Magnetoelectric effect and phase transitions in CuO in external magnetic fields

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of about 50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.Comment: 26 pages, 5 figure

Magnetoelectric effect and phase transitions in CuO in external magnetic fields

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of E50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions

Resolving spin currents and spin densities generated by charge-spin interconversion in systems with reduced crystal symmetry

The ability to control the generation of spins in arbitrary directions is a long-sought goal in spintronics. Charge to spin interconversion (CSI) phenomena depend strongly on symmetry. Systems with reduced crystal symmetry allow anisotropic CSI with unconventional components, where charge and spin currents and the spin polarization are not mutually perpendicular to each other. Here, we demonstrate experimentally that the CSI in graphene-WTe induces spins with components in all three spatial directions. By performing multi-terminal nonlocal spin precession experiments, with specific magnetic fields orientations, we discuss how to disentangle the CSI from the spin Hall and inverse spin galvanic effects.We acknowledge support of the European Union's Horizon 2020 FET-PROACTIVE project TOCHA under Grant No. 824140 and of the Spanish Research Agency (AEI), Ministry of Science and Innovation, under Contracts No. PID2019-111773RB-I00/AEI/10.13039/501100011033, and SEV-2017-0706 Severo Ochoa. J F S acknowledges support from AEIunder contract RYC2019-028368-I/AEI/10.13039/50110001103, W S T and M V C from the European Union Horizon 2020 research and innovation program, Grant No. 881603 (Graphene Flagship), and I F A of a fellowship from 'la Caixa' Foundation (ID 100010434) with code LCF/BQ/DI18/11660030 and of H2020 Marie Skłodowska-Curie Grant No. 713673. J S acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 754558

Magnetic response of YbMnO3 single crystal

3 pages, 3 figures.The low ac- and dc-field magnetic susceptibilities of hexagonal YbMnO3 single crystal have been measured along the ab plane and along the c axis (Xab,Xc). The antiferromagnetic Néel temperature TN is well signaled by a cusp in the temperature dependence of Xc but no features are visible in Xab. A remarkable anisotropy of paramagnetic susceptibility and the extrapolated Curie–Weiss temperature are observed and related to the quasibidimensional magnetic structure of YbMnO3. Below TN magnetic irreversibility sets, reflecting the presence of a weak ferromagnetic response. Possible scenarios for the origin of the weak ferromagnetism are discussed.This work has been supported by CSIC/BAS Project No. 2005BG0016. Financial support by the MEC of the Spanish Government (Project No. NAN2004-9094-C03, MAT2005- 5656-C04, and Nanoselec CSD 2007-00041), by Bulgarian Science Fund (Project No. BYX-308), and by the European Union (Projects MaCoMuFi: FP6-03321 and FEDER)is also acknowledged.Peer reviewe

Switchable photovoltaic response in hexagonal LuMnO3 single crystals

Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈ 1.5 eV). Using Pt electrodes, h LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent Jsc along the polar axis is modulated (up to 25 %) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization, and by polarization back switching. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc. This observation illustrates the dramatic role of the large optical absorption in hexagonal manganites. The accompanying optical dichroism shall challenge disentangling a genuine bulk photovoltaic response in h-LuMnO3 single crystal contributing to the non-switchable Jsc. Epitaxial thin films may offer a suitable alternative.Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (FUNFUTURE CEX2019-000917-S), MAT2017-85232-R (AEI/FEDER, EU), and PID2019-107727RB-I00 (MINECO/FEDER, EU) projects and from Generalitat de Catalunya (No. 2017 SGR 1377) is acknowledged. I.F. acknowledges RyC Contract No. RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. Y.S is financially supported by China Scholarship Council (CSC) with No. 201806410010. The work of Y.S. has been done as a part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona. A. Schankler and A. M. Rappe are acknowledged for the critical reading of the manuscript.Peer reviewe

Bulk photovoltaic effect modulated by ferroelectric polarization back-switching

Short-circuit photocurrent due to bulk photovoltaic effect displays an oscillatory dependence on the polarization state of light. Here, we explore how the ferroelectric polarization direction in h-LuMnO3 crystals affects the oscillating short-circuit photocurrent. It is shown that after prepoling the crystal at saturation, at remanence, the direction and amplitude of photocurrent oscillations are no longer dictated by prepoling voltage but are largely modulated by polarization back-switching, here ruled by the imprint field. Thus, the light polarization dependence of photocurrent is also ruled by the imprint field. The impact of these effects on the determination of the Glass coefficients of the material is discussed.Financial support from the Spanish Ministry of Science and Innovation (10.13039/501100011033), through the Severo Ochoa FUNFUTURE (No. CEX2019-000917-S); the TED2021-130453B-C21 (AEI/FEDER, EU), PID2020-118479RB-I00 (AEI/FEDER, EU), and PID2019-107727RB-I00 (AEI/FEDER, EU) projects; and from CSIC through the i-LINK (No. LINKA20338) program is acknowledged. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. Y.S. is financially supported by China Scholarship Council (CSC) through No. 201806410010. The work of Y.S. has been done as part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry

International audienceCuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated—both experimentally and theoretically—since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian

Anisotropic Optical Response of WTe2 Single Crystals Studied by Ellipsometric Analysis

In this paper we report the crystal growth conditions and optical anisotropy properties of Tungsten ditelluride (WTe2) single crystals. The chemical vapor transport (CVT) method was used for the synthesis of large WTe2 crystals with high crystallinity and surface quality. These were structurally and morphologically characterized by means of X-ray diffraction, optical profilometry and Raman spectroscopy. Through spectroscopic ellipsometry analysis, based on the Tauc–Lorentz model, we identified a high refractive index value (~4) and distinct tri-axial anisotropic behavior of the optical constants, which opens prospects for surface plasmon activity, revealed by the dielectric function. The anisotropic physical nature of WTe2 shows practical potential for low-loss light modulation at the 2D nanoscale level

Bulk photovoltaic effect in hexagonal LuMnO 3 single crystals

Hexagonal manganites, such as h-LuMnO3, are ferroelectric and have a narrow electronic band gap of ≈ 1.5 eV. Here we report on the photoresponse of h-LuMnO3 single crystals. It is found that the short circuit photocurrent density (Jsc) and the open circuit voltage (Voc) are dependent on the direction of the polarization plane of a linearly polarized impinging light. Its angular dependence indicates the contribution of bulk photovoltaic effect to the short circuit photocurrent. It is also observed that a switchable drift photocurrent, originating from the depoling field of the ferroelectric and thus tunable (<10%) by its polarization direction, also contributes to Jsc. Although its presence precludes accurate determination of the bulk photovoltaic tensor elements and Glass coefficients, some bounds can be established. The Glass coefficients are found to be significantly larger than those obtained in BiFeO3. We argue that the smaller band gap of h-LuMnO3, its distinctive bipyramidal crystal field, and electronic configuration (3d4 vs 3d5), account for the difference and suggest a path towards ferroelectrics of higher photoconversion efficiency.Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), PID2020-118479RBI00 (AEI/FEDER, EU) (AEI/FEDER, EU), and PID2019-107727RB-I00 (AEI/FEDER, EU) projects, and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges RyC Contract RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. The theoretical component of this work (A.M.S. and A.M.R.) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-FG02-07ER46431. Computational support was provided by the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy, Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02- 05CH11231.The experimental and theoretical contributions of Y.S. are financially supported by China Scholarship Council (CSC), respectively with No. 201806410010. The work of Y.S. has been done as a part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona.Peer reviewe

Synthesis and Characterization of Ti-Ta-Shape Memory Surface Alloys Formed by the Electron-Beam Additive Technique

The electron-beam cycling additive technique was proposed for the formation of shape memory Ti-Ta coatings on titanium substrate. On a commercially pure Ti plate, Ta film with a thickness of about 4 &mu;m was deposited by direct current (DC) magnetron sputtering. The sample was then subjected to an electron-beam surface alloying by a scanning electron beam. On the already-formed Ti-Ta surface alloy, a Ta coating with the same thickness was further deposited and the specimen was again subjected to electron-beam alloying for the second cycle. The same procedure was repeated for the third cycle. The structure obtained after each cycle Ti-Ta coatings was studied by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDX). The Young&rsquo;s modulus, hardness, and shape memory effect (SME) were studied by nanoindentation experiments. The results showed that the thickness of the Ti-Ta coatings is about 50 &mu;m in all cases, where the Ta content increases after each technological cycle. It was found that the obtained phase composition is in the form of a double-phase structure of &alpha;&rsquo; martensitic and &beta; phases, where the highest amount of beta is registered in the case of the Ti-Ta coating obtained after the third cycle. The results obtained for the Young&rsquo;s modulus and hardness showed that both mechanical characteristics decrease significantly after each cycle. Additionally, the elastic depth recovery ratio increases with an increase in the number of cycles