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Physical systems evolve from one state to another along paths of least energy barrier. Without a priori knowledge of the energy landscape, multidimensional search methods aim to find such minimum energy pathways between the initial and final states of a kinetic process. However in many cases, the user has to repeatedly provide initial guess paths, thus ensuring that the reliability of the final result is heavily user-dependent. Recently, the idea of "distortion symmetry groups" as a complete description of the symmetry of a path has been introduced. Through this, a new framework is enabled that provides a powerful means of classifying the infinite collection of possible pathways into a finite number of symmetry equivalent subsets, and then exploring each of these subsets systematically using rigorous group theoretical methods. The method is shown to lead to the discovery of new, previously hidden pathways for the case studies of bulk ferroelectric switching and domain wall motion in proper and improper ferroelectrics, as well as in multiferroic switching. This approach is applicable to a wide variety of physical phenomena ranging from structural, electronic and magnetic distortions, diffusion, and phase transitions in materials.

International audience; Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches blending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature ( T). Here, we establish a predictive and quantitative relationship between T and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) ABO as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (SrCa )SnO ( x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d and 6s) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T = 410 K for SrSnO. We also find that the T increases linearly up to 800 K upon increasing the Ca content, i.e., upon decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known ABO hybrid improper ferroelectrics, indicating that the T correlates with the simple crystal chemistry descriptor, t, based on the ionic size mismatch. This study provides a predictive guideline for estimating the T of a given material, which would complement the convergent group-theoretical and first-principles design approach.

Lattice volume expansion or amorphization renders EuTiO[3] ferromagnetic, although the stable phase of crystalline EuTiO[3] is an antiferromagnet. The lattice volume expansion is induced into the crystalline EuTiO[3] thin film by utilizing the lattice mismatch between the thin film and a substrate. The magnetization at low temperatures monotonically increases with an increase in lattice volume for the crystalline EuTiO3 thin film, coincident with the results of calculations based on the hybrid Hartree–Fock density functional approach. The ferromagnetic interaction between Eu[2+] ions is enhanced by the amorphization as well; the amorphous EuTiO[3] thin film becomes a ferromagnet, and the Curie temperature is higher for amorphous Eu[2]TiO[4] than for its crystalline counterpart. The phenomenon, that is, the volume expansion- and amophization-induced ferromagnetism, is explained in terms of the competition between ferromagnetic and antiferromagnetic interactions among Eu[2+] ions.

CdFe2O4 thin films have been prepared by a sputtering method, and their structural and magnetic properties have been examined. As-deposited film shows magnetic transition like that of cluster-spin glass or superspin glass, and possesses large magnetization even at room temperature. The magnetic behaviors are partly attributed to crystal structure with a random distribution of cations in CdFe2O4nanocrystals. Annealing of the as-deposited film at 200degC leads to an increase in magnetization while the magnetization is decreased when the film is annealed at 400degC. The effect of annealing on the magnetic properties has been discussed in terms of the change of microstructure as well as chemical structure of the thin films.

Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH3 treatment (800-1,300 °C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N(3-) species in the lattice place significant constraints on the composition and structure-and hence the properties-of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R3c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 μC/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe(3+) spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures.

Amorphous Eu[2]TiO[4] and EuTiO[3] have been studied by a combination of the Faraday effect in the visible region and polarization-dependent x-ray absorption spectroscopy at the Eu M[4, 5] and L[2, 3]edges to examine the role of Eu 4f-5d exchange interactions on the ferromagnetic behavior. The bulk-sensitive x-ray absorption spectra (XAS) for Eu L[2, 3] edges show that most of the europium ions are present as the divalent state in the amorphous Eu[2]TiO[4] and EuTiO[3]. The Eu M[4, 5] edge x-ray magnetic circular dichroism (XMCD) signals, measured for the amorphous Eu[2]TiO[4], dramatically increase upon cooling through the Curie temperature (16 K) determined by a superconducting quantum interference device (SQUID) magnetometer. Sum-rule analysis of the XMCD at Eu M4, 5edges measured at 10 K yields a 4f spin magnetic moment of 6.6μB per Eu[2+] ion. These results confirm that the ferromagnetic properties exclusively arise from 4f spins of Eu[2+]. In addition, for both the amorphous Eu[2]TiO[4] and EuTiO[3], the temperature and magnetic-field dependence of Eu L[2, 3]edge XMCD signals can be scaled with the corresponding magnetization measured by SQUID, indicating that the 5d magnetic polarization of Eu[2+] is involved in the process to cause the ferromagnetic interaction between Eu[2+] ions. We further discuss the origin of ferromagnetism in the amorphous system on the basis of the energy diagram of Eu 4f and 5d levels deduced from the Faraday effect in the visible region. From the wavelength dependence of Faraday rotation angles of the amorphous EuO-TiO[2] system in comparison with those of the divalent Eu chalcogenides as reported previously, it is found that the magnitude of crystal-field splitting of Eu 5d levels in the former is on the same order as that in the latter, which explains an enhanced ferromagnetic exchange interaction between Eu 4f and 5d states.

A superexchange mechanism between Eu^2+ 4f spins via the 3d states of nonmagnetic Ti^4+ ions is proposed through first-principles calculations based on a hybrid Hartree-Fock density functional approach to explain G-type antiferromagnetism in EuTiO3. This mechanism is supported by systematic calculations for related Eu2+-based perovskite oxides. In EuTiO3, the competition between the antiferromagnetic superexchange and an indirect ferromagnetic exchange via the Eu 5d states leads to a delicate balance between antiferromagnetic and ferromagnetic phases. The superexchange mechanism involving the Ti 3d states hints at the microscopic origin of the strong spin-lattice coupling in EuTiO3.

In most amorphous insulating magnets, the magnetic structure is dominated by the random distribution of magnetic moments as well as the predominant antiferromagnetic interaction among them, inevitably leading to a transition from high-temperature paramagnetic to low-temperature spin glass phase. In this paper, we report our discovery of ferromagnetic amorphous oxides with reentrant spin glass behavior. Unlike most oxide glasses, there is a strong tendency for the magnetic interaction of Eu2+ ions to be ferromagnetic in oxide glasses, as obviously indicated by the positive values of Weiss temperature. Comprehensive investigations of low-temperature magnetic properties for the present Eu2+-containing glasses have revealed a typical behavior of reentrant ferromagnets. We discuss the possible mechanisms behind the ferromagnetic interactions, as well as the origin of reentrant spin glass nature, based on the specific electronic structure of Eu2+ compounds.