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P D Rogers,
Y J Choi,
E C Standard,
T D Kang,
K H Ahn,
A Dubroka,
P Marsik,
Ch Wang,
C Bernhard, S Park,
S.-W Cheong,
M Kotelyanskii,
A A Sirenko
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ABSTRACT: Far-infrared spectra of magnetodielectric Dy 3 Fe 5 O 12 garnet were studied using a combination of transmittance, reflectivity, and rotating analyzer ellipsometry. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. Using 4 × 4 matrix formalism for materials with μ(ω) = 1, we modeled the experimental optical spectra and determined the far-infrared dielectric and magnetic permeability functions. The matching condition μ(ω h)S e = ε(ω h)S m for the oscillator strengths S e(m) explains the observed vanishing of certain hybrid modes at ω h in reflectivity.
Physical review. B, Condensed matter 02/2013; 83:174407.
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P. D. Rogers,
Y J Choi,
E. Standard,
T. D. Kang,
K. H. Ahn,
A Dubroka,
P. Marsik,
Ch. Wang,
C. Bernhard, S Park,
S-W Cheong,
M. Kotelyanskii,
A. A. Sirenko
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ABSTRACT: Far-infrared spectra of magneto-dielectric Dy3Fe5O12 garnet were studied
between 13 and 100 cm-1 and at low temperatures between 5 and 80 K. A
combination of transmission, reflectivity, and rotating analyzer ellipsometry
was used to unambiguously identify the type of the dipole activity of the
infrared modes. In addition to purely dielectric and magnetic modes, we
observed several hybrid modes with a mixed magnetic and electric dipole
activity. These modes originate from the superexchange between magnetic moments
of Fe and Dy ions. Using 4x4 matrix formalism for materials with Mu=/=1, we
modeled the experimental optical spectra and determined the far-infrared
dielectric and magnetic permeability functions. The matching condition
Mu(Wh)*Se=Eps(Wh)*Sm for the oscillator strengths Se(m) explains the observed
vanishing of certain hybrid modes at Wh in reflectivity.
01/2011;
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ABSTRACT: By correlating room temperature conductive atomic force microscopy with low temperature electrostatic force microscopy images of the same sample region, we demonstrate that nanoscale electric conduction between a sharp tip and the surface of ferroelectric HoMnO3 is intrinsically modulated by the polarization of ferroelectric domains. Conductance spectra reveal that the electric conduction is described by polarization-induced Schottky-like rectification at low bias, but dominated by a space-charge limited conduction mechanism at high bias. Our observation demonstrates visualization of ferroelectric domain structure by electric conduction, which may be used for nondestructive readout of nanoscale ferroelectric memories and/or ferroelectric sensors.
Physical Review Letters 05/2010; 104(21):217601. · 7.37 Impact Factor
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ABSTRACT: The spectra of far-infrared transmission in Tb3Fe5O12 magnetoelectric single crystals have been studied in the range between 15 and 100 cm-1, in magnetic fields up to 10 T, and for temperatures between 5 and 150 K. We attribute some of the observed infrared-active excitations to electric-dipole transitions between ligand-field split states of Tb3+ ions. Anticrossing between the magnetic exchange excitation and the ligand-field transition occurs at the temperature between 60 and 80 K. The corresponding coupling energy for this interaction is 6 cm-1. Temperature-induced softening of the hybrid IR excitation correlates with the increase of the static dielectric constant. We discuss the possibility for hybrid excitations of magnons and ligand-field states and their possible connection to the magnetoelectric effect in Tb3Fe5O12. Comment: submitted to Phys. Rev. B on May 15th, 2010
05/2010;
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ABSTRACT: We have explored giant magnetic coercivity (Hc≈7 tesla) in the highly anisotropic ferromagnet of Fe0.25TaS2 through harnessing order of Fe2+ ions intercalated in-between TaS2 layers. Fe ions order well in annealed crystals and form a (1/2,0,0) superlattice. However, a (1/3,1/3,0) superlattice, in addition to the (1/2,0,0) superlattice, can form in quenched crystals. These coexisting superlattices with nano-size domains result in significant change of magnetic domain configurations and huge enhancement of Hc, probably through efficient magnetic domain wall pinning by nano-size superlattice domains.
EPL (Europhysics Letters) 05/2009; 86(3):37012. · 2.17 Impact Factor
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Weida Wu,
V Kiryukhin,
H-J Noh,
K-T Ko,
J-H Park,
W Ratcliff,
P A Sharma,
N Harrison,
Y J Choi,
Y Horibe,
S Lee, S Park,
H T Yi,
C L Zhang,
S-W Cheong
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ABSTRACT: We have studied quasi-two-dimensional multiferroic LuFe2O4 with strong charge-spin-lattice coupling, in which low-temperature coercivity approaches an extraordinary value of 9 T in single crystals. The enhancement of the coercivity is connected to the collective freezing of nanoscale pancakelike ferrimagnetic domains with large uniaxial magnetic anisotropy ("Ising pancakes"). Our results suggest that collective freezing in low-dimensional magnets with large uniaxial anisotropy provides an effective mechanism to achieve enhanced coercivity. This observation may help identify novel approaches for synthesis of magnets with enhanced properties.
Physical Review Letters 10/2008; 101(13):137203. · 7.37 Impact Factor
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ABSTRACT: We employ neutron spherical polarimetry to determine the nature and population of the coexisting antiferromagnetic domains in multiferroic YMn2O5. By applying an electric field, we prove that reversing the electrical polarization results in the population inversion of two types of in-plane domains, related to each other by inversion. Our results are completely consistent with the exchange-striction mechanism of ferroelectricity, and support a unified model where cycloidal ordering is induced by coupling to the main magnetic order parameter.
Physical Review Letters 09/2008; 101(6):067205. · 7.37 Impact Factor
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ABSTRACT: We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn2O5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary points unambiguously at the exchange-striction mechanism as the primary driving force for ferroelectricity.
08/2008;
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ABSTRACT: The authors report the current-voltage characteristics of La0.5Ca0.5MnO3/Nb-SrTiO3 p-n junction. This junction shows remarkable thermal hysteresis, giant negative magnetoresistance (MR), remarkable differences of MR for the zero field cooling and field cooling processes, and memory effect of magnetic field. Magnetic force microscopy studies provide direct evidence of magnetic inhomogeneity in La0.5Ca0.5MnO3 film. These intriguing behaviors of our p-n junction can be explained by the phase separation in La0.5Ca0.5MnO3. This work demonstrates the principle of harnessing phase separation for highly tunable device applications.
Applied Physics Letters 06/2008; 92(23):232109-232109-3. · 3.84 Impact Factor
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ABSTRACT: The intriguing nano-structural properties of a ZnMnGaO4 film epitaxially grown on MgO (001) substrate have been investigated using synchrotron radiation-based x-ray diffraction. The ZnMnGaO4 film consisted of a self-assembled checkerboard (CB) structure with perfectly aligned and regularly spaced vertical nanorods. The lattice parameters of the orthorhombic and rotated tetragonal phases of the CB structure were analyzed using H-K, H-L, and K-L cross sections of the reciprocal space maps measured around various symmetric and asymmetric reflections of the spinel structure. We demonstrate that the symmetry of atomic displacements at the phases boundaries provides the means for coherent coexistence of two domains types within the volume of the film.
04/2008;
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ABSTRACT: The commensurate phase of multiferroic HoMn2O5 was studied by X-ray magnetic scattering, both off resonance and in resonant conditions at the Ho-L3 edge. Below 40 K, magnetic ordering at the Ho sites is induced by the main Mn magnetic order parameter, and its temperature dependence is well accounted for by a simple Curie-Weiss susceptibility model. A lattice distortion of periodicity twice that of the magnetic order is also evidenced. Azimuthal scans confirm the model of the magnetic structure recently refined from neutron diffraction data for both Mn and Ho sites, indicating that the two sublattices interact via magnetic superexchange.
03/2008;
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S Park,
Y Horibe,
T Asada,
L S Wielunski,
N Lee,
P L Bonanno,
S M O'Malley,
A A Sirenko,
A Kazimirov,
M Tanimura,
T Gustafsson,
S-W Cheong
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ABSTRACT: One of the central challenges of nanoscience is fabrication of nanoscale structures with well-controlled architectures using planar thin-film technology. Herein, we report that ordered nanocheckerboards in ZnMnGaO4 films were grown epitaxially on single-crystal MgO substrates by utilizing a solid-state method of the phase separation-induced self-assembly. The films consist of two types of chemically distinct and regularly spaced nanorods with mutually coherent interfaces, approximately 4 x 4 x 750 nm3 in size and perfectly aligned along the film growth direction. Surprisingly, a significant in-plane strain, more than 2%, from the substrate is globally maintained over the entire film thickness of about 820 nm. The strain energy from Jahn-Teller distortions and the film-substrate lattice mismatch induce the coherent three-dimensional (3D) self-assembled nanostructure, relieving the volume strain energy while suppressing the formation of dislocations.
Nano Letters 03/2008; 8(2):720-4. · 13.20 Impact Factor
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Physical Review B 01/2008; 77:134434. · 3.69 Impact Factor
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Physical Review Letters 01/2008; 101:067205. · 7.37 Impact Factor
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ABSTRACT: Linearly polarized spectra of far-infrared (IR) transmission in HoMn2O5 multiferroic single crystals have been studied in the frequency range between 8.5 and 105 cm-1 and for temperatures between 5 K and 300 K. Polarization of IR-active excitations depends on the crystallographic directions in HoMn2O5 and is sensitive to the magnetic phase transitions. We attribute some of the infrared-active excitations to electric-dipole transitions between ligand-field split states of Ho3+ ions. For light polarization along crystalline b-axis, the oscillator strength of electric dipoles at low frequencies (10.5, 13, and 18 cm-1) changes significantly at the commensurate-incommensurate antiferromagnetic phase transition at T3 = 19 K. This effect shows a strong correlation with the pronounced steps of the b-directional static dielectric function. We propose that the ligand field (LF) on Ho3+ connects the magnetism and dielectric properties of this compound through coupling with the Mn spin structure. We comment on the possibility for composite excitations of magnons and excited LF states. Comment: new figure
03/2007;
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ABSTRACT: We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) S=1/2 magnet of LiCu2O2. Electric polarization (P) emerges along the c direction below the spiral-magnetic order temperature, but changes from the c to a axis when magnetic fields (H) are applied along the b direction. We also found that P(c) increases with H(c), and P(a) appears with H(a). LiCu2O2 in zero field appears to be the first ferroelectric cuprate and also a prototypical example of the "1D spiral-magnetic ferroelectrics." However, the unexpected behavior in H may demonstrate the complexity of the ordered spin configuration, inherent in the 1D S=1/2 magnet of LiCu2O2.
Physical Review Letters 03/2007; 98(5):057601. · 7.37 Impact Factor
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ABSTRACT: The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.
Physical Review Letters 04/2006; 96(9):097601. · 7.37 Impact Factor
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ABSTRACT: The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn$_{2}$O$_{5}$, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magneto-elastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature-dependence of the spontaneous polarization, in particular its sign reversal at the commensurate-incommensurate transition. Comment: 10 pages, 3 figures, 1 table
11/2005;
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ABSTRACT: Materials exhibiting low-field magnetoelectric and magnetodielectric (MD) effects are necessary for utilization of these effects in multifunctional devices. Since large magnetic fields (H) or electric fields (E) are required to produce any significant effect in existing single-phase magnetoelectrics, recent efforts have been largely devoted to the investigation of laminates or thin film composites made of piezoelectric and magnetostrictive materials. In this work, we report large MD effect Δε/ε∼3% at remarkably low fields (H≪2 kOe ) in a single-phase material, terbium iron garnet. Our results suggest a route towards future applications of the MD effect in advanced devices.
Applied Physics Letters 08/2005; · 3.84 Impact Factor
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ABSTRACT: We have studied the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R=Tb,Ho,Dy) using neutron diffraction as a function of temperature. All three materials display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling. For R=Tb,Ho, a commensurate-incommensurate transition takes place at low temperatures. The commensurate magnetic structures have been solved and are discussed in terms of competing exchange interactions. The spin configuration within the ab plane is essentially the same for each system, and the radius of R determines the sign of the magnetic exchange between adjacent planes. The inherent magnetic frustration in these materials is lifted by a small lattice distortion, primarily involving shifts of the Mn3+ cations and giving rise to a canted antiferroelectric phase.
Phys. Rev. B. 06/2005; 71(21).
