Origin of Ferroelectricity in High-TcMagnetic Ferroelectric CuO

Abstract

"Magnetic ferroelectric" has been found in a wide range of spiral magnets. However, these materials all suffer from low critical temperatures, which are usually below 40 K, due to strong spin frustration. Recently, CuO has been found to be multiferroic at much higher ordering temperature (∼ 230K). To clarify the origin of the high ordering temperature in CuO, we investigate the structural, electronic and magnetic properties of CuO via first-principles methods. We find that CuO has very special nearly commensurate spiral magnetic structure, which is stabilized via the Dzyaloshinskii-Moriya interaction. The spin frustration in CuO is relatively weak, which is one of the main reasons that the compound have high ordering temperature. We propose that high Tc magnetic ferroelectric materials can be found in double sublattices of magnetic structures similar to that of CuO. PACS numbers: 75.85.+t, Magnetic ferroelectric materials in which ferroelectricity is induced by magnetic ordering, have attracted intensive interests [1,2]. The strong magnetoelectric (ME) coupling in these materials opens up a new path to the design of multifunctional devices that allow the control of charges by the application of magnetic fields or spins by applying voltages. So far, almost all magnetic ferroelectric materials are strongly frustrated magnets [2]. Frustrated magnets have very low ordering temperatures (∼ 30 -40 K), several times smaller than the temperatures expected from their spin interaction strengths. Low critical temperature is one of the major factors that limit the applications of these important materials. Therefore a new mechanism that allows high temperature magnetic ferroelectric materials is critical.Recently, CuO was found to be multiferroic at T c =230 K, which is much higher than the critical temperatures of all other magnetic ferroelectric materials [3]. However, the mechanism for the high ordering temperature were not clear. So far, CuO is the only binary compound that has been found to be multiferroic [3]. CuO undergoes two successsive magnetic phase transitions upon cooling from room temperature to near zero temperature. Neutron scattering experiments [4] show that below T N 1 =213 K, the spin structure is collinear antiferromagnetic (AFM1) [see Fig. 1(a)]. Between T N 1 and T N 2 =230 K, the spin structure becomes non-collinear and slightly incommensurate (AFM2) [see Fig. 1(b)], with a modulation vector of Q= (0.006, 0, 0.017). Remarkably, an electric polarization of 160 µC·m −2 , which can be reversed by applying an electric field of about 55 kV/m, develops in the AFM2 phase. The electric polarization was attributed to the spiral spin structure [3,5] which was assumed to result from spin frustration and whereas the high ordering * Email address: helx@ustc.edu.cn temperature is believed to come from the strong exchange interactions [3].To clarify the mechanism behind its high ordering temperature and the origin of its ferroelectricity, we carry out first-principles studies of the multiferroism of CuO. We find ...

Magnetic Phase Diagram of CuO via High-Resolution Ultrasonic Velocity Measurements

“…In contrast with the conclusions of Refs. [8][9][10], our analysis based on rigorous symmetry arguments indicates that there must be a collinear intermediate phase (AF3) between the paramagnetic and spiral AF2 states. Such a phase has been shown, both theoretically and experimentally, to occur in other geometrically frustrated antiferromagnets where symmetry allows for uniaxial anisotropy at second order [14,15] For the purpose of this study, a CuO sample was grown using a floating zone technique as described in Ref.…”

“…However, the authors of the neutron diffraction measurements [6] questioned the possibility of having a direct condensation from a paramagnetic (PM) phase to a spiral magnetic phase. Despite this remark, a recent Landau theory [8], as well as several Monte-Carlo simulations [9,10], appear to support this sequence of magnetic orderings.…”

Interpretation of magnetoelectric phase states using the praphase concept and exchange symmetry

“…Monte-Carlo studies based on first-principles calculations failed to reproduce the existence of the phase AF3 [49,50].…”

High-temperature electromagnons in the magnetically induced multiferroic cupric oxide driven by intersublattice exchange

“…Other measurements include |J 1 | ¼ 77 meV from susceptibility 44 , and |J 1 | ¼ 100 meV from mid-infrared conductivity 45 and Raman spectra 46 . Recent DFT calculations have yielded values for J 1 ranging from À 51 to À 127 meV 23,28 . Polarized neutron scattering was utilized to probe further the AF1 phase.…”

“…1c. This phase exhibits a static electric polarization P b B100 mC m À 2 along b 20,26,27 , the origin of which has been discussed in the context of weakly frustrated intersublattice spin interactions 23 , magnetic degeneracy and the spin-orbit interaction 28 and cross-coupled AFM order parameters 29 . While CuO exhibits ferroelectric hysteresis loops 20 and chiral magnetic domains that can be switched electrically 27 , no substantial uniform static magnetoelectric coupling has been found in polycrystalline CuO up to 7 T 26 , and the AF2 phase appears to be stable to at least 16 T 30 .…”