQUID measurements reveal strong magnetic order in the thin film. All the data suggests a coexistence of ferroelectricity and magnetic order in hexagonal LuFeO3 films.   

Enhancement of the Magnetic Moment in Ultrathin Fe-doped CoFe$_{2}$O$_{4}$

The magnetic properties of magnetic oxides can be drastically altered through a reduction in film thickness. It has previously been demonstrated that the magnetic moments of CoFe$_{2}$O$_{4}$ and NiFe$_{2}$O$_{4}$ are enhanced for ultrathin films; however, the physical mechanisms for this enhancement are still unknown. To determine the physical cause of this increased magnetic moment and to examine the effect of Fe doping, thin films of Co$_{1-x}$Fe$_{2+x}$O$_{4}$ (0 $\le x\le $ 0.8) are grown epitaxially on MgO (001) substrates by MBE at thicknesses ranging from 3 -- 20 nm. SQUID magnetometry measures the bulk magnetic properties of the samples and confirms that there is an increase in the magnetic moment for all stoichiometries as the film thickness is reduced. XAS, XMLD and XMCD measurements examine the cation-specific magnetic moments and spin directions to explain the physical mechanisms that lead to an enhanced magnetic moment in ultrathin Fe-doped CoFe$_{2}$O$_{4}$ films.   

Electronic states and magnetic structure at the Co$_{3}$O$_{4}$ (110) surface: a first principles study

Tricobalt tetraoxide (Co$_{3}$O$_{4})$ is an important catalyst and Co$_{3}$O$_{4}$(110) is a frequently exposed surface in Co$_{3}$O$_{4}$ nanomaterials. We employed Density-functional theory with on-site Coulomb repulsion U term to study the atomic structures, energetics, magnetic and electronic properties of the two possible terminations, A and B, of this surface. These calculations predict A as the stable termination in a wide range of oxygen chemical potentials, consistent with recent experimental observations. The Co$^{3+}$ ions do not have a magnetic moment in the bulk, but become magnetic at the surface, which leads to surface magnetic orderings different from the one in the bulk. Surface electronic states are present in the lower half of the bulk band gap and cause partial metallization of both surface terminations. These states are responsible for the charge compensation mechanism stabilizing both polar terminations. Furthermore, our calculations predict that the critical thickness for polarity compensation is 4 layers.