- Email: [email protected]
Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals
Current Applied Physics xxx (2017) 1e6
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals Jiyue Song a, b, Bangchuan Zhao a, *, Yanan Huang a, Yanfeng Qin a, b, Wenhai Song a, Yuping Sun a, c, d a
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China University of Science and Technology of China, Hefei 230026, People's Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People's Republic of China d Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People's Republic of China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2016 Received in revised form 29 December 2016 Accepted 15 February 2017 Available online xxx
The effect of Cu doping on the structural, magnetic, electrical and thermal transport properties of Ca3Co2xCuxO6 single crystals has been investigated systematically. Based on the analysis of the structural parameter and x-ray photoelectron spectroscopy spectra, the valence state of Cu is considered to be þ2. All samples undergo a long-range spin density wave (SDW) transition at TN and a glass-like magnetic transition at Tf with decreasing temperature. Both TN and Tf decrease monotonously with increasing Cudoping content. A series of magnetization steps can be observed in the M(H) curve of Ca3Co2O6, and the magnetization steps are sensitive to the sample cooling magnetic field and Cu doping content x. With the increase of x, the resistivity along c-axis decreases while the thermopower increases. As a result, the figure of merit (ZT) increases considerably and the room-temperature ZT value of the sample with x ¼ 0.6 is nearly 60 times larger than that of the un-doped crystal. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ca3Co2-xCuxO6 single crystal Magnetization steps The figure of merit
1. Introduction Coexistence of low dimensionality and geometric frustration can lead to exotic physical properties in a single compound. Among the family of geometrically frustrated materials, the quasi-onedimensional (Q1D) cobalt-based oxide Ca3Co2O6 has been attracted more attention in recent years due to its unusual physical properties, such as partially disordered antiferromagnetic (PDA) state [1], quantum tunneling effects of the magnetism [2], anisotropic magnetodielectric coupling [3], magnetic memory effect [4], high-temperature thermoelectric properties [5], and so on. The structure of Ca3Co2O6 consists of infinite CoO6 chains made up of alternating face-sharing octahedron (CoI site) and trigonal prism (CoII site) running along c-axis. Each chain is surrounded by six equally spaced chains forming a triangular lattice in ab plane. The chains are separated by Ca ions. According to the
* Corresponding author. E-mail addresses: [email protected] (J. Song), [email protected] (B. Zhao), [email protected] (Y. Huang), [email protected] (Y. Qin), [email protected] (W. Song), [email protected] (Y. Sun).
previous studies, the valence states of cobalt ions at both the octahedral and the trigonal prism sites are trivalent [6], but their spin states are different due to the different crystalline electric fields. The Co ions at CoI sites are in the low-spin (LS, S ¼ 0) state, while the Co ions at CoII sites are in high-spin (HS, S ¼ 2) state [7]. The magnetic interaction of Ca3Co2O6 is suggested to be ferromagnetic (FM) along c-axis and the nearest- and next-nearestneighbor chains are coupled through a weaker antiferromagnetic (AFM) interaction [8]. At low temperatures, a broad magnetization plateau can be clearly seen on the magnetic field dependent magnetization curve M(H) at about 1/3 of the saturation magnetization implying the appearance of the ferrimagnetic (FIM) state [9]. Additional weak magnetization steps can also be observed at regular field intervals (~1.2 T) on the M(H) curve. These weak magnetization steps depend on not only the temperature but also the sweep rate of the applied magnetic field, which can be considered as a result of the magnetic relaxation mechanism [10]. Besides the interesting magnetic properties, the high-temperature thermoelectric performance can be enhanced through Bi, Cu co-doping [11,12]. In addition, a significant decrease in resistivity due to the enhancement of grain size and
http://dx.doi.org/10.1016/j.cap.2017.02.008 1567-1739/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
2
J. Song et al. / Current Applied Physics xxx (2017) 1e6
the decrease of boundary scattering in Cu doped Ca3Co2O6 polycrystalline sample have been reported [13]. In order to further understand the mechanism of the magnetic correlation and improve the thermoelectric performance of Ca3Co2O6 system, we synthesized the Ca3Co2-xCuxO6 single crystals, and investigated the magnetic, electrical and thermal transport properties of Cu doped Ca3Co2O6 single crystals. 2. Experimental details Single crystals of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) were grown by flux method using K2CO3 as flux [14]. A mixture of CaCO3 (99%; Sinopharm Chemical Reagent Co., Ltd), Co3O4 (99.7%; Alfa Aesar), and CuO (99.9%; Alfa Aesar) powders were grinded and heated in air at 1173 K for 24 h. Single crystals were grown by heating a mixture of obtained pre-heated powder and K2CO3 (99%; Sinopharm Chemical Reagent Co., Ltd), in a weight ratio 1:7, up to 1153 K for 48 h in an alumina crucible in air and then cooling down to room temperature at 3 K/h. Needlelike single crystals with typical size of several millimeters along c-axis were obtained. The structure and phase purity of the samples were checked by a Philips X'pert PRO x-ray diffractometer using Cu Ka radiation at room temperature. The morphology and chemical composition of the studied samples were analyzed by a scanning electron microscope (SEM) and an energy dispersive spectroscope (EDS) attached to the SEM equipment (Oxford Instruments). The photoelectron spectra were measured in ESCALAB 250Xi spectrometer (Thermo) using Mg Ka X-rays at 1253.6eV as the excitation source. The electrical and thermal transport properties were measured in a Physical Property Measurement System (Quantum Design) from 5 to 350 K. The dc and ac magnetic measurements were carried out with a superconducting quantum interference device (SQUID) MPMS system (Quantum Design). The measurements of ac susceptibility were performed under a constant excitation field of Hac ¼ 3Oe, and frequencies range from 1 to 1000 Hz. In the measurement process of magnetic and transport properties, the magnetic field was all applied along the c-axis of the crystals.
3. Results and discussion 3.1. Structures The room temperature powder XRD patterns of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) samples are shown in Fig. 1(a). Only sharp (110), (220), (330) peaks are observed, demonstrating the high quality of the crystals. Moreover, all (ll0) peaks are double peaks, which can be clearly seen in the enlarged (330) peaks as shown in Fig. 1(b). The double peaks in the XRD patterns arise from the contributions of Cu Ka1 (1.5406 Å) and Ka2 (1.5444 Å) radiation with the different wavelengths [15]. With the increase of Cu doping content (x 0.6), the XRD diffraction peaks move to a lower angle position, indicating the lattice has an expanded trend. As shown in the inset of Fig. 1(a), the lattice parameter a increases from 9.0515 Å for the x ¼ 0.0 sample to 9.0694 Å for the sample with x ¼ 0.6. For the samples with x 0.8, both the strength and position of the diffraction peaks are almost the same with that of the x ¼ 0.6 sample. The result shows that x ¼ 0.6 may be the solubility threshold of Cu ions in Ca3Co2O6 crystals obtained by flux growth method. The result can be further proved by the EDS analysis result as shown in Table 1, where the actual contents of Co and Cu in the samples are given. From the table we can see that the actual Cu content in the grown crystals is much smaller than the nominal one and the actual composition of the x ¼ 0.8 crystal is almost the same with that of x ¼ 0.6 sample. To simplify, we still use the nominal content in the following Cu-doping effect discussion and only focus on the samples with x 0.6. The standard ionic radii [16] of Cu2þ and Cu3þ are 0.73 Å and 0.54 Å, respectively. However, both the HS (0.61 Å) and LS (0.545 Å) Co3þ ions in Ca3Co2O6 have larger ionic radii than 0.54 Å. Based on the expanded trend of the lattice with increasing x, we suggest that the doped Cu ions at Co-site should be in the form of Cu2þ. The suggestion of the oxidation state can be confirmed by the XPS measurement on the Ca3Co1.6Cu0.4O6 sample as shown in Fig. 1(c) and (d). In Fig. 1(c), the XPS peak located at ~780.7eV is attributed to Co 2p3/2 and it can be deconvoluted into two peaks. The two peaks corresponding to the higher binding energy (BE) and lower BE are
Fig. 1. (a) XRD patterns of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0). Inset shows the lattice parameter a as a function of x. (b) the enlarged (330) peaks for the samples; (c) the XPS result for Co ions; (d) the XPS result for Cu ions.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
J. Song et al. / Current Applied Physics xxx (2017) 1e6
3
Table 1 Structural, magnetic and transport parameters for Ca3Co2-xCuxO6 single crystals.
Co/f.u. Cu/f.u. TN(K) Tf (K) meff(mB/f.u.) q (K) T0 ( 105K) r300K(U cm) kch( 105W K1 m1) S300K(mV/K) ZT300K( 105)
x ¼ 0.0
x ¼ 0.2
x ¼ 0.4
x ¼ 0.6
x ¼ 0.8
2.16 0 24.4 9.5 6.28 82.2 10.4 52.3 1.4 105.2 1.6
1.98 0.12 22.9 8.8 6.36 75.5 7.8 14.4 5.1 81.8 2.0
1.97 0.14 20.8 7.7 6.35 58.7 7.1 11.5 6.4 125.4 5.7
1.84 0.18 19.8 6.8 6.42 55.0 6.1 1.8 4.2 198.1 92.0
1.85 0.17 e e e e e e e e e
attributed to Co3þ and Co4þ ions, respectively. The BE of Co3þ peak located at 781.3eV is consistent with that of Co 2p3/2 in Co2O3 [17]. In Fig. 1(d), the XPS peak at 935.2eV is corresponding to Cu 2p3/2, which is close to the BE of Cu 2p3/2 in Cu(OH)2 and CuO [18], implying the valence state of the doped Cu ions is þ2. It is consistent with the variation of lattice constant in this work and previous results in the similar structure compounds Ca3CuMnO6 [19] and Sr3CuIrO6 [20]. Based on the ratio of peak area between Co3þ and Co4þ, we can deduce that the valence state of the majority Co ions in the sample is þ3 and the ratio of Co4þ is small, which is a result of ion transfer from Co3þ to Co4þ induced by Cu-doping to keep the charge neutrality. Moreover, previous experimental studies on Ca3CuMnO6 [19] and Sr3CuIrO6 [20,21] with the similar structure of Ca3Co2O6 show that the Cu ions should occupy the trigonal prism sites in the lattice. The calculation result presented by S. Sarkar et al. [22]. shows that the Cu ions should locate at the trigonal prism sites in the energy-minimized structure of Sr3CuPtO6. The neutron diffraction refinement on Sr3NiIrO6 shows that the large size ions are located in the trigonal prisms and smaller ions should be in the octahedral sites [21]. Based on the calculation and experimental results on these similar structure materials and the fact that the ionic radius of Cu2þ is larger than Co3þ in Ca3Co2O6, we think that the doped Cu ions occupy the trigonal prism sites in the crystal lattice. It can be further proved by the magnetic and transport results as discussed below. 3.2. Magnetic properties The temperature dependence of dc magnetization M(T) of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6) is measured in the temperature range of 2.5e300 K and the low-temperature data below 50 K is shown in Fig. 2. The data in the figure was collected at both the zero-field-cooling (ZFC) and field-cooling (FC) modes with an applied magnetic field H ¼ 100Oe. In the high-temperature range, the un-doped sample behaves a typical paramagnetic material, its ZFC and FC curves almost overlap with each other and the magnetization increases monotonously with decreasing temperature. As temperature decreases to TN ¼ 24 K, both ZFC and FC curves increase abruptly, indicating the onset of long range SDW state originating from the antiferromagnetic inter-chain interaction of ferromagnetic chains [23,24]. As temperature decreases further, a cusp-like magnetic anomaly is observed at about Tf ¼ 9.5 K on both ZFC and FC curves. As can be seen from Fig. 2, the Cu-doping at Co site can change the magnetic properties of the system considerably: (1). The magnetization decreases with increasing Cu-doping level in the whole measured temperature range and the variation of the magnetization at low-temperatures below TN is more obvious than that in the high-temperature range. As aforementioned, the Co ions at the trigonal prism sites were substituted by Cu ions, resulting into the weakened FM interaction along c-axis and then the
Fig. 2. Temperature dependence of magnetization along c-axis of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6). Inset: The derivative of the magnetization for Ca3Co2-xCuxO6 at around TN.
decreased low-temperature magnetization. (2). As Cu-doped into the lattice, the cusp in the ZFC M(T) curve broadens and the irreversibility between the ZFC and FC curves becomes more obvious. In general, the existence of the discrepancy between ZFC and FC curves along with the cusp-like peak in the ZFC curve can be ascribed to the appearance of the spin-glass or cluster-glass state [25,26]. The detailed analysis of the glass-like magnetic state will be discussed below. (3). As partial Co3þ ions were substituted by Cu2þ ions, both TN and Tf move to a lower temperature position. TN and Tf decrease from 24.4 to 9.5 K for the x ¼ 0 sample to about 19.8 and 6.8 K for the sample with x ¼ 0.6, implying that both the long range SDW state and glass-like magnetic state can be suppressed by the substitution of Cu2þ for Co3þ in these 1D Co-based materials. The variation trend of TN is consistent with the decrease of Weiss temperature q obtained from the Curie-Weiss fitting to the M(T) curve and the fitting parameters are listed in Table 1. In addition, the effective magnetic moment meff increases slightly with the increase of x. The substitution of Cu2þ (s ¼ 1/2) for Co3þ (s ¼ 2) will lead to a part of Co3þ (s ¼ 0 or s ¼ 2) ions transfer into Co4þ (s ¼ 1/2 or s ¼ 5/2). If the high spin Co3þ ions transfer into Co4þ, it will lead to the decrease of meff, which is contrast to the increasing meff. So we think that a part of low spin Co3þ ions transfer into high spin Co4þ, which leads to the increase of meff. To further investigate the low-temperature glass-like magnetic state in Ca3Co2O6, we performed ac susceptibility c0 measurements for three selected crystals with x ¼ 0, 0.2 and 0.6, and the results are shown in Fig. 3. The used frequencies range from 1 to 1000 Hz and the ac driving field is 3Oe in the measurement progress. A large peak is clearly seen around a certain temperature (defined as the freezing temperature Tf) in all the three susceptibility curves. It can be seen that the peak of c0 (T) curve moves to the low temperature position and the intensity becomes weak with increasing x, which indicates the suppression of low-temperature spin freezing state. With increase of measurement frequency, the susceptibility peak shifts toward a high temperature position and the magnitude of the magnetic susceptibility decreases. Such a frequency dependence of the ac susceptibility demonstrates the slow dynamics of the system, which is a typical characteristic of a spin glass (SG) system. The quantitative shift of the frequency dependent freezing temperature (defined as p ¼ dTf =Tf dlog10 f ) is usually used to characterize a SG system. However, the value of p calculated from our data is 0.109,
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
4
J. Song et al. / Current Applied Physics xxx (2017) 1e6
Fig. 3. (a)e(c) Temperature dependent ac susceptibility for the samples with x ¼ 0.0, 0.2 and 0.6; (d) the fitted results using dynamical scaling theory.
0.119 and 0.161 for x ¼ 0.0, 0.2 and 0.6 samples, which is out of the typical value of canonical SG systems (0.0045 p 0.08) [27]. To understand the mechanism of this spin frozen state, we try to describe the susceptibility data to a standard dynamical scaling theory. The conventional result of dynamical scaling is related to the relaxation time t and the correlation lengthzf[T/T-Tc]zv [28]. For present studied system, the expression can be rewritten as t ¼ t0(Tf/Tg-1)zv, where t0 is the critical relaxation time, Tf is the frequency dependent freezing temperature determined by the 0 maximum in c ðTÞ curves, Tg is the spin freezing temperature and zv is the dynamics constant exponent. The critical relaxation time t0 obtained from the fitting is 3.4 105, 1.0 104 and 9.1 104s for the samples with x ¼ 0, 0.2 and 0.6, respectively. The values of t0 are significantly larger than that of the conventional spin glass material (1013
compared to the ZFC M(H) curve as shown in the inset (a) of Fig. 4, where a derivative magnetization to the magnetic field is plotted. As Cu-doped into the system, the magnetization steps become weaker and almost no magnetization step can be seen in the M(H) curve for the x ¼ 0.4 and 0.6 samples as shown in the inset (b) of Fig. 4, which is different to the Fe-doped Ca3Co2O6 system [30]. The result shows that Cu-doping can suppress the phenomenon of magnetization steps in Ca3Co2O6 crystal. 3.3. Electrical transport properties The temperature dependence of resistivity r(T) along c-axis for Ca3Co2-xCuxO6 single crystals at zero magnetic field is shown in Fig. 5. The resistivity perpendicular to the c-axis is too large to
Fig. 4. Different field cooling M-H curves of Ca3Co2O6 at 2.5 K. Inset (a) is the field dependence of the derivative magnetization dM/dH. Inset (b) is the relationship between M and H for Ca3Co2-xCuxO6 at 2.5 K.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
J. Song et al. / Current Applied Physics xxx (2017) 1e6
5
fitting parameter T0, which is a characteristic temperature related to the localization length x and the density of states N(EF) in the vicinity of Fermi level, i.e., kBT0z21/[x3N(EF)], is listed in Table 1. From Table 1, it is found that the T0 value decreases with the increase of Cu content, implying the increase of the localization length and the enhancement of carrier mobility, which is also benefit to the electric conductivity of the Cu-doped samples. 3.4. Thermoelectric performance
Fig. 5. Temperature dependence of resistivity along c-axis for Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6) crystals. Inset: the fitted curves using 1D-VRH model.
measure in the PPMS equipment, which indicates that the electric transport of all crystals in present study is highly anisotropic. From Fig. 5, we can see that all samples show a semiconducting transport behavior. The magnitude of resistivity decreases and the measured temperature range broadens considerably with increasing Cudoping level, indicating the samples become more conducting as Cu-ions doped into the system. The room-temperature resistivity of the x ¼ 0.6 sample is about one thirtieth of the un-doped sample. It is well known that, in Ca3Co2O6, the major carriers are holes and the valence state of Co ion is þ3 [6]. The substitution of Cu2þ for Co3þ can be considered as a hole-like type doping and induce the increase of the carrier concentration, and then the decrease of resistivity. To study the effect of Cu-doping on the electric transport mechanism of Ca3Co2O6 system, we try to fit the r(T) curves by the thermal activated law, the adiabatic small polaron hopping model and the Mott variable range hopping (VRH) models, respectively. It is found that all r(T) curves can be well described by the VRH model r ¼ r0 exp½ðT0 =TÞ1=ð1þnÞ , with n ¼ 1, showing the onedimensional transport property. The fitting results are shown in the inset of Fig. 5, where the relationship between lnr and T1/2 is plotted from 380 K to 210 K. The 1D-VRH transport character is consistent with the one-dimensional structure of Ca3Co2O6. The
The temperature dependence of thermal conductivity k(T) along c-axis for Ca3Co2-xCuxO6 single crystals are shown in Fig. 6(a). All samples have a similar temperature dependent thermal transport behavior. With decreasing temperature, k decreases slowly to about 200 K, and then increases rapidly in a finite temperature range. Finally, k decreases suddenly to a much low value and a sharp peak appeared at Tp in the kðTÞ. In general, the thermal conductivity can be expressed as k ¼ kph þ kch , where kph is the contribution from phonon, kch is the carrier component conductivity. The phonon transport can be expressed as kph fcv lv, where cv , l and y are the specific heat, the mean free path of phonons and sound velocity, respectively. In the high temperature region, the positive dk/dT can be ascribed to a local anharmonic distortions. In the temperature range from 200 K to 50 K (T≪QD; QD~415 K [31] for Ca3Co2O6), the phonon-phonon Umklapp scattering becomes important and the mean free path (l) can be described as: lfeQD =aT , where QD and a are the Debye temperature and coefficient. With the decrease of temperature, l increases exponentially, which leads to the increase of kph and then the total k. However, the combination of the boundary and point-defect scattering [32] will lead to the decrease of k at low temperatures and then the appearance of a sharp peak at Tp. With increasing x, the value of the thermal conductivity increases and the kðTÞ peak moves to a lower temperature position. Although the mismatched ionic substitution can unavoidable bring disorder and structural distortion in the material, which can scatter the phonon and decrease the thermal conductivity. In fact, the actual doping level is very low and the radius of Cu2þ is just slightly larger than that of Co3þ, the disorder and structural distortion is a less important factor in determining the thermal conductivity in the series samples. Instead, the doping induced carrier concentration variation may be a key factor, although carrier thermal conductivity is small in these semiconducting materials. The results of the temperature dependent thermopower measurements for Ca3Co2-xCuxO6 single crystals are shown in Fig. 6(b). As Fig. 6(b) shows, the positive value of S demonstrates the major
Fig. 6. Temperature dependences of: (a) thermal conductivity k(T); (b) thermopower S(T); (c) the figure-of-merit ZT(T) along c-axis for Ca3Co2-xCuxO6(x ¼ 0, 0.2, 0.4, and 0.6).
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
6
J. Song et al. / Current Applied Physics xxx (2017) 1e6
carriers in Ca3Co2-xCuxO6 are holes. Small doping content does not change the S(T) behavior significantly except for the x ¼ 0.6 sample. The x ¼ 0.6 sample has a much larger thermopower in a wide temperature range. With increasing Cu-doping content, S decreases firstly and then increases. In general, S can be expressed using Mott formula [15]:
S¼
Ce p2 k2B T vlnmðεÞ þ ; vε 3e n ε¼εF
(1) Acknowledgements
where Ce, n, mðεÞ, and kB are specific heat, carrier concentration, energy correlated carrier mobility and Boltzmann constant, respectively. In these Ca3Co2O6 crystals, the thermopower is dominated by the first term of this equation, which is similar to the Drude picture S∽Ce =n. As aforementioned, the carrier concentration increases with increasing Cu doping content. According to Drude picture, S should decreases monotonously. However, the experiment gives an inconsistent result, that is S decreases first and then increase. We think that the additional spin entropy may contribute to the increase of thermopower in the heavily doped samples. In general, the thermopower related to the spin entropy of Co ions can be ascribed to Heikes formula [33].
S¼
kB g y ; ln 3 e g4 1 y
during sample cooling process. With increase of Cu content, the resistivity along c-axis decreases while the thermopower increases. The enhancement of the electrical conductivity and thermopower induced by Cu-doping are considered to be related to the increase of carrier concentration and spin entropy respectively. The dimensionless figure of merit of Ca3Co2O6 can be improved considerably by Cu doping.
(2)
where y, g3 and g4 are the concentration of Co4þ, the spin orbital degeneracy for Co3þ and Co4þ, respectively. The substitution of Cu2þ for Co3þ will transfer partial Co3þ ions into Co4þ, which will bring additional spin entropy in this system and then the thermopower increases. That is to say, in Ca3Co2-xCuxO6 system, both the carrier concentration and spin entropy contribute to the thermopower variation. When Cu doping content is less than 0.2, the effect of carrier concentration is dominated, so the thermopower decreases. In the heavily doped samples, the role of the spin entropy originated from Co3þ/Co4þ becomes important, leading to the increase of thermopower. Based on the measured r、k and S data, the ZT of Ca3Co2-xCuxO6 can be obtained, which is plotted in Fig. 6(c). The ZT value at 300 K ZT300K for the x ¼ 0.6 sample is 9:2 104 , which is about 60 times larger than that of the undoped single crystal. Although the room-temperature ZT value of Ca3Co2O6 is quite small compared to the conventional layered cobaltite Ca3Co4O9 [34,35], its one-dimensional spin-chain structure makes it an important candidate in the low-dimensional TE investigation. Due to confinement of our experimental equipment, we can not obtain the high-temperature thermoelectric data at present. According to the variation trend of ZT value for Ca3Co2xCuxO6 samples, we can conclude that the improvement of the thermoelectric performance by Cu-doping at high-temperatures may be more effective. 4. Conclusion The Cu-doping effect on the structural, magnetic and transport properties of Ca3Co2-xCuxO6 has been investigated. The valence state of the doped Cu ions is determined to be þ2 and the doped site is suggested to be at the trigonal prism site. All samples undergo a long-range spin density wave (SDW) transition at TN and a glass-like magnetic transition at Tf with decreasing temperature. Both TN and Tf decrease with increasing Cu-doping level. A series of magnetization steps can be observed in the M(H) curves of Ca3Co2O6. In addition, the magnetization steps are found to be sensitive to the Cu doping content and the applied magnetic field
This work is supported by the National Key Basic Research under Contract No. 2011CBA00111, and the National Nature Science Foundation of China under Contract No. U1232140, 11504380 and 11174293. References [1] S. Takeshita, J. Arai, T. Goko, K. Nishiyama, K. Nagamine, J. Phys. Soc. Jpn. 75 (2006) 034712. [2] A. Maignan, V. Hardy, S. Hebert, M. Drillon, M.R. Lees, O. Petrenko, D.M. Paul, D. Khomskii, J. Mater. Chem. 14 (2004) 1231e1234. [3] T. Basu, K.K. Iyer, K. Singh, K. Mukherjee, P.L. Paulose, E.V. Sampathkumaran, Appl. Phys. Lett. 105 (2014) 102912. [4] M.Y. Ruan, Z.W. Ouyang, Y.M. Guo, Y.C. Sun, J.J. Cheng, Z.C. Xia, G.H. Rao, J. Alloys Compd. 620 (2015) 97e100. [5] M. Mikami, R. Funahashi, M. Yoshimura, Y. Mori, T. Sasaki, J. Appl. Phys. 94 (2003) 6579e6582. [6] K. Takubo, T. Mizokawa, S. Hirata, J.Y. Son, A. Fujimori, D. Topwal, D.D. Sarma, S. Rayaprol, E.V. Sampathkumaran, Phys. Rev. B 71 (2005) 073406. [7] S. Aasland, H. Fjellvag, B. Hauback, Solid State Commun. 101 (1997) 187e192. [8] N. Bellido, C. Simon, A. Maignan, Phys. Rev. B 77 (2008) 054430. [9] H. Kageyama, K. Yoshimura, K. Kosuge, M. Azuma, M. Takano, H. Mitamura, T. Goto, J. Phys. Soc. Jpn. 66 (1997) 3996e4000. [10] V. Hardy, M.R. Lees, O.A. Petrenko, D.M. Paul, D. Flahaut, S. Hebert, A. Maignan, Phys. Rev. B 70 (2004) 064424. [11] J. Takahashi, M. Shimada, K. Iwasaki, H. Yaname, J. Electroceram. 17 (2006) 95e98. [12] K. Iwasaki, H. Yamane, J. Takahashi, S. Kubota, T. Nagasaki, Y. Arita, Y. Nishi, T. Matsui, M. Shimada, J. Phys. Chem. Solids 66 (2005) 303e307. [13] M. Mikami, R. Funahashi, J. Solid State Chem. 178 (2005) 1670e1674. [14] A. Maignan, C. Michel, A.C. Masset, C. Martin, B. Raveau, Eur. Phys. J. B 15 (2000) 657e663. [15] Y.N. Huang, B.C. Zhao, S. Lin, J. Yang, W.H. Song, R. Ang, Y.P. Sun, J. Mater. Sci. 49 (2014) 4636e4642. [16] W. Yansen, A. Ismail, R. Rajagukguk, C.U. Jung, B.W. Lee, IEEE Trans. Magn. 47 (2011) 2709e2711. [17] B. Yuan, J. Yang, J. Chen, X.Z. Zuo, L.H. Yin, X.W. Tang, X.B. Zhu, J.M. Dai, W.H. Song, Y.P. Sun, Appl. Phys. Lett. 104 (2014) 062413. [18] S. Contarini, L. Kevan, J. Phys. Chem. 90 (1986) 1630e1632. [19] V.G. Zubkov, G.V. Bazuev, A.P. Tyutyunnik, I.F. Berger, J. Solid State Chem. 160 (2001) 293e301. [20] A. Niazi, P.L. Paulose, E.V. Sampathkumaran, Phys. Rev. Lett. 88 (2002) 107202. [21] T.N. Nguyen, H.C. Loye, J. Solid State Chem. 117 (1995) 300e308. [22] S. Sarkar, S. Kanungo, T. Saha-Dasgupta, Phys. Rev. B 82 (2010) 235122. [23] S. Agrestini, L.C. Chapon, A. Daoud-Aladine, J. Schefer, A. Gukasov, C. Mazzoli, M.R. Lees, O.A. Petrenko, Phys. Rev. Lett. 101 (2008) 097207. [24] D. Flahaut, A. Maignan, S. Hebert, C. Martin, R. Retoux, V. Hardy, Phys. Rev. B 70 (2004) 094418. [25] S.S. Sheng, Z.W. Ouyang, J. Chen, M.Y. Ruan, X.M. Shi, Z.C. Xia, J. Alloys Compd. 556 (2013) 287e291. [26] S. Takeshita, T. Goko, J. Arai, K. Nishiyama, J. Phys. Chem. Solids 68 (2007) 2174e2177. [27] B.S. Wang, P. Tong, Y.P. Sun, X.B. Zhu, Z.R. Yang, W.H. Song, J.M. Dai, Appl. Phys. Lett. 97 (2010) 042508. [28] W. Zhang, Y. Sun, H. Wang, Y. Li, X. Zhang, Y. Sui, H. Luo, F. Meng, Z. Qian, G. Wu, J. Alloys Compd. 589 (2014) 230e233. [29] M.D. Mukadam, S.M. Yusuf, P. Sharma, S.K. Kulshreshtha, G.K. Dey, Phys. Rev. B 72 (2005) 174408. [30] A. Jain, S. Singh, S.M. Yusuf, Phys. Rev. B 74 (2006) 174419. [31] V. Hardy, S. Lambert, M.R. Lees, D.M. Paul, Phys. Rev. B 68 (2003) 014424. [32] T.M. Tritt, Thermal Conductivity, Kluver Academic/Plenum Publishers, New York, 2004. [33] S. Biswas, S. Pal, Ceram. Int. 41 (2015) 14712e14717. [34] S. Demirel, E. Altin, E. Oz, S. Altin, A. Bayri, J. Alloys Compd. 627 (2015) 430e437. [35] S. Altin, A. Bayri, S. Demirel, M.A. Aksan, Curr. Appl. Phys. 14 (2014) 590e595.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals Jiyue Song a, b, Bangchuan Zhao a, *, Yanan Huang a, Yanfeng Qin a, b, Wenhai Song a, Yuping Sun a, c, d a
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China University of Science and Technology of China, Hefei 230026, People's Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People's Republic of China d Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People's Republic of China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2016 Received in revised form 29 December 2016 Accepted 15 February 2017 Available online xxx
The effect of Cu doping on the structural, magnetic, electrical and thermal transport properties of Ca3Co2xCuxO6 single crystals has been investigated systematically. Based on the analysis of the structural parameter and x-ray photoelectron spectroscopy spectra, the valence state of Cu is considered to be þ2. All samples undergo a long-range spin density wave (SDW) transition at TN and a glass-like magnetic transition at Tf with decreasing temperature. Both TN and Tf decrease monotonously with increasing Cudoping content. A series of magnetization steps can be observed in the M(H) curve of Ca3Co2O6, and the magnetization steps are sensitive to the sample cooling magnetic field and Cu doping content x. With the increase of x, the resistivity along c-axis decreases while the thermopower increases. As a result, the figure of merit (ZT) increases considerably and the room-temperature ZT value of the sample with x ¼ 0.6 is nearly 60 times larger than that of the un-doped crystal. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ca3Co2-xCuxO6 single crystal Magnetization steps The figure of merit
1. Introduction Coexistence of low dimensionality and geometric frustration can lead to exotic physical properties in a single compound. Among the family of geometrically frustrated materials, the quasi-onedimensional (Q1D) cobalt-based oxide Ca3Co2O6 has been attracted more attention in recent years due to its unusual physical properties, such as partially disordered antiferromagnetic (PDA) state [1], quantum tunneling effects of the magnetism [2], anisotropic magnetodielectric coupling [3], magnetic memory effect [4], high-temperature thermoelectric properties [5], and so on. The structure of Ca3Co2O6 consists of infinite CoO6 chains made up of alternating face-sharing octahedron (CoI site) and trigonal prism (CoII site) running along c-axis. Each chain is surrounded by six equally spaced chains forming a triangular lattice in ab plane. The chains are separated by Ca ions. According to the
* Corresponding author. E-mail addresses: [email protected] (J. Song), [email protected] (B. Zhao), [email protected] (Y. Huang), [email protected] (Y. Qin), [email protected] (W. Song), [email protected] (Y. Sun).
previous studies, the valence states of cobalt ions at both the octahedral and the trigonal prism sites are trivalent [6], but their spin states are different due to the different crystalline electric fields. The Co ions at CoI sites are in the low-spin (LS, S ¼ 0) state, while the Co ions at CoII sites are in high-spin (HS, S ¼ 2) state [7]. The magnetic interaction of Ca3Co2O6 is suggested to be ferromagnetic (FM) along c-axis and the nearest- and next-nearestneighbor chains are coupled through a weaker antiferromagnetic (AFM) interaction [8]. At low temperatures, a broad magnetization plateau can be clearly seen on the magnetic field dependent magnetization curve M(H) at about 1/3 of the saturation magnetization implying the appearance of the ferrimagnetic (FIM) state [9]. Additional weak magnetization steps can also be observed at regular field intervals (~1.2 T) on the M(H) curve. These weak magnetization steps depend on not only the temperature but also the sweep rate of the applied magnetic field, which can be considered as a result of the magnetic relaxation mechanism [10]. Besides the interesting magnetic properties, the high-temperature thermoelectric performance can be enhanced through Bi, Cu co-doping [11,12]. In addition, a significant decrease in resistivity due to the enhancement of grain size and
http://dx.doi.org/10.1016/j.cap.2017.02.008 1567-1739/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
2
J. Song et al. / Current Applied Physics xxx (2017) 1e6
the decrease of boundary scattering in Cu doped Ca3Co2O6 polycrystalline sample have been reported [13]. In order to further understand the mechanism of the magnetic correlation and improve the thermoelectric performance of Ca3Co2O6 system, we synthesized the Ca3Co2-xCuxO6 single crystals, and investigated the magnetic, electrical and thermal transport properties of Cu doped Ca3Co2O6 single crystals. 2. Experimental details Single crystals of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) were grown by flux method using K2CO3 as flux [14]. A mixture of CaCO3 (99%; Sinopharm Chemical Reagent Co., Ltd), Co3O4 (99.7%; Alfa Aesar), and CuO (99.9%; Alfa Aesar) powders were grinded and heated in air at 1173 K for 24 h. Single crystals were grown by heating a mixture of obtained pre-heated powder and K2CO3 (99%; Sinopharm Chemical Reagent Co., Ltd), in a weight ratio 1:7, up to 1153 K for 48 h in an alumina crucible in air and then cooling down to room temperature at 3 K/h. Needlelike single crystals with typical size of several millimeters along c-axis were obtained. The structure and phase purity of the samples were checked by a Philips X'pert PRO x-ray diffractometer using Cu Ka radiation at room temperature. The morphology and chemical composition of the studied samples were analyzed by a scanning electron microscope (SEM) and an energy dispersive spectroscope (EDS) attached to the SEM equipment (Oxford Instruments). The photoelectron spectra were measured in ESCALAB 250Xi spectrometer (Thermo) using Mg Ka X-rays at 1253.6eV as the excitation source. The electrical and thermal transport properties were measured in a Physical Property Measurement System (Quantum Design) from 5 to 350 K. The dc and ac magnetic measurements were carried out with a superconducting quantum interference device (SQUID) MPMS system (Quantum Design). The measurements of ac susceptibility were performed under a constant excitation field of Hac ¼ 3Oe, and frequencies range from 1 to 1000 Hz. In the measurement process of magnetic and transport properties, the magnetic field was all applied along the c-axis of the crystals.
3. Results and discussion 3.1. Structures The room temperature powder XRD patterns of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) samples are shown in Fig. 1(a). Only sharp (110), (220), (330) peaks are observed, demonstrating the high quality of the crystals. Moreover, all (ll0) peaks are double peaks, which can be clearly seen in the enlarged (330) peaks as shown in Fig. 1(b). The double peaks in the XRD patterns arise from the contributions of Cu Ka1 (1.5406 Å) and Ka2 (1.5444 Å) radiation with the different wavelengths [15]. With the increase of Cu doping content (x 0.6), the XRD diffraction peaks move to a lower angle position, indicating the lattice has an expanded trend. As shown in the inset of Fig. 1(a), the lattice parameter a increases from 9.0515 Å for the x ¼ 0.0 sample to 9.0694 Å for the sample with x ¼ 0.6. For the samples with x 0.8, both the strength and position of the diffraction peaks are almost the same with that of the x ¼ 0.6 sample. The result shows that x ¼ 0.6 may be the solubility threshold of Cu ions in Ca3Co2O6 crystals obtained by flux growth method. The result can be further proved by the EDS analysis result as shown in Table 1, where the actual contents of Co and Cu in the samples are given. From the table we can see that the actual Cu content in the grown crystals is much smaller than the nominal one and the actual composition of the x ¼ 0.8 crystal is almost the same with that of x ¼ 0.6 sample. To simplify, we still use the nominal content in the following Cu-doping effect discussion and only focus on the samples with x 0.6. The standard ionic radii [16] of Cu2þ and Cu3þ are 0.73 Å and 0.54 Å, respectively. However, both the HS (0.61 Å) and LS (0.545 Å) Co3þ ions in Ca3Co2O6 have larger ionic radii than 0.54 Å. Based on the expanded trend of the lattice with increasing x, we suggest that the doped Cu ions at Co-site should be in the form of Cu2þ. The suggestion of the oxidation state can be confirmed by the XPS measurement on the Ca3Co1.6Cu0.4O6 sample as shown in Fig. 1(c) and (d). In Fig. 1(c), the XPS peak located at ~780.7eV is attributed to Co 2p3/2 and it can be deconvoluted into two peaks. The two peaks corresponding to the higher binding energy (BE) and lower BE are
Fig. 1. (a) XRD patterns of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0). Inset shows the lattice parameter a as a function of x. (b) the enlarged (330) peaks for the samples; (c) the XPS result for Co ions; (d) the XPS result for Cu ions.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
J. Song et al. / Current Applied Physics xxx (2017) 1e6
3
Table 1 Structural, magnetic and transport parameters for Ca3Co2-xCuxO6 single crystals.
Co/f.u. Cu/f.u. TN(K) Tf (K) meff(mB/f.u.) q (K) T0 ( 105K) r300K(U cm) kch( 105W K1 m1) S300K(mV/K) ZT300K( 105)
x ¼ 0.0
x ¼ 0.2
x ¼ 0.4
x ¼ 0.6
x ¼ 0.8
2.16 0 24.4 9.5 6.28 82.2 10.4 52.3 1.4 105.2 1.6
1.98 0.12 22.9 8.8 6.36 75.5 7.8 14.4 5.1 81.8 2.0
1.97 0.14 20.8 7.7 6.35 58.7 7.1 11.5 6.4 125.4 5.7
1.84 0.18 19.8 6.8 6.42 55.0 6.1 1.8 4.2 198.1 92.0
1.85 0.17 e e e e e e e e e
attributed to Co3þ and Co4þ ions, respectively. The BE of Co3þ peak located at 781.3eV is consistent with that of Co 2p3/2 in Co2O3 [17]. In Fig. 1(d), the XPS peak at 935.2eV is corresponding to Cu 2p3/2, which is close to the BE of Cu 2p3/2 in Cu(OH)2 and CuO [18], implying the valence state of the doped Cu ions is þ2. It is consistent with the variation of lattice constant in this work and previous results in the similar structure compounds Ca3CuMnO6 [19] and Sr3CuIrO6 [20]. Based on the ratio of peak area between Co3þ and Co4þ, we can deduce that the valence state of the majority Co ions in the sample is þ3 and the ratio of Co4þ is small, which is a result of ion transfer from Co3þ to Co4þ induced by Cu-doping to keep the charge neutrality. Moreover, previous experimental studies on Ca3CuMnO6 [19] and Sr3CuIrO6 [20,21] with the similar structure of Ca3Co2O6 show that the Cu ions should occupy the trigonal prism sites in the lattice. The calculation result presented by S. Sarkar et al. [22]. shows that the Cu ions should locate at the trigonal prism sites in the energy-minimized structure of Sr3CuPtO6. The neutron diffraction refinement on Sr3NiIrO6 shows that the large size ions are located in the trigonal prisms and smaller ions should be in the octahedral sites [21]. Based on the calculation and experimental results on these similar structure materials and the fact that the ionic radius of Cu2þ is larger than Co3þ in Ca3Co2O6, we think that the doped Cu ions occupy the trigonal prism sites in the crystal lattice. It can be further proved by the magnetic and transport results as discussed below. 3.2. Magnetic properties The temperature dependence of dc magnetization M(T) of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6) is measured in the temperature range of 2.5e300 K and the low-temperature data below 50 K is shown in Fig. 2. The data in the figure was collected at both the zero-field-cooling (ZFC) and field-cooling (FC) modes with an applied magnetic field H ¼ 100Oe. In the high-temperature range, the un-doped sample behaves a typical paramagnetic material, its ZFC and FC curves almost overlap with each other and the magnetization increases monotonously with decreasing temperature. As temperature decreases to TN ¼ 24 K, both ZFC and FC curves increase abruptly, indicating the onset of long range SDW state originating from the antiferromagnetic inter-chain interaction of ferromagnetic chains [23,24]. As temperature decreases further, a cusp-like magnetic anomaly is observed at about Tf ¼ 9.5 K on both ZFC and FC curves. As can be seen from Fig. 2, the Cu-doping at Co site can change the magnetic properties of the system considerably: (1). The magnetization decreases with increasing Cu-doping level in the whole measured temperature range and the variation of the magnetization at low-temperatures below TN is more obvious than that in the high-temperature range. As aforementioned, the Co ions at the trigonal prism sites were substituted by Cu ions, resulting into the weakened FM interaction along c-axis and then the
Fig. 2. Temperature dependence of magnetization along c-axis of Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6). Inset: The derivative of the magnetization for Ca3Co2-xCuxO6 at around TN.
decreased low-temperature magnetization. (2). As Cu-doped into the lattice, the cusp in the ZFC M(T) curve broadens and the irreversibility between the ZFC and FC curves becomes more obvious. In general, the existence of the discrepancy between ZFC and FC curves along with the cusp-like peak in the ZFC curve can be ascribed to the appearance of the spin-glass or cluster-glass state [25,26]. The detailed analysis of the glass-like magnetic state will be discussed below. (3). As partial Co3þ ions were substituted by Cu2þ ions, both TN and Tf move to a lower temperature position. TN and Tf decrease from 24.4 to 9.5 K for the x ¼ 0 sample to about 19.8 and 6.8 K for the sample with x ¼ 0.6, implying that both the long range SDW state and glass-like magnetic state can be suppressed by the substitution of Cu2þ for Co3þ in these 1D Co-based materials. The variation trend of TN is consistent with the decrease of Weiss temperature q obtained from the Curie-Weiss fitting to the M(T) curve and the fitting parameters are listed in Table 1. In addition, the effective magnetic moment meff increases slightly with the increase of x. The substitution of Cu2þ (s ¼ 1/2) for Co3þ (s ¼ 2) will lead to a part of Co3þ (s ¼ 0 or s ¼ 2) ions transfer into Co4þ (s ¼ 1/2 or s ¼ 5/2). If the high spin Co3þ ions transfer into Co4þ, it will lead to the decrease of meff, which is contrast to the increasing meff. So we think that a part of low spin Co3þ ions transfer into high spin Co4þ, which leads to the increase of meff. To further investigate the low-temperature glass-like magnetic state in Ca3Co2O6, we performed ac susceptibility c0 measurements for three selected crystals with x ¼ 0, 0.2 and 0.6, and the results are shown in Fig. 3. The used frequencies range from 1 to 1000 Hz and the ac driving field is 3Oe in the measurement progress. A large peak is clearly seen around a certain temperature (defined as the freezing temperature Tf) in all the three susceptibility curves. It can be seen that the peak of c0 (T) curve moves to the low temperature position and the intensity becomes weak with increasing x, which indicates the suppression of low-temperature spin freezing state. With increase of measurement frequency, the susceptibility peak shifts toward a high temperature position and the magnitude of the magnetic susceptibility decreases. Such a frequency dependence of the ac susceptibility demonstrates the slow dynamics of the system, which is a typical characteristic of a spin glass (SG) system. The quantitative shift of the frequency dependent freezing temperature (defined as p ¼ dTf =Tf dlog10 f ) is usually used to characterize a SG system. However, the value of p calculated from our data is 0.109,
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
4
J. Song et al. / Current Applied Physics xxx (2017) 1e6
Fig. 3. (a)e(c) Temperature dependent ac susceptibility for the samples with x ¼ 0.0, 0.2 and 0.6; (d) the fitted results using dynamical scaling theory.
0.119 and 0.161 for x ¼ 0.0, 0.2 and 0.6 samples, which is out of the typical value of canonical SG systems (0.0045 p 0.08) [27]. To understand the mechanism of this spin frozen state, we try to describe the susceptibility data to a standard dynamical scaling theory. The conventional result of dynamical scaling is related to the relaxation time t and the correlation lengthzf[T/T-Tc]zv [28]. For present studied system, the expression can be rewritten as t ¼ t0(Tf/Tg-1)zv, where t0 is the critical relaxation time, Tf is the frequency dependent freezing temperature determined by the 0 maximum in c ðTÞ curves, Tg is the spin freezing temperature and zv is the dynamics constant exponent. The critical relaxation time t0 obtained from the fitting is 3.4 105, 1.0 104 and 9.1 104s for the samples with x ¼ 0, 0.2 and 0.6, respectively. The values of t0 are significantly larger than that of the conventional spin glass material (1013
compared to the ZFC M(H) curve as shown in the inset (a) of Fig. 4, where a derivative magnetization to the magnetic field is plotted. As Cu-doped into the system, the magnetization steps become weaker and almost no magnetization step can be seen in the M(H) curve for the x ¼ 0.4 and 0.6 samples as shown in the inset (b) of Fig. 4, which is different to the Fe-doped Ca3Co2O6 system [30]. The result shows that Cu-doping can suppress the phenomenon of magnetization steps in Ca3Co2O6 crystal. 3.3. Electrical transport properties The temperature dependence of resistivity r(T) along c-axis for Ca3Co2-xCuxO6 single crystals at zero magnetic field is shown in Fig. 5. The resistivity perpendicular to the c-axis is too large to
Fig. 4. Different field cooling M-H curves of Ca3Co2O6 at 2.5 K. Inset (a) is the field dependence of the derivative magnetization dM/dH. Inset (b) is the relationship between M and H for Ca3Co2-xCuxO6 at 2.5 K.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
J. Song et al. / Current Applied Physics xxx (2017) 1e6
5
fitting parameter T0, which is a characteristic temperature related to the localization length x and the density of states N(EF) in the vicinity of Fermi level, i.e., kBT0z21/[x3N(EF)], is listed in Table 1. From Table 1, it is found that the T0 value decreases with the increase of Cu content, implying the increase of the localization length and the enhancement of carrier mobility, which is also benefit to the electric conductivity of the Cu-doped samples. 3.4. Thermoelectric performance
Fig. 5. Temperature dependence of resistivity along c-axis for Ca3Co2-xCuxO6 (x ¼ 0, 0.2, 0.4, 0.6) crystals. Inset: the fitted curves using 1D-VRH model.
measure in the PPMS equipment, which indicates that the electric transport of all crystals in present study is highly anisotropic. From Fig. 5, we can see that all samples show a semiconducting transport behavior. The magnitude of resistivity decreases and the measured temperature range broadens considerably with increasing Cudoping level, indicating the samples become more conducting as Cu-ions doped into the system. The room-temperature resistivity of the x ¼ 0.6 sample is about one thirtieth of the un-doped sample. It is well known that, in Ca3Co2O6, the major carriers are holes and the valence state of Co ion is þ3 [6]. The substitution of Cu2þ for Co3þ can be considered as a hole-like type doping and induce the increase of the carrier concentration, and then the decrease of resistivity. To study the effect of Cu-doping on the electric transport mechanism of Ca3Co2O6 system, we try to fit the r(T) curves by the thermal activated law, the adiabatic small polaron hopping model and the Mott variable range hopping (VRH) models, respectively. It is found that all r(T) curves can be well described by the VRH model r ¼ r0 exp½ðT0 =TÞ1=ð1þnÞ , with n ¼ 1, showing the onedimensional transport property. The fitting results are shown in the inset of Fig. 5, where the relationship between lnr and T1/2 is plotted from 380 K to 210 K. The 1D-VRH transport character is consistent with the one-dimensional structure of Ca3Co2O6. The
The temperature dependence of thermal conductivity k(T) along c-axis for Ca3Co2-xCuxO6 single crystals are shown in Fig. 6(a). All samples have a similar temperature dependent thermal transport behavior. With decreasing temperature, k decreases slowly to about 200 K, and then increases rapidly in a finite temperature range. Finally, k decreases suddenly to a much low value and a sharp peak appeared at Tp in the kðTÞ. In general, the thermal conductivity can be expressed as k ¼ kph þ kch , where kph is the contribution from phonon, kch is the carrier component conductivity. The phonon transport can be expressed as kph fcv lv, where cv , l and y are the specific heat, the mean free path of phonons and sound velocity, respectively. In the high temperature region, the positive dk/dT can be ascribed to a local anharmonic distortions. In the temperature range from 200 K to 50 K (T≪QD; QD~415 K [31] for Ca3Co2O6), the phonon-phonon Umklapp scattering becomes important and the mean free path (l) can be described as: lfeQD =aT , where QD and a are the Debye temperature and coefficient. With the decrease of temperature, l increases exponentially, which leads to the increase of kph and then the total k. However, the combination of the boundary and point-defect scattering [32] will lead to the decrease of k at low temperatures and then the appearance of a sharp peak at Tp. With increasing x, the value of the thermal conductivity increases and the kðTÞ peak moves to a lower temperature position. Although the mismatched ionic substitution can unavoidable bring disorder and structural distortion in the material, which can scatter the phonon and decrease the thermal conductivity. In fact, the actual doping level is very low and the radius of Cu2þ is just slightly larger than that of Co3þ, the disorder and structural distortion is a less important factor in determining the thermal conductivity in the series samples. Instead, the doping induced carrier concentration variation may be a key factor, although carrier thermal conductivity is small in these semiconducting materials. The results of the temperature dependent thermopower measurements for Ca3Co2-xCuxO6 single crystals are shown in Fig. 6(b). As Fig. 6(b) shows, the positive value of S demonstrates the major
Fig. 6. Temperature dependences of: (a) thermal conductivity k(T); (b) thermopower S(T); (c) the figure-of-merit ZT(T) along c-axis for Ca3Co2-xCuxO6(x ¼ 0, 0.2, 0.4, and 0.6).
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008
6
J. Song et al. / Current Applied Physics xxx (2017) 1e6
carriers in Ca3Co2-xCuxO6 are holes. Small doping content does not change the S(T) behavior significantly except for the x ¼ 0.6 sample. The x ¼ 0.6 sample has a much larger thermopower in a wide temperature range. With increasing Cu-doping content, S decreases firstly and then increases. In general, S can be expressed using Mott formula [15]:
S¼
Ce p2 k2B T vlnmðεÞ þ ; vε 3e n ε¼εF
(1) Acknowledgements
where Ce, n, mðεÞ, and kB are specific heat, carrier concentration, energy correlated carrier mobility and Boltzmann constant, respectively. In these Ca3Co2O6 crystals, the thermopower is dominated by the first term of this equation, which is similar to the Drude picture S∽Ce =n. As aforementioned, the carrier concentration increases with increasing Cu doping content. According to Drude picture, S should decreases monotonously. However, the experiment gives an inconsistent result, that is S decreases first and then increase. We think that the additional spin entropy may contribute to the increase of thermopower in the heavily doped samples. In general, the thermopower related to the spin entropy of Co ions can be ascribed to Heikes formula [33].
S¼
kB g y ; ln 3 e g4 1 y
during sample cooling process. With increase of Cu content, the resistivity along c-axis decreases while the thermopower increases. The enhancement of the electrical conductivity and thermopower induced by Cu-doping are considered to be related to the increase of carrier concentration and spin entropy respectively. The dimensionless figure of merit of Ca3Co2O6 can be improved considerably by Cu doping.
(2)
where y, g3 and g4 are the concentration of Co4þ, the spin orbital degeneracy for Co3þ and Co4þ, respectively. The substitution of Cu2þ for Co3þ will transfer partial Co3þ ions into Co4þ, which will bring additional spin entropy in this system and then the thermopower increases. That is to say, in Ca3Co2-xCuxO6 system, both the carrier concentration and spin entropy contribute to the thermopower variation. When Cu doping content is less than 0.2, the effect of carrier concentration is dominated, so the thermopower decreases. In the heavily doped samples, the role of the spin entropy originated from Co3þ/Co4þ becomes important, leading to the increase of thermopower. Based on the measured r、k and S data, the ZT of Ca3Co2-xCuxO6 can be obtained, which is plotted in Fig. 6(c). The ZT value at 300 K ZT300K for the x ¼ 0.6 sample is 9:2 104 , which is about 60 times larger than that of the undoped single crystal. Although the room-temperature ZT value of Ca3Co2O6 is quite small compared to the conventional layered cobaltite Ca3Co4O9 [34,35], its one-dimensional spin-chain structure makes it an important candidate in the low-dimensional TE investigation. Due to confinement of our experimental equipment, we can not obtain the high-temperature thermoelectric data at present. According to the variation trend of ZT value for Ca3Co2xCuxO6 samples, we can conclude that the improvement of the thermoelectric performance by Cu-doping at high-temperatures may be more effective. 4. Conclusion The Cu-doping effect on the structural, magnetic and transport properties of Ca3Co2-xCuxO6 has been investigated. The valence state of the doped Cu ions is determined to be þ2 and the doped site is suggested to be at the trigonal prism site. All samples undergo a long-range spin density wave (SDW) transition at TN and a glass-like magnetic transition at Tf with decreasing temperature. Both TN and Tf decrease with increasing Cu-doping level. A series of magnetization steps can be observed in the M(H) curves of Ca3Co2O6. In addition, the magnetization steps are found to be sensitive to the Cu doping content and the applied magnetic field
This work is supported by the National Key Basic Research under Contract No. 2011CBA00111, and the National Nature Science Foundation of China under Contract No. U1232140, 11504380 and 11174293. References [1] S. Takeshita, J. Arai, T. Goko, K. Nishiyama, K. Nagamine, J. Phys. Soc. Jpn. 75 (2006) 034712. [2] A. Maignan, V. Hardy, S. Hebert, M. Drillon, M.R. Lees, O. Petrenko, D.M. Paul, D. Khomskii, J. Mater. Chem. 14 (2004) 1231e1234. [3] T. Basu, K.K. Iyer, K. Singh, K. Mukherjee, P.L. Paulose, E.V. Sampathkumaran, Appl. Phys. Lett. 105 (2014) 102912. [4] M.Y. Ruan, Z.W. Ouyang, Y.M. Guo, Y.C. Sun, J.J. Cheng, Z.C. Xia, G.H. Rao, J. Alloys Compd. 620 (2015) 97e100. [5] M. Mikami, R. Funahashi, M. Yoshimura, Y. Mori, T. Sasaki, J. Appl. Phys. 94 (2003) 6579e6582. [6] K. Takubo, T. Mizokawa, S. Hirata, J.Y. Son, A. Fujimori, D. Topwal, D.D. Sarma, S. Rayaprol, E.V. Sampathkumaran, Phys. Rev. B 71 (2005) 073406. [7] S. Aasland, H. Fjellvag, B. Hauback, Solid State Commun. 101 (1997) 187e192. [8] N. Bellido, C. Simon, A. Maignan, Phys. Rev. B 77 (2008) 054430. [9] H. Kageyama, K. Yoshimura, K. Kosuge, M. Azuma, M. Takano, H. Mitamura, T. Goto, J. Phys. Soc. Jpn. 66 (1997) 3996e4000. [10] V. Hardy, M.R. Lees, O.A. Petrenko, D.M. Paul, D. Flahaut, S. Hebert, A. Maignan, Phys. Rev. B 70 (2004) 064424. [11] J. Takahashi, M. Shimada, K. Iwasaki, H. Yaname, J. Electroceram. 17 (2006) 95e98. [12] K. Iwasaki, H. Yamane, J. Takahashi, S. Kubota, T. Nagasaki, Y. Arita, Y. Nishi, T. Matsui, M. Shimada, J. Phys. Chem. Solids 66 (2005) 303e307. [13] M. Mikami, R. Funahashi, J. Solid State Chem. 178 (2005) 1670e1674. [14] A. Maignan, C. Michel, A.C. Masset, C. Martin, B. Raveau, Eur. Phys. J. B 15 (2000) 657e663. [15] Y.N. Huang, B.C. Zhao, S. Lin, J. Yang, W.H. Song, R. Ang, Y.P. Sun, J. Mater. Sci. 49 (2014) 4636e4642. [16] W. Yansen, A. Ismail, R. Rajagukguk, C.U. Jung, B.W. Lee, IEEE Trans. Magn. 47 (2011) 2709e2711. [17] B. Yuan, J. Yang, J. Chen, X.Z. Zuo, L.H. Yin, X.W. Tang, X.B. Zhu, J.M. Dai, W.H. Song, Y.P. Sun, Appl. Phys. Lett. 104 (2014) 062413. [18] S. Contarini, L. Kevan, J. Phys. Chem. 90 (1986) 1630e1632. [19] V.G. Zubkov, G.V. Bazuev, A.P. Tyutyunnik, I.F. Berger, J. Solid State Chem. 160 (2001) 293e301. [20] A. Niazi, P.L. Paulose, E.V. Sampathkumaran, Phys. Rev. Lett. 88 (2002) 107202. [21] T.N. Nguyen, H.C. Loye, J. Solid State Chem. 117 (1995) 300e308. [22] S. Sarkar, S. Kanungo, T. Saha-Dasgupta, Phys. Rev. B 82 (2010) 235122. [23] S. Agrestini, L.C. Chapon, A. Daoud-Aladine, J. Schefer, A. Gukasov, C. Mazzoli, M.R. Lees, O.A. Petrenko, Phys. Rev. Lett. 101 (2008) 097207. [24] D. Flahaut, A. Maignan, S. Hebert, C. Martin, R. Retoux, V. Hardy, Phys. Rev. B 70 (2004) 094418. [25] S.S. Sheng, Z.W. Ouyang, J. Chen, M.Y. Ruan, X.M. Shi, Z.C. Xia, J. Alloys Compd. 556 (2013) 287e291. [26] S. Takeshita, T. Goko, J. Arai, K. Nishiyama, J. Phys. Chem. Solids 68 (2007) 2174e2177. [27] B.S. Wang, P. Tong, Y.P. Sun, X.B. Zhu, Z.R. Yang, W.H. Song, J.M. Dai, Appl. Phys. Lett. 97 (2010) 042508. [28] W. Zhang, Y. Sun, H. Wang, Y. Li, X. Zhang, Y. Sui, H. Luo, F. Meng, Z. Qian, G. Wu, J. Alloys Compd. 589 (2014) 230e233. [29] M.D. Mukadam, S.M. Yusuf, P. Sharma, S.K. Kulshreshtha, G.K. Dey, Phys. Rev. B 72 (2005) 174408. [30] A. Jain, S. Singh, S.M. Yusuf, Phys. Rev. B 74 (2006) 174419. [31] V. Hardy, S. Lambert, M.R. Lees, D.M. Paul, Phys. Rev. B 68 (2003) 014424. [32] T.M. Tritt, Thermal Conductivity, Kluver Academic/Plenum Publishers, New York, 2004. [33] S. Biswas, S. Pal, Ceram. Int. 41 (2015) 14712e14717. [34] S. Demirel, E. Altin, E. Oz, S. Altin, A. Bayri, J. Alloys Compd. 627 (2015) 430e437. [35] S. Altin, A. Bayri, S. Demirel, M.A. Aksan, Curr. Appl. Phys. 14 (2014) 590e595.
Please cite this article in press as: J. Song, et al., Magnetic properties and enhanced thermoelectric performance in Cu-doped Ca3Co2O6 single crystals, Current Applied Physics (2017), http://dx.doi.org/10.1016/j.cap.2017.02.008