Journal of Magnetism and Magnetic Materials 365 (2014) 138–144
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Frequency dependent magneto-transport in charge transfer Co(II) complex Bikash Kumar Shaw, Shyamal K. Saha n Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2013 Received in revised form 26 March 2014 Available online 2 May 2014
A charge transfer chelated system containing ferromagnetic metal centers is the ideal system to investigate the magneto-transport and magneto-dielectric effects due to the presence of both electronic as well as magnetic properties and their coupling. Magneto-transport properties in materials are usually studied through dc charge transport under magnetic field. As frequency dependent conductivity is an essential tool to understand the nature of carrier wave, its spatial extension and their mutual interaction, in the present work, we have investigated frequency dependent magneto-transport along with magnetization behavior in [Co2(II)-(5-(4PhMe)-1,3,4-oxadiazole-H þ -2-thiolate)5](OAc)4 metal complex to elucidate the nature of above quantities and their response under magnetic field in the transport property. We have used the existing model for ac conduction incorporating the field dependence to explain the frequency dependent magneto-transport. It is seen that the frequency dependent magneto-transport could be well explained using the existing model for ac conduction. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ac conduction Magneto-transport Weak localization Magneto-dielectric
1. Introduction Multifunctionality in magnetic materials has attracted considerable attention recently due to the ability of producing new materials in which the magnetism can be tailored due to coupling with other properties [1–3]. Most of these solid materials are generally made by chelating an inorganic magnetic center with a conducting organic network [4,5]. In recent times, a large number of molecule-based solids which mingle electrical conductivity with magnetism have been reported [6,7]. Over the past years, coordination chemistry has controlled the structural framework of the metal complexes, whose constituent units like molecular bridges used to interlock them in the solid-state govern the physical properties of the system. The augmentation of these new classes of binuclear metal complexes incorporating suitable bridging ligands has shown interesting magneto-transport behavior over past years [3,8,9]. The intermetallic coupling operated through the s and π orbital pathways of the bridging ligand results in the magnetic super-exchange interaction. In the previous work, we have investigated giant magnetoresistance and magneto-dielectric effects in a charge transfer complex using magnetic field dependent impedance spectroscopy
n
Corresponding author. Tel.: þ 91 33 2473 4971x227; fax: þ91 33 2473 2805. E-mail address:
[email protected] (S.K. Saha).
http://dx.doi.org/10.1016/j.jmmm.2014.04.048 0304-8853/& 2014 Elsevier B.V. All rights reserved.
(frequency dependent conductivity). These giant positive magnetoresistance and magneto-dielectric effects were explained on the basis of weak localization and electron–electron interaction phenomena [10]. In the present work, we have investigated the magnetic interaction between the two metal centers and frequency dependent magneto-transport in the binuclear [Co2(II)(5-(4-PhMe)-1,3,4-oxadiazole-H þ -2-thiolate)5](OAc)4 system. So far in most cases frequency dependent conductivity has been investigated in amorphous semiconductors and several models for ac charge transport such as hopping and tunneling of electrons and polarons have been developed [11]. The variation of frequency exponent with temperature determines the charge transport precisely in these materials. In the present work, we have applied this technique to investigate frequency dependent conductivity and extended our measurements to understand its response under application of magnetic field. As polaron formation is the common feature in charge transfer systems, we believe that these models will also be suitable to explore the frequency dependent magneto-transport. For polaronic systems, there are two models one small polaron tunneling and the other overlapping large polaron tunneling which are commonly used to investigate the frequency dependent conductivity. In the present case, we have considered the overlapping large polaron tunneling model to explain our frequency dependent magneto-transport data. Our motivation for this study is to understand the response of the frequency dependent conductivity in the presence of magnetic field.
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2. Experimental methods 2.1. Preparation of metal complex To synthesize the binuclear Co(II) complex, 0.498 g of cobalt acetate tetrahydrate (in excess) is taken in a 250 mL beaker containing 30 mL of chloroform and stirred for half an hour with a magnetic stirrer. 0.192 g (1 mM) of 5-(4-Methylphenyl)-1,3,4oxadiazole-2-thiol is added to it in steps under continuous stirring. After few minutes the solution becomes bluish and finally turns intense blue to give [Co2(II)-(5-(4-PhMe)-1,3,4-oxadiazole-H þ -2thiolate)5](OAc)4. After one hour stirring the excess metal precursor is filtered out and the blue solution left undisturbed for one day. Polycrystalline blue compound is collected for several characterizations viz. UV–vis, FTIR, EPR, etc. Yield: 0.483 g (69%). Anal. Calcd. for C53H52N10Co2O13S5: C, 45.82; H, 3.05; N, 11.08; Found: C, 45.76%; H, 2.95%; N, 10.98%. 2.2. Characterizations Elemental (C, H, and N) analyses were performed using a Perkin-Elmer model 2400 series II CHNS analyzer. The molecular properties are characterized by a spectroscopic method with a Cary 5000 UV–vis NIR Spectrophotometer in diffuse reflectance accessory (DRS) using BaSO4 matrix measured in the range of 200– 1100 nm and a Fourier Transform Infrared Spectrometer using KBr pellets (NICOLET MAGNA IR 750 System). The thermodynamic stability of the metal complex is analyzed by a TG–DTA technique (model SDT Q600) in the range of 25–900 1C. XRD pattern is investigated using powdered sample with an X-ray diffractometer (RICH SEIFERT-XRD 3000P, wavelength 1.54 Å). Electrospray ionization mass spectrometry (ESI) is carried out using a Micromass Q-Tof micromass spectrometer with an electrostatic ion source in CH2Cl2. Cyclic voltammograms are obtained using a CH Instrument (Model chi720d). A Jeol (JES FA200) spectrophotometer is used to get the EPR spectrum. We have used a SQUID magnetometer (Quantum Design MPMS) to investigate the magnetic properties (FC–ZFC and M–H measurements) and the magnetoresistance is measured using a PPMS system (Model no. J 2468). Magnetotransport measurements have been carried out using the Agilent LCR meter (Model: E4980A). To carry the magneto-transport measurements, we kept the orientation of magnetic field parallel to the applied electric field. Magnetic field is generated by a large water-cooled electromagnet supplied by M/S Control Systems & Devices, Mumbai, India. Temperature dependent resistance, magnetoresistance and magnetic field dependent ac conductivity were measured by the standard two-probe technique using powdered sample pressed in the form of a pellet with 0.5 cm2 area and 0.06 cm in thickness. Good contact was made by highly conducting silver adhesive and fine copper wires as electrodes of 5 cm length.
3. Results and discussion
Fig. 1. (a) Diffuse reflectance UV–vis spectra of the metal complex using BaSO4 as matrix. Inset shows the spectroscopic transitions occurred in trigonal bipyramidal geometry (only the S¼ 3/2 states are considered). (b) Electrospray ionization mass spectra (ESI-MS) of the metal complex. (c) Derivative signal of the EPR spectra obtained at room temperature.
3.1. UV visible spectra To investigate the stereochemistry of the metal complex, diffuse reflectance UV–vis spectra using BaSO4 matrix have been carried out as shown in Fig. 1a. Combining the diffuse reflectance spectra with molecular orbital (MO) theory, it is concluded that the bivalent metal–ligand chelated system exists in trigonal bipyramidal configuration [12,13]. The 4F and 4P levels of S¼ 3/2 high spin Co(II) system split into several sub-levels in trigonal bipyramidal geometry shown in the inset of this figure. The bands appeared at 20,280, 16,500 and 15,360, and 12,260 cm 1 are referred to the electronic transitions from 4A2 ðFÞ-4E0 ðFÞ as (1),
4A2 ðFÞ-4E ðPÞ and 4A1″ ðFÞ-4E0 ðFÞ as (2), and 4A2 ðFÞ-4A20 ðPÞ as (3) respectively. The low energy transitions occurred from 4A2 ðFÞ-4E ðFÞ; 4A1} ðFÞ; 4A2″ ðFÞ are in the far ultraviolet region (41100 nm); as a result no peaks are observed. 3.2. Mass spectra In order to know the actual mass of the complex, formed from the bivalent cobalt and 5-(4-Methylphenyl)-1,3,4-oxadiazole-2-thiol in non-aqueous chloroform solution we have analyzed the mass spectrum using an electrospray ionization mass spectrometer at
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field cooled (FC) magnetization data which indicate antiferromagnetic Neel temperature (TN) at around 51 K, above which it follows Curie–Weiss paramagnetic behavior. Another interesting point to note that in the magnetization data is the sharp increase below 15 K which is due to paramagnetic impurity at lower temperature. The values of χT as a function of T are also plotted in Fig. 2b and fitted by the conventional van Vleck model [20,21], generally applied for the S¼1/2 and 3/2 binuclear systems as expressed by Eq. (1). The spin Hamiltonian is defined by H ¼ 2J Si Sj:
atmospheric pressure. The spectrum of the complex (Fig. 1b) shows the molecular ion (Mþ ) peak at m/z¼1075 a.m.u., pointing toward the molecular ion of first sphere Co2(II)-(5-(4-PhMe)-1,3,4-oxadiazole-H þ -2-thiolate)5. The parent ion peak suggests a binuclear nature of the complex in which two cobalt centers each of which is attached to two ligands are linked via a bridging ligand in trigonal bipyramidal fashion as discussed earlier by ultraviolet spectroscopy. The peak at m/z 883 is generated due to the fissure of one ligand from the [Co2(L)L4] (OAc)4 moiety to [Co2(L)L3]4 þ (where L¼5-(4PhMe)-1,3,4-oxadiazole-H þ -2-thiolate) with two major peak fragments of relative intensities of one around m/z 441 and 633 (refer to CoL2þ and CoL3þ ) and the other around m/z 404 and 671, observed as a consequence of the collision-induced dissociation [14,15].
χT ¼
2Ng 2 β e 2 x þ 5Ae 6 x þ 14Be 12 x þ 30Ce 20 x k 1 þ 3e 2 x þ 5Ae 6 x þ 7Be 12 x 2
ð1Þ
where x ¼ J=kT, J is exchange coupling constant, N is Avogadro number, g is Lande factor and k is Boltzmann constant, A ¼B ¼1 and C¼ 0 for S¼ 3/2 Co(II). The value of exchange coupling constant (J ¼ 54 cm 1) extracted from the fitting procedure indicates the antiferromagnetic exchange interactions operated between interacting cobalt centers. The value of χT at 300 K is 3.83 emu mol 1 K greater than the spin-only value (3.74 emu mol 1 K) usually occurred due to orbital contribution for two magnetically isolated high-spin CoII ions [8]. The very low χT value (0.08 emu mol 1 K) at temperature 2 K is also an indication for strong antiferromagnetic interactions [22,23]. The anisotropic Lande factor g ¼2.18 and 2.09 obtained from the fitting data is equivalent to the experimental g value from EPR measurement (g ¼2.28 and 2.01). The variation of magnetization with magnetic field at temperatures 2, 10, 20, 50, 100, and 300 K is shown in Fig. 2c. Interestingly we have observed MH loops at temperatures 2, 10, and 20 K with coercive field around 315, 75, and 25 Oe respectively. The appearance of hysteresis loop at low temperature region arises due to canting of the moments which vanishes at higher temperature. Therefore, a strong antiferromagnetic interaction between the two cobalt centers in the bridged dimeric compound and a sharp increase in moment at lower temperature due to canted spin interaction are observed in the present chelated complex. The co-existence of these two magnetic spin interactions in a single system results in exchange bias field creating asymmetry in the MH loop and values of such asymmetric coercivity at different temperatures are summarized in Table 1.
3.3. EPR study The first derivative X-band ( 9.5 GHz) EPR spectrum of polycrystalline complex in solid state gives a typical signal of a high spin (S¼3/2) non-interacting Co(II) species at room temperature, shown in Fig. 1c. The resonance is characterized by an anisotropic low intense signal at g ¼2.28 and 2.01 generally shown for five coordinated bivalent cobalt complexes with unresolved hyperfine coupling interactions (59Co, I¼ 7/2) [16–19]. This decrease in EPR intensity in the present Co(II) system might be due to antiferromagnetic coupling or due to high spin-lattice relaxation time. However, it is seen from the SQUID data (given below) that the molecule possesses an antiferromagnetic coupling which is prominent at lower temperature. Therefore, it is concluded from both the EPR and SQUID data that this decrease in intensity is due to a coupling phenomenon mediated by spin exchange via bridging ligand in this binuclear Co(II) system. In accordance with this strong antiferromagnetic coupling (S ¼S1 þS2 ¼ 3, integer spin-EPR silent), no signal appears when the measurement is carried out in liquid nitrogen temperature, 77 K (close to TN). The bivalent oxidation state of the metal as predicted from the UV–vis and EPR spectra is also confirmed from the cyclic voltammetry (given in S.I.). On the basis of above spectral studies (including CHN) the following scheme (Scheme 1) is suggested for the formation of the complex. 3.4. Magnetic study
3.5. Temperature-dependent resistivity and magnetoresistance study The magnetic measurements have been carried out over the temperature range from 2 to 300 K at 100 Oe using polycrystalline powdered sample. Fig. 2a shows the zero-field cooled (ZFC) and N
In order to study the charge transport in the absence of magnetic field, we have measured the temperature dependence
N
N
N
N
CHCl3 H
Co(CH3COO)2.4H2O +
HS
S
R
O
R
O
S
HN
OAc
R
coordinates in tbp fashion
Co Co S
HN
O
N
S
S R O
N
S
HN
N
O
O
NH
R
4
R
O
S
O
S
N
N
N
NH
R
O
(R = -PhMe)
HN
N
H
HN
R
S
Co2+
R
[Co2(L4)L] Scheme 1. Reaction mechanism proposed for the formation of chelated complex.
N
O
R
B.K. Shaw, S.K. Saha / Journal of Magnetism and Magnetic Materials 365 (2014) 138–144
Fig. 2. (a) The temperature dependence of magnetization data (ZFC-FC) with Neel temperature at 51 K. (b) Susceptibility-temperature (χT) versus temperature fitted graph gives the value of exchange coupling constant (J). (c) Magnetization-field hysteresis curves at different temperatures. Inset shows the variation of coercivity of hysteresis loops at different temperatures.
141
Fig. 3. (a) Temperature-dependent resistivity at zero magnetic field. Data are plotted in ln ρ versus T 1/4 form. Straight line fits with the respective slopes (T0)1/4 indicate Mott-VRH conductivity mechanism of localization theory. (b) Change of DC resistance with magnetic field (MR) at different temperatures. Percentage change of magnetoresistance at various temperatures is shown in the inset. (c) The H2 and H1/ 2 reliance of the MC with the localization-interaction model.
lower activation energy at low temperature region Table 1 The exchange bias fields at different temperatures. T (K)
þ ve Field (Oe)
–ve Field (Oe)
2 10 20
315 75 25
135 51 30
of resistivity of pelleted sample from 255 to 300 K, shown in Fig. 3a. From the figure it is seen that the charge transport follows Mott's variable-range hopping Model based on weak localization theory. The curve is fitted using Eq. (2) of variable-range hopping (VRH) expression [24] with two different slopes: one with higher activation energy at high temperature region and the other with
ln ρ ¼ ln ρ0 þðT 0 =TÞ1=4
ð2Þ
where ρ is the resistivity, T is the variable temperature, ρ0 is constant, and T0 is the localization temperature. From the fitting procedure, the extracted values of two slopes are T0 ¼16 K and 1.29 103 K. T0 is the localization temperature which depends upon the localization length and the density of states at the Fermi level. The origin of two values of T0 lies with the fact of two types of charge transport: one among the localized states within the grain (lower localization length) giving high value of T0 and the other through the grain boundary (higher localization length) corresponds to the low value of T0. The magnetoresistance is measured at temperatures 255, 275, and 300 K using a pelleted sample. Fig. 3b shows the changes in magnetoresistance with field from which it is seen that at 255 K
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initially negative MR (10%) is observed in the low field region and gradually changes from negative to positive as magnetic field increases (15%). At higher temperatures 275 and 300 K, positive MR results throughout the whole field range with 25% and 60% changes respectively as shown in the inset of Fig. 3b. Since magnetoresistance in charge transfer systems is sensitive to the nature of carrier wave, its spatial extension and their mutual interactions, it serves as an useful probe to elucidate the details of charge transport in the present complex. Generally MR depends on three factors: weak localization which gives negative MR, positive MR due to electron–electron interaction and strongly localized states due to wave function shrinkage under magnetic field resulting positive MR. From our previous study it is seen that the negative and positive magnetoresistances appear mainly due to weak localization and electron–electron interaction effect respectively. In case of weak localization, the negative MR is generally observed in low temperature region; however electron–electron correlation occurs at high temperature. In present case, at temperature 255 K negative MR is observed up to field 0.6 T and at higher field (5 T) positive MR is dominated due to a strong localization effect as a result of wave function shrinkage under application of high magnetic field. At higher temperatures viz. 275 and 300 K the electronic wave functions are more delocalized to dominate the electron–electron correlation effect resulting in the positive MR even at lower field region. In the higher field region, field induced strong localization is superimposed to get positive MR throughout the whole region. Therefore by using this localization-interaction model the experimental results are fitted and obtained a nice bond between the theoretical fittings and the experimental results [10,25]. The magnetic field dependence of the MC can be written as —Δ∑ðH; TÞ ¼ sðH; TÞ sð0; TÞ. At low magnetic field regime (up to 0.6 T), the negative MR appeared at lower temperature (255 K) is
fitted using Eq. (3), where a weak localization effect is operated (green dotted points are the experimental data and the red line is the theoretical curve). At higher temperature (275 and 300 K) electron–electron interaction leading to give positive MR is fitted by Eq. (4) respectively (blue and black dotted points are the experimental data and the red line is the theoretical curve). The H1/2 dependency at high fields for electron–electron interaction and H2 reliance at low fields for the weak localization effect to the MC merge well with the experimental curves shown in Fig. 3c
Δ∑ðH; TÞ ¼ 0:041αðgμB =kB Þ2 γ F s T 3=2 H 2 þ ð1=12π 2 ÞÞðe=cħÞ2 G0 ðlin Þ3 H 2
Δ∑ðH; TÞ ¼ αγ F s T 1=2 0:77αðgμB =kB Þ1=2 γ F s H 1=2
ð3Þ ð4Þ
where the Hartree factor (F) is the screened interaction, α is a parameter that depends on the diffusion coefficient (D), γF is the interaction parameter and lin is the inelastic scattering length. 3.6. Frequency-dependent magneto-dielectric study Although magneto-transport is a well studied subject in the area of pi conjugated polymeric systems, its frequency dependence has not been investigated in pi conjugated systems as well as charge transfer metal complexes. As polaron formation in organic molecular crystals is a well established fact [26], it is desirable to explore the magneto-transport behavior in polaronic metal organic frameworks. In addition to that in the present study we have extended for the first time our measurements of magneto-transport using Impedance spectroscopy to understand the magnetic response of the frequency dependent conductivity in the polaronic molecular crystals. To investigate the frequency dependent magneto-transport in this system we have measured the impedance spectra at room
Fig. 4. (a) The variation of conductivity as a function of frequency at different magnetic fields. Solid lines are the theoretical curves obtained using Eq. 5 and points are the experimental data. (b) Fitted curves of conductivity with frequency using Eq. 6 giving rise to the values of parameters (WHO, r0). (c) Variation of frequency exponent with magnetic field. (d) Values of dielectric-permittivity as a function of frequency at different temperatures. Change of permittivity with field at different frequencies is shown in the inset.
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Table 2 Extracted values of the parameters contained in the frequency dependent conductivity expression. Field (T) WHO (J,10 19) r0 (Å) 0 2αr0 (r0)
0.0 0.09 6.35 3.42
0.01 0.41 6.54 3.18
0.05 1.38 6.30 2.94
0.10 0.62 6.14 2.92
0.25 1.03 6.17 2.88
Table 3 Percentage change in dielectric permittivity with field at higher frequencies. Frequency (kHz)
10
50
100
500
1000
Permittivity change % change
891-279 69
383-175 54
299-142 53
174-92 47
138-79 43
temperature over the frequency range from 20 Hz to 2 MHz in the presence of magnetic field. The total measured conductivity at a given frequency (ω) is given by
st ðωÞ ¼ sdc þ Aωs
ð5Þ
where sdc is the d.c. conductivity, s is frequency exponent and A is a constant. In the present case the frequency dependent data are fitted by Eq. (5) to extract the values of exponent for different magnetic fields. In Fig. 4a the points are the experimental data and the solid lines are the theoretical curves obtained using Eq. (5). The conduction process and the form of charge carrier can be obtained from the values of s and its field variation. The extracted values of frequency exponent (s) with magnetic field are plotted in Fig. 4c which show a decreasing trend of s value from 0.64 to 0.57 up to 1.75 T. The frequency dependent conductivity for overlapping large polaron tunneling is given by [27]
sðωÞ ¼
π4
ωR4ω e2 ðkTÞ2 N 2 ðEF Þ 12 2αkT þ W HO r 0 =R2ω
ð6Þ
0.50 1.61 6.04 2.84
0.75 1.15 5.79 2.78
1.00 1.20 5.73 2.70
1.25 1.47 4.90 2.32
1.50 1.19 5.08 2.38
1.75 1.40 5.03 2.30
decreasing trend with increasing magnetic field. Similar behavior has also been observed previously in 1-dimensional conjugated polymers [25]. The values of s increase to attain 1.0 as magnetic field reaches to zero. So, the conduction process can be attributed to either correlated barrier hopping or polaron tunneling. As the frequency dependent conductivity data matches well with the OLPT model it is assumed that the frequency exponent also behaves in the same manner which shows an exponential decreasing trend with increase in magnetic field. Therefore from the above analysis it is seen that the polarons are formed in the present chelated system. Like other amorphous semiconductors, in the present case also the frequency dependent conductivity has been analyzed on the basis of existing models and we have extended our investigation in the presence of magnetic field to understand the frequency dependent magnetotransport at room temperature. Besides synthesis, characterization, magnetism and magnetoresistance, the major finding in this work is the existing model of overlapping large polaron tunneling used for the variation of magnetic field instead of temperature. Values of dielectric permittivity as a function of frequency are also plotted at different magnetic fields as shown in Fig. 4d. Significantly large changes in permittivity values due to increase in magnetic field up to 1.75 T are observed. Percentage change in permittivity values at different frequencies is summarized in Table 3. It is noticed from the inset of this figure that the dielectric permittivity value decreases with increasing magnetic field. This is in agreement with the reduction of polaron size with increasing magnetic field which essentially decreases the polarization in the present complex.
where )1=2 ( 1 1 W HO 1 W HO 2 8αr 0 W HO þ ln Rω ¼ ln þ – – 4α ωτ0 kT ωτ0 kT kT being the tunneling distance, NðEF Þ is the density of states, W HO is the potential barrier or the polaron hopping energy, τ0 is the relaxation time, r 0 is the polaron radius and 2αr 0 is the reduced polaron radius. At temperature 300 K, the frequency dependent conductivity data have been fitted as shown in Fig. 4b. In this figure the solid lines are the theoretical curves and the points are the experimental data. The values of the parameters extracted from the fitting procedure are summarized in Table 2, from which it is observed that the size of r0 is comparable to large polaron. The values of r0, i.e., size of polaron radius show the decreasing trend and simultaneous increase of polaron hopping energy (WHO) as a consequence of decrease in overlapping area of potential well with increase in magnetic field. The degree of fitting is quite satisfactory and it is seen that the frequency dependent conductivity data are well fitted with the overlapping large polaron tunneling model [11]. As mentioned above that in the present study we have extracted the values of frequency exponent as a function of magnetic field instead of temperature. In the present work our objective is to understand whether the values of exponent as a function of magnetic field agree well with the OLPT model which gives the variation of frequency exponent with temperature [28,29]. From Table 2, it is noted that the polaron radius has a
4. Conclusions In summary, the frequency dependent magneto-transport along with the magnetization behavior in charge transfer [Co2(II)-(5-(4-PhMe)-1,3,4-oxadiazole-H þ -2-thiolate)5](OAc)4 complex has been investigated on the basis of existing models for ac charge transport with a modification incorporating field dependence to understand the nature of charge carrier, its size and response under magnetic field. It is seen that the existing models are well applicable to understand the frequency dependent magneto-transport.
Acknowledgments BKS acknowledges CSIR (Grant no. 09/080(0764)/2011-EMR-1), New Delhi, for awarding fellowship. SKS acknowledges financial support from the DST, New Delhi, Govt. of India, Project no. SR/ NM/NS-1089/2011.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jmmm.2014.04. 048.
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References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Multiferroic and magnetoelectric materials, Nature 442 (2006) 759–765. [2] T. Enoki, A. Miyazaki, Magnetic TTF-based charge-transfer complexes, Chem. Rev. 104 (2004) 5449–5477. [3] M. Clemente-Leon, E. Coronado, C. Marti-Gastaldoz, F.M. Romero, Multifunctionality in hybrid magnetic materials based on bimetallic oxalate complexes, Chem. Soc. Rev. 40 (2011) 473–497. [4] J.S. Miller, A.J. Epstein, W.M. Reiff, Ferromagnetic molecular charge-transfer complexes, Chem. Rev. 88 (1988) 201. [5] L.L. Li, K.J. Lin, C.J. Ho, C.P. Sunb, H.D. Yang, A coordination π–π framework exhibits spontaneous magnetization, Chem. Commun. 12 (2006) 1286. [6] H. Kobayashi, H. Tomita, T. Naito, A. Kobayashi, F. Sakai, T. Watanabe, P. Cassoux, New BETS conductors with magnetic anions (BETS ¼ bis(ethylenedithio)tetraselenafulvalene, J. Am. Chem. Soc. 118 (1996) 368. [7] M. Kurmoo, A.W. Graham, P. Day, S.J. Coles, M.B. Hursthouse, J.L. Caulfield, J. Singleton, F.L. Pratt, W. Hayes, L. Ducasse, P. Guionneaul, Superconducting and semiconducting magnetic charge transfer salts: (BEDT-TTF)4AFe(C2O4)3. C6H5CN (A ¼ H2O, K, NH4), J. Am. Chem. Soc. 117 (1995) 12209. [8] M. Zeng, W. Zhang, X. Sun, X. Chen, Spin canting and metamagnetism in a 3D homometallic molecular material constructed by interpenetration of two kinds of cobalt(II)-coordination-polymer sheets, Angew. Chem. 117 (2005) 3139–3142. [9] S. Triki, F. Thetiot, F. Vandevelde, J. Sala-Pala, C.J. Gomez-Garcia, New magnetic copper(II) coordination polymers with the polynitrile ligand (C[C(CN)2]3)2 and N-donor Co-ligands, Inorg. Chem. 44 (11) (2005) 4086–4093. [10] B.K. Shaw, S.K. Saha, Giant magnetoresistance and magneto-dielectric effects in a charge transfer metal complex for multiferroic applications, J. Phys. Chem. C 116 (39) (2012) 20700–20707. [11] S.R. Elliott, A.c. conduction in amorphous chalcogenide and pnictide semiconductors, Adv. Phys. 36 (2) (1987) 135–218. [12] M. Ciampolini, N. Nardi, Five-coordinated high- spin complexes of bivalent cobalt, nickel, and copper with tris(2-dimethylaminoethyl)amine, Inorg. Chem. 5 (1966) 42–44. [13] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed., Elsevier, Amsterdam, Oxford, New York, Tokyo (1984) 1984478–505. [14] A. Matsumoto, T. Fukumoto, H. Adachi, H. Watarai, Electrospray ionization mass spectrometry of metal complexes. Gas phase formation of a binuclear copper(II)-5-Br-PADAP complex, Anal. Chim. Acta 390 (1999) 193–199. [15] I. Spasojevic, I.B. Haberle, R.D. Stevens, P. Hambright, A.N. Thorpe, J. Grodkowski, P. Neta, I. Fridovich, Manganese(III) biliverdin IX dimethyl
[16]
[17]
[18]
[19] [20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
ester: a powerful catalytic scavenger of superoxide employing the Mn(III)/Mn (IV) redox couple, Inorg. Chem. 40 (2001) 726–739. A. Bencini, C. Benelli, D. Gatteschi, C. Zanchini, ESR spectra of low-symmetry high-spin cobalt(II) Complexes. 70 trigonal-bipyramidai pentakis (pico1ine N-oxide) cobalt(II) perchlorate, Inorg. Chem. 19 (12) (1980) 3839–3841. M. Baumgarten, C.J. Winscom, W. Lubitz, Probing the surrounding of a cobalt (II) porphyrin and its superoxo complex by EPR techniques, Appl. Magn. Reson. 20 (2001) 1–36. B.B. Wayland, A.E. Sherry, A.G. Bunn, EPR studies of 1:1 complexes of rhodium (II) and cobalt(II) porphyrins with s donor and π acceptor ligands: origins of rhodium(II) metalloradical reactivity, J. Am. Chem. Soc. 115 (1993) 7675–7684. M. Ikeda-Saito, H. Fujii, T. Yoshida, Heme oxygenase. Active site structure studied by EPR of cobalt(II) porphyrin-enzyme complex, RIKEN Rev. 24 (1999) 53–55. O. Kahn, Molecular Magnetism, VCH, New York, 1993. T. Ishida, T. Kawakami, S. Mitsubori, T. Nogami, K. Yamaguchi, H. Iwamura, Antiferromagnetic coupling of transition metal spins across pyrimidine and pyrazine bridges in dinuclear manganese(II), cobalt(II), nickel(II) and copper (II) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate complexes, J. Chem. Soc. Dalton Trans. 16 (2002) 3177–3186. D.P. Mills, F. Moro, J. McMaster, J. Slageren, W. Lewis, A.J. Blake, S.T. Liddle, A delocalized arene-bridged diuranium single-molecule magnet, Nat. Chem. 3 (2011) 454–460. F. Lloret, M. Julve, J. Cano, R. Ruiz-Garcia, E. Pardo, Magnetic properties of sixcoordinated high-spin cobalt(II) complexes: theoretical background and its application, Inorg. Chim. Acta 361 (1) (2008) 3432–3445. L. Shlyk, S. Kryukov, V. Durairaj, S. Parkin, G. Cao, L.E. De Long, Magnetic, and electronic properties of a Ba3InRu2O9 single crystal, J. Magn. Magn. Mater. 319 (2007) 64–68. T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds, Handbook of Conducting Polymers, 2nd revised ed., Marcel Dekker, New York, Basel, Hong Kong (1998) 199827–58. E.A. Silinsh, A. Klimkans, S. Larsson, V. Capek, Molecular polaron states in polyacene crystals. Formation and transfer processes, Chem. Phys. 198 (1995) 311–331. Muhammed Tan, Yuksel Koseoglu, Furkan Alan, Erdogan Senturk, Overlapping large polaron tunneling conductivity and giant dielectric constant in Ni0.5Zn0.5Fe1.5Cr0.5O4 nanoparticles (NPs), J. Alloys Compd. 509 (2011) 9399–9405. M. Bouhassoune, R. Charrour, M. Fliyou, D. Bria, A. Nougaoui, Polaronic and magnetic field effects on the binding energy of an exciton in a quantum well wire, J. Appl. Phys. 91 (1) (2002) 232–236. G.E. Pike, Ac conductivity of scandium oxide and a new hopping model for conductivity, Phys. Rev. B 6 (4) (1972) 1572–1580.
