Journal of Physics and Chemistry of Solids 72 (2011) 601–603
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Characterization of multiferroic LiTMxCu2 xO2 (TM ¼ Ni and Zn) single crystals Wei-Hsiang Chao a,b, Kuo-Wei Yeh b, Ta-Kun Chen b, Chi-Liang Chen b, Li Zhao b, Chung-Chieh Chang b, Tzu-Wen Huang b, Chung-Ting Ke b, Mau-Kuen Wu b, a b
Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
a r t i c l e in f o
abstract
Available online 14 October 2010
We investigate a series of single crystalline samples of LiTMxCu2 xO2 (TM ¼ Ni and Zn) grown by floatingzone technique. As-grown crystals showed a clear X-ray diffraction pattern of pure(single) phase. These crystals exhibit varied magnetic and electrical properties owing to successful incorporation of Ni and Zn dopants. & 2010 Elsevier Ltd. All rights reserved.
Keywords: B. Crystal growth C. Photoelectron spectroscopy D. Magnetic properties
1. Introduction
2. Experimental procedure
Recent discovery of low-dimensional quantum spin systems in compound LiCu2O2 has drawn considerable attentions because of uncommon magnetic order at low temperature [1–4]. LiCu2O2 is the quasi-one-dimensional (quasi-1D) frustrated spin-1/2 magnet. It contains equal amounts of s¼1/2 magnetic Cu2 + and nonmagnetic Cu + ions. LiCu2O2 belongs to an orthorhombic crystal structure of the space group Pnma (62) with lattice ˚ b¼2.86 A, ˚ and c¼12.41 A˚ [5]. The Cu2 + parameter a ¼5.72 A, locates at the center of the CuO4 squares in ab-plane, and the edge-sharing CuO4 squares form the magnetic backbone running along the b axis, which is well separated by double chains of Li + ions and layers of Cu + ions. The bond angle of edge-sharing CuO2 chains(Cu–O–Cu) is about 901. From the angle change of Cu–O–Cu bond in the CuO4 squares, the magnetic interactions can be described which is known to be driven by the quasi-1D chains of spin-1/2 Cu2 + ions with nearest-neighbor (J1) and next-nearestneighbor (J2) interactions. As the temperature is reduced continuously for LiCu2O2, the spiral magnetic order presents a complex rearrangement of spin structure which is associated with competition of ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions between neighboring Cu2 + ions [1,2,6]. Floating-zone (FZ) technique provides a possibility to grow single crystals in near equilibrium condition with a well-controlled solid–liquid interface. In this paper, we used an infrared radiation furnace with two 1.5 kW halogen lamps (model 15HD, NEC Nichiden Machinary) to carry out LiTMxCu2 xO2 (TM ¼Ni and Zn) crystal growth. The nominal composition of x is immovable, where x equaled to 0.1. The influence of the dopants on magnetic ordering temperatures is discussed.
Single crystals of LiCu2O2 and LiTMxCu2 xO2 (TM ¼ Ni and Zn) were grown by FZ technique under Ar:O2 ¼4:1 gas mixture at atmospheric pressure. Powders of Li2CO3, CuO, NiO, and ZnO were weighted in selected ratio (x¼0.1) and thoroughly mixed. The mixture was calcined at 800 1C in air and maintained for 20 h. The reacted bulk was reground into fine powder, and then pressed into a rubble tube under a hydrostatic pressure of 350 kg/cm2 and subsequently sintered at 820 1C for 20 h. Powder X-ray diffraction (XRD) measurements were carried out with X-ray diffractometer (X’Pert Powder PW3040/60) using CuKa radiation source. Structure parameters of single crystals were obtained by Rietveld refinement from the powder XRD data. The crystal quality was further characterized by a three-circle diffractometer (X’Pert PRO MRD PW3040/60) on the (1 0 1) phi-scan of Bragg reflection. The inductively coupled plasma atomic emission spectroscopy analyzer (ICP-AES) was used to determine the chemical compositions of the crystals. The DC magnetic susceptibility w was measured by superconducting quantum interference device magnetometer (SQUID-VSM). The O(1s), Cu(2p), and C(1s) core-level spectra were collected by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) using micro-focused AlKa radiation at 901.
Corresponding author.
E-mail address:
[email protected] (M.-K. Wu). 0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.10.042
3. Results and discussion Large and highly crystalline single crystals of LiTMxCu2 xO2 (TM ¼ Ni and Zn) have been obtained using FZ method by carefully optimizing their growth conditions. As shown in left inset of Fig. 1(a), crystals with dimensions typically 5 mm 5 mm can be easily cleaved from the crystallized boule of LiCu2O2. The main panel of Fig. 1(a) displays a typical (1 0 1) phi-scan of LiCu2O2 crystal, clearly indicating the untwined characteristic of our
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W.-H. Chao et al. / Journal of Physics and Chemistry of Solids 72 (2011) 601–603
ions, such as Zn2 + (LiZn0.07Cu1.93O2) ions. The lattice expansion or shrinkage depends on the ionic radii clearly indicating a successful incorporation of impurity ions in these crystals. Fig. 2 shows the XPS core-level spectra of Cu(2p) and O(1s) for LiCu2O2, LiNi0.08Cu1.92O2, and LiZn0.07Cu1.93O2 crystals. Clearly, the Cu and O peaks of LiZn0.07Cu1.93O2 crystal shift slightly to lower binding energy, while the spectra of Ni-doped crystal positioned at the same energy as that of LiCu2O2. As shown in Fig. 2(a), additional tiny peaks form at the binding energy of 939–945 and 959–964 eV after doping of Ni and Zn, respectively. The O(1s) XPS spectrum was shown in Fig. 2(b). There were shown two main peaks situated at binding energy of 529.1/531.2 and 528.9/531.0 eV for LiNi0.08Cu1.92O2 and LiZn0.07Cu1.93O2, respectively. The lower binding-energy peak was consistent with the spectral feature of LiCu2O2, attributing to the normal lattice oxygen, and the peak with higher binding energy was most likely to be the oxygen atoms in the hydroxyl groups, trapped nitrate ions, or adsorbed carbonates, which was not presented within the crystals. Nevertheless, the incorporation of foreign atoms seems insensitive to the chemical potentials of species in LiCu2O2. Fig. 3 reveals the temperature derivative of magnetic susceptibility dwðTÞ=dT of LiTMxCu2 xO2 (TM ¼Ni and Zn) as a function of temperature under the condition of zero-field cooling (ZFC). As seen in this plot, the temperature derivative of magnetic
Fig. 1. (a) A typical (1 0 1) phi-scan of LiCu2O2 crystal. Undoped LiCu2O2 crystal with (0 0 l) face of several mm2 areas is shown in inset. (b) (1 0 1) phi-scan of LiZn0.07Cu1.93O2 crystal which includes two different domains (A+ B). The right inset shows crystal flakes of LiZn0.07Cu1.93O2 (left) and LiNi0.08Cu1.92O2 (right). (c) Lattice variations of LiTMxCu2 xO2 (nominal x ¼0.1) crystals.
undoped sample. The full-width half-maximum (FWHM) of the X-ray rocking curves of (1 0 1) Bragg-reflections is only 0.041, indicating that high in-plane and out-of-plane crystallinity of undoped crystals. The crystal size and the quality are degraded for LiNi0.08Cu1.92O2 and LiZn0.07Cu1.93O2 crystal, as shown in inset of Fig. 1(b). The combination of two domains (A +B) is clearly seen in Fig. 1(b) indicating that the doped LiZn0.07Cu1.93O2 crystals are slightly twined. The minor domain is perpendicular to the main domain and the existence is less than 20% contribution. Apparently, the crystal quality of LiCu2O2 is degraded while doped with transition metal impurities; therefore, further lowering the growth rate is required to avoid the constitutional supercooling and enlarge the grain size. Cell parameters are estimated by Rietveld refinement from experiments of powder X-ray diffraction. The fitting is satisfactory with low Rwp values from 1.95% to 2.51% and those lattice constants of undoped crystal are found consistent with previous results [7]. The dependence of a, b and c cell parameters with the 3d element number is shown in Fig. 1(c). The ionic radii of Ni2 + , Cu2 + , and Zn2 + ˚ with six coordination are known equaling to 0.69, 0.73, and 0.74 A, respectively. Apparently, lattices a, b shrink and c expands as doped with smaller Ni2 + ions (LiNi0.08Cu1.92O2); on the contrary, the lattice expands in a and b but contracts in c while doped with larger
Fig. 2. XPS spectra of (a) Cu-2p and (b) O-1s for LiTMxCu2 xO2 crystals (TM ¼ Ni and Zn). Line positions are obtained by performing peak deconvolution.
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4. Conclusion We have demonstrated the successful growth of LiTMxCu2 xO2 (TM ¼ Ni and Zn) crystals utilizing floating-zone technique. Because of the TM/Cu mutual substitutions, the structure deviations correlate well with the varied temperatures of antiferromagnetic to helimagnetic transition in different doped samples. Detailed measurements on valence states which are expected to provide more information about the crystals of LiTMxCu2 xO2 (TM ¼Ni and Zn) are currently under investigation.
Acknowledgment We gratefully acknowledge the support from National Science Council (NSC) of the Republic of China Grant no. NSC98-2119-M001-025. Fig. 3. Temperature dependence of dwðTÞ=dT for LiTMxCu2 xO2 (TM ¼Ni and Zn) with the magnetic field applied perpendicular to the a b plane. The transition temperatures of different crystals are indicated by arrows.
References susceptibility ðdwðTÞ=dTÞ indicates that two anomalies at TN1 25 K and TN2 ¼23.2 K for LiCu2O2 crystal, in agreement with previous report [2]. Nonetheless, only one and smaller magnitude anomaly exists for LiNi0.08Cu1.92O2 (TN2 ¼17 K) and LiZn0.07Cu1.93O2 (TN2 ¼19 K). Although the dopings with Ni and Zn exerts a contrary influence on structure, the lowering of the magnetic ordering temperature in a similar manner indicates that dopant ions replace spin-1/2 Cu2 + within quasi-1D chains and they turn out to weaken the exchange interactions between neighboring Cu2 + ions. This dependence of the temperature while the magnetic order sets in with the doped ions is clearly demonstrated relating to the influence of excess Li in LixCu2O2 ðx 41Þ solid solutions [8].
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