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3 D$ Interactions and Spin-Induced Ferroelectricity in the Green Phase

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3 D$ Interactions and Spin-Induced Ferroelectricity in the Green Phase PHYSICAL REVIEW RESEARCH 2, 023271 (2020) Interplay of 4 f -3d interactions and spin-induced ferroelectricity in the green phase Gd2BaCuO5 Premakumar Yanda,1 I. V. Golosovsky,2 I. Mirebeau,3 N. V. Ter-Oganessian,4 Juan Rodríguez-Carvajal,5 and A. Sundaresan 1 1School of Advanced Materials and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O. 560064, India 2National Research Center, Kurchatov Institute, B. P., Konstantinov Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia 3Université Paris-Saclay, CNRS, CEA, Laboratoire Léon Brillouin, 91191 Gif-sur-Yvette, France 4Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia 5Institut Laue-Langevin, 71, Avenue des Martyrs, CS 20156, 38042 Grenoble Cedex 9, France (Received 19 December 2019; revised manuscript received 31 March 2020; accepted 29 April 2020; published 3 June 2020) In most of the spin-induced multiferroics, the ferroelectricity is caused by inversion symmetry breaking by complex spin structures of the transition-metal ions. Here, we report the importance of interplay of 4 f -3d magnetic interactions in inducing ferroelectricity in the centrosymmetric (Pnma) green phase compound 3+ 2+ Gd2BaCuO5. With decreasing temperature, a long-range incommensurate ordering of both Gd and Cu spins at TN = 11.8 K occurs with the modulation vector k = (0, 0, g) and a lock-in transition to a strongly 1 noncollinear structure with kc = (0, 0, /2)atTloc ∼ 6 K. Both spin structures induce electric polarization consistent with the polar magnetic space groups Pm1 (α,0, g)ss and Paca21, respectively. Based on the symmetry analysis of magnetoelectric interactions, we suggest that the ferroelectricity in both commensurate and incommensurate phases is driven by a complex interplay of two-spins and single-spin contributions from magnetic ions located in noncentrosymmetric environments. Our study demonstrates that the green phase family of compounds may serve as a playground for studying the multiferroic phenomena, where the interplay of 4 f -3d interactions demonstrates an alternative route to find magnetoelectric materials. DOI: 10.1103/PhysRevResearch.2.023271 I. INTRODUCTION are the exchange striction, the spin current, or the inverse Dzyaloshinskii-Moriya interaction, and in some cases p-d Magnetoelectric multiferroics, which allow magnetic-field hybridization [3,7,20]. control of electric polarization and electric-field control The green phase compounds R BaCuO , where R = of magnetization, have been the subject of great inter- 2 5 Sm–Lu and Y, having a centrosymmetric (Pnma) crystal est in the field of condensed-matter physics due to their structure exhibit a wide range of magnetism and ground- fundamental physics and applications in spintronics [1–5]. state spin structures due to strong 4 f and 3d interactions Along this line, the spin-induced multiferroics, in which [21–25]. In these compounds, the magnetic interactions be- certain magnetic orders break inversion symmetry and thus + + + + tween Cu2 ions occur through Cu2 –O–R3 –O–Cu2 su- induce electric polarization, attracted much attention be- perexchange path and therefore the magnetic interactions be- cause of strong coupling between electric and magnetic or- + + tween Cu2 :3d and R3 :4 f sublattice moments are important ders [2,3]. For example, the well-known rare-earth man- in understanding their magnetic properties. In general, the ganites RMnO , RMn O ,CaCoMnO ,NiV O ,MnWO, 3 2 5 3 6 3 2 8 4 Cu- and R-sublattice moments in these compounds undergo CoCr O ,Gd. Dy . MnO , delafossites, and the recently 2 4 0 5 0 5 3 long-range antiferromagnetic ordering at different tempera- reported aeschynite family of oxides, RFeWO (R = Eu, Tb, + + 6 tures except Gd BaCuO where the Cu2 and Gd3 moments Dy, and Y), etc. are known to exhibit spin-induced mul- 2 5 order simultaneously around 12 K [25–28]. Previous neutron- tiferroicity [6–14]. While the layered copper oxides were diffraction study on Gd BaCuO reveals that this compound known for the high-temperature superconductivity, several 2 5 exhibits incommensurate magnetic structure with a modula- complex copper oxides such as LiCu O ,LiCuVO,CuO, 2 2 4 tion vector k = (0, 0, g) below 12 K and undergoes lock-in Bi CuO ,GeCuO , and CuFeO have been reported to ex- 2 4 2 4 2 transition at 5 K to a commensurate magnetic structure with hibit multiferroic properties [13,15–19]. In all these materials, k = (001/2) [29]. Considering that the magnetic ions are the microscopic mechanisms responsible for ferroelectricity c located at the local noncentrosymmetric crystallographic sites [30], we thought that these magnetic structures may induce magnetoelectric properties. In this paper, we report the observation of ferroelectricity Published by the American Physical Society under the terms of the in both commensurate and incommensurate spin states of Creative Commons Attribution 4.0 International license. Further the compound Gd2BaCuO5. Reinvestigation of the previous distribution of this work must maintain attribution to the author(s) neutron-diffraction data [29] reveals that the incommensurate and the published article’s title, journal citation, and DOI. ordering corresponds to elliptical cycloidal structure with the 2643-1564/2020/2(2)/023271(7) 023271-1 Published by the American Physical Society PREMAKUMAR YANDA et al. PHYSICAL REVIEW RESEARCH 2, 023271 (2020) polar magnetic point group m1, and the low-temperature commensurate phase is strongly noncollinear with the po- lar magnetic space group Paca21 (point group mm21 ). We suggest that a complex interplay of two spins and single- spin contributions from ions located in noncentrosymmetric environments are responsible for the multiferroicity. II. EXPERIMENT Polycrystalline sample of Gd2BaCuO5 was prepared by heating the stoichiometric mixture of high-purity Gd2O3 (pre- ◦ heated), BaCO3, and CuO at 950 C in the air as described in Ref. [21]. Magnetization measurements were performed by a superconducting quantum interference device magne- tometer (MPMS, Quantum Design). The specific heat (Cp) was measured in the physical property measurement system (PPMS, Quantum Design). To measure the dielectric con- stant and pyrocurrent (electric polarization), we used 0.46- mm-thickness hardened pellet of polycrystalline Gd2BaCuO5 sample covered with an area 25 mm2 of silver paste, while the temperature and magnetic field control were provided by PPMS. The dielectric constant as a function of tempera- ture under different magnetic fields was recorded using the Agilent E4980A LCR meter. The temperature dependence of pyrocurrent was measured with a Keithley 6517A elec- trometer and electric polarization was obtained by integrat- ing the pyrocurrent with respect to time. Low-temperature neutron-diffraction data collected at the G61 diffractometer FIG. 1. (a) Schematics of the crystal structure of Gd2BaCuO5. (λ = 4.76 Å) in the Laboratory Léon Brillouin (Saclay) were (b) Temperature-dependent dc magnetization measured under vari- used to analyze the crystal and magnetic structure refinements ous magnetic fields in field-cooled sequence (left axis) and specific performed by using the FULLPROF program [31]. heat data measured under zero magnetic field. III. RESULTS AND DISCUSSION seen in Fig. 1(b), where a small anomaly at 6 K indicates the Figure 1(a) depicts the crystal structure of Gd2BaCuO5 lock-in transition. viewed along the b axis. The refined powder x-ray-diffraction We observe dielectric anomalies at both the TN and Tloc (XRD) pattern and complete structural details are provided in temperatures under zero magnetic field as shown in Fig. 2(a). Fig. S1 and Table S1, respectively, in Supplemental Material The low-temperature anomaly at Tloc is suppressed gradually [32]. In this structure, the oxygen coordination polyhedra of with applied magnetic field and disappears above 0.7 T. On two Gd3+ sites differ slightly but the local environments differ the other hand, the high-temperature anomaly shows a small significantly. The Gd2 ion is bonded to six nearby copper ions shift to low temperature and becomes broad. A notable mag- through oxygens, five of the six Gd2–O–Cu bond angles being netodielectric effect is observed below the magnetic ordering close to 180◦, whereas Gd1 is bonded to only three copper temperature with the value of ∼0.05% on an average [32]. ions at bond angles close to 90◦. To explore whether the dielectric peaks are associated with The peak of magnetization at 11.8 K at low field (0.01 T) ferroelectricity, we have recorded temperature-dependent py- in Fig. 1(b) confirms the long-range antiferromagnetic or- roelectric current under different magnetic fields which are dering of Cu2+ and Gd3+ moments, which is suppressed shown in Fig. 2(b). Prior to the measurement, we have poled under applied magnetic fields indicating possible change in the sample where a magnetic field was applied parallel to the the magnetic structure. Upon further cooling, we observe a poling electric field. A clear asymmetric peak is seen at the small anomaly at Tloc ∼ 6 K which is consistent with the TN = 11.8 K under zero magnetic field, indicating the emer- lock-in transition [29]. This anomaly shifts to high tem- gence of spontaneous electric polarization. At Tloc, another perature with applied magnetic fields. The Curie-Weiss fit pyrocurrent peak appears in the same direction, indicating the and field-dependent magnetization are presented in Fig. S2, appearance of a ferroelectric state below Tloc with an addi- Supplemental Material [32]. The effective magnetic moment tional polarization. The corresponding polarization is shown obtained from the fit is μeff = 11.56 μB which is close to the in Fig. 2(c), where the appearance of spontaneous polarization theoretical value of 11.36 μB. The negative Curie-Weiss con- at the onset of magnetic ordering and the enhanced polar- stant is θCW =−4 K, indicating that dominant interaction is ization at Tloc demonstrate the type-II multiferroic nature of 2 antiferromagnetic. The behavior of M(H ) data are consistent Gd2BaCuO5.
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