Parasitic Ferromagnetism in Few-Layered Transition-Metal Chalcogenophosphate.

Since the rise of two-dimensional (2D) semiconductors, it seems that electronic devices will soon be upgraded with spintronics, in which the manipulation of spin degree of freedom endows it obvious advantages over conventional charge-based electronics. However, as the most crucial prerequisite for the above-mentioned expectation, 2D semiconductors with adjustable magnetic interaction are still rare, which has greatly hampered the promotion of spintronics. Recently, transition metal phosphates have attracted tremendous interest due to their intrinsic antiferromagnetism and potential applications in spintronics. In the work described herein, parasitic ferromagnetism is achieved for the first time by exfoliating an antiferromagnetic chalcogenophosphate to a few layers. Taking the transition metal chalcogenophosphate Mn2P2S6 as an example, the antiferromagnetic transition at the Néel temperature is completely suppressed, and the magnetic behaviors of the as-obtained few-layered Mn2P2S6 are dominated by parasitic ferromagnetism. We experimentally verify an electron redistribution by which part of the Mn 3d electrons migrate and redistribute on P atoms in few-layered Mn2P2S6 due to the introduced Mn vacancies. The results demonstrated here broaden the tunability of the material's magnetic properties and open up a new strategy to rationally design the magnetic behaviors of 2D semiconductors, which could accelerate the applications of spintronics.

Competing spin fluctuations and trace of vortex dynamics in the two-dimensional triangular-lattice antiferromagnet AgCrS2.

The spin dynamics of the two-dimensional triangular-lattice antiferromagnet AgCrS2 is investigated by electron spin resonance (ESR) spectroscopy. The g-factor is found to show an unusual non-monotonously temperature dependent behavior, which, along with the super-Curie behavior observed in the ESR intensity data, provides clear evidence for the competition between ferromagnetic and antiferromagnetic fluctuations at temperatures well above T N. On approaching the Néel temperature T N from above, the linewidth is found to diverge. Such a divergent behavior could be well described by the Kawamura-Miyashita model due to Z2 type magnetic vortex-antivortex pairing, which is consistent with the expectation for a 2D Heisenberg magnetic system.

Local Electric Field Facilitates High-Performance Li-Ion Batteries.

By scrutinizing the energy storage process in Li-ion batteries, tuning Li-ion migration behavior by atomic level tailoring will unlock great potential for pursuing higher electrochemical performance. Vacancy, which can effectively modulate the electrical ordering on the nanoscale, even in tiny concentrations, will provide tempting opportunities for manipulating Li-ion migratory behavior. Herein, taking CuGeO3 as a model, oxygen vacancies obtained by reducing the thickness dimension down to the atomic scale are introduced in this work. As the Li-ion storage progresses, the imbalanced charge distribution emerging around the oxygen vacancies could induce a local built-in electric field, which will accelerate the ions' migration rate by Coulomb forces and thus have benefits for high-rate performance. Furthermore, the thus-obtained CuGeO3 ultrathin nanosheets (CGOUNs)/graphene van der Waals heterojunctions are used as anodes in Li-ion batteries, which deliver a reversible specific capacity of 1295 mAh g-1 at 100 mA g-1, with improved rate capability and cycling performance compared to their bulk counterpart. Our findings build a clear connection between the atomic/defect/electronic structure and intrinsic properties for designing high-efficiency electrode materials.

Critical behavior of the van der Waals bonded high T C ferromagnet Fe3GeTe2.

Fe3GeTe2 is a promising candidate for van der Waals bonded ferromagnet because of its high Curie temperature and the prediction that its ferromagnetism can maintain upon exfoliating down to single layer. Here, we have reported the critical behavior to understand its ferromagnetic exchange. Based on various techniques including modified Arrott plot, Kouvel-Fisher plot, and critical isotherm analysis, a set of reliable critical exponents (ß = 0.327 ± 0.003, γ = 1.079 ± 0.005, and δ = 4.261 ± 0.009) has been obtained. The critical behavior suggests a three-dimensional long-range magnetic coupling with the exchange distance decaying as J(r) ≈ r -4.6 in Fe3GeTe2. The possible origin of three-dimensional magnetic characteristics in van der Waals bonded magnets is discussed.

Highly Sensitive Detection of Ionizing Radiations by a Photoluminescent Uranyl Organic Framework.

Precise detection of low-dose X- and γ-radiations remains a challenge and is particularly important for studying biological effects under low-dose ionizing radiation, safety control in medical radiation treatment, survey of environmental radiation background, and monitoring cosmic radiations. We report here a photoluminescent uranium organic framework, whose photoluminescence intensity can be accurately correlated with the exposure dose of X- or γ-radiations. This allows for precise and instant detection of ionizing radiations down to the level of 10-4  Gy, representing a significant improvement on the detection limit of approximately two orders of magnitude, compared to other chemical dosimeters reported up to now. The electron paramagnetic resonance analysis suggests that with the exposure to radiations, the carbonyl double bonds break affording oxo-radicals that can be stabilized within the conjugated uranium oxalate-carboxylate sheet. This gives rise to a substantially enhanced equatorial bonding of the uranyl(VI) ions as elucidated by the single-crystal structure of the γ-ray irradiated material, and subsequently leads to a very effective photoluminescence quenching through phonon-assisted relaxation. The quenched sample can be easily recovered by heating, enabling recycled detection for multiple runs.

Critical behavior of the quasi-two-dimensional semiconducting ferromagnet CrSiTe3.

The semiconducting ferromagnet CrSiTe3 is a promising candidate for two-dimensional magnet simply by exfoliating down to single layers. To understand the magnetic behavior in thin-film samples and the possible applications, it is necessary to establish the nature of the magnetism in the bulk. In this work, the critical behavior at the paramagnetic to ferromagnetic phase transition in single-crystalline CrSiTe3 is investigated by bulk magnetization measurements. We have obtained the critical exponents (ß = 0.170 ± 0.008, γ = 1.532 ± 0.001, and δ = 9.917 ± 0.008) and the critical temperature TC = 31.0 K using various techniques such as modified Arrott plot, Kouvel-Fisher plot, and critical isotherm analysis. Our analysis suggests that the determined exponents match well with those calculated from the results of renormalization group approach for a two-dimensional Ising model coupled with long-range interaction.

De Hass-van Alphen and magnetoresistance reveal predominantly single-band transport behavior in PdTe2.

Research on two-dimensional transition metal dichalcogenides (TMDs) has grown rapidly over the past several years, from fundamental studies to the development of next generation technologies. Recently, it has been reported that the MX2-type PdTe2 exhibits superconductivity with topological surface state, making this compound a promising candidate for investigating possible topological superconductivity. However, due to the multi-band feature of most of TMDs, the investigating of magnetoresistance and quantum oscillations of these TMDs proves to be quite complicated. Here we report a combined de Hass-van Alphen effect and magnetoresistance studies on the PdTe2 single crystal. Our high-field de Hass-van Alphen data measured at different temperature and different tilting angle suggest that though these is a well-defined multi-band feature, a predominant oscillation frequency has the largest oscillation magnitude in the fast Fourier transformation spectra, which is at least one order of magnitude larger than other oscillation frequencies. Thus it is likely that the transport behavior in PdTe2 system can be simplified into a single-band model. Meanwhile, the magnetoresistance results of the PdTe2 sample can be well-fitted according to the single-band models. The present results could be important in further investigation of the transport behaviors of two-dimensional TMDs.

Electric-Field-Driven Dual Vacancies Evolution in Ultrathin Nanosheets Realizing Reversible Semiconductor to Half-Metal Transition.

Fabricating a flexible room-temperature ferromagnetic resistive-switching random access memory (RRAM) device is of fundamental importance to integrate nonvolatile memory and spintronics both in theory and practice for modern information technology and has the potential to bring about revolutionary new foldable information-storage devices. Here, we show that a relatively low operating voltage (+1.4 V/-1.5 V, the corresponding electric field is around 20,000 V/cm) drives the dual vacancies evolution in ultrathin SnO2 nanosheets at room temperature, which causes the reversible transition between semiconductor and half-metal, accompanyied by an abrupt conductivity change up to 10(3) times, exhibiting room-temperature ferromagnetism in two resistance states. Positron annihilation spectroscopy and electron spin resonance results show that the Sn/O dual vacancies in the ultrathin SnO2 nanosheets evolve to isolated Sn vacancy under electric field, accounting for the switching behavior of SnO2 ultrathin nanosheets; on the other hand, the different defect types correspond to different conduction natures, realizing the transition between semiconductor and half-metal. Our result represents a crucial step to create new a information-storage device realizing the reversible transition between semiconductor and half-metal with flexibility and room-temperature ferromagnetism at low energy consumption. The as-obtained half-metal in the low-resistance state broadens the application of the device in spintronics and the semiconductor to half-metal transition on the basis of defects evolution and also opens up a new avenue for exploring random access memory mechanisms and finding new half-metals for spintronics.

Gapless quantum spin liquid ground state in the two-dimensional spin-1/2 triangular antiferromagnet YbMgGaO4.

Quantum spin liquid (QSL) is a novel state of matter which refuses the conventional spin freezing even at 0 K. Experimentally searching for the structurally perfect candidates is a big challenge in condensed matter physics. Here we report the successful synthesis of a new spin-1/2 triangular antiferromagnet YbMgGaO4 with symmetry. The compound with an ideal two-dimensional and spatial isotropic magnetic triangular-lattice has no site-mixing magnetic defects and no antisymmetric Dzyaloshinsky-Moriya (DM) interactions. No spin freezing down to 60 mK (despite θw ~ -4 K), the power-law temperature dependence of heat capacity and nonzero susceptibility at low temperatures suggest that YbMgGaO4 is a promising gapless (≤|θw|/100) QSL candidate. The residual spin entropy, which is accurately determined with a non-magnetic reference LuMgGaO4, approaches zero (<0.6%). This indicates that the possible QSL ground state (GS) of the frustrated spin system has been experimentally achieved at the lowest measurement temperatures.

Ultrathin Co3S4 nanosheets that synergistically engineer spin states and exposed polyhedra that promote water oxidation under neutral conditions.

Development of efficient and affordable electrocatalysts in neutral solutions is paramount importance for the renewable energy. Herein, we report that the oxygen evolution reaction (OER) performance of Co3 S4 under neutral conditions can be enhanced by exposed octahedral planes and self-adapted spin states in atomically thin nanosheets. A HAADF image clearly confirmed that the active octahedra with Jahn-Teller distortions were exposed exclusively. Most importantly, in the atomically thin nanosheets, the spin states of Co(3+) in the octahedral self-adapt from low-spin to high-spin states. As a result, the synergistic effect endow the Co3 S4 nanosheets with superior OER performance, with exceptional low onset overpotentials of circa 0.31 V in neutral solutions, which is state-of-the-art among inorganic non-noble metal compounds.

Unusual ferromagnetic critical behavior owing to short-range antiferromagnetic correlations in antiperovskite Cu(1-x)NMn(3+x) (0.1 ≤ x ≤ 0.4).

For ferromagnets, varying from simple metals to strongly correlated oxides,the critical behaviors near the Curie temperature (T(C)) can be grouped into several universal classes. In this paper, we report an unusual critical behavior in manganese nitrides Cu(1-x)NMn(3+x) (0.1 ≤ x ≤ 0.4). Although the critical behavior below T(C) can be well described by mean field (MF) theory, robust critical fluctuations beyond the expectations of any universal classes are observed above T(C) in x = 0.1. The critical fluctuations become weaker when x increases, and the MF-like critical behavior is finally restored at x = 0.4. In addition, the paramagnetic susceptibility of all the samples deviates from the Curie-Weiss (CW) law just above T(C). This deviation is gradually smeared as x increases. The short-range antiferromagnetic ordering above T(C) revealed by our electron spin resonance measurement explains both the unusual critical behavior and the breakdown of the CW law.

Anisotropic paramagnetism of monoclinic Nd2Ti2O7 single crystals.

The anisotropic paramagnetism and specific heat in Nd(2)Ti(2)O(7) single crystals are investigated. The angular dependence of the magnetization and Weiss temperatures show the dominant role of the crystal field effect in the magnetization. By incorporating the results from the diluted samples, contributions to the Weiss temperature from exchange interactions and crystal field interactions are isolated. The exchange interactions are found to be ferromagnetic, while the crystal field contributes a large negative part to the Weiss temperature, along all three crystallographic directions. The specific heat under a magnetic field reveals a two-level Schottky ground state scheme, due to the Zeeman splitting of the ground state doublet, and the g-factors are thus determined. These observations provide solid foundations for further investigations of Nd(2)Ti(2)O(7).