Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D16: Focus Session: Multiferroics II |
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Sponsoring Units: DMP GMAG Chair: Gavin Lawes, Wayne State University Room: Colorado Convention Center Korbel 4F |
Monday, March 5, 2007 2:30PM - 2:42PM |
D16.00001: Manipulation of the ferromagnetic domains of a manganite using an electric field. Tara Dhakal, Sinan Selcuk, Arthur F. Hebard, Amlan Biswas We have measured the response of the fluid like phases$^{1}$ of ferromagnetic metal (FMM) and charge ordered insulator (COI) in thin films of the manganite (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) to an external electric field. The electric field (set by applying a voltage difference across the material) alters the fluid phases and increases the conductivity of the material by about 2 orders of magnitude above a threshold voltage$^{2}$. To check if the enhanced conductivity is associated with an increase in the size of the FMM domains, we measured the magnetization of the thin films using a SQUID magnetometer with and without an applied electric field. The saturation magnetization remained the same in either case showing that the FMM domains do not increase in size, which led us to hypothesize that the domains are just reoriented by the electric field. This hypothesis was verified by measuring the transverse resistance while a voltage difference was applied longitudinally across the material. At a threshold voltage when the longitudinal resistance decreased by about 2 orders of magnitude, the transverse resistance showed a small increase. This increase in resistance was attributed to the FMM domains being stretched in the direction of the electric field. [1] P. A. Sharma et al., Phys. Rev. B 71, 224416 (2005), [2] Tara Dhakal, et. al, Cond-mat/0607502. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D16.00002: Controlling the magnetic phase of a hole-doped manganite with an out-of-plane electric field. Sung Hee Yun, Rajiv Misra, A. F. Hebard, Amlan Biswas We have measured the effect of an electric field applied perpendicular to the plane of a (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) manganite thin film. Since the thickness of the film is 30 nm, the electric field across the manganite is about an order of magnitude greater than when the voltage is applied in the plane of the film. The films are grown on an insulating substrate NdGaO$_{3 }$(NGO). Hence, we deposited a 30 nm LPCMO film on top of a 60 nm La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) film on an NGO substrate using pulsed laser deposition (PLD). The LCMO film is metallic below 250 K and is used as the bottom electrode to apply the voltage perpendicular to the plane of the LPCMO film. We deposited gold contacts on the LPCMO film as the top electrode. We then etched the LPCMO film, using ion plasma etching or chemical etching, leaving only a small LPCMO pillar beneath the gold contact. An indium contact was pressed on the exposed LCMO film. We then measured current-voltage (I-V) characteristics of this structure. Due to the small interelectrode distance of 30 nm the resultant electric field is high and we were able to measure the effect of an electric field on LPCMO at temperatures well above the insulator to metal transition temperature, for voltages less than 10 V. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D16.00003: Colossal piezoresistance in manganites. Jacob Tosado, Josymir Lopez, Tara Dhakal, Amlan Biswas We have studied the effect of the application of direct mechanical stress on thin films of the hole-doped manganese oxide (manganite) (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO). The two competing phases in manganites are the pseudocubic ferromagnetic metallic (FMM) phase and the orthorhombic charge-ordered insulating (COI) phase. Due to the different structures of the FMM and COI phases, manganites are susceptible to mechanical stresses. The traditional methods of applying stress on oxide thin films result in different growth modes which makes it difficult to quantify the strain in the thin film. Using a calibrated screw we applied direct mechanical stress on the substrate and measured the change in the phase diagram of the manganite as a function of strain. Our results show that the effect of strain is the largest in the fluid phase separated region of the phase diagram$^{2}$, where we observe a strain-induced change in resistance of about 5 orders of magnitude. [2] Tara Dhakal, et. al, Cond-mat/0607502. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D16.00004: Spin filtering with magnetic oxide tunnel barriers Invited Speaker: Interesting physical phenomena and potential new devices arise when the barrier of magnetic tunnel junctions is made of a ferroic material. In the most studied case of ferromagnetic barriers, carriers are spin-polarized by the spin-filter effect, which gives rise to tunnel magnetoresistance (TMR). We will present results on the use of ultrathin ferrimagnetic and ferromagnetic layers as tunnel barriers. We have use thin films of NiFe$_{2}$O$_{4}$ (NFO), T$_{C}$=850K, to filter electrons according to their spin with an efficiency of $\sim $25{\%}, as evidenced by a TMR of up to 60{\%} in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$(LSMO)/NFO/Au junctions. We will discuss these results in the frame of a model describing tunneling through epitaxial magnetic barriers. We will also show results on ferromagnetic films of BiMnO$_{3}$ and La$_{0.1}$Bi$_{0.9}$MnO$_{3}$ (LBMO) and their use as spin-filter barriers. Interestingly, LBMO films are also ferroelectric and therefore exhibit a multiferroic character, that is retained down to thicknesses of only 2 nm. Accordingly, LSMO/LBMO/Au junctions exhibit four different resistance states, instead of two with conventional spin-filters. We will discuss the origin of this behavior on the basis on the combination of the spin-filter effect and the influence of ferroelectricity on tunneling. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D16.00005: Ferroelectricity in a Collinear Magnetic Phase of Orthorhombic Perovskites Ivan Sergienko, Cengiz Sen, Elbio Dagotto Below $T_c$ = 30 K, a number of perovskite manganites (such as HoMnO$_3$) and nickelates order magnetically in the so-called E-type phase with zigzag chains of parallel spins. We demonstrate that this magnetic phase is also ferroelectric. We discuss the magnetoelectric coupling based on the symmetry arguments of the Landau theory of phase transitions. We also explore a microscopic mechanism of ferroelectricity induced by the collinear spin arrangement and address the order of magnitude of the ferroelectric polarization. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D16.00006: Ferroelectricity in perovskite HoMnO$_{3}$ and YMnO$_{3}$ induced by magnetic order. B. Lorenz, Y. Q. Wang, C. W. Chu Ferroelectricity is observed in orthorhombic HoMnO$_{3}$ and YMnO$_{3 }$at the magnetic lock-in transitions into an E-type structure or an incommensurate phase with a temperature independent wave vector, respectively. In HoMnO$_{3}$ the ferroelectric polarization strongly depends on the external magnetic field indicating the involvement of the rare earth moment order in this compound. The results are discussed within the framework of recent theoretical models, in particular the double exchange driven polar displacements predicted for E-type magnetic structures. The ferroelectricity observed in YMnO$_{3}$ cannot be explained within the current picture of the magnetic order and a refinement of the magnetic structure seems to be necessary. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D16.00007: Ferroelectricity driven by zig-zag magnetic chains: orthorhombic HoMnO3 as a multiferroic? Silvia Picozzi, Kunihiko Yamauchi, Biplab Sanyal The search for multiferroic materials showing a coexistence of magnetic and ferroelectric long-range order has recently triggered enormous interests due to their potential use in magnetic recording industry. Based on first-principles density functional calculations including electron-electron correlations explicitly in the framework of Hubbard model, we show that a peculiar arrangement of Mn spins can induce a sizeable ferroelectric polarization in distorted rare-earth manganites. In particular, our calculated ground state magnetic structure of orthorhombic HoMnO3 is AFM-E type in which the zig-zag ferromagnetic chains in the MnO2 planes are antiferromagnetically coupled to their neighboring chains. This unconventional magnetic configuration and the different values of the Mn-O-Mn bond angle for parallel and antiparallel Mn spins lead to a coherent displacement of the O center of mass with respect to the Mn center of mass giving rise to a permanent dipole i.e. ferroelectricity. A quantitative analysis of the ferroelectric polarization, as well as trends in the structural, magnetic and ferroelectric properties as a function of the strength of Coulomb correlation parameter will be presented. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D16.00008: Multiferroicity induced by phase modulated spin-density waves Jeroen Vandenbrink, Joseph Betouras, Gianluca Giovannetti Materials in which magnetic and ferroelectric order coexist--termed multiferroics--have recently become the focus of much interest. From a technological point of view the possibility to control magnetic properties by electric fields and, vice versa, ferroelectric order by magnetic fields, is very attractive. But despite the possible coexistence of ferroelectricity and magnetism, materials with a pronounced interplay between these properties are very rare. Here we report on a novel route to generate such an interdependence: we show that symmetry arguments allow a finite magnetoelectric coupling for any spin-density wave that is phase modulated and commensurate, even if the spin ordering is collinear. It is this new coupling that drives the formation of multiferroic phases at the magnetic commensurability transitions, for instance the one of YMn$_2$O$_5$ at 23 K. This example makes clear that materials with phase modulated spin-density wave ordering are a new class of multiferroics with a strong interdependence of magnetization and ferroelectric polarization. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D16.00009: High Magnetic Tunability of Dielectric Properties in Magnetically-Driven Ferroelectricity Sang-Wook Cheong, S. Park, Y. J. Choi, C. L. Zhang, S. Guha Lattice relaxation in magnetically-ordered states with broken inversion symmetry through exchange-striction can induce non- centrosymmetric lattice distortions, leading to the presence of electric polarization. In these magnetically-driven ferroelectrics, dielectric properties turn out to be highly susceptible to applied magnetic fields. Both symmetric and antisymmetric exchange coupling can be involved in the exchange- striction. Magnetically-driven ferroelectrics with the symmetric coupling are associated with acentric spin density wave (SDW) states, and the antisymmetric coupling, relevant to the Dzyaloshinskii-Moriya-type interaction, becomes active when ferroelectricity is induced by spiral magnetic orders. A few examples of magnetically-driven ferroelectrics, exhibiting high tunability of dielectric properties in magnetic fields, will be discussed. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D16.00010: A Microscopic Model of Multiferroics RMn$_2$O$_5$ Chen Fang, Jiangping Hu A microscopic model is developed to explain the phase diagram and the mechanism of magnetoelectric coupling in RMn$_2$O$_5$. We show that frustrated magnetic structure drives the system to a commensurate-incommensurate phase trasition which can be understood as a competition between a collinear order, which stems from the `order by disorder' mechanism, and a chiral symmetry order. The magnetoelectric interaction couples the collinear order to the electric polarization. The low energy excitation and the effect of external magnetic field are also analyzed. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D16.00011: Direct evidence of spin-phonon coupling in multiferroic DyMn$_2$O$_5$ via magneto-infrared measurements J. Cao, J. L. Musfeldt, Y. J. Wang, S. Park, S.-W. Cheong The infrared active phonons in multiferroic DyMn$_2$O$_5$ are investigated as a function of magnetic field and temperature. Both field-induced frequency shifts and oscillator strength redistributions are observed in three important modes (the $f$-manifold crystal field splitting of Dy$^{3+}$ near $\sim$150 cm$^{-1}$, the Mn-O bending mode at $\sim$270 cm$^{-1}$, and the Mn-O stretching mode near $\sim$630 cm$^{-1}$), indicating strong spin-phonon coupling in this material. The crystal field levels of Dy$^{3+}$ are weakly sensitive to temperature induced magnetic phase transitions, whereas the Mn-O bending and stretching modes are insensitive to these phase boundaries. These measurements provide direct proof of spin-lattice interactions in DyMn$_2$O$_5$. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D16.00012: Magnetic-field-induced quantum phase transition in multiferroic BiMn$_{2}$O$_{5}$ J.W. Kim, S.Y. Ham, Y.S. Oh, Kee Hoon Kim, S. Park, S.-W. Cheong, P. Sharma, M. Jaime, N. Harrison Multiferroic BiMn$_{2}$O$_{5}$ exhibits both antiferromagnetic and ferroelectric ordering below $\sim $40 K. We have systematically investigated the electric/magnetic phase of BiMn$_{2}$O$_{5}$ by magnetization (M), dielectric constant ($\varepsilon )$, electric polarization (P) and specific heat (C$_{p})$ measurements down to 0.6 K and magnetic field (H) up to 45 tesla. At 4 K, BiMn$_{2}$O$_{5}$ shows a single magnetic-field-induced transition near H$_{c}\sim $18 T as evidenced by a sharp increase in M. Interestingly, $\varepsilon $ vs H shows a sharp peak at H$_{c}$, of which magnitude systematically increases as critical temperature T$_{c}$ approaches proximity to 0 K. Furthermore, P changes its sign with increasing H from positive to negative near H$_{c}$ with no hysteresis. The trajectory of which above three transitions occur follows the scaling relation T$_{c}$(H)$\sim $(H-H$_{c})^{1/2}$. The shape of C$_{p}$ vs H curve indicates that this transition is 2$^{nd}$ order down to 0.6 K, consistent with the absence of hysteresis in M, $\varepsilon $, and P measurements. Temperature dependent $\varepsilon $ measurements under fixed H near H$_{c}$ reveal that $\varepsilon $ increases on cooling to 5 K and slightly decreases down to 0.6 K, as similarly observed in a quantum paraelectric SrTiO$_{3}$. All of these observations support an interesting possibility that BiMn$_{2}$O$_{5}$ can be the first system to exhibit quantum fluctuation of ferroelectricity tuned by magnetic field. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D16.00013: Local Structure of the Multiferroic System RMn$_{2}$O$_{5}$ T. A. Tyson, M. DeLeon, Q. Qian, A. Ignatov, S. Park, S.-W. Cheong In order to understand the origin of the coupled magnetic and ferroelectric properties observed in RMn$_{2}$O$_{5}$ temperature and magnetic field dependent local structure measurements have been performed. The local structure about the rare earth (R) and the Mn sites have been examined. The atomic correlations involved in the spin-lattice coupling are identified. [Preview Abstract] |
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