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Single Crystal Growth of Multiferroic Double Perovskites: Yb2comno6 and Lu2comno6

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Single Crystal Growth of Multiferroic Double Perovskites: Yb2comno6 and Lu2comno6 crystals Article Single Crystal Growth of Multiferroic Double Perovskites: Yb2CoMnO6 and Lu2CoMnO6 Hwan Young Choi, Jae Young Moon, Jong Hyuk Kim, Young Jai Choi * and Nara Lee * Department of Physics and IPAP, Yonsei University, Seoul 120-749, Korea; [email protected] (H.Y.C.); [email protected] (J.Y.M.); [email protected] (J.H.K.) * Correspondence: [email protected] (Y.J.C.); [email protected] (N.L.); Tel.: +82-2-2123-5613 (Y.J.C. & N.L.) Academic Editor: Iwan Kityk Received: 29 January 2017; Accepted: 24 February 2017; Published: 27 February 2017 Abstract: We report on the growth of multiferroic Yb2CoMnO6 and Lu2CoMnO6 single crystals which were synthesized by the flux method with Bi2O3. Yb2CoMnO6 and Lu2CoMnO6 crystallize in a double-perovskite structure with a monoclinic P21/n space group. Bulk magnetization measurements of both specimens revealed strong magnetic anisotropy and metamagnetic transitions. We observed a dielectric anomaly perpendicular to the c axis. The strongly coupled magnetic and dielectric states resulted in the variation of both the dielectric constant and the magnetization by applying magnetic fields, offering an efficient approach to accomplish intrinsically coupled functionality in multiferroics. Keywords: double perovskite; multiferroic; single crystal growth; flux method 1. Introduction A multiferroic is a material that simultaneously exhibits ferroelectricity and magnetism [1–4]. Strong interplay between the electric and magnetic order parameters in multiferroics provides opportunities for novel device applications, such as magnetoelectric data storage and sensors [5–10]. In particular, in type-II multiferroics, ferroelectricity originates from the lattice relaxation via exchange strictions in a particular spin order with broken spatial inversion symmetry, which naturally leads to strong controllability of the dielectric properties via external magnetic fields [11–13]. Recently, a new type-II multiferroic was discovered: a double-perovskite structure of Lu2CoMnO6 [14–17]. From the polycrystalline work, the ferroelectricity was predicted to be along the crystallographic c axis originating from the symmetric exchange striction of the up-up-down-down (""##) spin arrangement with alternating charge valences [15,17,18]. However, previous studies on single crystals revealed ferroelectricity perpendicular to the c axis, which can be explained by the net polarization induced by the uniform oxygen displacements perpendicular to the c axis on neighboring ""## spin chains when the symmetric exchange striction is activated [16–18]. Lu2CoMnO6 belongs to the double-perovskite RE2CoMnO6 series (RE = La, ... , Lu). These materials crystallize in a monoclinic 2+ 4+ perovskite structure (space group P21/n) with alternating Co and Mn ions in a corner-shared oxygen octahedra [19]. In these compounds, additional antiferromagnetic clusters can arise from another valence state of Co3+-Mn3+ and antisites of ionic disorders and/or antiphase boundaries leading to Co2+-Co2+ or Mn4+-Mn4+ pairs [20]. As the size of rare earth ions decreases, the magnetic transition temperature decreases from 204 K for La2CoMnO6 [21] to 48 K for Lu2CoMnO6 [22]. We successfully grew single crystals of multiferroic Yb2CoMnO6 and Lu2CoMnO6 using the flux method with Bi2O3 flux. X-ray diffraction (XRD) confirmed the double-perovskite structure with a monoclinic P21/n space group. The up-up-down-down (""##) spin order arose below Tc = 52 and 48 K, respectively, leading to a dielectric anomaly perpendicular to the c axis because of cooperative displacements of oxygen ions. Crystals 2017, 7, 67; doi:10.3390/cryst7030067 www.mdpi.com/journal/crystals Crystals 2017, 7, 67 2 of 8 48Crystals K, respectively,2017, 7, 67 leading to a dielectric anomaly perpendicular to the c axis because of cooperative2 of 8 displacements of oxygen ions. 2.2. Experimental Experimental Methods Methods WeWe synthesized single crystals of Yb2CoMnO6 (YCMO)(YCMO) and and Lu Lu22CoMnOCoMnO66 (LCMO)(LCMO) by by utilizing utilizing a a conventionalconventional flux flux method method with with Bi 2OO33 fluxflux in in air air [22]. [22]. Before Before the the growth, growth, the the polycrystalline polycrystalline specimens specimens werewere prepared prepared by the solid-state reactionreaction method.method. HighHigh puritypurity powderspowders ofof YbYb22OO33 (Lu(Lu22O3),), MnO MnO22 andand ◦ CoCo33OO4 4werewere mixed mixed and and ground ground in in a a mortar, mortar, followed followed by by calcining calcining at at 1000 1000 °CC for for 12 12 h h in in a a box box furnace. furnace. ◦ TheThe calcined calcined compound compound was was finely finely re-ground re-ground and and si sinteredntered at at 1100 1100 °CC for for 24 24 h. The The same same sintering sintering ◦ procedureprocedure after after regrinding regrinding was was done done at at 1200 1200 °CC fo forr 48 48 h. h. A A mixture mixture of of pre-sintered pre-sintered YCMO YCMO (LCMO) (LCMO) ◦ polycrystallinepolycrystalline powder powder and Bi 2OO33 fluxflux with with a a ratio ratio of of 1:12 1:12 ratio ratio was was heated heated to to 1300 1300 °CC in in a a Pt Pt crucible, crucible, cooledcooled slowly toto 985985◦ °C,C, and and then then cooled cooled in in a furnacea furnace after after the powerthe power was was turned turned off. The off. grown The grown single singlecrystals crystals are shown are shown in Figure in Figure1. 1. FigureFigure 1. ImagesImages of of representative single crystalscrystals ofof YbYb22CoMnO6 (YCMO)(YCMO) ( a,,b)) and LuLu22CoMnO66 (LCMO)(LCMO) ( (cc,,dd)) viewed viewed from from the the cc axisaxis and and from from the the direction direction perpendicular perpendicular to to the the cc axis.axis. The The length length of of eacheach side side for for the the squares squares of of grid grid pattern pattern is is 2 2 mm. mm. TheThe crystallographiccrystallographic structures structures of bothof both crystals cr wereystals confirmed were confirmed by a powder by X-raya powder diffractometer X-ray diffractometer(Ultima IV, Rigaku, (Ultima Tokyo, IV, Rigaku, Japan) Tokyo, using Japan) Cu-K using radiation Cu-K at radiation room temperature. at room temperature. The detailed The detailedcharacterization characterization for the structuresfor the structures was performed was performed by Rietveld by Rietveld refinement refi bynement applying by applying the FullProf the FullProfsoftware software to the measured to the data.measured The temperaturedata. The temperature and magnetic-field and magnetic-field dependence ofdependence magnetization of magnetizationfor the single crystalsfor the weresingle measured crystals were at temperatures measured at of temperaturesT = 2–300 K underof T = applied2–300 K magneticunder applied fields ⊥ magneticup to 9 T fields along up (H //to c9) T and along perpendicular (H//c) and perpendicular (H?c) to the c(Haxisc) usingto thea c VSMaxis using technique a VSM in technique a Physical inProperties a Physical Measurement Properties Measur Systemement (PPMS, System Quantum (PPMS, Design, Quantum San Diego, Design, CA, San USA). Diego, The temperatureCA, USA). The and temperaturemagnetic-field and dependence magnetic-field of the dependence dielectric constant of the di andelectric the tangentialconstant and loss the under tangential various loss magnetic under variousfields were magnetic measured fields using were a measured LCR meter using (E4980, a LCR Agilent, meter Santa (E4980, Clara, Agilent, CA, USA).Santa Clara, CA, USA). 3.3. Results Results FigureFigure 2 shows the XRD pattern for ground single crystals of YCMO and LCMO. Resulting from thethe Rietveld Rietveld refinement refinement [23], [23], the the crystal crystal structure structure of of YCMO YCMO (LCMO) (LCMO) was was refined refined as as a a monoclinic monoclinic 1 2 double-perovskitedouble-perovskite structurestructure ( P21(/nP2space/n space group) withgroup) good with agreement good factors,agreementχ = 3.18factors, (4.89), 2 = χRp = 3.18 7.78 (4.89), (9.04)%, RpR =wp 7.78= 6.62 (9.04)%, (8.23)%, Rwp and= 6.62Rexp (8.23)%,= 3.71 (3.72)%.and Rexp Figure= 3.71 3(3.72)%. shows theFigure crystallographic 3 shows the crystallographicstructure of YCMO structure viewed of from YCMO the viewedc and a axes,from respectively.the c and a axes, Co2+ respectively.and Mn4+ ions Co2+ were and alternatelyMn4+ ions werelocated alternately in corner-shared located in octahedral corner-shared environments. octahedral environments. Crystals 2017, 7, 67 3 of 8 Crystals 2017, 7, 67 3 of 8 Crystals 2017, 7, 67 3 of 8 (a) (a) Yb2CoMnO6 Yb2CoMnO6 (arb. units) (arb. units) Intensity Intensity 20 40 60 80 100 20 40 60 80 100 2θ (deg.) 2θ (deg.) (b) (b) Lu2CoMnO6 Lu2CoMnO6 (arb. units) (arb. units) Intensity Intensity 20 40 60 80 100 120 2θ (deg.) 2θ (deg.) Figure 2. Observed (open circles) and calculated (solid line) powder XRD patterns for YCMO (a) and Figure 2. Observed (open (open circles) circles) and and calculated (solid (solid line) line) powder powder XRD XRD patterns patterns for for YCMO YCMO ( aa)) and LCMO (b). The single phase of the monoclinic perovskite structure (space group P21/n) was identified. LCMO ( (bb).). The The single single phase phase of of the the monoclinic monoclinic perovskite perovskite structure structure (space (space group group P21P/n2)1 /nwas) was identified. identified. Figure 3. Views of the crystallographic structure of YCMO from the c axis (a) and the a axis (b). Green, Figure 3. 3. ViewsViews of of the the crystallographic crystallographic structure structure of YCMO
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