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The temperature-dependent spin-reorientation transition (SRT) and spin interaction mechanism of bulk TmFeO3 were studied by the electron paramagnetic resonance (EPR) method. The combined experimental results of magnetic curves and EPR spectra confirmed that there is an antiferromagnetic transition at 85 K with a reentering ferromagnetic state due to the spin-reorientation behavior. In the high-temperature region of T > 90 K, there are three distinct resonance peaks in the EPR spectrum, which indicates the presence of multiple magnetic phases (canted antiferromagnetic, weak ferromagnetic, and paramagnetic phases). In the low-temperature region (T 85 K, with SRT between 85-65 K and ferromagnetic interaction at the lower temperature, respectively. Above 90 K, we find that the spin relaxation mechanism is determined by the mixture of spin-spin and spin-lattice interactions. Below 85 K, the transverse relaxation rate increases with the decrease in temperature, which is consistent with the weakening of the fluctuating internal field in this temperature region. This EPR detection provides a new method to clarify the strong spin coupling in antiferromagnetic materials.

Understanding of the intimate cross-coupling between electric and magnetic degrees of freedom in solids usually requires sophisticated models and time-consuming calculation methods. Instead of macroscopic symmetry analysis, we present a simple but general approach to explore the microscopic mechanism of magnetoelectric (ME) effects in magnetic ordered materials based on local spin and lattice symmetry analysis. Our methods are successfully applied to Cr$_2$O$_3$ and orthorhombic RMnO$_3$ with linear and quadratic ME effects, respectively. We revealed all the possible microscopic origins of every non-zero ME coefficients that cannot be easily fulfilled by other theoretical methods. Moreover, the contribution from each mechanism can be located down to specific spins or spin pairs. Our theoretical approach is capable of providing a detailed guide to explore rich spin-induced magnetoelectrics with strong ME effects., 3 figures

We studied the effects of multiband electronic structure on the thermoelectric and electrical transport properties in the normal state of kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs). In all three members, the multiband nature is manifested by sign changes in the temperature dependence of the Seebeck and Hall resistivity, together with sublinear response of the isothermal Nernst and Hall effects to external magnetic fields in the charge ordered state. Moreover, ambipolar transport effects appear ubiquitously in all three systems, giving rise to sizable Nernst signal. Finally, possible origins of the sign reversal in the temperature dependence of the Hall effect are discussed., Comment: 8 pages, 5 figures. To appear in New Journal of Physics

Collective spin excitations of ordered magnetic structures o er great potential for the development of novel spintronic devices. The present approach is to rely on micromagnetic models to explain the origins of dynamic modes observed by ferromagnetic resonance (FMR) studies, since experimental tools to directly reveal the origins of the complex dynamic behavior are lacking. Here we demonstrate a new approach which combines resonant magnetic x-ray diffraction with FMR, thereby allowing for a reconstruction of the real-space spin dynamics of the system. This new diffractive FMR (DFMR) technique builds on x-ray detected FMR (XFMR) that allows for element-selective dynamic studies, giving unique access to specific wave components of static and dynamic coupling in magnetic heterostructures. In combination with diffraction, FMR is elevated to the level of a modal spectroscopy technique, potentially opening new pathways for the development of spintronic devices.

Transtor and memtranstor are the fourth basic linear and memory elements, which allows direct coupling of charge (q) to magnetic flux ({\phi}) via linear and non-linear ME effects, respectively. It is found here that large variation of magnetization by electric field is realized in both linear and nonlinear hysteretic styles in a magnetoelectric Y-type hexaferrite Ba0.5Sr1.5Zn2(Fe0.92Al0.08)12O22 single-crystal. Moreover, based on the spin current model, the underlying microscopic mechanisms for generating the two types of linear and nonlinear M vs E curves are understood as E induced changes of cone angle and sign of P respectively, establishing the charge-driven transtor and memtranstor in the Y-type hexaferrite system. This work points to a promising pathway to develop unique circuit functionalities using the magnetoelectric materials.

We report a comprehensive magnetization and magnetotransport study on a metallic-type of ${\mathrm{EuMnSb}}_{2}$ single crystals. A large Hall signal up to room temperature is found in our metallic ${\mathrm{EuMnSb}}_{2}$, which is dominated by hole carriers with very low carrier density and high mobility. Clear Shubnikov--de Haas oscillations are observed at low temperatures (T $l$ 30 K), from which a dominant tiny hole pocket with small effective mass is deduced. The interplay of carrier density and Mn and Eu moments results in complex magnetic ground states with two successive antiferromagnetic (AFM) transitions appearing at ${T}_{N1}=24 \mathrm{K}$ and ${T}_{N2}=9 \mathrm{K}$. In line with the rich magnetic phases, the transport properties, including magnetoresistance, quantum oscillation, and Berry phase, experience abrupt disturbances when crossing the magnetic phase boundaries. The Dirac state is likely quenched once the AFM orders set in, indicating a strong coupling between the AFM magnetic order and the band topology. Our results suggest that metallic-type ${\mathrm{EuMnSb}}_{2}$ is an ideal platform for the study of the interplay between magnetism, charge transport, and band topology.

Magnetic topological semimetals provide new opportunities for power generation and solid-state cooling based on thermoelectric (TE) effect. The interplay between magnetism and nontrivial band topology prompts the magnetic topological semimetals to yield strong transverse TE effect, while the longitudinal TE performance is usually poor. Herein, it is demonstrated that the magnetic Weyl semimetal TbPtBi has high value for both transverse and longitudinal thermopower with large power factor (PF). At 300 K and 13.5 Tesla, the transverse thermopower and PF reach up to 214 µV K

Capsule: Eurasian Bitterns Botaurus stellaris caught in winter in eastern China summered in the Russian Far East. Aims: To identify migration timing, duration and routes, as well as stopover sites, used by Eurasian Bitterns in the Russian Far East flyway and obtain basic information on behaviour and ecology from tracking data. Methods: We tracked two Eurasian Bitterns caught in China with global positioning system/mobile communications loggers for one and three years respectively, to identify their migration routes and schedules. We used the distance moved between successive fixes to determine their diurnal activity patterns. Results: The two individuals wintered in eastern China and travelled a mean of 4221 ± 603 km (in 2015–17) and 3844 km (2017) to summer in the Russian Far East. Results from one bird have shown that in all three years, the bird was significantly more active during the day than at night, although the absolute differences varied with season, being most nocturnally active in summer. The most surprising result from this bird was the flexibility in spring migration and the lack of summer site fidelity. Conclusion: The study identified previously unknown migration routes of the Eurasian Bittern in East Asia, and suggested that the species is generally more active during the day throughout the year.

Giant magnetostriction in insulating magnetic materials is highly required for applications but is rarely observed. Here we show that giant magnetostriction $(g1500\phantom{\rule{0.16em}{0ex}}\mathrm{ppm})$ can be achieved in an insulating transition metal oxide $\mathrm{CaBa}{\mathrm{Co}}_{4}{\mathrm{O}}_{7}$ where the ferrimagnetic ordering at ${T}_{\mathrm{C}}\ensuremath{\sim}62\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is associated with a huge change in the lattice. Moreover, because this material is pyroelectric with a nonswitchable electric polarization ($P$), the giant magnetostriction results in a pronounced magnetoelectric effect---a huge change of electric polarization $(\mathrm{\ensuremath{\Delta}}P\ensuremath{\sim}1.6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2})$ in response to the applied magnetic field up to 60 T. Geometric frustration as well as the orbital instability of ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ ions is believed to play a crucial role in the giant magnetostriction. Our study provides insights on how to achieve both giant magnetostriction and pronounced magnetoelectric effect in insulating transition metal oxides.

We present out-of-plane dielectric and magnetodielectric measurements of single crystallines $\alpha$-RuCl$_3$ with various degrees of stack faults. A frequency dependent, but field independent, dielectric anomaly appears at $T_{A}\:(f=100\:\mathrm{kHz})\sim$ 4 K once both magnetic transitions at $T_{N1}\sim$ 7 K and $T_{N2}\sim$ 14 K set in. The observed dielectric anomaly is attributed to the emergency of possible local electric polarizations whose inversion symmetry is broken by inhomogeneously distributed stacking faults. A field-induced intermediate phase is only observed when a magnetic field is applied perpendicular to the Ru-Ru bonds for samples with minimal stacking faults. Less pronounced in-plane anisotropy is found in samples with sizable contribution from stacking imperfections. Our findings suggest that dielectric measurement is a sensitive probe in detecting the structural and magnetic properties, which may be a promising tool especially in studying $\alpha$-RuCl$_3$ thin film devices. Moreover, the stacking details of RuCl$_3$ layers strongly affect the ground state both in the magnetic and electric channels. Such a fragile ground state against stacking faults needs to be overcome for realistic applications utilizing the magnetic and/or electric properties of Kitaev based physics in $\alpha$-RuCl$_3$., Comment: 8 pages, 5 figures. Accepted for publication in Physical Review B

In the present study, we investigated magnetodielectric, magnetoelectric (ME), and angular-dependent polarization in single-crystal ${\mathrm{Fe}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$. The magnetodielectric effects in ${\ensuremath{\varepsilon}}_{x}$ ($x//[100]$), ${\ensuremath{\varepsilon}}_{y}$ ($y//[120]$), and ${\ensuremath{\varepsilon}}_{z}$ ($z//[001]$) are found to be significant only around ${T}_{\mathrm{N}}\ensuremath{\approx}95\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ when magnetic fields are applied along three orthogonal x-, y- ($y//[120]$), and $z$ directions. The finite polarization ${P}_{x}, {P}_{y}$, and ${P}_{z}$ of 70, 100, and $30\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$, respectively, can be induced in the antiferromagnetic phase when a finite magnetic field up to 9 T is applied along the three orthogonal directions. The angular-dependent polarization measurements verify the dominating linear ME effects below ${T}_{\mathrm{N}}$. From the above experimental results, a linear ME tensor ${a}_{ij}$ with all nine nonzero components can be inferred, demonstrating a much lower magnetic point group of $\ensuremath{-}{\mathbf{1}}^{\ensuremath{'}}$ for the canted antiferromagnetic configuration.

Achieving high thermoelectric power factor in thin film heterostructures is essential for integrated and miniaturized thermoelectric device applications. In this work, we demonstrate a mechanism to enhance thermoelectric power factor through coupling the interfacial confined two-dimensional electron gas (2DEG) with thin film conductivity in a transition metal dichalcogenides-SrTiO3 heterostructure. Owing to the formed conductive interface with two-dimensional electron confinement effect and the elevated conductivity, the ZrTe2/SrTiO3 (STO) heterostructure presents enormous thermoelectric power factor as high as 4×10^5 μW cm^(-1) K^(-2) at 20 K and 4800 μW cm^(-1) K^(-2) at room temperature. Interfacial reaction induced degradation of Ti cations valence number from Ti4+ to Ti3+ is attributed to be responsible for the formation of the quasi-two-dimensional electrons at the interface which results in very large Seebeck coefficient; and the enhanced electrical conductivity is suggested to be originated from the charge transfer induced doping in the ZrTe2. By taking the thermal conductivity of STO substrate as a reference, the effective zT value of this heterostructure can reach 15 at 300 K. This superior thermoelectric property makes this heterostructure a promising candidate for future thermoelectric device, and more importantly, paves a new pathway to design promising high-performance thermoelectric systems.

Achieving high thermoelectric power factor in thin film heterostructures is essential for integrated and miniatured thermoelectric device applications. In this work, we demonstrate a mechanism and device performance of enhanced thermoelectric power factor through coupling the interfacial confined two-dimensional electron gas (2DEG) with thin film conductivity in a transition metal dichalcogenides-SrTiO3 heterostructure. Owing to the formed conductive interface with two-dimensional electron confinement effect and the elevated conductivity, the ZrTe2/SrTiO3 (STO) heterostructure presents enormous thermoelectric power factor as high as 4×10^5 μW/cmK^2 at 20 K and 4800 μW/cmK^2 at room temperature. Formation of quasi-two-dimensional electrons gas at the interface is attributed to the giant Seebeck coefficient, and enhanced electrical conductivity is suggested to be originated from charge transfer induced doping in the ZrTe2, which leads to extremely large thermoelectric power factor. By taking the thermal conductivity of STO substrate as a reference, the effective zT value of this heterostructure can reach 1.5 at 300 K. This high thermoelectric figure of merit is demonstrated by a prototype device based on this heterostructure which results in 3K temperature cooling by passing through a current of 100 mA. This superior thermoelectric property makes this heterostructure a promising candidate for future thermoelectric device, and more importantly, paves a new pathway to design promising high-performance thermoelectric systems.

MnBi$_2$Te$_4$, a two-dimensional magnetic topological insulator with a uniaxial antiferromagnetic structure, is an ideal platform to realize quantum anomalous Hall effect. However, the strength of magnetic interactions is not clear yet. We performed systematic studies on the magnetization and angle dependent magnetotransport of MnBi$_2$Te$_4$ single crystal. The results show that the direction of the magnetic field has significant effects on the critical field values and magnetic structure of this compound, which leads to different magnetotransport behaviors. The field-driven reorientation transitions can be utilized to estimate the AFM interlayer exchange interaction coupling and uniaxial magnetic anisotropy D. The obtained Hamiltonian can well explain the experimental data by Monte Carlo simulations. Our comprehensive studies on the field-driven magnetic transitions phenomenon in MnBi$_2$Te$_4$ provide a general approach for other topological systems with antiferromagnetism., Comment: 6 figures

We report discovery of new antiferromagnetic semimetal EuZnSb$_2$, obtained and studied in the form of single crystals. Electric resistivity, magnetic susceptibility and heat capacity indicate antiferromagnetic order of Eu with $T_N$ = 20 K. The effective moment of Eu$^{2+}$ inferred from the magnetization and specific heat measurement is 3.5 $\mu_B$, smaller than the theoretical value of Eu$^{2+}$ due to presence of both Eu$^{3+}$ and Eu$^{2+}$. Magnetic field-dependent resistivity measurements suggest dominant quasi two dimensional Fermi surfaces whereas the first-principle calculations point to the presence of Dirac fermions. Therefore, EuZnSb$_2$ could represent the first platform to study the interplay of dynamical charge fluctuations, localized magnetic 4$f$ moments and Dirac states with Sb orbital character., Comment: 9 pages, 4 figures

The temperature-dependent spin-reorientation transition (SRT) and spin interaction mechanism of bulk TmFeO3 were studied by the electron paramagnetic resonance (EPR) method. The combined experimental results of magnetic curves and EPR spectra confirmed that there is an antiferromagnetic transition at 85 K with a reentering ferromagnetic state due to the spin-reorientation behavior. In the high-temperature region of T90 K, there are three distinct resonance peaks in the EPR spectrum, which indicates the presence of multiple magnetic phases (canted antiferromagnetic, weak ferromagnetic, and paramagnetic phases). In the low-temperature region (T85 K), the temperature dependence of the EPR linewidth, effective g-factor, and intensity can be used to infer a strong spin-lattice correlation. Different magnetic interactions such as Fe3+-Fe3+, Fe3+-Tm3+, and Tm3+-Tm3+ lead to a paramagnetic-canted antiferromagnetic phase at T85 K, with SRT between 85-65 K and ferromagnetic interaction at the lower temperature, respectively. Above 90 K, we find that the spin relaxation mechanism is determined by the mixture of spin-spin and spin-lattice interactions. Below 85 K, the transverse relaxation rate increases with the decrease in temperature, which is consistent with the weakening of the fluctuating internal field in this temperature region. This EPR detection provides a new method to clarify the strong spin coupling in antiferromagnetic materials.

We present the significant spin-driven dielectric anomaly ($\ensuremath{\sim}40%$ drop) and magnetoelectric coupling near the magnetic ordering temperature in single crystal ${\mathrm{Fe}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$. By combining neutron and x-ray single crystal diffraction techniques, we unambiguously determined its magnetic symmetry and studied the structural phase transition at ${T}_{S}$ = 70 K. The temperature-dependent static dielectric constant is strongly anisotropic, rendering two dielectric anomalies along the $a$ axis in the hexagonal lattice with the first one coupled to the magnetic ordering around ${T}_{N}$ = 97 K and the second one accompanying with a first-order structural transition around ${T}_{S}$ = 70 K. Below ${T}_{N}$, we found that the anomalous dielectric constant is practically proportional to the square of the magnetic moment from neutron diffraction data, indicating that the exchange striction is likely responsible for the strong spin-lattice coupling. Magnetic-field-induced magnetoelectric coupling was observed and is compatible with the determined magnetic structure that is characteristic of antiferromagnetically arranged ferromagnetic chains in the honeycomb plane. We propose that such magnetic symmetry should be immune to external magnetic fields to some extent favored by the freedom of rotation of moments in the honeycomb plane, laying out a promising system to control the magnetoelectric properties by magnetic fields.

It remains a daunting task to quantify the configurational entropy of a material from atom‐revolved electron microscopy images and correlate the results with the material's lattice thermal conductivity, which strides across statics, dynamics, and thermal transport of crystal lattice over orders of magnitudes in length and time. Here, a proof‐of‐principle study of atomic‐scale visualization and quantification of configurational entropy in relation to thermal conductivity in single crystalline trigonal GeSb2Te4 (aka t‐GeSb2Te4) with native atomic site disorder is reported. A concerted effort of large t‐GeSb2Te4 single crystal growth, in‐lab developed analysis procedure of atomic column intensity, the visualization and quantification of configurational entropy including corresponding modulation, and thermal transport measurements enable an entropic “bottom‐up” perspective to the lattice thermal conductivity of t‐GeSb2Te4. It is uncovered that the configurational entropy increases phonon scattering and reduces phonon mean free path as well as promotes anharmonicity, thereby giving rise to low lattice thermal conductivity and promising thermoelectric performance. The current study sheds lights on an atomic scale bottom‐up configurational entropy design in diverse regimes of structural and functional materials research and applications., A proof‐of‐principle study of atomic‐scale visualization and quantification of configurational entropy in relation to thermal conductivity in single crystalline trigonal GeSb2Te4 with native atomic site disorder is reported. The configurational entropy spontaneously intensifies phonon scattering and promotes anharmonicity, which leads to significantly reduced phonon mean free path, enabling an entropic “bottom‐up” perspective to the lattice thermal conductivity.

We investigated polarized infrared and Raman spectra of the multiferroic hexaferrites BaSrCoZnFe11AlO22 (Y-type) and (Ba0.5Sr0.5)3Co2Fe24O41 (Z-type) on cooling from room temperature down to 4 K, a...

With increasing demands toward device miniaturization, Pb-free magnetoelectric laminates (MELs) with a small lateral dimension of ∼3 × 3 mm2 have been synthesized by the tape-casting method. The MELs are composed of alternating layers of magnetostrictive NiFe2O4 and piezoelectric BaTiO3 with a uniform single layer thickness of t = 50 and 15 μm, respectively. Both laminates exhibit much larger longitudinal ME voltage coefficient αE33 than the transverse ME voltage coefficient αE31 at both off-resonant and resonant frequencies, which is attributed to the preferential easy-plane alignment of the magnetic moments. Furthermore, enhancement in αE33 by more than 5 times has been achieved upon decreasing t from 50 to 15 μm in both resonance and off-resonance conditions. The enhanced αE33 values indicate that nearly ideal interface coupling between the ferromagnetic and piezoelectric layers is realized in the miniaturized, thinner MEL, pointing to practical application potential towards developing mass-produced, low-cost ME devices.

In order to explain unique magnetoelectric (ME) coupling behaviors found in a trilayer ME laminate having a piezoelectric crystal particularly with anisotropic planar piezoelectric properties, a theoretical model based on the average field method is developed. New analytical expressions could be derived to predict the transverse ME voltage coefficients at off- and in-resonance frequencies respectively. It is predicted that transverse ME voltage coefficients should be anisotropic under in-plane magnetic fields at both off- and in-resonance frequencies. Furthermore, numerical simulations based on material parameters of a representative 2-2 trilayer, composed of Metglas/[0 1 1] Pb(Mg1/3Nb2/3)O3–PbTiO3 crystal/Metglas, prove the emergence of multiple resonance frequencies and characteristic phase difference in the complex ME voltages at each resonant frequency. All these theoretical predictions are in good agreement with the experimental results both at off- and in-resonant frequencies. The theoretical expressions developed here could be broadly applicable to the various types of layered ME laminates with a piezoelectric material with or without anisotropic piezoelectric coefficients.

Solid state electrochemical cells with synaptic functions have important applications in building smart-terminal networks. Here, the essential synaptic functions including potentiation and depression of synaptic weight, transition from short- to long-term plasticity, spike-rate-dependent plasticity, and spike-timing-dependent plasticity behavior were successfully realized in an Ag/MoOx/fluorine-doped tin oxide (FTO) cell with continual resistance switching. The synaptic plasticity underlying these functions was controlled by tuning the excitatory post-synaptic current (EPSC) decay, which is determined by the applied voltage pulse number, width, frequency, and intervals between the pre- and post-spikes. The physical mechanism of the artificial synapse operation is attributed to the interfacial electrochemical reaction processes of the MoOx films with the adsorbed water, where protons generated by water decomposition under an electric field diffused into the MoOx films and intercalated into the lattice, leading to the short- and long-term retention of cell resistance, respectively. These results indicate the possibility of achieving advanced artificial synapses with solid state electrochemical cells and will contribute to the development of smart-terminal networking systems.

Hexaferrites with noncollinear conical magnetic structures are among the most interesting and promising single-phase multiferroics. One puzzle remaining unsolved is why the spin-induced electric polarization can be either reversible or nonreversible by applying magnetic fields. We have unraveled a solution to this puzzle by a systematic study in the multiferroic hexaferrite ${\mathrm{Ba}}_{0.3}{\mathrm{Sr}}_{1.7}{\mathrm{Co}}_{2}{\mathrm{Fe}}_{11}{\mathrm{AlO}}_{22}$ where the electric polarization is reversible at low temperatures but nonreversible at high temperatures. Neutron diffraction results reveal that the rotation of spin cones with applied $ab$-plane magnetic field takes distinct paths at 150 and 305 K: in-plane and out-of-plane rotation, respectively. A theoretical analysis based on the spin current model confirms that the reversal of electric polarization is caused by the in-plane rotation. Our study clarifies the mechanism underlying the reversibility of spin-induced electric polarization in multiferroic hexaferrites.

$\mathrm{F}{\mathrm{e}}_{4}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{9}$ was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at ${T}_{N}\ensuremath{\approx}90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for an antiferromagnetic transition and ${T}_{\mathrm{str}}\ensuremath{\approx}77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below ${T}_{N}$ and uncovered the origin of the second dielectric anomaly as a structural phase transition across ${T}_{\mathrm{str}}$. In the antiferromagnetically ordered state below ${T}_{N}$, both Fe1 and Fe2 magnetic moments lying within the weakly and strongly buckled honeycomb layers are arranged in a fashion that the three nearest neighbors are directed oppositely. Upon cooling below ${T}_{\mathrm{str}}$, the symmetry of crystal structure is lowered from trigonal $P\ensuremath{-}3c1$ to monoclinic $C2/c$, in which a weak sliding of the metal octahedral planes introduces a monoclinic distortion of $\ensuremath{\sim}1.{7}^{\ensuremath{\circ}}$. The magnetic structure is preserved in the low-temperature monoclinic phase, and the Fe magnetic moment increases from $2.1(1){\ensuremath{\mu}}_{B}$ at 95 K to $3.83(4){\ensuremath{\mu}}_{B}$ at 10 K assuming an equal moment configuration at Fe1 and Fe2 sites. The magnetic point group and linear magnetoelectric tensor at each temperature region are determined. From a symmetry-related tensor analysis, the microscopic origins of the magnetoelectric effects between ${T}_{N}$ and ${T}_{\mathrm{str}}$ are proved to be spin-current and $d\ensuremath{-}p$ hybridization mechanisms.

[EN] All the magnetoelectric properties of scheelite-type DyCrO are characterized by temperature- and field-dependent magnetization, specific heat, permittivity, electric polarization, and neutron diffraction measurements. Upon application of a magnetic field within ±3 T, the nonpolar collinear antiferromagnetic structure leads to a large linear magnetoelectric effect with a considerable coupling coefficient. An applied electric field can induce the converse linear magnetoelectric effect, realizing magnetic field control of ferroelectricity and electric field control of magnetism. Furthermore, a higher magnetic field (>3 T) can cause a metamagnetic transition from the initially collinear antiferromagnetic structure to a canted structure, generating a large ferromagnetic magnetization up to 7.0 μ f.u.. Moreover, the new spin structure can break the space inversion symmetry, yielding ferroelectric polarization, which leads to coupling of ferromagnetism and ferroelectricity with a large ferromagnetic component., This work was supported by the National Key R&D Program of China (Grant Nos. 2018YFE0103200, 2018YFA0305700), the National Natural Science Foundation of China (Grant Nos. 11574378, 51772324, 11674384), the Chinese Academy of Sciences (Grant Nos. YZ201555, QYZDB-SSW-SLH013, GJHZ1773), and Spanish MINECO (Grant MAT2013-41099-R). Research conducted at ORNL’s High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy.

It remains a daunting task to quantify the configurational entropy of a material from atom‐revolved electron microscopy images and correlate the results with the material's lattice thermal conductivity, which strides across statics, dynamics, and thermal transport of crystal lattice over orders of magnitudes in length and time. Here, a proof‐of‐principle study of atomic‐scale visualization and quantification of configurational entropy in relation to thermal conductivity in single crystalline trigonal GeSb2Te4 (aka t‐GeSb2Te4) with native atomic site disorder is reported. A concerted effort of large t‐GeSb2Te4 single crystal growth, in‐lab developed analysis procedure of atomic column intensity, the visualization and quantification of configurational entropy including corresponding modulation, and thermal transport measurements enable an entropic “bottom‐up” perspective to the lattice thermal conductivity of t‐GeSb2Te4. It is uncovered that the configurational entropy increases phonon scattering and reduces phonon mean free path as well as promotes anharmonicity, thereby giving rise to low lattice thermal conductivity and promising thermoelectric performance. The current study sheds lights on an atomic scale bottom‐up configurational entropy design in diverse regimes of structural and functional materials research and applications.

An important potential application of solid state electrochemical reactions is in redox-based resistive switching memory devices. Based on the fundamental switching mechanisms, the memory has been classified into two modes, electrochemical metallization memory (ECM) and valence change memory (VCM). In this work, we have investigated a solid state electrochemical cell with a simple Ag/MoO3-x/fluorine-doped tin oxide (FTO) sandwich structure, which shows a normal ECM switching mode after an electroforming process. While in the lower voltage sweep range, the switching behavior changes to VCM-like mode with the opposite switching polarity to the ECM mode. By current-voltage measurements under different ambient atmospheres and X-ray photoemission spectroscopy analysis, electrochemical anodic passivation of the Ag electrode and valence change of molybdenum ions during resistance switching have been demonstrated. The crucial role of moisture adsorption in the switching mode transition has been clarified based on the Pourbaix diagram for the Ag-H2O system for the first time. These results provide a fundamental insight into the resistance switching mechanism model in solid state electrochemical cells.

We report on the temperature-dependence Raman modes of in-plane B 3 g , A g 2 and out-of-plane A g 1 , A g 3 in single crystals of tin selenide that grown by a modified home-made Bridgeman method. The samples are characterized by single crystal X-ray diffraction and high-resolution transmission electron microscopy to confirm their crystal structure. The temperature-dependence of Raman spectroscopy illustrates that A g 1 , B 3 g , A g 2 and A g 3 modes are red-shifted and softened with increasing temperature from 77 K to 400 K. Large temperature coefficients for in-plane Raman modes are observed, say −4.13 × 10−2 cm−1/K and −5.20 × 10−2 cm−1/K for B 3 g and A g 2 modes, respectively. The values for out-of-plane modes are much lower, say −0.91 × 10−2 cm−1/K and −1.38 × 10−2 cm−1/K for A g 1 and A g 3 modes, respectively. The result indicates intrinsic anisotropic Gruneisen parameter β in SnSe, revealing the strong anisotropic lattice anharmonicity. Besides, coefficients of optical-phonon frequency with incident laser-power were (−0.87, −2.77, −2.18 and −1.08) cm−1/mW for A g 1 , B 3 g , A g 2 and A g 3 , respectively for the laser-power dependent Raman spectroscopy experiments. Our work offers a new insight to understand the lattice anharmonicity via Raman spectroscopy and is conductive to search the new thermoelectric materials with intrinsic low lattice thermal conductivity.

The scheelite-type DyCrO4 shows a large linear magnetoelectric effect and field-induced ferromagnetism and ferroelectricity. In this article, we investigate the Bi-doping effects on the crystal structure, magnetism, dielectric constant, and electric polarization for a Dy1−xBixCrO4 (x = 0, 0.05, 0.1) solid solution. The substitution of Bi for Dy up to 10% does not change the structure and antiferromagnetic ordering temperature very much. However, it remarkably suppresses the magnetodielectric effect from 14% in the parent phase to 3.2% in the x = 0.1 sample. More sharply, the maximum field-induced electric polarization is reduced from 320 μC/m2 (x = 0) to 10 μC/m2 (x = 0.1). The results reveal that Dy plays a crucial role in the origin of the large linear magnetoelectric effect of DyCrO4, so chemical doping on this site provides an effective method to manipulate the magnetodielectric and magnetoelectric properties.

Electric field reversal of large magnetization has been achieved in the multiferroic hexaferrite $\mathrm{B}{\mathrm{a}}_{0.4}\mathrm{S}{\mathrm{r}}_{1.6}\mathrm{M}{\mathrm{g}}_{2}\mathrm{F}{\mathrm{e}}_{12}{\mathrm{O}}_{22}$ with conical spin structures. The variation of magnetization depends on the sweeping rate of the applied electric field, leading to a significant fatigue effect. Such a slow dynamical behavior is attributed to the hysteresis magnetization process facilitated by the electric field. Moreover, the $M\text{\ensuremath{-}}H$ hysteresis loop can be manipulated by external electric fields, causing either a positive or negative exchange bias phenomenon. The role of the electric field acting as a bias magnetic field on the magnetic hysteresis loop is discussed in terms of the pinning of magnetic domains by conjugated ferroelectric domains.