Renewing Halogen Substitution Strategy for the Rational Design of High-Curie Temperature Metal-Free Molecular Antiferroelectrics.

Metal-free molecular antiferroelectric (AFE) holds a promise for energy storage on account of its unique physical attributes. However, it is challenging to explore high-curie temperature (Tc) molecular AFEs, due to the lack of design strategies regarding the rise of phase transition energy barriers. By renewing the halogen substitution strategy, we have obtained a series of high-Tc molecular AFEs of the halogen-substituted phenethylammonium bromides (x-PEAB, x=H/F/Cl/Br), resembling the binary stator-rotator system. Strikingly, the p-site halogen substitution of PEA+ cationic rotators raises their phase transition energy barrier and greatly enhances Tc up to ~473 K for Br-PEAB, on par with the record-high Tc values for molecular AFEs. As a typical case, the member 4-fluorophenethylammonium bromide (F-PEAB) shows notable AFE properties, including high Tc (~374 K) and large electric polarization (~3.2 µC/cm2). Further, F-PEAB also exhibits a high energy storage efficiency (η) of 83.6 % even around Tc, catching up with other AFE oxides. This renewing halogen substitution strategy in the molecular AFE system provides an effective way to design high-Tc AFEs for energy storage devices.

Designing Two-Dimensional Versatile Room-Temperature Ferromagnets via Assembling Large-Scale Magnetic Quantum Dots.

Two-dimensional (2D) ferromagnets possess astonishing potential in new-concept spintronics. However, most of the reported intrinsic 2D ferromagnets show a low Curie temperature far below room temperature. Here, we propose a series of 2D magnetic covalent and metal organic frameworks (COFs/MOFs) by assembling triangular zigzag graphene quantum dots (TZGDs) with various linkages, involving small-sized TZGDs, nonmetal atoms, magnetic metal atoms, and molecules. Upon first-principles calculations, we demonstrate 2D magnetic semiconductors with an enhanced Curie temperature of up to 472 K can be realized through the strong p(d)-p direct exchange interaction between TZGDs and linkages. Particularly, the TZGD size hardly affects the Curie temperature, whereas linkages can modulate the Curie temperature significantly. The TZGD size and linkages can regulate the electronic and magnetic properties of TZGD-based 2D ferromagnets. Our results confirm the possibility of designing 2D ferromagnets based on TZGDs and motivate the research of 2D ferromagnets on magnetic quantum dots and molecular magnets.

High Temperature Ferromagnetism in a GdAg2 Monolayer.

Materials that exhibit ferromagnetism, interfacial stability, and tunability are highly desired for the realization of emerging magnetoelectronic phenomena in heterostructures. Here we present the GdAg2 monolayer alloy, which possesses all such qualities. By combining X-ray absorption, Kerr effect, and angle-resolved photoemission with ab initio calculations, we have investigated the ferromagnetic nature of this class of Gd-based alloys. The Curie temperature can increase from 19 K in GdAu2 to a remarkably high 85 K in GdAg2. We find that the exchange coupling between Gd atoms is barely affected by their full coordination with noble metal atoms, and instead, magnetic coupling is effectively mediated by noble metal-Gd hybrid s,p-d bands. The direct comparison between isostructural GdAu2 and GdAg2 monolayers explains how the higher degree of surface confinement and electron occupation of such hybrid s,p-d bands promote the high Curie temperature in the latter. Finally, the chemical composition and structural robustness of the GdAg2 alloy has been demonstrated by interfacing them with organic semiconductors or magnetic nanodots. These results encourage systematic investigations of rare-earth/noble metal surface alloys and interfaces, in order to exploit them in magnetoelectronic applications.

Chemical Vapor Deposition Synthesis of Intrinsic High-Temperature Ferroelectric 2D CuCrSe2.

Ultrathin 2D ferroelectrics with high Curie temperature are critical for multifunctional ferroelectric devices. However, the ferroelectric spontaneous polarization is consistently broken by the strong thermal fluctuations at high temperature, resulting in the rare discovery of high-temperature ferroelectricity in 2D materials. Here, a chemical vapor deposition method is reported to synthesize 2D CuCrSe2 nanosheets. The crystal structure is confirmed by scanning transmission electron microscopy characterization. The measured ferroelectric phase transition temperature of ultrathin CuCrSe2 is about ≈800 K. Significantly, the switchable ferroelectric polarization is observed in ≈5.2 nm nanosheet. Moreover, the in-plane and out-of-plane ferroelectric response are modulated by different maximum bias voltage. This work provides a new insight into the construction of 2D ferroelectrics with high Curie temperature.

Intrinsic Out-Of-Plane and In-Plane Ferroelectricity in 2D AgCrS2 with High Curie Temperature.

2D ferroelectric materials have attracted extensive research interest due to potential applications in nonvolatile memory, nanoelectronics and optoelectronics. However, the available 2D ferroelectric materials are scarce and most of them are limited by the uncontrollable preparation. Herein, a novel 2D ferroelectric material AgCrS2 is reported that are controllably synthesized in large-scale via salt-assist chemical vapor deposition growth. By tuning the growth temperature from 800 to 900 °C, the thickness of AgCrS2 nanosheets can be precisely modulated from 2.1 to 40 nm. Structural and nonlinear optical characterizations demonstrate that AgCrS2 nanosheet crystallizes in a non-centrosymmetric structure with high crystallinity and remarkable air stability. As a result, AgCrS2 of various thicknesses display robust ferroelectric polarization in both in-plane (IP) and out-of-plane (OOP) directions with strong intercorrelation and high ferroelectric phase transition temperature (682 K). Theoretical calculations suggest that the ferroelectricity in AgCrS2 originates from the displacement of Ag atoms in AgS4 tetrahedrons, which changes the dipole moment alignment. Moreover, ferroelectric switching is demonstrated in both lateral and vertical AgCrS2 devices, which exhibit exotic nonvolatile memory behavior with distinct high and low resistance states. This study expands the scope of 2D ferroelectric materials and facilitates the ferroelectric-based nonvolatile memory applications.

Switchable Bulk Photovoltaic Effect in Intrinsically Ferroelectric 3D All-Inorganic CsPbBr3 Perovskite Nanocrystals.

Ferroelectric all-inorganic halide perovskite nanocrystals with both spontaneous polarization and visible light absorption are promising candidates for designing ferroelectric photovoltaic applications. It remains a challenge to realize ferroelectric photovoltaic devices with all-inorganic halide perovskites that can be operated in the absence of an external electric field. Here we report that a popular all-inorganic halide perovskite nanocrystal, CsPbBr3, exhibits a ferroelectricity-driven photovoltaic effect under visible light in the absence of an external electric field. Pristine CsPbBr3 nanocrystals exhibit intrinsic ferroelectric key properties with a notable saturated polarization of ∼0.15 µC/cm2 and a high Curie temperature of 462 K, driven by the stereochemical activity of the Pb(II) lone pair. Furthermore, application of an external electric field allows the photovoltaic effect to be enhanced and the spontaneous polarization to be switched with the direction of the electric field. CsPbBr3 nanocrystals exhibit a robust fatigue performance and a prolonged photoresponse under continuous illumination in the absence of an external electric field. These findings establish all-inorganic halide perovskite nanocrystals as potential candidates for designing photoferroelectric devices by coupling optical functionalities and ferroelectric responses.

Ferroelectricity and High Curie Temperature in a 2D Janus Magnet.

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3 with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tc low-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.

Cr2XTe4(X = Si, Ge) monolayers: a new type of two-dimensional high-TCIsing ferromagnetic semiconductors with a large magnetic anisotropy.

Two-dimensional (2D) ferromagnetic semiconductor (FMS) provides the ideal platform for the development of quantum information technology in nanoscale devices. However, most of them suffer from low Curie temperature and small magnetic anisotropic energy (MAE), severely limiting their practical application. In this work, by using first-principles calculations, we predicted two stable 2D materials, namely, Cr2SiTe4and Cr2GeTe4monolayers. Interestingly, both of them are intrinsic direct band gap FMSs (∼1 eV) with a large magnetization (8µBf.u.-1) and sizable MAE (∼500µeV Cr-1). Monte Carlo simulations based on Heisenberg model suggest markedly high Curie temperatures of these monolayers (∼200 K). Besides, their high mechanical, dynamical, and thermal stabilities are further verified by elastic constants, phonon dispersion calculations, andab initiomolecular dynamics simulations. The outstanding attributes render Cr2XTe4(X = Si, Ge) monolayers broadening the candidates of 2D FMS for a wide range of applications.

Realization of a Half Metal with a Record-High Curie Temperature in Perovskite Oxides.

Half metals, in which one spin channel is conducting while the other is insulating with an energy gap, are theoretically considered to comprise 100% spin-polarized conducting electrons, and thus have promising applications in high-efficiency magnetic sensors, computer memory, magnetic recording, and so on. However, for practical applications, a high Curie temperature combined with a wide spin energy gap and large magnetization is required. Realizing such a high-performance combination is a key challenge. Herein, a novel A- and B-site ordered quadruple perovskite oxide LaCu3 Fe2 Re2 O12 with the charge format of Cu2+ /Fe3+ /Re4.5+ is reported. The strong Cu2+ (↑)Fe3+ (↑)Re4.5+ (↓) spin interactions lead to a ferrimagnetic Curie temperature as high as 710 K, which is the reported record in perovskite-type half metals thus far. The saturated magnetic moment determined at 300 K is 7.0 µB f.u.-1 and further increases to 8.0 µB f.u.-1 at 2 K. First-principles calculations reveal a half-metallic nature with a spin-down conducting band while a spin-up insulating band with a large energy gap up to 2.27 eV. The currently unprecedented realization of record Curie temperature coupling with the wide energy gap and large moment in LaCu3 Fe2 Re2 O12 opens a way for potential applications in advanced spintronic devices at/above room temperature.

Neutron diffraction andab initiostudies on the fully compensated ferrimagnetic characteristics of Mn2V1-xCoxGa Heusler alloys.

Neutron diffraction andab initiostudies were carried out on Mn2V1-xCoxGa (x= 0, 0.25, 0.5, 0.75, 1) Heusler alloys which exhibits highTCfully compensated ferrimagnetic characteristics forx= 0.5. A combined analysis of neutron diffraction andab initiocalculations revealed the crystal structure and magnetic configuration which could not be determined from the x-ray diffraction and magnetic measurements. As reported earlier, Rietveld refinement of neutron diffraction data confirmedL21structure for Mn2VGa andXastructure for Mn2CoGa. The alloys withx= 0.25 and 0.5 possessL21structure with Mn(C)-Co disorder. As the Co concentration reaches 0.75, a structural transition has been observed from disorderedL21to disorderedXa. Detailedab initiostudies also confirmed this structural transition. The reason for the magnetic moment compensation in Mn2(V1-xCox)Ga was identified to be different from that of the earlier reported fully compensated ferrimagnet (MnCo)VGa. With the help of neutron diffraction andab initiostudies, it is identified that the disorderedL21structure with antiparallel coupling between the ferromagnetically aligned magnetic moments of (Mn(A)-Mn(C)) and (V-Co) atom pairs enables the compensation in Mn2V1-xCoxGa.

Strain-Controllable High Curie Temperature, Large Valley Polarization, and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Janus VSeTe Monolayer.

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are important for spintronics. A highly stable ML (monolayer) Janus 2H-VSeTe with intrinsic ferromagnetism is investigated by density functional theory. The biaxial strain could effectively tune the magnetic and electronic properties of Janus VSeTe. Specifically, the magnetic moment, band gap, Curie temperature (Tc), and valley splitting (Δ) could be modulated, as the states near the Fermi level are mainly contributed by the in-plane atomic orbitals. The VSeTe could be switched from ferromagnetic (FM) order to antiferromagnetic (AFM) ground state, under biaxial strains. And the corresponding Tc is tuned from 360 K (4%) to 0 K (-10.7%). However, VSeTe can be modulated from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), half-metal (HM), and even normal metal as the biaxial strain varies from -13 to 10%. Moreover, the easy and hard axes could be switched from each other, and the magnetocrystalline anisotropy (MCA) energy is also controlled by the strains. The Δ is also increased from 158 to 169 meV as the strain varies from 3.3 to -3.0%. The magnetic and electronic phase transitions in the strained VSeTe are observed, which could help researchers to investigate the controllable electronic and magnetic properties in electronics, spintronics, and valleytronics.

Increased Curie Temperature Induced by Orbital Ordering in La0.67Sr0.33MnO3/BaTiO3 Superlattices.

Recent theoretical studies indicated that the Curie temperature of perovskite manganite thin films can be increased by more than an order of magnitude by applying appropriate interfacial strain to control orbital ordering. In this work, we demonstrate that the regular intercalation of BaTiO3 layers between La0.67Sr0.33MnO3 layers effectively enhances ferromagnetic order and increases the Curie temperature of La0.67Sr0.33MnO3/BaTiO3 superlattices. The preferential orbital occupancy of eg(x 2 -y 2 ) in La0.67Sr0.33MnO3 layers induced by the tensile strain of BaTiO3 layers is identified by X-ray linear dichroism measurements. Our results reveal that controlling orbital ordering can effectively improve the Curie temperature of La0.67Sr0.33MnO3 films and that in-plane orbital occupancy is beneficial to the double exchange ferromagnetic coupling of thin-film samples. These findings create new opportunities for the design and control of magnetism in artificial structures and pave the way to a variety of novel magnetoelectronic applications that operate far above room temperature.

Above 400-K robust perpendicular ferromagnetic phase in a topological insulator.

The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal-doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced Tc by exchange coupling TIs to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.