conventional displacive-type ferroelectricity in a 3D space, e.g., relative shifts of the transition-metal cation or lone-pair ions, geometric ferroelectricity does not require strong hybridization between the transition-metal and oxygen ions. [7][8][9][10] This removes the constraint of d 0 -ness of the transition-metal ion, thereby allowing magnetism to coexist with ferroelectricity. In addition, while the soft-phonon mode at the Brillouin zone-center manifests displacive ferroelectricity, [11,12] the collective distortion of the 2D network results in zone-boundary soft-phonon modes for geometric ferroelectrics. [3] What about in 1D networks? Spatial inversion symmetry breaking can be intuitively envisioned in 1D chain networks as well, especially with an MO 4 tetrahedral unit. An MO 4 tetrahedron has one of the lowest symmetries among the MO x polyhedra and can form corner-shared 1D chains within crystals (Table 1). More interestingly, a collective distortion of the 1D network can introduce an unconventional polar state resulting from both the displacement of ions and the rotation of the tetrahedra.Ferroelectricity occurs in crystals with broken spatial inversion symmetry. In conventional perovskite oxides, concerted ionic displacements within a 3D network of transition-metal-oxygen polyhedra (MO x ) manifest spontaneous polarization. Meanwhile, some 2D networks of MO x foster geometric ferroelectricity with magnetism, owing to the distortion of the polyhedra. Because of the fundamentally different mechanism of ferroelectricity in a 2D network, one can further challenge an uncharted mechanism of ferroelectricity in a 1D channel of MO x and estimate its feasibility. Here, ferroelectricity and coupled ferromagnetism in a 1D FeO 4 tetrahedral chain network of a brownmillerite SrFeO 2.5 epitaxial thin film are presented. The result provides a new paradigm for designing low-dimensional MO x networks, which is expected to benefit the realization of macroscopic ferro-ordering materials including ferroelectric ferromagnets. Multiferroic MaterialsComplex transition-metal oxides can be analytically viewed as a network of transition-metal-oxygen polyhedra (MO x ) ( Table 1). A collective distortion of such networks in a (quasi) 2D space can lead to inversion symmetry breaking and geometric ferroelectricity through trilinear coupling among distortions and electric polarization. [1][2][3][4][5][6] In comparison with the

Here we report a new quasi-one-dimensional S = 1 chain compound NiTe2O5. From the comprehensive study of the structure and magnetic properties on high quality single crystalline NiTe2O5, it's revealed that NiTe2O5 undergoes a transition into an intriguing long-range antiferromagnetic order at TN = 30.5 K, in which longitudinal magnetic moments along the chain direction are ferromagnetically ordered, while their transverse components have an alternating ferromagnetic-antiferromagnetic coupling. Even though the temperature dependence of magnetic susceptibility represents an archetypal anisotropic antiferromagnetic order, we found that critical behavior of unconventional nature with ′~0 .25 and ~0.18 is accompanied by the temperature evolution of the antiferromagnetic order parameter. Table I. Unit cell parameters, reliability factors, and atomic positional parameters for NiTe2O5.

A one‐dimensional tetrahedral chain network in a brownmillerite structure gives rise to a coupled ferroelectric and ferromagnetic behavior. In article number https://doi.org/10.1002/adma.201808104, Woo Seok Choi and co‐workers describe the discovery of a room‐temperature ferroelectric ferromagnet in a SrFeO2.5 epitaxial thin film, which exhibits an unconventional combined polar distortion with the mechanism of the coupled ferro‐orderings.

Magnetoelectrically active multiferroics have been of a significant interest and have instigated intense research and development owing to their potential applications. [1][2][3][4][5] From the perspective of fundamental physics, 6-10 multiferroic properties are often grouped into two categories, that is, Type I, where a ferromagnetism and a ferroelectricity

We demonstrate the effect of the crystallinity of ceramic targets on the electronic properties of LaNiO3 (001) thin films epitaxially grown by pulsed laser deposition (PLD). We prepared two kinds of LaNiO3 targets with different crystallinity by manipulating calcination temperature (i.e., 300 and 1000 °C) in the solid state reaction for ceramic synthesis. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) experiments of the as-sintered LaNiO3 ceramic targets clearly show that the LaNiO3 target sintered after high-temperature (1000 °C, high crystallinity) calcination is more oxidized to Ni3+ with better crystallinity than the LaNiO3 target sintered after low-temperature (300 °C, poor crystallinity) calcination. Using these two LaNiO3 ceramics as PLD targets, we fabricated epitaxial LaNiO3/LaAlO3 (001) thin-film heterostructures to examine how target crystallinity affects the physical properties of LaNiO3 films. Intriguingly, the electrical transport properties of the as-grown LaNiO3 thin films are quite different depending on crystallinity of the LaNiO3 ceramic target used for film deposition. In conjunction with subsequent XPS analyses of our LaNiO3 thin films, it appears that LaNiO3 (001) films deposited from the high-temperature-calcined target with better crystallinity are less disproportionate in Ni charge valency with more Ni3+ oxidation states compared with LaNiO3 (001) films deposited from the low-temperature-calcined target with poor crystallinity. This difference in degree of charge disproportionation can induce a discrepancy in the metal-to-insulator transition temperature of ultrathin LaNiO3 (001) films and in their electrical conductance.