Strain control and spontaneous phase ordering in vertical nanocomposite heteroepitaxial thin films

MacManus‐Driscoll, Judith L.; Zerrer, Patrick; Wang, Haiyan; Yang, Hao; Yoon, Jungho; Fouchet, Arnaud; Yu, Rong; Blamire, M. G.; Jia, Q. X.

doi:10.1038/nmat2124

Cited by 348 publications

(354 citation statements)

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“…Subtle changes in d occupancy and overlap-and thereby phase transitions-can be induced by variations in temperature, by external fields, through doping, and lattice distortions. Especially the strong coupling of the electronic properties with structural parameters allows controlling the physical characteristics of nanoarchitectures through strain at interfaces of layered and nanocomposite heterostructures [4][5][6][7][8]. Here we show that the angular dependence of the x-ray magnetic circular dichroism (XMCD) signal provides unique insights into the impact of strain on the electronic structure of magnetic transition metal oxides.…”

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confidence: 89%

Strain-Induced Changes in the Electronic Structure ofMnCr2O4Thin Films Probed by X-Ray Magnetic Circular Dichroism

et al. 2010

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We show that the angular dependence of x-ray magnetic circular dichroism (XMCD) is strongly sensitive to strain induced electronic structure changes in magnetic transition metal oxides. We observe a pronounced dependence of the XMCD spectral shape on the experimental geometry as well as non-vanishing XMCD with distinct spectral features in transverse geometry in compressively strained MnCr2O4 films. The XMCD can be described as a linear combination of an isotropic and an angular dependent anisotropic contribution, the latter linearly proportional to the axial distortion due to strain. The XMCD spectra are well reproduced by atomic multiplet calculations.PACS numbers: 68.60. Bs, 78.70.Dm, 78.20.Bh, 75.50.Gg The delicate balance between charge, spin, orbital, and lattice degrees of freedom in transition metal oxides leads to unique phenomena such as colossal magnetoresistance [1], high temperature superconductivity, [2] as well as a remarkable diversity of charge, spin, and orbital ordered phases. The rich phase diagrams are determined by the strong local interaction of electrons in transition metal d orbitals [3]. Subtle changes in d occupancy and overlap-and thereby phase transitions-can be induced by variations in temperature, by external fields, through doping, and lattice distortions. Especially the strong coupling of the electronic properties with structural parameters allows controlling the physical characteristics of nanoarchitectures through strain at interfaces of layered and nanocomposite heterostructures [4][5][6][7][8]. Here we show that the angular dependence of the x-ray magnetic circular dichroism (XMCD) signal provides unique insights into the impact of strain on the electronic structure of magnetic transition metal oxides.For an isotropic system, magnetically saturated by an external field, the XMCD signal scales with the angle, θ, between the field and x-ray beam as cos θ.[9] Consequently, in transverse geometry, i.e., perpendicular orientation of x rays and field (θ = 90 • ), the XMCD signal vanishes completely. In systems with cubic magnetic anisotropy the XMCD spectrum is slightly different along ⟨001⟩ and ⟨111⟩ directions [10]. The angular dependence of the intensity of the XMCD spectral features can be well described by the lowest order term for the cubic anisotropy [10], however, the XMCD signal still disappears for θ = 90 • . This is no longer the case for systems with axial magnetic anisotropy, such as uniaxial, tetragonal, or trigonal symmetry of the lattice. In this case a non-vanishing integral of the XMCD signal indicates a non-zero component of the orbital magnetic moment perpendicular to the spin moment, a situation encountered in, e.g., thin films with strong uniaxial magnetic anisotropy. Whereas changes in the integrated intensity, proportional to the orbital moment, have been observed [11,12], less attention has been paid to the detailed spectral shape of the XMCD and its correlation with structural distortions.Here we determine the strain induced electronic structure changes...

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confidence: 89%

Strain-Induced Changes in the Electronic Structure ofMnCr2O4Thin Films Probed by X-Ray Magnetic Circular Dichroism

et al. 2010

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“…We therefore argue that the intrinsic stacking faults caused by boron vacancies are the microscopic origin of the increased impurity scattering rate, which enhances the upper critical field and the other high-field properties. 38 The role of carbon for the void reduction Even though it is generally argued that carbon substitutes for boron, it was not possible to find substituted carbon inside the MgB 2 phase of the malic acid-doped sample, whereas boron vacancies were abundant. A natural question is then where the carbon is.…”

Section: Microscopic Origin For the Enhanced High-field Propertiesmentioning

confidence: 99%

Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi

et al. 2012

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Increasing dissipation-free supercurrent has been the primary issue for practical application of superconducting wires. For magnesium diboride, MgB 2 , carbon is known to be the most effective dopant to enhance high-field properties. However, the critical role of carbon remains elusive, and also low-field critical current density has not been improved. Here, we have undertaken malic acid doping of MgB 2 and find that the microscopic origin for the enhancement of high-field properties is due to boron vacancies and associated stacking faults, as observed by high-resolution transmission electron microscopy and electron energy loss spectroscopy. The carbon from the malic acid almost uniformly encapsulates boron, preventing boron agglomeration and reducing porosity, as observed by three-dimensional X-ray tomography. The critical current density either exceeds or matches that of niobium titanium at 4.2 K. Our findings provide atomic-level insights, which could pave the way to further enhancement of the critical current density of MgB 2 up to the theoretical limit.

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“…8 In the case of CFO-BFO nanostructures, the only system where the reversal of magnetization by electric field at room temperature has been demonstrated, 2 only out-ofplane unit cell parameter values are reported. 9,10 In contrast, a detailed study of lattice strains was reported for BFO-Sm 2 O 3 columnar nanocomposite heteroepitaxial films on SrTiO 3 ͑001͒ ͑STO͒, 11 showed that the vertical strain of both phases was determined by the mechanical interaction between BFO and Sm 2 O 3 , rather than by the stress caused by the substrate.…”

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confidence: 96%

“…16͒ compared to CFO, 17 in agreement with recent observations of vertical strain in columnar nanocomposite films of other complex oxides. 11 To definitively determine if the CFO residual strain is elastic and caused by the BFO phase, we removed the BFO matrix by selective chemical etching. First, the removal of the matrix was confirmed by SEM.…”

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confidence: 99%

On the strain coupling across vertical interfaces of switchable BiFeO3–CoFe2O4 multiferroic nanostructures

Dix¹,

Muralidharan²,

Guyonnet³

et al. 2009

Applied Physics Letters

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In magnetoelectrically coupled CoFe 2 O 4 -BiFeO 3 nanostructures vertical and lateral lattice parameters of both phases are determined. We find that the in-plane lattice parameter of CoFe 2 O 4 is fully relaxed whereas it presents compressive strain along the out-of-plane direction. Although the CoFe 2 O 4 -BiFeO 3 interface is semicoherent, CoFe 2 O 4 out-of-plane lattice strain is not relaxed after selective removal of the matrix and thus it is of nonelastic origin. In spite of the absence of elastic residual strain caused by CoFe 2 O 4 -BiFeO 3 interfaces, the two phases are mechanically coupled as demonstrated by the electrical switching of the magnetization. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3204464͔Magnetoelectric coupling in biferroic self-assembled epitaxial nanostructures occurs indirectly via the elastic coupling. [1][2][3][4] Although theoretical models 4-6 point out the importance of the residual strains in the magnetoelectric coupling, there is limited information on the lattice strains in ferromagnetic nanostructures in a ferroelectric matrix. It was reported 7 compressive residual strain of PbTiO 3 ͑PTO͒ in PTO-CoFe 2 O 4 ͑CFO͒, and elastic coupling was evidenced by deformation of the ferromagnetic phase during heating across the Curie temperature of PTO. In a high resolution transmission electron microscopy ͑HRTEM͒ study of NiFe 2 O 4 -BiFeO 3 ͑BFO͒ nanostructures, the interfaces between the two nanocomposite phases were found to be fully relaxed. 8 In the case of CFO-BFO nanostructures, the only system where the reversal of magnetization by electric field at room temperature has been demonstrated, 2 only out-ofplane unit cell parameter values are reported. 9,10 In contrast, a detailed study of lattice strains was reported for BFO-Sm 2 O 3 columnar nanocomposite heteroepitaxial films on SrTiO 3 ͑001͒ ͑STO͒, 11 showed that the vertical strain of both phases was determined by the mechanical interaction between BFO and Sm 2 O 3 , rather than by the stress caused by the substrate.We have measured all strains in CFO-BFO nanostructures. CFO is in-plane relaxed but out-of-plane strained, with semicoherent CFO-BFO interfaces. CFO does not relax after selective removal of the BFO phase and thus its residual strain is of nonelastic nature. We show switching of the magnetization of the CFO nanopillars by appropriate poling of the ferroelectric phase. This indicates that magnetoelectric coupling can occur in nanocomposites with semicoherent ferroelectric-ferromagnetic interfaces and absence of elastic residual strain between the two phases. Nanocomposites with thickness in the 100-200 nm range were deposited at a rate of around 0.9 Å/s on STO substrates at temperatures in the 625-675°C range by pulsed laser deposition ͑KrF laser, 5 Hz͒ using a BFO-CFO target with molar ratio of 65:35. 10 The lattice strain was analyzed by x-ray diffractometry ͑XRD͒ with Cu K␣ radiation and by HRTEM. Some samples were etched with HCl ͑10%, 75 s͒ to selectively remove the BFO phase. Atomic force microscopy...

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Strain control and spontaneous phase ordering in vertical nanocomposite heteroepitaxial thin films

Cited by 348 publications

References 36 publications

Strain-Induced Changes in the Electronic Structure ofMnCr2O4Thin Films Probed by X-Ray Magnetic Circular Dichroism

Strain-Induced Changes in the Electronic Structure ofMnCr2O4Thin Films Probed by X-Ray Magnetic Circular Dichroism

Microscopic role of carbon on MgB2 wire for critical current density comparable to NbTi

On the strain coupling across vertical interfaces of switchable BiFeO3–CoFe2O4 multiferroic nanostructures

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