-
P Yu,
W Luo,
D Yi,
J X Zhang,
M D Rossell,
C-H Yang,
L You,
G Singh-Bhalla, S Y Yang,
Q He,
Q M Ramasse,
R Erni,
L W Martin,
Y H Chu,
S T Pantelides,
S J Pennycook,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.
Proceedings of the National Academy of Sciences 05/2012; 109(25):9710-5. · 9.68 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A reversal of magnetization requiring only the application of an electric field can lead to low-power spintronic devices by eliminating conventional magnetic switching methods. Here we show a nonvolatile, room temperature magnetization reversal determined by an electric field in a ferromagnet-multiferroic system. The effect is reversible and mediated by an interfacial magnetic coupling dictated by the multiferroic. Such electric-field control of a magnetoelectric device demonstrates an avenue for next-generation, low-energy consumption spintronics.
Physical Review Letters 11/2011; 107(21):217202. · 7.37 Impact Factor
-
Susanne C. Kehr,
Yongmin M. Liu,
L W Martin,
P Yu,
M Gajek, S Y Yang,
C H Yang,
M T Wenzel,
R. Jacob,
H-G von Ribbeck,
M Helm,
X Zhang,
L M Eng,
R. Ramesh
[show abstract]
[hide abstract]
ABSTRACT: A planar slab of negative index material works as a superlens with
sub-diffraction-limited imaging resolution, since propagating waves are focused
and, moreover, evanescent waves are reconstructed in the image plane. Here, we
demonstrate a superlens for electric evanescent fields with low losses using
perovskites in the mid-infrared regime. The combination of near-field
microscopy with a tunable free-electron laser allows us to address precisely
the polariton modes, which are critical for super-resolution imaging. We
spectrally study the lateral and vertical distributions of evanescent waves
around the image plane of such a lens, and achieve imaging resolution of
wavelength/14 at the superlensing wavelength. Interestingly, at certain
distances between the probe and sample surface, we observe a maximum of these
evanescent fields. Comparisons with numerical simulations indicate that this
maximum originates from an enhanced coupling between probe and object, which
might be applicable for multifunctional circuits, infrared spectroscopy, and
thermal sensors.
03/2011;
-
S.C. Kehr,
Y.M. Liu,
L.W. Martin,
P. Yu,
M. Gajek, S.-Y. Yang,
C.-H. Yang,
M.T. Wenzel,
R. Jacob,
H.-G. von Ribbeck,
M. Helm,
X. Zhang,
L.M. Eng,
R. Ramesh
Nature Communications 03/2011; 2:249. · 7.40 Impact Factor
-
Q He,
Y-H Chu,
J T Heron, S Y Yang,
W I Liang,
C Y Kuo,
H J Lin,
P Yu,
C W Liang,
R J Zeches,
W C Kuo,
J Y Juang,
C T Chen,
E Arenholz,
A Scholl,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO₃. Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.
Nature Communications 03/2011; 2:225. · 7.40 Impact Factor
-
S C Kehr,
Y M Liu,
L W Martin,
P Yu,
M Gajek, S-Y Yang,
C-H Yang,
M T Wenzel,
R Jacob,
H-G von Ribbeck,
M Helm,
X Zhang,
L M Eng,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: A planar slab of negative-index material works as a superlens with sub-diffraction-limited resolution, as propagating waves are focused and, moreover, evanescent waves are reconstructed in the image plane. Here we demonstrate a superlens for electric evanescent fields with low losses using perovskites in the mid-infrared regime. The combination of near-field microscopy with a tunable free-electron laser allows us to address precisely the polariton modes, which are critical for super-resolution imaging. We spectrally study the lateral and vertical distributions of evanescent waves around the image plane of such a lens, and achieve imaging resolution of λ/14 at the superlensing wavelength. Interestingly, at certain distances between the probe and sample surface, we observe a maximum of these evanescent fields. Comparisons with numerical simulations indicate that this maximum originates from an enhanced coupling between probe and object, which might be applicable for multifunctional circuits, infrared spectroscopy and thermal sensors.
Nature Communications 03/2011; 2:249. · 7.40 Impact Factor
-
J X Zhang,
B Xiang,
Q He,
J Seidel,
R J Zeches,
P Yu, S Y Yang,
C H Wang,
Y-H Chu,
L W Martin,
A M Minor,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: Piezoelectric materials exhibit a mechanical response to electrical inputs, as well as an electrical response to mechanical inputs, which makes them useful in sensors and actuators. Lead-based piezoelectrics demonstrate a large mechanical response, but they also pose a health risk. The ferroelectric BiFeO(3) is an attractive alternative because it is lead-free, and because strain can stabilize BiFeO(3) phases with a structure that resembles a morphotropic phase boundary. Here we report a reversible electric-field-induced strain of over 5% in BiFeO(3) films, together with a characterization of the origins of this effect. In situ transmission electron microscopy coupled with nanoscale electrical and mechanical probing shows that large strains result from moving the boundaries between tetragonal- and rhombohedral-like phases, which changes the phase stability of the mixture. These results demonstrate the potential of BiFeO(3) as a substitute for lead-based materials in future piezoelectric applications.
Nature Nanotechnology 02/2011; 6(2):98-102. · 27.27 Impact Factor
-
J Seidel,
P Maksymovych,
Y Batra,
A Katan, S-Y Yang,
Q He,
A P Baddorf,
S V Kalinin,
C-H Yang,
J-C Yang,
Y-H Chu,
E K H Salje,
H Wormeester,
M Salmeron,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO3) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.
Physical Review Letters 11/2010; 105(19):197603. · 7.37 Impact Factor
-
P Yu,
J-S Lee,
S Okamoto,
M D Rossell,
M Huijben,
C-H Yang,
Q He,
J X Zhang, S Y Yang,
M J Lee,
Q M Ramasse,
R Erni,
Y-H Chu,
D A Arena,
C-C Kao,
L W Martin,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with ferromagnet La(0.7)Sr(0.3)MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L(2,3) edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge.
Physical Review Letters 07/2010; 105(2):027201. · 7.37 Impact Factor
-
P Yu,
J S Lee,
S Okamoto,
M D Rossell,
M. Huijben,
C H Yang,
Q He,
J X Zhang, S Y Yang,
M J Lee,
Q M Ramasse,
R Erni,
Y H Chu,
D. A. Arena,
C. -C. Kao,
L W Martin,
R. Ramesh
[show abstract]
[hide abstract]
ABSTRACT: We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with La0.7Sr0.3MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L2,3-edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related with an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at oxygen K-edge. Comment: 17 pages, 4 figures, PRL in press
06/2010;
-
S Y Yang,
J Seidel,
S J Byrnes,
P Shafer,
C-H Yang,
M D Rossell,
P Yu,
Y-H Chu,
J F Scott,
J W Ager,
L W Martin,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.
Nature Nanotechnology 02/2010; 5(2):143-7. · 27.27 Impact Factor
-
S. Y. Yang,
L. W. Martin,
S. J. Byrnes,
T. E. Conry,
S. R. Basu,
D. Paran,
L. Reichertz,
J. Ihlefeld,
C. Adamo,
A. Melville,
Y.-H. Chu,
C.-H. Yang,
J. L. Musfeldt,
D. G. Schlom,
J. W. Ager,
R. Ramesh
[show abstract]
[hide abstract]
ABSTRACT: We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼ 0.8–0.9 V and external quantum efficiencies up to ∼ 10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.
Applied Physics Letters 08/2009; 95(6):062909-062909-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The size and shape evolution of embedded ferromagnetic α-Fe nanowires is discussed. The α-Fe nanowires are formed by pulsed-laser deposition of La0.5Sr0.5FeO3−x on single-crystal SrTiO3 (001) substrate in reducing atmosphere. The average diameter of the nanowires increases from d ≈ 4 to 50 nm as the growth temperature increases from T = 560 to 840 °C. Their in-plane shape evolves from circular to octahedral and square shape with [110] facets dominating as the growth temperature increases. A fitting to a theoretical calculation shows that the circular shape is stable when the diameter of the nanowires is smaller than 8 nm.
Applied Physics Letters 11/2005; 87(20):203110-203110-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Piezoelectric force microscopy is employed to study the ferroelectric domain structure in a 600 nm thick epitaxial BiFeO3 film. In the as-grown film, a mosaic-like domain structure is observed. Scans taken with the cantilever pointing along the principal crystallographic directions enabled us to reconstruct the polarization direction. By combining the perpendicular and in-plane piezoresponse data, we found that the ferroelectric domain structure is mainly described by four polarization directions. These directions point oppositely along two body diagonals, which form an angle of ∼ 71°. The other variants are also occasionally observed.
Applied Physics Letters 10/2005; 87(18):182912-182912-3. · 3.84 Impact Factor
-
F Zavaliche,
H Zheng,
L Mohaddes-Ardabili, S Y Yang,
Q Zhan,
P Shafer,
E Reilly,
R Chopdekar,
Y Jia,
P Wright,
D G Schlom,
Y Suzuki,
R Ramesh
[show abstract]
[hide abstract]
ABSTRACT: We present direct evidence for room-temperature magnetization reversal induced by an electric field in epitaxial ferroelectric BiFeO3-ferrimagnetic CoFe2O4 columnar nanostructures. Piezoelectric force microscopy and magnetic force microscopy were used to locally image the coupled piezoelectric-magnetic switching. Quantitative analyses give a perpendicular magnetoelectric susceptibility of approximately 1.0 x 10(-2) G cm/V. The observed effect is due to the strong elastic coupling between the two ferric constituents as the result of the three-dimensional heteroepitaxy.
Nano Letters 10/2005; 5(9):1793-6. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The experimental domain size scaling law in epitaxial BiFeO3 films shows a different behavior from predictions of the conventional elastic domains: the (101)-type 71° domains are much wider than that of (100)-type 109° despite the larger domain-wall energy in (100) boundary. A phenomenological analysis for rhombohedral BiFeO3 film is proposed, and it reveals that both the depolarizing energy and the elastic energy are indispensable for the equilibrium domain structures. With the increase in the asymmetrical electrostatic boundary on the film surfaces, the dominant domain scaling mechanism changes from electrostatic-dependent domain structure to elastic-dependent one, which is consistent with the experimental data. The present results highlight the general role of depolarizing field in rhombohedral domain structures.
Phys. Rev. B. 80(14).
