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Register a new userDesign of a Mott Multiferroic from a Non-Magnetic Polar Metal
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Added by
Gianluca Giovannetti GG
Publication date
2015
fields
Physics
and research's language is
English
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We examine the electronic properties of newly discovered ferroelectric metal LiOsO$_3$ combining density-functional and dynamical mean-field theories. We show that the material is close to a Mott transition and that electronic correlations can be tuned to engineer a Mott multiferroic state in 1/1 superlattice of LiOsO$_3$ and LiNbO$_3$. We use electronic structure calculations to predict that the (LiOsO$_3$)$_1$/(LiNbO$_3$)$_1$ superlattice is a type-I multiferroic material with a ferrolectric polarization of 41.2~$mu$C cm$^{-2}$, Curie temperature of 927,K, and Neel temperature of 671,K. Our results support a route towards high-temperature multiferroics, emph{i.e.}, driving non-magnetic emph{polar metals} into correlated insulating magnetic states.
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The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarging the limit of the spin and orbital sum rules, which usually holds for materials where the magnetic ions exhibit a unique crystal field symmetry This finding translates in a sign-reversal between the average Fe magnetic orbital and spin moments. Large Fe-displacements, among some of the octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the magnetic orbital moment. The latter may lead to a partial re-orientation of the magnetic orbital moment under an external magnetic field that, combined to the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign-reversal of the XMCD integral. The results suggest that a control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.
Competing interactions and geometric frustration provide favourable conditions for exotic states of matter. Such competition often causes multiple phase transitions as a function of temperature and can lead to magnetic structures that break inversion symmetry, thereby inducing ferroelectricity [1-4]. Although this phenomenon is understood phenomenologically [3-4], it is of great interest to have a conceptually simpler system in which ferroelectricity appears coincident with a single magnetic phase transition. Here we report the first such direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric polarization when the symmetry of the magnetic order changes and ferroelectricity is only observed when the magnetic structure has chirality and breaks inversion symmetry. Multiferroic behaviour in RFMO provides a theoretically tractable example of ferroelectricity from competing spin interactions. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that the chiral magnetic order of the triangular lattice antiferromagnet gives rise to a pseudoelectric field, whose temperature dependence agrees with that observed experimentally.
We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn2O5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary points unambiguously at the exchange-striction mechanism as the primary driving force for ferroelectricity.
Pb$_2$CoOsO$_6$ is a newly synthesized polar metal in which inversion symmetry is broken by the magnetic frustration in an antiferromagnetic ordering of Co and Os sublattices. The coupled magnetic and structural transition occurs at 45 K at ambient pressure. Here we perform transport measurements and first-principles calculations to study the pressure effects on the magnetic/structural coupled transition of Pb$_2$CoOsO$_6$. Experimentally we monitor the resistivity anomaly at $T_N$ under various pressures up to 11 GPa in a cubic anvil cell apparatus. We find that $T_N$ determined from the resistivity anomaly first increases quickly with pressure in a large slope of $dT_N/dP$ = +6.8(8) K/GPa for $P < 4$ GPa, and then increases with a much reduced slope of 1.8(4) K/GPa above 4 GPa. Our first-principles calculations suggest that the observed discontinuity of $dT_N/dP$ around 4 GPa may be attributed to the vanishing of Os magnetic moment under pressure. Pressure substantially reduces the Os moment and completely suppresses it above a critical value, which relieves the magnetic frustration in the antiferromagnetic ordering of Pb$_2$CoOsO$_6$. The Co and Os polar distortions decrease with the increasing pressure and simultaneously vanish at the critical pressure. Therefore above the critical pressure a new centrosymmetric antiferromagnetic state emerges in Pb$_2$CoOsO$_6$, distinct from the one under ambient pressure, thus showing a discontinuity in $dT_N/dP$.
InMnO$_3$ is a peculiar member of the hexagonal manganites h-RMnO$_3$ (where R is a rare earth metal element), showing crystalline, electronic and magnetic properties at variance with the other compounds of the family. We have studied high quality samples synthesized at high pressure and temperature by powder neutron diffraction. The position of the Mn ions is found to be close to the threshold $it{x}=1/3$ where superexchange Mn-Mn interactions along the $it{c}$ axis compensate. Magnetic long range order occurs below $T_{rm N}$= 120(2) K with a magnetic unit cell doubled along $it{c}$, whereas short range two dimensional dynamical spin correlations are observed above $T_{rm N}$. We propose that pseudo-dipolar interactions are responsible for the long period magnetic structure.
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