A study of magnetic, specific heat and resistivity properties in Ca0.85Eu0.15MnO3 around the phase transition temperature

Journal of Magnetism and Magnetic Materials 357 (2014) 24–28

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A study of magnetic, specific heat and resistivity properties in Ca0.85Eu0.15MnO3 around the phase transition temperature Momin Hossain Khan a, Sudipta Pal a,n, Esa Bose b a b

Department of Physics, University of Kalyani, Kalyani, Nadia 741235, WB, India Department of Engineering Physics, B. P. P. I. M.T., Kolkata 700052, WB, India

art ic l e i nf o

a b s t r a c t

Article history: Received 27 November 2013 Received in revised form 10 January 2014 Available online 18 January 2014

Correlation between the magnetic, specific heat and resistivity properties around the phase transition temperature has been investigated in electron doped polycrystalline Ca0.85Eu0.15MnO3 (CEMO) manganite. The magnetic data show the existence of the magnetic phase transition around TC ¼122 K under cooling from the paramagnetic (PM) to ferromagnetic (FM) state. The critical exponents determined from the Arrott plot are higher than the value expected from the Heisenberg model because of short range ferromagnetic ordering in the sample. The resistivity sharply rises at the magnetic phase transition temperature. Specific heat measurement exhibits a distinct peak near TC that can be interpreted in terms of the magnetic phase transition. & 2014 Elsevier B.V. All rights reserved.

Keywords: Perovskite manganite Magnetic property Electrical property

1. Introduction Physical properties such as electrical resistivity, thermoelectric power, thermal conductivity, magnetization and specific heat have so far been studied extensively for the hole-doped manganites with predominating Mn3 þ [1–4] whereas there are very few studies carried out on electron doped manganites with predominating Mn4 þ ions [5–7]. Nevertheless, they are of considerable interest because the phase diagrams of hole and electron doped manganites are qualitatively different [8,9]. Most of the authors have reported on electron-doped polycrystalline CaMnO3 manganites doped by Tb, Yb, Nd, Ho [10,11], Lu [12], La, Y, and Ce [13,14]. Some of the publications on electron-doped manganites were devoted to studying the magnetic and transport properties of polycrystalline Ca1 xEuxMnO3 (0rxr0.3) [15,16], but these works have not been continued. Recently Naumov et al. studied Ca0.85Eu0.15MnO3 single crystal and discussed their results with the properties of polycrystalline sample [17]. The Ca0.85Eu0.15MnO3 single crystal becomes a C type antiferromagnet with monoclinic crystal structure exhibiting charge/orbital ordering at 150 K. However Pnma crystal structure of polycrystalline Ca0.85Eu0.15MnO3 shows the paramagnetic to ferromagnetic phase transition [17]. Differences in the properties of single crystal and polycrystalline sample with the same content of Eu in CEMO are associated with the ordering of oxygen vacancies that appear during the crystal growth. Studies on magnetic property are significant in this system as the phase transition largely affects the electrical transport properties [18,19]. In this temperature

n

Corresponding author. Tel.: þ 91 33 2582 2505; fax: þ 91 33 2582 8282. E-mail address: [email protected] (S. Pal).

0304-8853/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2014.01.013

dependent magnetic study, the critical exponents β, γ and δ provide important information concerning the interaction mechanisms near the paramagnetic-to-ferromagnetic (PM–FM) phase transition. These parameters also provide valuable information regarding the lattice dimension, dimension of order parameter and range of interaction. The phase transition in polycrystalline manganites can be correlated to the value of critical exponents to know whether they follow the mean-field theory (long range interaction) or the Heisenberg model (short range interaction) originates from magnetic polaron. Further, the observed entropy change associated with the specific heat peak could provide information about the presence of magnetic inhomogeneties. In this paper, we report a systematic study of magnetization, specific heat, and electrical resistivity as a function of temperature in Ca0.85Eu0.15MnO3 polycrystalline sample. 2. Experimental procedure The polycrystalline sample, CEMO was prepared by using a standard solid state reaction method. The precursors CaO, MnO2 and Eu2O3 (each of purity 99.9%) were mixed in proper stoichiometric ratio, grounded and then preheated at 1170 K for 24 h. The mixed powder thus obtained was regrounded, pelletized in bar shape and sintered at 1520 K for 12 h. Finally the pellets were sintered at 1770 K for another 12 h and cooled systematically at the rate 21/min till 800 K and then furnace cooled to room temperature. Resistivity measurement was performed by standard four-probe method. The specific heat measurement was carried out by means of the relaxation method and the data were collected during the cooling process at room temperature under

M.H. Khan et al. / Journal of Magnetism and Magnetic Materials 357 (2014) 24–28

zero fields, using the specific heat option of the Quantum Design physical property measurement system. The magnetic properties as a function of temperature T and field H were measured using a Quantum Design superconducting quantum interfering device.

In order to make sure that the sample is single phase CEMO compound, X-ray diffraction (XRD) data was analyzed. Fig. 1 shows the XRD pattern of the sample taken at room temperature. All the reflection peaks of the X-ray profile were determined from Rietveld analysis of the XRD data using fullprof suite. Good agreement between the observed and calculated interplanar spacing (d-values) suggests that the structure of the compound is orthorhombic with space group Pnma at room temperature with a ¼5.2852 Å, b¼ 7.4796 Å, c¼5.3103 Å and the cell volume¼ 209.9224 Å3. 3.2. Magnetic properties Fig. 2 shows the temperature dependence of the magnetization M (T) of the sample. Both the zero-field cooled (ZFC) and field cooled (FC) data undergo a magnetic transition from paramagnetic to ferromagnetic state at TC ¼ 122 K defined as the inflection point of temperature dependence of the magnetization curve, which can be found from the peak position of the dM/dT curve. This transition is markedly sharp, as revealed by the narrow width of the dM/dT peak (shown in the inset of Fig. 2). The FC and ZFC dc magnetization bifurcates very close to TC. Further, the ZFC magnetization shows a gradual drop at 44.23 K, which can be attributed to partially canted spins in the ferromagnetic state of the sample. Fig. 3 shows M vs H curves for CEMO recorded at temperatures 5 K and 300 K. In the temperature range T oTC, magnetization increases nonlinearly with increasing magnetic field; however the saturation magnetization is not attained. Meanwhile, in the region T 4TC the M vs H curves become linear. Such behavior has been observed in the ferromagnetic and paramagnetic regimes of perovskite manganites [20–22]. A hysteresis loop observed at 5 K indicates ferromagnetism at low temperature whereas the

dM/dT (emu/gm/K)

0.3

5

M (emu/gm)

3.1. Structural properties

6

4 3

0.2

0.1

0.0

ZFC FC

2

0

50 100 150 200 250 300

T(K)

1 0 0

50

100

150

200

250

300

T (K) Fig. 2. The temperature dependence magnetization of Ca0.85Eu0.15MnO3 at the magnetic field H¼ 50 kOe in the ZFC and FC regimes. Inset shows the variation of dM/dT vs T.

60

5K 300 K

40 20

M (emu/gm)

3. Results and discussion

25

0 -20 -40 -60 -80000

-40000

0

40000

80000

H (Oe) Fig. 3. Field dependence of the magnetization of Ca0.85Eu0.15MnO3 at T ¼ 5 K and 300 K.

increase in linearly increasing magnetization with the field indicates paramagnetism near room temperature as shown in Fig. 3. To get a deep insight into the magnetic phase transition and the influence of the interplay among various degrees of freedom at TC in CEMO, we have done a critical analysis of magnetic properties in terms of the Arrott plot, M 2 vs H/M shown in Fig. 4. The positive slope on the M 2 vs H/M plot indicates second-order magnetic transition in the present sample [22]. Mathematically, the second order transition can be described by the critical exponents β, γ and δ. The exponents are obtained through asymptotic relations [22]

Fig. 1. X-ray diffraction pattern and the corresponding Rietveld refinement for Ca0.85Eu0.15MnO3 manganites.

M S ðT; 0Þ ¼ M 0 ð  εÞβ

ð1Þ

χ 0 1 ðTÞ ¼ ðh0 =M 0 Þεγ

ð2Þ

M ¼ DH1=δ

ð3Þ

where M0, h0, and D are the critical amplitudes, and ε¼(TTC)/TC is the reduced temperature. It was observed that the values of the critical exponent embrace the mean-field theory (β¼0.5, δ¼1.0), 3D

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M.H. Khan et al. / Journal of Magnetism and Magnetic Materials 357 (2014) 24–28

25000 T = 117 K T = 118 K T = 119 K T = 120 K = 122 K T = 123 K T = 124 K T = 126 K T = 127 K

M2 (emu/gm)2

20000

15000

10000

5000

0 0

100

200

300

400

500

600

H/M (Oe-gm/emu) Fig. 4. Isotherms of M2 vs H/M (Arrott plot) at different temperatures close to TC.

isotropic nearest-neighbor Heisenberg model (β¼0.365, δ¼1.33), and 3D Ising model (β¼ 0.325, δ¼1.24). However, to obtain exactly these parameters, we used the modified Arrott plot [17] by plotting MS vs T and χ 0 1 ðTÞ vs T as shown in Fig. 5(a) (left and right panel). We have obtained β¼0.38870.023 and γ¼ 0.98170.007, by best fitting M S ðTÞ to Eq. (1) and χ 0 1 ðTÞ to Eq. (2), respectively. Concurrently, two TC values obtained from Eqs. (1) and (2) are 126.537 0.71 K and 120.2970.54 K, respectively. Their average value is thus TC ¼123.41 K. In addition, the magnetization data was also analyzed in terms of the Kouvel–Fisher (K–F) method and the values of the critical exponent were compared with the values obtained from the Arott plot. According to Kouvel and Fisher the equation of state can be written as [23] M S ðTÞ½dM S =dT  1 ¼ ðT  T C Þ=β

ð4Þ

χ o 1 ðTÞ½dχ o 1 =dT  1 ¼ ðT  T C Þ=γ

ð5Þ

According to these equations, the plots of M S ðTÞ½dM S =dT  1 and as a function of T should yield a straight line with slope 1/β and 1⧸γ, respectively and the intercepts on T axes equal to TC. The results are shown in Fig. 5(b). We have found the critical exponent β¼ 0.3897 0.003 and γ¼0.9917 0.004. The isotherm magnetization data near TC have been plotted according to Eq. (3) in log–log scale as shown in Fig. 5(c). From the best fit with Eq. (3) at high magnetic field, we have obtained δ¼3.337 0.005. The critical exponents from the scaling analysis are related to the Widom scaling relation δw ¼ 1 þ γ=β [24]. It is determined that δw to be about 3.54. Thus, the critical exponents obtained from K–F method obey the Widom scaling relation. The value of critical exponent for our CEMO is reliable and comparable with other manganites [25–28]. Gehring and Coombes [29] found that a Mnion triplet containing one hole, i.e., a Mn3 þ –Mn4 þ –Mn3 þ cluster has a significant binding energy of about half the binding energy of bulk. These large spin moments enhance the dipole–dipole interaction in the case of Heisenberg model. The obtained value of β in our sample is higher than that expected in the Heisenberg model. This might be due to the above mentioned dipole–dipole interactions, which reveal an existence of short-range ferromagnetic order in CEMO. The magnetic equation of state in the critical region can be written as

χ o 1 ðTÞ½dχ o 1 =dT  1

M S ðH; εÞ ¼ jεjβ f 7 ðH=jεjβ þ γ Þ

ð6Þ

Fig. 5. (a) Temperature dependence of the spontaneous magnetization MS and the inverse initial susceptibility χ0. The solid lines are the best fit to Eqs. (1) and (2), respectively. (b) Kouvel–Fisher plots for the spontaneous magnetization and the inverse initial susceptibility. The solid lines are the best fit to Eqs. (4) and (5), respectively. (c) The critical isotherms on a log–log scale and the solid line is the best fit to Eq. (3). The inset shows scaling plots for below and above TC using β and γ determined by the Kouvel–Fisher method.

where f þ for T4TC and f  for T oTC, respectively, are regular functions. Eq. (6) implies that M|ε|  β as a function of H|ε|  (β þ δ) produces two universal curves: one for temperatures below TC and the other for temperatures above TC. In order to check whether our data in the critical region obey the magnetic equation of state as described by Eq. (6), M|ε|  β as a function of H|ε|  (β þ δ) is plotted in the inset of Fig. 5(c). Interestingly, all the points fall on two curves, one for T oTC and the other for T 4TC. This suggests that the values of the critical exponents and TC are reasonably accurate and in agreement with the scaling hypothesis

M.H. Khan et al. / Journal of Magnetism and Magnetic Materials 357 (2014) 24–28

18

3.3. Specific heat

1.65 1.60

15

1.55

12

ρ (Ω cm)

The specific heat measurement Cp(T) has been carried out from 80 to 300 K for CEMO and the results are shown in Fig. 6. The sample shows a distinct peak in its Cp(T) at PM–FM transition. This indicates that the specific heat peak arises due to magnetic ordering in the sample, i.e. the magnetization data could be correlated with the specific heat data. In order to investigate the nature of the magnetic transition at TC, we estimated the excess specific heat jump ΔC at TC. So we excluded the phonon contribution in specific heat. To estimate ΔC due to the magnetic ordering, a smooth background was drawn tangential to both end points and is shown as a solid line (Fig. 6). ΔC has been found to be 2.22kB at TC, which is very close to 2.2kB for S¼ 3/2 for the Heisenberg system [30]. Similar behavior has also been observed in many electron doped manganites [31]. Further, the excess entropy (ΔS) can be evaluated by integrating the area under ΔC/T vs T curve (not shown here). We have found that the value of ΔS is also less than the theoretical value Rln 2 for a PM–FM transition (where R is the ideal gas constant), which indicates the magnetic inhomogeneity or partially canted spins in the ferromagnetic state of the sample, once again corroborating the results obtained from magnetic data analysis.

27

1.50

9 1.45

230 240 250 260 270 280 290

6 3 0

90

120

150

180

210

240

270

300

T (K) Fig. 7. Variation of resistivity with temperature for Ca0.85Eu0.15MnO3 manganites. Inset shows the enlarged view of resistivity at high temperature.

3.4. Resistivity properties Fig. 7 shows resistivity ρ vs T curves for CEMO sample while cooling from 300 K to 80 K. In contrast to the insulating behavior of resistivity in the paramagnetic phase of hole doped compounds [32], ρ (T) of the electron-doped compound decreases as temperature decreases from 300 K and shows metal-insulator transition at TMI ¼238 K. Hence, the temperature dependent resistivity behavior of the samples can be distinctly separated into two regions over the temperature region 80–300 K: PM metallic region 238–300 K and FM semiconducting region 80–238 K. In the semiconducting region, the resistivity data sharply rises near 122 K, i.e. there is an apparent change in the slope of resistivity near the magnetic ordering temperature TC (Fig. 7). It indicates a close correlation between the magnetic state and the transport behavior and justifies that the onset of the ferromagnetic exchange interaction promotes the transfer of charge carriers.

The sample shows second order PM–FM phase transition around 122 K. The obtained value of critical exponent β calculated using the Arrott plots is higher than that expected in the Heisenberg model due to the dipole–dipole interactions. The field dependent magnetization data fall on two master curves: one for ToTC and the other for T4TC indicating that the values of the exponents and TC are reasonably accurate and confirms PM–FM phase transition in CEMO manganite. Specific heat measurement exhibits a distinct peak near TC due to the magnetic phase transition. The resistivity data sharply rises at TC ¼122 K. So, the magnetic, specific heat and resistivity data indicate towards a second order PM–FM phase transition in this CEMO manganite.

Acknowledgment

4. Conclusion

The work is funded by DST Fast Track, Govt. of India Project no. SR/FTP/PS-101/2010. Author M. H. Khan would like to acknowledge the University of Kalyani for providing the fellowship.

The magnetic, specific heat and resistivity properties of polycrystalline CEMO have been studied over a wide range of temperature.

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105

75 0.12

60

ΔC/T (J mol K )

CP (J mol-1 K-1)

90

45

0.09 0.06 0.03 0.00

30

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50

100

150

200

100

150 200 T (K)

250

250

300

T(K) Fig. 6. Temperature dependent specific heat. Inset shows ΔCP/T vs T curve.

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