Relativistic effects, including the Rashba effect, are increasingly seen as key ingredients in spintronics. A link between Rashba physics and the field of ferroelectrics is established by predicting giant Rashba spin-splitting in bulk GeTe (see the Figure showing the band-structure as well as in-plane and out- of-plane spin polarization for a constant energy cut).

Metal-organic frameworks (MOFs) are hybrid crystalline compounds comprised of extended ordered networks made up of organic linkers and metal cations, often forming porous materials at the interface between molecular coordination chemistry and materials science. They show unique properties arising from organic-inorganic duality [ 1 ] which has already resulted in an unprecedented variety of physical properties in a single class of materials and applications, such as gas storage, exchange or separation, catalysis, drug delivery, optics, and magnetism. [ 2 , 3 ] An additional feature is the possibility of creating ideally infinite new MOFs by varying inorganic/organic components, molecular topologies, organic linkers, etc. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] All these degrees of freedom can be exploited for a rational design of new materials with enhanced functionalities.MOFs with ABX 3 perovskite structure or closely related superstructures of this chemical chameleon have attracted much attention since they show promising properties in areas that have traditionally been dominated by inorganic materials, for example magnetism and ferroelectricity. [24][25][26][27][28] Combination of spontaneous magnetic and ferroelectric order in a single material, i.e., multiferroicity (MF), is of great technological and fundamental importance, in particular when both orders are coupled through a sizeable magneto-electric (ME) coupling. Despite the large activity devoted to multiferroics, [ 29 ] most of the past and current studies have been focused on inorganic compounds, though mainly in the family of perovskite-like oxides. Among these, a strong ME coupling is expected in magnetically driven improper ferroelectrics, such as rare-earth manganites or delafossite oxides, where the magnetic ordering is responsible for spontaneous electric polarization. [ 30 ] Unfortunately, the symmetry-breaking is usually associated with frustrated magnetism displaying complex antiferromagnetic or spiral order, and both measured electric polarization and critical temperatures are usually too small for any device applications. [ 30 ] The large ferroelectric polarization of proper multiferroics such as BiFeO 3 , on the other hand, originates from a polar lattice instability that is generally not coupled to any magnetic instability leading to a net magnetization. [ 31 ] Very recently, a third class of magnetoelectric multiferroics has been suggested, where a lattice instability is responsible for both ferroelectricity and the appearance of a weak-ferromagnetic (WFM) ordering, thus allowing for a potentially large ME coupling. The key ingredient is a trilinear coupling between a (stable) polar mode and two nonpolar instabilities, usually octahedron tilting and rotations in layered or double perovskites, as recently found in some inorganic compounds, where cation ordering leads to the required symmetry breaking. [32][33][34][35][36][37][38] The mechanism has been called "hybrid improper ferroelectricity", implying t...

Ferroelectricity is a potentially crucial issue in halide perovskites, breakthrough materials in photovoltaic research. Using density functional theory simulations and symmetry analysis, we show that the lead-free perovskite iodide (FA)SnI 3 , containing the planar formamidinium cation FA, (NH 2 CHNH 2 ) þ , is ferroelectric. In fact, the perpendicular arrangement of FA planes, leading to a 'weak' polarization, is energetically more stable than parallel arrangements of FA planes, being either antiferroelectric or 'strong' ferroelectric. Moreover, we show that the 'weak' and 'strong' ferroelectric states with the polar axis along different crystallographic directions are energetically competing. Therefore, at least at low temperatures, an electric field could stabilize different states with the polarization rotated by p/4, resulting in a highly tunable ferroelectricity appealing for multistate logic. Intriguingly, the relatively strong spin-orbit coupling in noncentrosymmetric (FA)SnI 3 gives rise to a co-existence of Rashba and Dresselhaus effects and to a spin texture that can be induced, tuned and switched by an electric field controlling the ferroelectric state.

Forget me not: In a new multiferroic metal–organic framework (see structure, Cu green, O red, C black, N blue, H gray; arrows show spin configuration), Jahn–Teller and antiferro‐distortions induce a switchable ferroelectric polarization, which is coupled to a weak ferromagnetic component. This true magnetoelectric multiferroic should be very attractive for advanced memory devices.

The coexistence of both electric and magnetic orders in some metal-organic frameworks (MOFs) has yielded a new class of multiferroics beyond inorganic materials. However, the coupling between two orders in multiferroic MOFs has not been convincingly verified yet. Here we present clear experimental evidences of cross coupling between electric and magnetic orders in a multiferroic MOF [(CH3)2NH2]Fe(HCOO)3 with a perovskite structure. The dielelectric constant exhibit a hump just at the magnetic ordering temperature TN. Moreover, both the direct (magnetic field control of dielectric properties) and converse (electric field control of magnetization) magnetoelectric effects have been observed in the multiferroic state. This work opens up new insights on the origin of ferroelectricity in MOFs and highlights their promise as magnetoelectric multiferroics.