Physical Properties Of Eu3Ba2Mn2Cu2O12 N. Kumar1, S. Rayaprol2,*, A. Dogra3, S. D. Kaushik2, K. Singh1, N. K. Gaur1, G. Anjum4, Y. Kumar5, Ravi Kumar6, K. K. Iyer7 and E. V. Sampathkumaran7 1
2
Department of Physics, Barkatullah University, Bhopal – 462026 UGC-DAE Consortium for Scientific Research, Mumbai Centre, R-5 Shed, BARC, Mumbai - 400085 3 National Physical Laboratory, New Delhi – 110012 4 Department of Physics, Aligarh Muslim University, Aligarh - 202002 5 Inter University Accelerator Centre, New Delhi - 110067 6 National Institute of Technology, Hamirpur - 177005 7 Tata Institute of Fundamental Research, Mumbai – 400005 *Corresponding author email:
[email protected] /
[email protected]
Abstract. The physical properties of the manganocuprate compound, Eu3Ba2Mn2Cu2O12 (Eu-3222) are investigated using X-ray diffraction, resistivity, magnetization and dielectric measurements. The structure of the Eu-3222 compound is made by the intermixing of Eu-123 (oxygen deficient triple perovskite block) and Eu-214 (K2NiF4 type single rocksalt like block). Electrical resistivity data show insulating behavior down to the measurable temperature and exhibits a conduction mechanism which can be explained on the basis of 3D variable range hopping model. Magnetization data shows a antiferromagnetic behavior at 5 K, however without any clear signature of magnetic ordering in the temperature range 5–300 K. Dielectric (Capacitance), measured as a function of temperature at various frequencies exhibits frequency dependence, indicating glassy relaxation of dipole moments. Keywords: Manganocuprate, Eu-3222, resistivity, magnetization, dielectric PACS: 75.50.Lk, 75.47.Lx, 77.22.-d
and Mn can give rise to interesting physical properties. In this short communication, we present the physical properties of Eu-3222 compounds obtained from electrical resistivity (ρ), magnetization (M) and dielectric (C) measurements.
INTRODUCTION The discovery of high temperature superconductivity and colossal magnetoresistance were among two prominent achievements of material science in last two decades. The discovery of superconductivity in a layered cuprate compound s initiated a continuous search for novel compound with properties desirable for applications in devices.1 Another class of compound which commanded considerable attention is manganites for their colossal magnetoresistance properties (CMR).2 The manganocuprate, Eu3Ba2Mn2Cu2O12 provides a unique opportunity to investigate the physical properties. This compound has a coordination of Cu similar to that in high Tc cuprates (R123 type structure). In the triple perovskite block, one layer of Cu is replaced by Mn. From the resistivity measurements and by charge balance, the mixed valence state of Cu can be ruled out; leaving Mn in the mixed valence state of Mn 3+/4+ is 1:1 ratio.3-5 This is uncommon situation in the manganite compounds, which gives rise to the charge-ordering. Therefore the presence of both Cu
SAMPLE PREPARATION Polycrystalline sample was prepared by solid state reaction of high purity starting compounds, Eu2O3, BaCO3, MnO2 and CuO taken in stoichiometric quantities. Details of the sample preparation are similar to the one reported in Ref.6
RESULTS AND DISCUSSIONS X-ray Diffraction and Crystal Structure Powder X-ray diffractogram was analyzed for phase formation of Eu-3222, based on the paper by Field et.al .4 Fig. 1 shows the XRD pattern of Eu-3222
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refined using Rietveld method. The refinement parameters and lattice constants are shown in the figure. The crystal structure shown in Fig. 1 clearly exhibits the R-123 type triple perovskite block, and the K2NiF4 type single rock-salt type block. Our results of XRD structural analysis of Eu-3222 are in good agreement with the neutron diffraction and XRD / HRTEM studies by earlier groups.3,4
Magnetic Measurements
Eu-123 triple perovskite block
Eu2MnO4 block
Magnetic measurements were carried out with the help of a SQUID magnetometer (Quantum Design, USA). The magnetic susceptibility (χ = M/H) measured as a function of temperature in a field of 5kOe is shown in the main panel of Fig. 3. χ(T) increases as temperature is decreased and does not exhibit any kind of magnetic ordering down to the lowest temperature. Susceptibility was measured in a small field of 20 Oe and in zero field cooled (ZFC) and field cooled (FC) state of the sample. A downturn in the plot of χZFC(T) around 5 K is seen, indicating weak antiferromagnetism at this temperature. There is no bifurcation between the ZFC and FC curves. The magnetic behavior of Eu-3222 is thus different from its isostructural counterpart, Gd3Ba2Mn2Cu2O12.6 Magnetization was measured as a function of varying magnetic field at 5 K and is shown as second inset in Fig. 3. M increases non-linearly with increasing H and does not saturate till 50 kOe. However, M(H) does not follow the Brillouin function, therefore existence of though weak antiferromagnetism in Eu-3222 at this temperature is proposed. Fig. 3 also shows χ-1(T) for Eu-3222 for χ(T) measured in H = 5 kOe. The linear behavior of χ-1(T) is seen till 150 K, below which it deviates from linearity. From the fit to the Curie-Weiss law, χ = C/(T-θp), the paramagnetic Curie temperature, θp is found to be 11.5 K. The effective Bohr magnetron number is µeff = 8.8 µB/fu, consistent with the mixed valence state for and divalent state for Cu. However, there are complexities in determining absolute valence state for Cu and Mn from magnetization studies without the knowledge of Eu valence. Spectroscopic studies such as Eu Mössbauer, L3 edge spectroscopy are therefore more desirable. It is worth mentioning here that our preliminary Eu Mössbauer studies indicate that the Eu remains in trivalent state till 5 K.10
FIGURE 1. Rietveld refined XRD pattern for Eu3Ba2Mn2Cu2O12. A crystal structure of Eu-3222 is also depicted in the figure.
Resistivity studies The resistivity was measured as a function of temperature using a closed cycle refrigerator and employing four-probe method, on a rectangular piece of a well sintered pellet of Eu-3222. Resistance increases with decreasing temperature and goes beyond the measurement limit below 110 K. ρ(T) exhibits semi-conducting behavior in the entire temperature range. Fig. 2 shows the plot of resistivity as a function of temperature. The fit of the Arrhenius law, ρ = ρ0exp(-Ea/kBT) yields an activation energy (Ea) of 0.17 eV, which is close to the value obtained for charge-ordered manganite of the type CaMn7O12, Ca2-xPrxMnO4, La2-2xCa1+2xMn2O7.7-9 The inset of Fig.2 shows the plot of lnρ vs. T-0.25. The straight line drawn as a guide to the eye, exhibits the linearity in the plot suggesting 3D-variable range hopping (VRH) type conduction mechanism in Eu-3222.
FIGURE 2. ρ vs. T for Eu3Ba2Mn2Cu2O12. The inset shows the plot of lnρ vs. T-0.25 indicating the 3-D VRH type conduction mechanism.
FIGURE 3. χ vs. T for Eu3Ba2Mn2Cu2O12 measured in H = 5 kOe. The plot also shows the behavior of χ-1(T). The straight (red) line passing through the data points is the
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Where, ν0 is the pre-exponential factor or also known as the characteristic frequency (usually of the order of 1011 – 1012 Hz), Ea/kB represents the activation barrier and Ta is known as the freezing temperature. Tp is the temperature of maxima in C(T, ν) plot. In the inset to
Curie-Weiss fit. The top inset shows the plot of χ(T) measured in ZFC-FC state of the sample in a field of 20 Oe. The bottom inset shows the M vs. H behavior at T = 5 K.
Dielectric Properties The resistivity and magnetization measurements show that Eu-3222 is a semi-conductor and exhibits properties similar to charge ordered manganites, therefore it would be interesting to investigate the dielectric properties of these compounds. Keeping in view this objective the dielectrics was measured in the temperature range 100 – 300K using a HP LCR meter (model no. HP4192). The capacitance for Eu3222 is shown in Fig. 4. Starting from the lowest frequency of the present study, 75 kHz, we find an increase in the capacitance (C, in pF) with increasing temperature.
CONCLUSIONS The physical properties of a manganocuprate compound, Eu3Ba2Mn2Cu2O12 have been investigated using resistivity, magnetization, and dielectric studies. The compound exhibits semi-conducting behavior and a 3D-VRH type conduction mechanism. Magnetically it is a weak antiferromagnet at very low temperatures and exhibits glassy type relaxation of dipole moments at higher temperatures. Detailed investigations to understand the correlation between the mixed valence state of Mn and / or Cu to gain more insight into the physics of the manganocuprates are currently under progress.
REFERENCES 1.
2.
3.
FIGURE 4. C vs. T for Eu3Ba2Mn2Cu2O12 measured at various frequencies (ν). The inset shows the plot of Tp vs. 1/ln(ν0/ν) - the Vogel-Fulcher relationship. The straight line passing through the data points is the linear fit to the VF-law.
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C increases by five times from 100 pF at 100 K to 500 pF at 300 K. In the derivative plot, there is a peak seen around for 135 K. This peak is frequency dependent, and with increasing the driving frequency (ν) the peak shift to higher temperatures. The frequency dependence of C(T) exhibits dielectric relaxation, and this can be explained by Vogel-Fulcher (VF) law, given as: Fig. 4, we have plotted the VF relation for Eu3222. The straight line passing through the data lines is the linear fit of the data points, revealing that the dielectric relaxation in the manganocuprates is glassy type.
6.
7.
8.
9.
10.
J. C. Philips, Physics of High Tc superconductors, London: Academic Press Inc. 1989. Y. Tokura, Colossal Magnetoresistive Oxides, Amsterdam: Gordon and Breach Publishers, 2000. M. Hervieu, C. Michael, R. Genouel, A. Maignan and B. Raveau, J. Sold State Chem. 115, 1 (1995). M. A. L. Field, C. S. Knee, and M. T. Weller, J. Solid State Chem. 167, 237 (2002). Matsubara, N. Kida, R. Funahashi, K. Ueno, H. Ishikawa, and N. Ohno, J. Solid State Chem. 141, 546 (1998). S. Rayaprol, K. Singh, S. Jammalamadaka, N. Mohapatra, N. K. Gaur and E. V. Sampathkumaran, Solid State Commun. 147, 353 (2008). Castro-Couceiro, S. Vilar- Yáñez-Vilar, M. SánchezAndújar, B. Rivas-Murias, J. Rivas, M. A. SeñarĩsRodríguez, Prog. Solid State Chem. 35, 379 (2007). S. Yáñez Vilar, A. Castro-Couceiro, B. Rivas-Murias, A. Fondado, J. Mira, J. Rivas, M. A. Señarís-Rodríguez, Z. Anorg. Allg. Chem. 631, 2192 (2005). A. Castro-Couceiro, M. Sánchez-Andújar, B. RivasMurias, J. Mira, J. Rivas, M.A. Señarís-Rodríguez, Solid State Sci. 7, 905 (2005). Rayaprol et. al (unpublished result).
− Ea ( − ) k T T B p a
ν = ν 0 exp
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