Magnetic phase diagrams of multiferroic hexagonal R MnO 3 ( R = Er, Yb, Tm, and Ho)
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M. Gospodinov Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
C.W. Chu Department of Physics and Texas Center for Superconductivity at the University of Houston, University of Houston, Houston, Texas 77204-5002; Lawrence Berkeley National Laboratory, Berkeley, California 94720; and Hong Kong University of Science and Technology, Hong Kong, People’s Republic of China (Received 23 December 2006; accepted 8 May 2007)
The magnetic phase diagrams of RMnO3 (R ⳱ Er, Yb, Tm, Ho) are investigated up to 14 T via magnetic and dielectric measurements. The stability range of the atomic force microscopy order below the Néel temperature of the studied RMnO3 extends to far higher magnetic fields than previously assumed. Magnetic irreversibility indicating the presence of a spontaneous magnetic moment is found near 50 K for R ⳱ Er, Yb, and Tm. At very low temperatures and low magnetic fields, the phase boundary defined by the ordering of the rare-earth moments is resolved. The sizable dielectric anomalies observed along all phase boundaries are evidence for strong spin-lattice coupling in the hexagonal RMnO3. In HoMnO3, the strong magnetoelastic distortions are investigated in more detail via magnetostriction experiments up to 14 T. The results are discussed based on existing data on magnetic symmetries and the interactions among the Mn-spins, the rare-earth moments, and the lattice.
I. INTRODUCTION
Multiferroic hexagonal rare-earth manganites have attracted special attention because of the coexistence of ferroelectric (FE) and magnetic orders. A phenomena resulting from the coupling and mutual interference of magnetism and ferroelectricity is a large magnetoelectric (ME) effect. This effect allows the magnetic field to tap into and control the dielectrical properties of the system1 and vice versa, with the electric field allowed to control the magnetic properties of the system.2 Information may thus be stored both in the magnetic and electrical polarization instead of just the magnetic polarization, and it may also be retrieved by sensing the magnetic moment or the FE polarization. A thorough understanding of the underlying physics between the coupling of these two orders may thus yield the possibility of developing novel devices that will improve memory storage densities. For the hexagonal RMnO3 (R ⳱ Sc, Y, In, Ho–Lu), there are two main structural properties that are essential for the multiferroicity. One is the FE order that sets in at high temperatures and results in ionic displacements,
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0271 J. Mater. Res., Vol. 22, No. 8, Aug 2007
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breaking the inversion symmetry of the lattice.3 The other feature is the presence of geometrical frustration caused by the antiferromagnetic (AFM) ordering of the Mn-spins within a planar triangular lattice with ordering temperatures ranging from 70 to 130 K.4 The linear ME effect in the hexagonal RMnO3 i
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