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P J Ryan,
J-W Kim,
T Birol,
P Thompson,
J-H Lee,
X Ke,
P S Normile,
E Karapetrova,
P Schiffer,
S D Brown,
C J Fennie,
D G Schlom
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ABSTRACT: Intrinsic magnetoelectric coupling describes the interaction between magnetic and electric polarization through an inherent microscopic mechanism in a single-phase material. This phenomenon has the potential to control the magnetic state of a material with an electric field, an enticing prospect for device engineering. Here, we demonstrate 'giant' magnetoelectric cross-field control in a tetravalent titanate film. In bulk form, EuTiO(3), is antiferromagnetic. However, both anti and ferromagnetic interactions coexist between different nearest europium neighbours. In thin epitaxial films, strain was used to alter the relative strength of the magnetic exchange constants. We not only show that moderate biaxial compression precipitates local magnetic competition, but also demonstrate that the application of an electric field at this strain condition switches the magnetic ground state. Using first-principles density functional theory, we resolve the underlying microscopic mechanism resulting in G-type magnetic order and illustrate how it is responsible for the 'giant' magnetoelectric effect.
Nature Communications 01/2013; 4:1334. · 7.40 Impact Factor
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Journal of The Electrochemical Society 01/2012; 159:F436. · 2.59 Impact Factor
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J Chakhalian,
J M Rondinelli,
Jian Liu,
B A Gray,
M Kareev,
E J Moon,
N Prasai,
J L Cohn,
M Varela,
I C Tung,
M J Bedzyk,
S G Altendorf,
F Strigari,
B Dabrowski,
L H Tjeng, P J Ryan,
J W Freeland
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ABSTRACT: Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.
Physical Review Letters 09/2011; 107(11):116805. · 7.37 Impact Factor
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ABSTRACT: Oxygen octahedral rotations have been measured in short-period (LaNiO3)n/(SrMnO3)m superlattices using synchrotron diffraction. The in-plane and out-of-plane bond angles and lengths are found to systematically vary with superlattice composition. Rotations are suppressed in structures with m>n, producing a nearly unrotated form of LaNiO3. Large rotations are present in structures with m<n, leading to reduced bond angles in SrMnO3. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single-layer films. This result demonstrates that superlattice structures can be used to stabilize nonequilibrium octahedral behavior in a manner distinct from epitaxial strain, providing a novel means to engineer the electronic and ferroic properties of oxide heterostructures.
Phys. Rev. B. 04/2011; 83(15).
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J. Chakhalian,
J. M. Rondinelli,
Jian Liu,
B. A. Gray,
M. Kareev,
E. J. Moon,
N. Prasai,
J. L. Cohn,
M. Varela,
I. C. Tung,
M. J. Bedzyk,
S. G. Altendorf,
F. Strigari,
B. Dabrowski,
L. H. Tjeng, P. J. Ryan,
J. W. Freeland
[show abstract]
[hide abstract]
ABSTRACT: Using resonant X-ray spectroscopies combined with density functional
calculations, we find an asymmetric bi-axial strain-induced $d$-orbital
response in ultra-thin films of the correlated metal LaNiO$_3$ which are not
accessible in the bulk. The sign of the misfit strain governs the stability of
an octahedral "breathing" distortion, which, in turn, produces an emergent
charge-ordered ground state with an altered ligand-hole density and bond
covalency. Control of this new mechanism opens a pathway to rational orbital
engineering, providing a platform for artificially designed Mott materials.
08/2010;
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[show abstract]
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ABSTRACT: We report spin reorientation transitions in a Fe3O4/BaTiO3 heterostructure driven by strain at the structural phase transitions of BaTiO3. These spin reorientations result from the emergence of an in-plane uniaxial magnetic anisotropy. The magnetoelastic response of Fe3O4 to the variations in epitaxial strain that occur at the BaTiO3 phase transitions gives rise to the uniaxial anisotropy. The anisotropy energies calculated from the in-plane strain are in quantitative agreement with a change in the Zeeman energy.
Applied Physics Letters 03/2010; 96(9):092510-092510-3. · 3.84 Impact Factor
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ABSTRACT: 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 LaNiO3 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. Comment: 23 pages, 11 figures
02/2010;
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S J May, P J Ryan,
J L Robertson,
J-W Kim,
T S Santos,
E Karapetrova,
J L Zarestky,
X Zhai,
S G E te Velthuis,
J N Eckstein,
S D Bader,
A Bhattacharya
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ABSTRACT: The disorder inherent to doping by cation substitution in the complex oxides can have profound effects on collective-ordered states. Here, we demonstrate that cation-site ordering achieved through digital-synthesis techniques can dramatically enhance the antiferromagnetic ordering temperatures of manganite films. Cation-ordered (LaMnO3)m/(SrMnO3)2m superlattices show Néel temperatures (TN) that are the highest of any La(1-x)Sr(x)MnO3 compound, approximately 70 K greater than compositionally equivalent randomly doped La(1/3)Sr(2/3)MnO3. The antiferromagnetic order is A-type, consisting of in-plane double-exchange-mediated ferromagnetic sheets coupled antiferromagnetically along the out-of-plane direction. Through synchrotron X-ray scattering, we have discovered an in-plane structural modulation that reduces the charge itinerancy and hence the ordering temperature within the ferromagnetic sheets, thereby limiting TN. This modulation is mitigated and driven to long wavelengths by cation ordering, enabling the higher TN values of the superlattices. These results provide insight into how cation-site ordering can enhance cooperative behaviour in oxides through subtle structural phenomena.
Nature Material 11/2009; 8(11):892-7. · 32.84 Impact Factor
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ABSTRACT: 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 Co-O 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.
Phys. Rev. B. 85(2).
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M. K. Stewart,
C.-H. Yee,
Jian Liu,
M. Kareev,
R. K. Smith,
B. C. Chapler,
M. Varela, P. J. Ryan,
K. Haule,
J. Chakhalian,
D. N. Basov
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ABSTRACT: An optical study of fully strained ultrathin LaNiO3 films is presented and compared with LDA + DMFT calculations. LaNiO3 films were grown by pulsed laser deposition on LaAlO3 and SrTiO3 substrates which provide compressive and tensile strain, respectively. Optical conductivity data show a Drude peak with a spectral weight that is significantly reduced compared to that obtained from LDA calculations. The extended Drude analysis reveals the presence of a pseudogap around 80 meV for the film on SrTiO3 and near 40 meV, at low temperature only, for the film on LAO. An unusual temperature dependence of the optical conductivity is observed, with the Drude plasma frequency increasing by up to 20% at low temperature due to spectral weight transfer from bands lying 2–4 eV below the Fermi energy. Such a strong temperature dependence of the Drude spectral weight has previously been reported for correlated electron systems in which a phase transition is present. In LaNiO3, however, no phase transition is observed.
Phys. Rev. B. 83(7).
