Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D7: Giant Spin-Lattice Coupling in Multiferroics |
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Sponsoring Units: GMAG DMP Chair: Sang-Wook Cheong, Rutgers University Room: LACC 408B |
Monday, March 21, 2005 2:30PM - 3:06PM |
D7.00001: GMAG Dissertation Award: Magnetoelectric and Magnetodielectric effects in REMn$_{2}$O$_{5}$ Invited Speaker: Recently there have been renewed interests in multiferroics, simultaneously possessing ferroelectricity and magnetic ordering. The understanding of this remarkable occurrence remains a scientific challenge. Despite the possible coexistence of ferroelectricity and magnetism, any profound interplay between them has been rarely observed. This fact has largely prevented the realization of devices with a previously unavailable functionality, which these multiferroics could make possible. We have disovered astonishing interplay between ferroelectricity and magnetism, demonstrated by polarization reversal by magnetic fields and colossal magneto-dielectric effect in REMn$_{2}$O$_{5}$ (RE: rare earths) as well as intriguing magnetoelectric coupling effects in other multiferroics. Our results point to new device applications such as magnetically recorded ferroelectric memory and important means to tune dielectric properties with external parameters. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:42PM |
D7.00002: Giant Magnetoelectric Effect in Hexagonal Multiferroic Manganites Invited Speaker: Recently an enormous interest in magnetoelectric (ME) phenomena is observed because composite materials and multiferroics exhibit structural and giant ME effects that exceed previous effects by orders of magnitude and control phase transitions. I will discuss 3 manifestations of giant ME coupling in ferroelectric (FEL) antiferromagnetic (AFM) RMnO$_3$ (R = Sc, Y, In, Ho-Lu). (i) External magnetic or electric fields induce ferromagnetic R$^{3+}$ ordering which is reversibly switched on or off. The process is monitored by magneto-optical techniques (second harmonic generation, Faraday rotation). Its microscopy is disclosed by neutron and x-ray powder diffraction. The ME phase control is driven by an asymmetry in the R$^{3+}$-Mn$^{3+}$ superexchange which originates in the ferroelectric distortion and lowers the free energy. [Nature {\bf 430}, 541 (2004)] (ii) Interaction of FEL and AFM domain walls which clamps the AFM to the FEL domains. The coupling is revealed by simultaneous imaging of FEL, AFM, and ME $180^{\circ}$ domains by second harmonic generation. It roots in a piezomagnetic interaction between the magnetization of the AFM walls and strain in the FEL walls which lowers the free energy. [Nature {\bf 419}, 818 (2002)] (iii) Massive formation of {\it spin-rotation domains} which supplement {\it spin-reversal domains} in the course of a Mn$^{3+}$ spin reorientation. This leads to a local ME effect that is only allowed because of the low symmetry of the domain walls. [Phys.\ Rev.\ B, (2005)] From our results basic requirements for other candidate materials to exhibit magnetoelectric phase control are identified. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D7.00003: Quasi-frustrated spin systems as magnetoelectrics with strong spin-lattice coupling Invited Speaker: The magnetoelectric effect --i.e. the induction of a magnetization by means of an electric field and the induction of a polarization by means of a magnetic field- was already presumed to exist at the end of 19$^{th}$ century, and subsequently attracted a great deal of interest in the 1960s and 1970s. Whereas the symmetry-conditioned properties and phenomenological aspects of magnetoelectrics are rather well understood, there is a great paucity of studies of the microscopic mechanisms in specific compounds and no devices have been made so far because of small interaction coefficients. The recent observation of gigantic magnetoelectric and magnetocapacitive effects in rare-earth manganites, TbMnO$_{3}$ and DyMnO$_{3}$ [1,2], provides a novel approach to the mutual control of magnetization and electric polarization in magnetic ferroelectrics. We can control the magnitude and/or direction of the electric polarization vector by the application of magnetic field in these materials. In comparing the results from the both manganites, we noticed that a characteristic common to the both materials is that they possess modulated magnetic structures with long wavelengths (as compared to the chemical unit cell) which arise from competing magnetic interactions (i.e. \textit{quasi-frustrated spin systems}). Ferroelectricity in these materials appears to originate from the competing magnetic interactions which cause lattice modulations through magnetoelastic coupling. In this talk, we propose that quasi-frustrated spin systems provide new route to design magnetoelectrics with strong spin-lattice coupling. [1] T. Kimura, T.Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, Nature 426, 55 (2003). [2] T. Goto, T. Kimura, G. Lawes, A. P. Ramirez, and Y. Tokura, Phys. Rev. Lett. 92, 257201 (2004). [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:54PM |
D7.00004: Magnetocapacitance effect in EuTiO$_{3}$ and related compounds Invited Speaker: Perovskite titanates, EuTiO$_{3}$, contains Ti$^{4+}$ ions, similarly with BaTiO$_{3}$ and SrTiO$_{3}$, and is expected to show ferroelectric instability. In addition, this compound contains Eu$^{2+}$ ions with S=7/2 spin, which order antiferromagnetically at 5.5 K. We measured the dielectric constant of this compound [1], and found that large dielectric constants ($>400$) critically decrease with antiferromagnetic ordering of the Eu spins at 5.5 K. We also found a large change of the dielectric constant under magnetic field (magnetocapacitance) by 7 \% with 1.5 T at 2 K. From a comparison with a mean-field calculation, it was shown that the variation of dielectric constants scales with the pair correlation of the nearest-neighbor Eu spins. We also measured the magnetocapacitance of pyrochlore titanates, R$_{2}$Ti$_{2}$O$_{7}$ (R=rare earth) [2], having the same Ti$^{4+}$, but the magnetic moment is located on a pyrochlore lattice, and thus is dominated by geometrical frustration. By comparing the magnitude of mangetocapacitance with the square of magnetization, evidence of ferromagnetic (R=Ho) and antiferromagnetic (R=Gd) fluctuation was obtained. [1] T. Katsufuji et al., Phys. Rev. B 64, 054415 (2001). [2] T. Katsufuji et al., Phys. Rev. B 69 064422 (2004). [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:30PM |
D7.00005: Complex phase diagram in HoMnO$_3$ due to large spin-lattice coupling Invited Speaker: The coexistence of ferroelectric and magnetic orders in multiferroic hexagonal rare-earth manganites is of fundamental and practical interest. The magnetic transitions in these compounds result in sharp anomalies of the dielectric constant indicating an interesting correlation between magnetic and ferroelectric orders. We derive the T-H phase diagram of hexagonal HoMnO$_{3 }$from magnetic, thermodynamic, and dielectric measurements and find several reentrant phases, a tetracritical point, and first order phase transitions reflecting a wealth of physical phenomena due to the correlation between Mn-spin arrangements, Ho-moment order, ferroelectricity, and frustration. We show that the magnetic phase transitions in HoMnO$_{3}$ are accompanied by large anomalies of the thermal expansion coefficients along the different crystallographic directions the origin of which lies in extraordinarily strong magnetic correlations and spin-lattice coupling. We propose that the magneto-dielectric coupling observed in hexagonal rare earth manganites results from the lattice strain induced by the magnetoelastic effect. [Preview Abstract] |
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