ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 272–276 (2004) 933–934 Magnetic exchange in a low-dimensional complex oxide ðCu; ZnÞ2V2O7 V. Kataeva,b,*, J. Pommerc, K.-Y. Choic, P. Lemmensd, A. Ionescuc, c . f, A. Freimutha, G. Guntherodt Yu. Pashkeviche, K. Lamonovae, A. Moller . II. Physikalisches Institut, Universitat . zu Koln, . Zuelpicherstr. 77, Koln . 50937, Germany Kazan Physical Technical Institute, Russian Academy of Sciences, Kazan 420111, Russia c II. Physikalisches Institut, RWTH Aachen, Aachen 52056, Germany d Max-Planck-Institut fur Stuttgart 70569, Germany . Festkorperforschung, . e Donetsk Phystech NASU, Donetsk 83114, Ukraine f Institut fur . Anorganische Chemie, Universitat . zu Koln, . Koln . 50939, Germany a b Abstract Copper-divanadate Cu2y Zny V2 O7 comprises trans-edge sharing chains of CuO5 polyhedra where Cu spins S ¼ 12 are antiferromagnetically coupled. Zn substitution for Cu induces a structural phase transition which affects strongly the magnetic properties. In particular, a substantial magnetic anisotropy found in one of the structural phases is strongly suppressed in the other one, owing to the changes of the local bonding geometry. r 2003 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 75.30.Et; 76.30.Fc Keywords: Low-dimensional spin system; ESR; Magnetic anisotropy Copper oxides with low dimensional structural elements are considered as the best experimental realizations of isotropic Heisenberg magnets owing to the quenching of the orbital momentum of Cu2þ : However, the remaining spin–orbit coupling may yield a surprisingly strong magnetic anisotropy for certain bonding geometries. In copper-divanadate, Cu2y Zny V2 O7 ; CuO5 polyhedra form trans-edge sharing chains . We ﬁnd that Cu2y Zny V2 O7 crystallizes in the orthorhombic a-phase (space group Fdd2) for yo0:15 and in the monoclinic bphase ðC2=cÞ for yX0:15: Magnetic susceptibility wðTÞ of a-Cu2 V2 O7 follows closely the Curie–Weiss (CW) law wCW ¼ Cmol =ðT YÞ with Y ¼ 78 K; indicating an effective antiferromagnetic (AF) exchange JB100 K *Corresponding author. Leibniz Institute for Solid State and Materials Research Dresden, P.O. Box 270116, D-01171 Dresden, Germany. Tel.: +49-351-4659-328; fax: +49-3514659-313. E-mail address: [email protected] (V. Kataev). between Cu spins S ¼ 12 (Fig. 1). We observe the AF order at TN ¼ 34 K with a small spontaneous magnetization Ms C0:04 mB ; consistently with results of Ref. . The occurrence of Ms is responsible for a sharp drop of 1=wðTÞ at TN (Fig. 1). Zn doping results in a smooth decrease of Y and TN (lower inset in Fig. 1), owing to an increase of the mean distance between Cu spins, whereas Ms remains constant. Strong changes occur at y ¼ 0:15: wðTÞ follows the CW law only at high T with an appreciably larger value of Y; whereas at lower T the susceptibility shows a dome-shaped feature characteristic of a 1D-Heisenberg antiferromagnet  (Fig. 1). The onset of 3D-AF order can be identiﬁed from the jump of the derivative dwðTÞ=dT: No spontaneous magnetization at ToTN is observed. wðTÞ for yX0:15 can be ﬁtted as a sum of the Bonner–Fisher (BF) susceptibility of a 1D-AF chain , the Van-Vleck contribution of Cu2þ and the CW susceptibility of quasifree ‘‘defect’’ spins responsible for a small Curie-like tail of wðTÞ at low T: The increase of y above 0.15 results in a further decrease of Y and TN ; whereas the intra-chain 0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.527 ARTICLE IN PRESS 25 10 1/χ (0.01 mole Cu/emu) 20 Zn (y) 0 6 0.2 0.4 0.6 5 0 H=1T 2.0 2.16 2.12 1.6 2.08 0 1.2 2.1 H||orient. axis H⊥orient. axis g-factor 15 2.2 g|| = 2.23 g⊥ = 2.09 2.20 ESR linewidth (kOe) 40 Ndefect (%) 60 20 J (K) 8 2.24 2.4 80 g-factor V. Kataev et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 933–934 934 Cu2-yZnyV2O7 2.0 y = 0.2 y = 0.4 1.9 30 60 90 120 0 θ (deg) 100 200 T (K) 300 250 300 Cu2V2O7 Cu1.85Zn0.15V2O7 4 0.2 0.4 T (K) 2 60 0.6 0 Zn (y) 80 50 100 150 T (K) 200 40 TN 0 0 50 Fig. 2. ESR parameters of Cu2y Zny V2 O7 : -Θ 20 0 0.8 100 150 200 T (K) 250 300 Fig. 1. Static susceptibility of Cu2y Zny V2 O7 : AF coupling JB70 K deduced from the BF ﬁt remains almost constant (upper inset in Fig. 1). Remarkably, the number of quasi-free spins Ndefect increases almost linear with y suggesting that the removal of one spin from the AF correlated background creates an uncorrelated spin localized around the Zn-vacancy. The occurrence of Ms in the AF-ordered state of acopper-divanadate (yo0:15) indicates the canting of spins due to the Dzyaloshinsky–Moriya (DM) interaction , which strength amounts to B7% of J: A close inspection of the bonding geometry in the Cu–O chain reveals that indeed the DM interaction is allowed in the a-phase. However, owing to the changes in the local structure, it is expected to vanish in the b-phase. An elaborated symmetry analysis of the possible spin structures conﬁrms this scenario (see Ref. ). Measurements of electron spin resonance (ESR) at n ¼ 9:47 GHz corroborate these ﬁndings. At ToTN a strong resonance occurs in the a-phase at Ho150 Oe: This mode in the low-symmetry lattice is expected only if the DM interaction is active . No bulk signal is observed above TN : Indeed, a conventional estimate of the ESR linewidth suggests that the DM exchange should broaden the signal in the paramagnetic a-phase very strongly. In contrast, in the b-phase no resonance mode is found in the AF-ordered state, indicating the absence of the DM interaction. A strong signal is observed above TN : The anisotropy of the g-factor of the powder sample oriented in a strong magnetic ﬁeld (left inset in Fig. 2) is typical for Cu2þ in the x2 y2 orbital ground state, which is predicted for copperdivanadate by the angular overlap model. Broadening of the signal and shift of the effective g-factor below 30 K (Fig. 2) are the signatures of the short-range AF order . At high T the ESR linewidth can be explained by an intra-chain magnetic anisotropy  which does not require the occurrence of the DM interaction. We acknowledge support by the Deutsche Forschungsgemeinschaft through SFB 608 and SPP 1073 and by NATO Grant PST.CLG.977766 and INTAS Grant 01-0278. A.F. acknowledges support by the VolkswagenStiftung. V.K. acknowledges support of the RAS through Project. No. OFN03/032061/020703-996. References  D. Mercurio-Lavaud, B. Rit, Acta. Crystallogr. B 29 (1973) 2731; D. Mercurio-Lavaud, B. Rit, Acad. Sci. Paris, Ser. C 277 (1973) 1101.  L.A. Ponomarenko, et al., Physica B 284–288 (2000) 1459.  J.C. Bonner, M.E. Fisher, Phys. Rev. 135 (1964) A640.  I. Dzyaloshinsky, Phys. Chem. Solids 4 (1958) 241; T. Moriya, Phys. Rev. 120 (1960) 91.  J. Pommer, et al., Phys. Rev. B 67 (2003) 214410.  E.A. Turov, Physical Properties of Magnetically Ordered Crystals, Academic Press, New York, 1965.  V. Kataev, et al., Phys. Rev. Lett. 86 (2001) 2882.
© Copyright 2021 ExpyDoc