Abstract
Combining ferroelectricity and magnetism in the same material remains a challenge because it involves complex crystal chemistry and stringent symmetry requirements. In conventional ferroelectrics, the polarization arises from the second-order Jahn–Teller effect associated with cations of d0 or s2 lone pair electronic configuration. In contrast, the magnetism arises from cations with partially filled d or f electrons. Materials that incorporate these two kinds of cations in different crystallographic sites exhibit multiferroic properties but with weak coupling between magnetism and ferroelectricity. On the other hand, a strong cross-coupling occurs in some materials, where specific spin structures induce weak ferroelectricity below the magnetic ordering temperature. In this article, we discuss a new class of multiferroics where the polar distortion results from chemical ordering. These polar oxides are mainly pyroelectric in the entire temperature range and exhibit magnetoelectric coupling below the magnetic ordering temperatures.
Highlights
Inorganic oxide materials crystallize in the highest possible symmetry that includes inversion center because of the efficient packing of ions and to attain the lowest electrostatic energy
Most common ferroelectric materials are perovskite oxides, halides, and sulfides where a structural phase transition occurs from high symmetry nonpolar structure to the low symmetry polar structure, arising from the second-order Jahn– Teller effect associated with cations of d0 and s2 lone pair electronic configuration, as observed in BaTiO3 and PbZr0.5Ti0.5O3.2,3 These ferroelectric materials are already applied in devices in the form of components such as capacitors, transducers, and actuators.[4,5,6,7]
In this Perspective, we propose a new class of multiferroics based on polar magnetic oxides, where the polar distortion driven by the chemical ordering of cations is stabilized right at the formation temperature of these compounds
Summary
Inorganic oxide materials crystallize in the highest possible symmetry that includes inversion center because of the efficient packing of ions and to attain the lowest electrostatic energy. A typical example is the orthorhombic (Pnma) perovskite TbMnO3, which undergoes a collinear sinusoidal antiferromagnetic ordering at TN1 ∼ 42 K, followed by a cycloidal spin ordering (TN2 ∼ 28 K) that breaks the inversion symmetry and induces electric polarization.8,25A large number of materials belonging to these two kinds of classes have been reviewed.[23,26–28] In this Perspective, we propose a new class of multiferroics based on polar magnetic oxides, where the polar distortion driven by the chemical ordering of cations is stabilized right at the formation temperature of these compounds. The extent of magnetoelectric coupling depends on the magnetic spin structure and, the microscopic mechanisms that couple the magnetism with electric polarization This behavior is similar to type-II multiferroics, the inversion symmetry in the polar materials is broken in the paramagnetic state and does not involve complex magnetic structures to induce electric polarization at the magnetic ordering temperature. The chemical order includes the following mechanisms: (i) chemical occupancy of different atoms/ions splitting the symmetry equivalent crystallographic sites into several inequivalent positions; (ii) atoms/ions of the same charge with different chemical coordination environments; and (iii) charge ordering of the same or different ions
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