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Register a new userIsing magnetism and ferroelectricity in Ca$_3$CoMnO$_6$
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The origin of both the Ising chain magnetism and ferroelectricity in Ca$_3$CoMnO$_6$ is studied by $ab$ $initio$ electronic structure calculations and x-ray absorption spectroscopy. We find that Ca$_3$CoMnO$_6$ has the alternate trigonal prismatic Co$^{2+}$ and octahedral Mn$^{4+}$ sites in the spin chain. Both the Co$^{2+}$ and Mn$^{4+}$ are in the high spin state. In addition, the Co$^{2+}$ has a huge orbital moment of 1.7 $mu_B$ which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed $uparrowuparrowdownarrowdownarrow$ antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.
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Layered perovskites $A_3M_2$O$_7$ are known to exhibit the so-called hybrid improper ferroelectricity. Despite experimentally confirmed cases (e.g. nonmagnetic $M$=Ti and Sn), the ferroelectricity in magnetic Ca$_3$Mn$_2$O$_7$ remains a puzzle. Here, the structural, ferroelectric, magnetoelectric, and optical properties of Ca$_3$Mn$_2$O$_7$ are systematically investigated. Switchable polarization is directly measured, demonstrating its ferroelectricity. In addition, magnetoelectric response is also evidenced, implying the coupling between magnetism and ferroelectricity. Furthermore, strong visible light absorption is observed, which can be understood from its electronic structure. Its direct and appropriate band gap, as well as wide conducting bands, makes Ca$_3$Mn$_2$O$_7$ a potential candidate for ferroelectric photoelectric applications.
Charge dipole moment and spin moment rarely coexist in single-phase bulk materials except in some multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO$_3$/PbTiO$_3$ superlattices. Although neither of the components is magnetic, magnetic moments emerge at the ferroelectric domain walls of PbTiO$_3$ in these superlattices. Such a twist between ferroelectric domain and local magnetic moment, not only manifests an interesting type of multiferroicity, but also is possible useful to pursuit the electrical-control of magnetism in nanoscale heterostructures.
We report a systematic study of the structure, electric and magnetic properties of Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ single crystals with $x =$ 0.72 and 0.26. The DC and AC magnetic susceptibilities display anomalies with characteristic of the spin freezing. The crystals show ferroelectric transition at 40 K and 35 K ($T_{FE}$) for $x =$ 0.72 and 0.26, respectively, with a large value of 1400 $mu$C/m$^2$ at 8 K for electric polarization ($P_c$) along the spin-chain ($c$-axis) direction. Interestingly, the electric polarization perpendicular to the chain direction ($P_{ab}$) can also be detected and has value of 450 and 500 $mu$C/m$^2$ at 8 K for the $x =$ 0.72 and 0.26 samples, respectively. The specific heat and magnetic susceptibility show no anomaly around $T_{FE}$, which means that the electric polarization of these samples has no direct relationship with the magnetism. The X-ray diffraction and the Raman spectroscopy indicate that these samples may undergo Jahn-Teller distortions that could be the reason of electric polarization.
Tungstates $A$WO$_4$ with the wolframite structure characterized by the $A$O$_6$ octahedral zigzag chains along the $c$-axis, can be magnetic if $A$=Mn, Fe, Co, Cu, Ni. Among them, MnWO$_4$ is a unique member with a cycloid Mn$^{2+}$ spin order developed at low temperature, leading to an interesting type-II multiferroic behavior. However, so far no other multiferroic material in the tungstate family has been found. In this work, we present the synthesis and the systematic study of the double tungstate LiFe(WO$_4$)$_2$. Experimental characterizations including structural, thermodynamic, magnetic, neutron powder diffraction, and pyroelectric measurements, unambiguously confirm that LiFe(WO$_4$)$_2$ is the secondly found multiferroic system in the tungstate family. The cycloidal magnetism driven ferroelectricity is also verified by density functional theory calculations. Although here the magnetic couplings between Fe ions are indirect, namely via the so-called super-super-exchanges, the temperatures of magnetic and ferroelectric transitions are surprisingly much higher than those of MnWO$_4$.
The emergence of magnetic reconstructions at the interfaces of oxide heterostructures are often explained via subtle modifications in the electronic densities, exchange couplings, or strain. Here an additional possible route for induced magnetism is studied in the context of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_n$ superlattices using a hybrid tight-binding model. In the LaNiO$_3$ region, the induced magnetizations decouple from the intensity of charge leakage from Mn to Ni, but originate from the spin-filtered quantum confinement present in these nanostructures. In general, the induced magnetization is the largest for the (111)-stacking and the weakest for the (001)-stacking superlattices, results compatible with the exchange bias effects reported by Gibert et al. Nat. Mater. 11, 195 (2012).
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