Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides III |
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Sponsoring Units: DMP GMAG Chair: Ichiro Takeuchi, University of Maryland Room: LACC 150C |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P43.00001: Magnetoelectric measurements of multiferroic thin film materials using microwave microscopy Ichiro Takeuchi, Chen Gao, Makoto Murakami, Kao-Shuo Chang We have developed a technique to quantitatively measure the magnetoelectric (ME) coupling effect in multiferroic materials using microwave microscopy operating at 1 GHz. The technique is used to measure the piezovoltage induced by an external magnetic field through the non-linear dielectric constant. The unique tip geometry of the microscope allows measurement of the ME coupling from a sample region as small as 1 micron cube without the use of a bottom electrode. AC magnetic field up to $\sim $ 10 Oe is used. We have demonstrated the measurement on different types of multiferroic films. In particular, PbTiO$_{3}$-CoFe$_{2}$O$_{4}$ nanocomposite thin films consisting of nanograins of PbTiO$_{3}$ embedded in the matrix CoFe$_{2}$O$_{4}$ was found to exhibit enhanced ferroelectric properties and robust ferromagnetism. The ME coefficient as large as 4 V/cmOe was observed in the nanocomposites at room temperature. Enhancement in the ME effect due to the mechanical resonance of the substrate was observed at $\sim $100 KHz. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P43.00002: Large Second Harmonic Kerr rotation in GaFeO3 thin films on YSZ buffered Silicon Ernst Knoesel, Keith F. McDonald, Tim Osedach, Samuel E. Lofland, S.B. Ogale, Sankar Dhar, Sanjay Shinde, Darshan Kundaliya, T. Venkatesan GaFeO$_{3}$ (GFO) simultaneously exhibits ferromagnetic and pyroelectric properties. Recently, magnetization-induced second harmonic generation (MSHG) have been investigated in single crystal GFO (Y. Ogawa et al., Phys. Rev. Lett. 92, (2004) 047401), where a remarkably large Kerr rotation has been found. We report on MSHG measurements from GFO thin films on YSZ buffered Silicon at room temperature (above $T_{C})$ and at 100 K (below $T_{C})$, where a Kerr rotation of $\sim $15 degrees in the ferromagnetic state is observed. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P43.00003: All-Optical Subpicosecond Magnetic Switching in NiO(001) W. H\"{u}bner, G. Lefkidis, K. Satitkovitchai, O. Ney All-optical switching scenarios have so far been demonstrated in atomic and molecular systems only, thus being relatively slow. Here we show, for the first time, the potential of intragap states at a solid surface, viz. antiferromagnetic NiO(001), a notoriously strongly correlated electron system of both high spin density as well as separated multiplet states, for \textit{magnetic} switching on the subpicosecond time scale. Employing a previously designed four-level scenario and computing the electronic structure from a high-level \textit{ab initio} quantum chemical method QCISD(T) with the additional inclusion of spin-orbit coupling yields: (1) The fine structure splitting of the $^3T_{2g}$ states of 70 meV found in experiment (optical second harmonic generation) is well reproduced, (2) Using gaussian pulses of 60-120 fs duration switching the magnetic state of the system back and forth is possible with high fidelity on time scales as short as 150 fs per single switch [1]. The decisive roles of the shape, frequency, and duration of the applied laser pulse are analyzed. The switched state can be reached by tuning the laser parameters to any of the multiplets within the gap. [1] R. G\'{o}mez-Abal, O. Ney, K. Satitkovitchai, and W. H\"{u}bner, PRL \textbf{90}, 227402 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:27PM |
P43.00004: Optical Magnetoelectric Effects in Multiferroics Invited Speaker: Multiferroics show peculiar magneto-optical properties: Optical refractive index and absorption change with the reversal of the propagation vector \textbf{k} of the electromagnetic wave. This magneto-optic effect is clearly distinct from the conventional magneto-optics like Faraday effect and named optical magneto-electric effect, because it can be considered as the high-frequency extension of the linear magneto-electric effect in multiferroics. We have recently succeeded in detecting the optical/x-ray magneto-electric effect in a polar ferrimagnet GaFeO$_{3}$, where spontaneous polarization \textbf{P}$_{0}$ and magnetization \textbf{M}$_{0}$ are parallel to the $b$ and $c$ axes, respectively. Optical magneto-electric effects are expected to show up for the electromagnetic wave with \textbf{k}//$a$, as the difference in absorption and refractive index with the sign reversal of the triple product of \textbf{P}$_{0}$, \textbf{M}$_{0}$, and \textbf{k}. X-ray magneto-electric absorption shows large enhancement at Fe 1$s$-3$d$ transition.$^{1}$ The obtained spectra are well explained by the interference between electric dipole and electric quadrupole transitions of Fe 1$s$ electrons in an FeO$_{6}$ cluster. Optical magneto-electric absorption of the order of 10$^{-3}$ was observed at around Fe intra-atomic d-d transition.$^{2}$ *Measurements of x-ray spectroscopy were performed at BL-1A, KEK-PF, Japan. $^{1}$M. Kubota et al., Phys. Rev. Lett. \textbf{92} (2004) 137401. $^{2}$J. H. Jung et al., Phys. Rev. Lett. \textbf{93} (2004) 037403. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P43.00005: Enhanced Tc in a High Temperature Superconductor, Proximate to an Antiferromagnetic Insulator S. Park, C. N. Zhang, S. Guha, S.-W. Cheong Placing a high Tc superconductor physically close to an antiferromagnetic insulator brings fundamental scientific interests. Not only the non-superconducting component of the heterostructure plays the role of an artificially created barrier for a Josephson junction network, but also the heterostructure-interface may highlight the magnetic proximity effect or the strain-induced pressure effect on the superconductivity. Electrical transport and percolation behavior of a polycrystalline high Tc superconductor-antiferromagnetic insulator composite has been studied by resistivity, magnetic susceptibility, x-ray diffraction and the polarized optical microscopic experiment. We discovered that Tc can be significantly enhanced in a particular choice of an antiferromagnetic insulator. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P43.00006: Anomalous Effect of Chromium on Enhancing Ferromagnetism of SrRuO$_3$ F. Huang, W. Dmowski, Y-D. Wang, I-W. Chen The ferromagnetism in SrRuO$_{3}$ is generally thought to be due to Stoner mechanism. It is also known that essentially all modifications, by A-site or B-site substitution, lower the Curie temperature of SrRuO$_{3}$. An anomalous effect, howerer, is found with Cr substitution. When Cr replaces Ru in SrRuO$_{3}$, the Curie temperature increases due to the ferromagnetic superexchange interaction between tetrahedral Cr$^{4+}$ and octahedral Ru$^{4+}$. This case provides the first example that the Stoner ferromagnetism of itinerant electrons can be augmented by superexchange ferromagnetism of localized electrons. On the other hand, the ferromagnetic enhancement effect of Cr is completely suppressed by the simultaneous La substitution of Sr, turning Cr into octahedral Cr$^{3+}$, which leads to antiferromagnetic Ru-Cr and Cr-Cr interactions. Magnetic, transport and XANES data are reported to depict these phenomena. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P43.00007: Anomalous Transport in Ca$_{1-y}$Sr$_y$MnO$_3$ ($0\leq y\leq 0.75$)~$^{\ast}$ Corneliu Chiorescu, Joshua Cohn, John Neumeier Electron-doped manganites such as Ca$_{1-x}$La$_x$MnO$_3$ have
attracted considerable interest in recent years due
to their inhomogeneous magnetic ground state, consisting of
nanoscale ferromagnetic (FM) droplets and/or spin
canted clusters within a G-type antiferromagnetic matrix.$^1$
The nominally undoped (Mn$^{4+}$)
Ca$_{1-y}$Sr$_y$MnO$_3$ compounds [for which $T_N$ increases
from 125 K (y=0) to 200 K (y=0.75)] have a small
electron concentration, $n\sim 10^{18} {\rm cm}^{-3}$,
associated with native defects. We report transport
measurements on these materials that reveal an unusual
transition at a characteristic temperature, $T^{\ast}\sim
(0.5-0.8)T_N$. For $T |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P43.00008: Ferroelectric microdomains and magnetocapacitance in Ti-doped YMnO3 S. Mori, Y. Horibe, Y. Aikawa, K. Tsuda, T. Arima, T. Katsufuji We have investigated microstructure giving rise to the magnetocapacitance (MC) in Ti-doped YMn$_{1-x}$Ti$_{x}$O$_{3}$ by electron microscopy, combining with conventional magnetic measurement We found characteristic diffuse scatterings elongated along both the [001] and [110] directions in x=0.175, which shows the maximum value of the MC. In addition, the microdomains with the size of 10-20nm consisting of ferroelectric (FE) phase appear in x=0.175. As the Ti concentration (x) is increasing, the paraelectric phase grows up at the expense of the FE phase and the paraelectric phase is dominated above x=0.2. We will discuss the relation between the MC and FE microdomains in this compound. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P43.00009: Magnetic domains in the ferromagnetic and ferroelectric mixture of (La, Lu, Sr)MnO$_3$ Alex de Lozanne, Weida Wu, Casey Israel, N. Hur, S. Park, S.-W. Cheong Single crystals of (La$_{5/8}$Sr$_{3/8}$MnO$_3$)$_x$(LuMnO$_3$)$_ {1-x}$ (LSMO)(LMO) synthesized by the floating-zone method were studied by Magnetic Force Microscopy (MFM). Samples were mechanically cut and polished with the surface normal to the growth direction. Polarized optical microscopy shows that LSMO and LMO are separated in a stripe-like pattern due to the chemical immiscibility.\footnote{S. Park, et al., PRL 92, 167206 (2004)} MFM images show magnetic domains ($\sim$1$\mu$m) in the LSMO stripes and no magnetic signal in the LMO phase. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P43.00010: Super-Colossal Magnetoresistance in Magnetic / Superconducting Oxides Carlos Sa de Melo I discuss the super-colossal magneto-resistive effect in complex magnetic/superconducting oxides. In nanometer scale manganite/cuprate heterostructures it is possible to change the magneto-resistance of the nano-heterostructure by several orders of magnitude through the application of a small parallel magnetic field. When consecutive manganite layers are coupled antiferromagneticaly, superexchange fields cancel out in the sandwiched cuprate atomic layer and the heterostructure is superconducting at low temperatures. When a small parallel field is applied to the heterostructure the magnetizations in the manganite layers align and the superexchange fields add up in the cuprate layers, thus destroying its superconductivity. This results in the ``super-colossal magneto-resistive effect''[1]. In my talk I will describe the microscopic theory that produces such effect. \newline [1] C. A. R. Sa de Melo, submitted (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P43.00011: Effect of adiabatic breathing and half-breathing phonons on striped ground states. Yucel Yildirim, Adriana Moreo A spin-fermion model for high Tc cuprates is studied using numerical simulations. For certain dopings, stripes are observed in the ground state [1]. Different modes of adiabatic phonons are added to the Hamiltonian, among them the breathing and half-breathing modes. Diagonal and off-diagonal couplings are also considered. It is observed that increasing diagonal electron-phonon couplings tends to stabilize the stripes, while the off-diagonal terms destabilize them creating inhomogeneous ground states [2]. References: [1] C. Buhler, S. Yunoki and A. Moreo, Phys.Rev.Lett. 84, 2690 (2000). [2] Y. Yildirim and A. Moreo, in preparation. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P43.00012: Soliton like excitations and inconmensurate phases in half doped manganites. Luis Brey, P.B. Littlewood In half doped manganites, the Jahn-Teller effect, the antiferromagnetic coupling between Mn spins and the directionality in the hopping amplitude conspire to create an exotic spin, charge and orbital ordered phase (CE). In this phase the $x-y$ plains are coupled antiferromagnetically and into the layers the structure consists of zigzag chains with steps formed by three Mn ions. Along the chains the occupied Mn $e_g$ orbitals are ordered in the form $...(2x^2-r^2)\! - \! (x^2-y^2) \! - \! (3y^2-r^2) \! - \!(x^2-y^2)...$. Using a pseudospin notation, $|x^2-y^2>$=$\uparrow$ and $|3z^2 -r ^2>$=$\downarrow$, the order along the chain is described by the $x$-component of the pseudospin $\tau _x (i) \sim \cos (\frac{\pi}{4} i )$. This state is degenerated with a state with pseudospin order $\tau _x (i) \sim \cos (\frac {\pi}{ 4} i+\pi )$, and associated with this degeneracy, we expect there to exist soliton-like excitations. The solitons can have charge states $Q$=0, $\pm n \frac{e}{2}$. We study the dependence of the soliton properties on the Jahn-Teller coupling and Hubbard repulsion and we conclude that the solitons could be the low energy charged excitations of the CE phase. When a finite density of charge excitations exists, the ground state is an array of solitons, that develop an incommensurate charge, orbital and spin modulation. [Preview Abstract] |
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