Dielectric measurements on formamidinium lead halide perovskites, FAPbCl3 and FAPbBr3, compared to those of MAPbCl3 and previously reported MAPbBr3, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX3 system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results.

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Pyro-current measurements have been widely used to study ferroelectric properties in multiferroic materials. However, determination of intrinsic polarization by this method is not straightforward because of leakage current and trapped charge carriers. Here, we demonstrate the formation of internal electric field due to thermally stimulated charge carriers and its influence on ferroelectric polarization in a polycrystalline sample of the well known multiferroic TbMnO 3 . While an electric field (E ext ) poling across the ferroelectric transition (T C ∼ 26 K) is essential to obtain depolarization current at T C , the sample poled only in the paraelectric state (T pole = 130 − 50 K) also exhibits a pyro-current peak at T C but with the same polarity (− I pyro ) as that of the external field (− E ext ).We demonstrate that these unusual behavior of pyro-current are caused by a positive internal electric field (+ E int ) which in turn is created by thermally stimulated free charge carriers during the poling process in the paraelectric state. We also show that a combination of DC-biased current and pyro-current measurements is a promising method to study the intrinsic ferroelectric properties in multiferroic materials.

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time-resolved pump-probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure.

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We report an unusual sign reversal of exchange bias (EB) across a magnetic compensation point in an orthorhombic perovskite SmFeO3. A conventional negative EB with a positive vertical magnetization shift is observed below a cluster-glass freezing temperature (Tg ∼ 150 K). Upon further lowering of the temperature, the EB disappears at the magnetic compensation point before reversing its sign to a positive exchange bias below 4 K. The EB effect originates from an interfacial exchange interaction within a cluster glass phase, whereas its sign reversal arises from the reversal of the direction of the net magnetic moment as a result of dominance of Sm3+ over Fe3+ below the compensation temperature. The existence of a multi-glass state is demonstrated by ac-susceptibility and electrical permittivity measurements. A phenomenological model is presented to understand the EB effect and its sign reversal across the compensation point.

non-vdW crystals (such as Mn 3 Si 2 Te 6, [12,13] ) has led to an interesting arena for studying fundamental 2D magnetism and provided numerous opportunities for 2D magnetic, magnetoelectric, and magnetooptic applications. [14,15] Electrical control of both charge and spin degrees of freedom in 2D ferromagnetic semiconductors is an important step in creating novel spintronic devices. As a single integrated platform, the spintronic devices based on these materials are expected to complement or outperform (in some cases) the conventional semiconductor-based devices. [16] However, compared to binary 2D metal chalcogenides, synthesis of ternary 2D metal chalcogenides is far more challenging, so the proof-of-concept studies on the magnetoelectric phenomena of the ferromagnetic ternary 2D metal chalcogenides have been performed on thin flakes mechanically exfoliated from bulk crystals. [9,17] There have been limited reports on the synthesis of ferromagnetic 2D ternary metal chalcogenide thin films. [18,19] The synthesis from pure elemental mixtures in an evacuated quartz ampule at 900-1100 °C through the self-flux of elements took 5-20 days. [7,13,17,20] Only two types of thin films have been synthesized via molecular beam epitaxy (MBE) in an ultra-high vacuum condition. [21][22][23] Very recently, a few CVD-based synthesis have been reported for binary 2D metal chalcogenide magnetic crystals such as CrSe and FeTe. [24,25] However, to the best of our knowledge, the conventional CVD-based and Following the first experimental realization of intrinsic ferromagnetism in 2D van der Waals (vdW) crystals, several ternary metal chalcogenides with unprecedented long-range ferromagnetic order have been explored. However, the synthesis of large-area 2D ternary metal chalcogenide thin films is a great challenge, and a generalized synthesis has not been demonstrated yet. Here, a quick and scalable synthesis of epitaxially aligned ferromagnetic ternary metal chalcogenide thin films (Cr 2 Ge 2 Te 6 , Cr 2 Si 2 Te 6 , Mn 3 Si 2 Te 6 ) is reported. The synthesis is based on the flux-controlled surface diffusion of Te on metal (Cr, Mn)-deposited wafer (Ge, Si) substrates. Magnetic anisotropy study of the epitaxial ternary thin films reveals the intrinsic magnetic easy axis; out-of-plane direction for Cr 2 Ge 2 Te 6 and Cr 2 Si 2 Te 6 , and in-plane direction for Mn 3 Si 2 Te 6 . In addition to the synthesis, this work creates an opportunity for transfer-free device fabrication for realizing magnetoelectronics based on the electrical control of both charge and spin degrees of freedom in 2D ferromagnetic semiconductors.

The ordered perovskites, NaLnMnWO6 (Ln = La, Nd, Tb), are reported to exhibit simultaneous ordering of A-site cations (Na and Ln) in layered arrangement and B-site cations (Mn and W) in rock salt structure. They have been shown to crystallize in a monoclinic structure with the polar space group P21. Based on density functional calculations and group theoretical analysis, it has recently been proposed that NaLaMnWO6 should be ferroelectric with a relatively large polarization (16 μC cm(-2)). Contrary to this prediction, our electrical measurements such as conventional P-E loop, Positive-Up and Negative-Down (PUND), piezoelectric response and Second Harmonic Generation (SHG) reveal the absence of ferroelectric polarization in NaLnMnWO6 (Ln = La, Nd, Tb). A dielectric anomaly is observed just below room temperature (∼270 K) for all the three compounds, which is related to the change in conductivity as revealed by temperature dependent ac and dc resistivity. A pyrocurrent peak is also observed at the same temperature. However, its origin cannot be attributed to a ferroelectric transition.