Formamidinium [FA, HC­(NH2)2 +] lead iodide and its cation mixture have attracted interest as potentials in applications for efficient solar cells superior to well-known methylammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA1–x Cs x PbI3 for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α-FA0.9Cs0.1PbI3 as well as α-FAPbI3 crystallize in a cubic Pm3̅m structure with orientationally disordered FA molecules confined in the nondistorted Pb–I framework. In FAPbI3, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between β- and γ-phases instead of the previously known single β–γ transition. After doping with 10% Cs, the multiple first-order transitions disappeared, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.

Preparation of ordered CoPt on Si substrates is significant for expanding future applications of spintronic devices. In this study, ordered CoPt alloys including the L1 0 phase with a maximum coercivity of 2.1 kOe are formed in electron-beam-deposited 11.4 nm thick Pt/Co bilayer thin films on Si/SiO 2 substrates via interdiffusion during rapid thermal annealing (RTA). The effects of RTA temperature on the magnetic properties, crystal structures, cross-sectional elemental profiles, and surface morphologies of the films are analyzed by vibrating sample magnetometer (VSM), grazing incidence x-ray diffraction (GI-XRD), energydispersive x-ray spectroscopy (EDX), and scanning electron microscope (SEM), respectively. For the asdeposited film, polycrystalline Pt was confirmed by uniform Debye-Scherrer rings of Pt. At 200°C, interdiffusion between Co and Pt atoms in the film started to be observed by EDX elemental maps, and at 300°C, alloying of Co and Pt atoms was confirmed by diffraction peaks corresponding to A1-disordered CoPt. At 400°C, the in-plane coercivity of the film began to increase. At 700°C, ordered CoPt alloys were confirmed by superlattice diffraction peaks. At 800°C, a graded film containing L1 0 -ordered CoPt was found to be formed and a maximum coercivity of 2.1 kOe was observed by VSM, where the easy axis of magnetization was oriented along the in-plane direction. At 900°C, deformation of the ordered CoPt alloys was observed by GI-XRD, and the grain size of the film reached a maximum.

The effects of high magnetic fields applied perpendicular to the spontaneous ferroelectric polarization on single crystals of BiFeO3 were investigated through magnetization, magnetostriction, and neutron diffraction measurements. The magnetostriction measurements revealed lattice distortion of 2 × 10 −5 , during the reorientation process of the cycloidal spin order by applied magnetic fields. Furthermore, anomalous changes in magnetostriction and electric polarization at a larger field demonstrate an intermediate phase between cycloidal and canted antiferromagnetic states, where a large magnetoelectric effect was observed. Neutron diffraction measurements clarified that incommensurate spin modulation along [110] Recently, multiferroic materials have been widely investigated due to their coupling between magnetic and ferroelectric ordering. BiFeO 3 is perhaps the most extensively studied multiferroic material as it possesses robust multiferroicity at room temperature as well as various possible applications [1][2][3][4][5][6][7][8][9][10]. The effects of magnetic fields on the coupled multiple degrees of freedom behind these phenomena are not fully understood.BiFeO 3 exhibits a cycloidal magnetic order below 640 K [11]. This state is known to exhibit marginal quadratic magnetoelectric (ME) effect at low magnetic fields [12]. High magnetic fields of ∼20 T stabilize the canted antiferromagnetic (CAFM) phase as opposed to the cycloidal phase [13][14][15][16][17]. Although several groups succeeded in realizing the CAFM phase at zero field [18][19][20][21][22][23], the ME effect in this phase has not been clarified. In this study, several features were observed when a magnetic field was applied normal to the trigonal c-axis, and they indicated that a third magnetic phase emerged in bulk BiFeO 3 between the cycloidal and CAFM phases at approximately room temperature.BiFeO 3 has a crystal structure with the polar space group of R3c. A large switchable spontaneous electric polarization emerges along the c-axis of the trigonal cell [24][25][26]. Degeneracy exists in selecting the polarization direction from eight 111 directions in the cubic unit. Hence, depending on synthesis methods, BiFeO 3 crystals can contain multiple ferroelectric domains [27].Magnetic domains can be present even in single ferroelectric domain crystals. The magnetic propagation vector Q points in one of the 110 directions of the trigonal cell [11] in the cycloidal spin ordered state below ∼640 K. The three-fold rotational symmetry around the c-axis (Z direction in this paper) leads to three equivalent Q i (i = 1, 2, 3), as shown in Fig. 1(d). The spins rotated primarily in the Q i -Z plane in a magnetic domain with a given Q i .Recently, Tokunaga et al. indicated the emergence of an electric polarization perpendicular to the Z direction that was controlled by magnetic fields [17]. Theoretical calculations suggested that the cycloidal spin order in BiFeO 3 could involve electric polarization perpendicular to the Q i -Z plane as illustrated as P T in...

The spin-driven component of electric polarization in a single crystal of multiferroic BiFeO3 was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal c axis (Pt) in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of Pt is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by changing the direction of the applied magnetic field. This magnetoelectric coupling is reasonably reproduced by the metal-ligand hybridization model.

Material properties depend largely on the dimensionality of the crystal structures and the associated electronic structures. If the crystal-structure dimensionality can be switched reversibly in the same material, then a drastic property change may be controllable. Here, we propose a design route for a direct three-dimensional (3D) to 2D structural phase transition, demonstrating an example in (Pb1−xSnx)Se alloy system, where Pb2+ and Sn2+ have similar ns2 pseudo-closed shell configurations, but the former stabilizes the 3D rock-salt-type structure while the latter a 2D layered structure. However, this system has no direct phase boundary between these crystal structures under thermal equilibrium. We succeeded in inducing the direct 3D-2D structural phase transition in (Pb1−xSnx)Se alloy epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic property change was attained at x ~ 0.5 originating in the abrupt band structure switch from gapless Dirac-like state to semiconducting state.