On the basis of first‐principles calculations, we design a novel Cr‐based metal‐organic framework to be both multiferroic and magnetoelectric. The compound shows a “double‐hybrid” nature: it is a ...hybrid organic‐inorganic compound and it shows hybrid improper ferroelectricity. Here, the coupling of non‐polar distortions, such as Jahn‐Teller pseudo‐rotations and tilting, pave the way to a polar behavior, with the coupling being realized through hydrogen bonds.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present a detailed study of the structural, electronic, magnetic and ferroelectric properties of prototypical
proper
and
improper
multiferroic (MF) systems such as BiFeO
3
and orthorhombic HoMnO
3
..., respectively, within density functional theory (DFT) and using the Heyd-Scuseria-Ernzerhof hybrid functional (HSE). By comparing our results with available experimental data as well as with state-of-the-art GW calculations, we show that the HSE formalism is able to account well for the relevant properties of these compounds and it emerges as an accurate tool for predictive first-principles investigations on multiferroic systems. We show that effects beyond local and semilocal DFT approaches (as provided by HSE) are necessary for a realistic description of MFs. For the electric polarization, a decrease is found for MFs with magnetically-induced ferroelectricity, such as HoMnO
3
, where the calculated polarization changes from ∼6 μC cm
−2
using Perdew-Burke-Ernzerhof (PBE) to ∼2 μC cm
−2
using HSE. However, for
proper
MFs, such as BiFeO
3
, the polarization slightly increases upon introduction of exact exchange. Our findings therefore suggest that a general trend for the HSE correction to bare density functional cannot be extracted; rather, a specific investigation has to be carried out on each compound.
Towards a realistic description of proper and improper multiferroics using the Heyd-Scuseria-Ernzerhof hybrid functional.
The behaviour of electrons and holes in a crystal lattice is a fundamental quantum phenomenon, accounting for a rich variety of material properties. Boosted by the remarkable electronic and physical ...properties of two-dimensional materials such as graphene and topological insulators, transition metal dichalcogenides have recently received renewed attention. In this context, the anomalous bulk properties of semimetallic WTe2 have attracted considerable interest. Here we report angle- and spin-resolved photoemission spectroscopy of WTe2 single crystals, through which we disentangle the role of W and Te atoms in the formation of the band structure and identify the interplay of charge, spin and orbital degrees of freedom. Supported by first-principles calculations and high-resolution surface topography, we reveal the existence of a layer-dependent behaviour. The balance of electron and hole states is found only when considering at least three Te-W-Te layers, showing that the behaviour of WTe2 is not strictly two dimensional.
The recent research on multiferroics has provided solid evidence that the breaking of inversion symmetry by spin order can induce ferroelectric polarization P. This type of multiferroics, called ...spin-driven ferroelectrics, often show a gigantic change in P on application of a magnetic field B. However, their polarization (<~0.1 μC cm(-2)) is much smaller than that in conventional ferroelectrics (typically several to several tens of μC cm(-2)). Here we show that the application of external pressure to a representative spin-driven ferroelectric, TbMnO3, causes a flop of P and leads to the highest P (≈ 1.0 μC cm(-2)) among spin-driven ferroelectrics ever reported. We explain this behaviour in terms of a pressure-induced magnetoelectric phase transition, based on the results of density functional simulations. In the high-pressure phase, the application of B further enhances P over 1.8 μC cm(-2). This value is nearly an order of magnitude larger than those ever reported in spin-driven ferroelectrics.
Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO₃ system, only tiny changes of the ...interfacial Fe magnetic moment upon reversal of the BaTiO₃ dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO₃ system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO₃ polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO₃ produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices.
Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization ...observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Spin-orbit coupling is increasingly seen as a rich source of novel phenomena, as shown by the recent excitement around topological insulators and Rashba effects. We here show that the addition of ...ferroelectric degrees of freedom to a semiconductor featuring topologically nontrivial properties, such as SnTe, merges the intriguing field of spin-orbit-driven physics with nonvolatile functionalities appealing for spintronics. By using a variety of modeling techniques, we show that a strikingly rich sequence of phases can be induced in SnTe, when going from a room-temperature cubic phase to a low-temperature ferroelectric structure, ranging from a topological crystalline insulator to a time-reversal-invariant Z sub(2) topological insulator to a "ferroelectric Rashba semiconductor," exhibiting a huge electrically controllable Rashba effect in the bulk band structure.
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Based on density functional theory calculations and group theoretical analysis, we have studied NaLaMnWO(6) compound which has been recently synthesized G. King, A. Wills and P. M. Woodward, Phys. ...Rev. B: Condens. Matter, 2009, 79, 224428 and belongs to the AA'BB'O(6) family of double perovskites. At low temperature, the structure has monoclinic P2(1) symmetry, with layered ordering of the Na and La ions and rocksalt ordering of Mn and W ions. The Mn atoms show an antiferromagnetic collinear spin ordering, and the compound has been reported as a potential multiferroic. By comparing the low symmetry structure with a parent phase of P4/nmm symmetry, two distortion modes are found dominant. They correspond to MnO(6) and WO(6) octahedron tilt modes, often found in many simple perovskites. While in the latter these common tilting instabilities yield non-polar phases, in NaLaMnWO(6) the additional presence of the A-A' cation ordering is sufficient to make these rigid unit modes a source of the ferroelectricity. Through a trilinear coupling with the two unstable tilting modes, a polar distortion is induced, although the system has no intrinsic polar instability. The calculated electric polarization resulting from this polar distortion is as large as ∼16 μC cm(-2). Despite its secondary character, this polarization is coupled with the dominant tilting modes and its switching is bound to produce the switching of one of two tilts, enhancing in this way a possible interaction with the magnetic ordering. The transformation of common non-polar purely steric instabilities into sources of ferroelectricity through a controlled modification of the parent structure, as done here by the cation ordering, is a phenomenon to be further explored.
Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of “improper electronic ferroelectrics”, i.e. correlated ...materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a
coexistence
of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics to organic-inorganic hybrids.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Abstract
Next-generation spintronic devices will benefit from low-dimensionality, ferromagnetism, and half–metallicity, possibly controlled by electric fields. We find these technologically–appealing ...features to be combined with an exotic microscopic origin of magnetism in doped CdOHCl, a van der Waals material from which 2D layers may be exfoliated. By means of first principles simulations, we predict homogeneous hole–doping to give rise to
p
-band magnetism in both the bulk and monolayer phases and interpret our findings in terms of Stoner instability: as the Fermi level is tuned via hole–doping through singularities in the 2D-like density of states, ferromagnetism develops with large saturation magnetization of 1
µ
B
per hole, leading to a half-metallic behaviour for layer carrier densities of the order of 10
14
cm
−2
. Furthermore, we put forward electrostatic doping as an additional handle to induce magnetism in monolayers and bilayers of CdOHCl. Upon application of critical electric fields perpendicular to atomically–thin-films (as low as 0.2 and 0.5 V Å
−1
in the bilayer and monolayer case, respectively), we envisage the emergence of a magnetic half-metallic state. The different behaviour of monolayer vs bilayer systems, as well as an observed asymmetric response to positive and negative electric fields in bilayers, are interpreted in terms of intrinsic polarity of CdOHCl atomic stacks, a distinctive feature of the material. In perspective, given the experimentally accessible magnitude of critical fields in bilayer of CdOHCl, one can envisage
p
band magnetism to be exploited in miniaturized spintronic devices.
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