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High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound
Accepted Manuscript High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound A. Coşkun, E. Taşarkuyu, A.E. Irmak, S. Aktürk PII:
S0925-8388(16)30231-6
DOI:
10.1016/j.jallcom.2016.01.230
Reference:
JALCOM 36581
To appear in:
Journal of Alloys and Compounds
Received Date: 13 December 2015 Revised Date:
29 January 2016
Accepted Date: 29 January 2016
Please cite this article as: A. Coşkun, E. Taşarkuyu, A.E. Irmak, S. Aktürk, High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.230. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound A. Coşkun, E. Taşarkuyu, A.E. Irmak and S. Aktürk Department of Physics, Faculty of Sciences, Mugla Sitki Kocman University, 48000 Mugla, Turkey Magnetic Materials Laboratory, Research Laboratories Center, Mugla Sitki Kocman University, 48000 Mugla, Turkey
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Corresponding author e-mail:[email protected]
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Abstract
In this work, the structural, electrical, magnetic and magneto-caloric properties of the La0.70Ca0.21Ag0.09MnO3 compound were investigated. The compound was prepared by the sol gel method and sintered at 1000 oC for 24
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hours. Powder X-ray diffraction pattern (XRD) analyzes reveals a single phase orthorhombic structure with the Pnma space group as shown by the Rietveld analysis. The grainy morphology was confirmed with a grain size variation of 0.5-1.5 µm by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. High resolution transmission electron microscopy (HR-TEM) results strongly support an orthorhombic structure with the parameters calculated from the XRD data. Ferromagnetic-paramagnetic phase transitions occur at ~263 K (Curie temperature, TC) and ~265 K (metal-insulator, TMI). The sample undergoes a sharp metal– insulator transition at TMI following the sharp paramagnetic–ferromagnetic transition TC. From the isothermal
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magnetization measurements (M-H), the maximum magnetic entropy change |∆SM| was calculated to be ~4.8 J/kgK for an applied field change of 1 T, which is greater than that of pure Gd.
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1. Introduction
Due to the intrinsic problems of conventional vapor compression in refrigeration
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technology (such as environmental pollution and low efficiency), there is intense ongoing research on materials that have a potential use in magnetic cooling or magnetic refrigeration technology. Manganite-based compounds (generally in the form of Ln1-xTmxMnO3, where usually Ln is a lanthanide and Tm is a transition metal) which exhibit magnetic phase transitions near room temperature with a large magneto-caloric effect (MCE) are good candidates for use in magnetic refrigeration technology because of their relative low cost compared to counterparts containing expensive rare earth elements [1-6]. Lanthanum manganite doped with a monovalent or divalent element are known to have a perovskite structure, and exhibit a transition in their magnetic behavior, such as paramagnetic (PM) to ferromagnetic (FM) phase transition at the Curie temperature, TC. In
ACCEPTED MANUSCRIPT order for a material to be useful in magnetic cooling technologies (for a wide commercial use, such as in household appliances), it has to have a magnetic phase transition around room temperature and also high magnetic MCE in response to a moderate applied magnetic field change, µoH. To achieve this goal, manganite-based perovskite compounds are currently being investigated with monovalent and divalent element substitution.
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In the literature, many reports can be found on monovalent and divalent element doped LaMnO3 (La1-xAxMnO3) compounds. The Ca-doped LaMnO3 (La1-xCaxMnO3) series is especially attractive because of its high magnetic entropy change around room temperature [7-9] and for understanding the underlying physics behind it.[10-12] It is known that the
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structural, electrical and magnetic properties of doped LaMnO3 compound depend on concentration, oxidation state, and ionic radius of dopant elements and resulting Mn+4/Mn+3 ratio in the Mn-site. Ionic radius differences between dopant elements and La in the A-site
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lead a change in average ionic radius, known as the mismatch effect. Additionally, monovalent or divalent element doping in the A-site also changes the number of Mn3+ and Mn4+ ions, resulting a change in B-site average ionic radius because of the differences in ionic radii of Mn3+ and Mn4+. Even small variations in the crystal structure causes distortion MnO6 octahedra, leading a variation in Mn-O-Mn bond angle and Mn-O bond length, in turn,
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affecting almost all physical properties of these compounds.
Notably, La1-xCaxMnO3 compounds prepared by different techniques and applied different heat treatment procedure have diversity in their physical properties even for the same composition. Hence the crystal structures, magnetic phase transition temperatures, and
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maximum magnetic entropy change values are reported differently of this compound [7, 1214]. In the literature, the recorded MCE value of La0.70Ca0.30MnO3 compound is found to be
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6.25 J/kgK at TC=216 K for a magnetic field change of 1 T [7]. This value is nearly two times larger than MCE value of pure gadolinium. In our previous work, we found an increased maximum magnetic entropy change (4.1 J/kg K) at 273 K at a low applied magnetic field for the La0.65Ca0.35MnO3 compound [6]. In another study, we found maximum magnetic entropy change (2.1 J/kg K) at 290 K in a low applied magnetic field for the La0.75Ag0.25MnO3 compound [15]. It is well known that the magnetic properties of the compounds are mostly influenced by the average ionic radius of the A and B-sites and the Mn3+/Mn4+ ratio. The substitution of the monovalent metal ions (K+, Na+, Ag+) for Ca2+ changes the Mn3+ and Mn4+ ratio. It should be emphasized that as a result of Ag doping in A-site, while the average ionic radius of the A-site increases, the average ionic radius of the B-site decreases. This
ACCEPTED MANUSCRIPT happens by altering the valance state of Mn ions from Mn3+ to Mn4+, since the ionic radius of Mn4+ is smaller than Mn3+. In previous works, we had showed that the La0.65Ca0.35MnO3 compound has high MCE value at low temperature and La0.75Ag0.25MnO3 has low MCE value at room temperature. Therefore, in order to achieve the optimization of TC and ∆SM parameters of the LaCaMnO3
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and LaAgMnO3 based compounds, we have studied the structural, electrical, magnetic and magnetocaloric properties of La0.70Ca0.21Ag0.09MnO3 compound in which divalent (Ca2+) and monovalent (Ag+) element substitutions for La3+ were made simultaneously. The nature of the double-exchange and the super-exchange interactions and the strength of these interactions
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depend on Mn–O bond distance and the Mn–O–Mn bond angle. The Mn+4/Mn+3 ratio of the studied compound is 0.39/0.61=0.6393 that is different from La0.70Ca0.30MnO3 compound for 0.33/0.67=0.4285. It is expected that the possible changes in the Mn–O bond length and in the
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Mn–O–Mn bond angle due to the change in average ionic radius of B-site and Mn+4/Mn+3 ratio give rises to a local distortion (the Jahn–Teller distortion) of the MnO6 octahedra which affects the magnetic properties of the materials. 2. Experimental Procedure
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La0.70Ca0.21Ag0.09MnO3 compound was prepared by the sol-gel method, sintered at 1000 oC for 24 hours in air and cooled down to room temperature in the furnace. The details of the sol-gel preparation procedure can be found elsewhere [15, 16]. Powder X-ray diffraction at room temperature (XRD) was performed by a Bruker D8
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advanced x-ray diffractometer using CuKα1. Scanning electron microscopy (SEM) investigations were performed using a JEOL SEM 7700F, equipped with an energy dispersive
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spectrometer (EDS) and back scatter electron (BSE). Atomic force microscopy (AFM) studies were carried out by Solver Pro model AFM from NT-MDT. The high resolution transmission electron microscopy (HR-TEM) images of the sample were taken using a JEOL JEM 2100F. The magnetic properties of the sample were investigated using a quantum design PPMS with a closed cycle helium cryostat from 5 K to 320 K with magnetic fields up to 5 T. The TC value of the compound was determined from the temperature dependent magnetization measurements (M-T) at an applied field of 100 Oe. In the measurement sequence, the sample was first cooled down to 5 K under zero fields, and magnetization was measured while warming up the sample to 320 K with an applied field of 100 Oe (ZFC-FW). Then, under the same magnetic field, the magnetization was measured again while cooling down to 5 K (FC).
ACCEPTED MANUSCRIPT For magnetic entropy change determination, field dependent magnetization (M-H) measurements were performed by changing µ oH from 0 to 5 T around TC value exhibited by the sample with constant temperature intervals of 4 K between 260 K and 320 K. The temperature dependent resistivity (R-T) measurement was performed from 320 K down to 10 K with a standard four probe technique using a closed cycle helium cryostat from Cryo
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Industries. 3. Results and Discussions
AFM and SEM studies were used to investigate the surface morphology and grain size
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distribution of the compound. AFM images (2-D and 3-D) of the sample surface with a size of 10x10 µm2 which taken by contact mode are given in Figure 1a (2-D) and Figure 1b (3-D). In
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Figure 1c, a line profile spectrum of the surface shows the grain size distribution corresponding to the diagonal line in the Figure 1a. The grain size varies between 0.5 to 1.5 µm and these grains constitute grain clusters with a size of nearly 10 µm. A backscattered electron micrograph (BSE) with a magnification of 5000 is showing in Figure 2 to emphasize the compositional differences due to the contrast discrepancy in the
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image. The sample surface has almost identical contrast except the dark gray areas indicating the depth. In many works in which Ag is used as a dopant element, it has been observed that Ag dissociates from the perovskite phase and accumulates at inter-grain vacancies as metallic Ag, resulting in formation of impurity phases [16-18]. In this work, we did not observe
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metallic Ag impurity phases, as can be seen from the image. This is an indicator that the chosen heat treatment temperature was well suited for the Ag-doped compound.
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In Figure 3, the EDS profile and corresponding weight fractions of the elements are given. The actual chemical composition was calculated assuming that the La content remains unchanged (that is, the same as the targeted La content), as La0.70Ca0.21Ag0.088Mn0.99O3+x, which is almost the same as the targeted composition. The room-temperature XRD pattern is shown in Figure 4. The crystal structure of the
sample was refined by the Rietveld profile matching method, using the freely available FullProf Suit software. It was found that the sample has an orthorhombic crystal structure with the Pnma space group, and contains no impurity phase, neither manganese oxides nor metallic Ag, in agreement with BSE results mentioned above. The lattice parameters of the sample were calculated to be a=5.469280 Å, b=7.727343 Å and c=5.468515 Å.
ACCEPTED MANUSCRIPT The microstructure of the sample was investigated using HR-TEM. An image of the sample is given in Figure 5a. In Figure 5b, it was acquired by two successive Fourier Transforms of the real image (Figure 5a) in order to filter out unwanted noise on the images. The line profiles
are also given in Figure 6a and 6b to emphasize inter-atomic distances. The average distances between the most apparent atoms (electron clouds) are found to be 2.80 ± 0.02 Å and 3.90 ±
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0.02 Å. It can also be found that the planes (lines in 2D image) with separations of 5.60 ± 0.02 and 7.80 ± 0.02 Å (see Figure 6a and 6b) and the obliquity between the planes strongly support the orthorhombic structure with the parameters calculated from the XRD data. Figure 7 shows the zero magnetic field electrical resistivity,
R(T), of
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La0.70Ca0.21Ag0.09MnO3 as a function of temperature (320 K to 10 K). As the temperature falls, the resistivity of the sample increases (dR/dT < 0) until it reaches a maximum value at the TMI transition temperature, 265 K. Here it begins changing from an insulator or
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semiconductor state to a more metallic state. Further cooling decreased the resistivity (dR/dT > 0) of the sample to metallic conductivity.
In Figure 8, the graph of temperature-dependent magnetization measurement is given both zero field cooled (ZFC) and field cooled processes (FC). The main purpose of low field ZFC and FC measurements is characterizing the long-range or short-range spin order behavior
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in the compound. The Curie temperature, TC, of the La0.70Ca0.21Ag0.09MnO3 compound was determined from dM/dT and it was found to be 263 K, which is very close to 265 K, the TMI temperature of the sample. For every Ag+ ion substitution for Ca2+ in La-site, two Mn3+ ions oxidized to Mn4+ ions, resulting in the formation of rich Mn4+ and Mn3+ regions in the
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compound. This causes electronic inhomogeneity and possible phase separation into antiferromagnetic insulator (AFI), ferromagnetic insulator (FMI) and ferromagnetic metal (FMM) phases. It can be seen from Figure 8 that the FC and ZFC curves separate from each
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other at the irreversibility temperature. Below TC, the ZFC magnetization is much lower than the FC magnetization. This large difference between ZFC and FC curve below TC implies mixed magnetic states (ferromagnetic, antiferromagnetic, spin-glass) after the magnetic ordering
temperature.
The
remarkable
feature
of
magnetic
properties
of
La0.70Ca0.21Ag0.09MnO3 is the occurrence of short-range ferromagnetic interaction. According to the double-exchange model (DE), ferromagnetism is accompanied by metallic conduction and vice versa. Therefore, the temperature corresponding to ferromagnetic-paramagnetic transition, TC, coincides with that of metallic-insulating transition, TMI. The sharpness or narrowness of PM-FM transition region and the smoothness of the curve in FM region indicates, once again, that the prepared compound is pure and has a single phase.
ACCEPTED MANUSCRIPT The field-dependent magnetization curves (M-H) were taken at various fixed temperatures up to 5 T and are presented in Figure 9. The family of curves consists of characteristic paramagnetic and ferromagnetic behavior well above and below TC. Near TC, the curves gain an S-like character, which is a desirable indicator for a large magnetic entropy change (∆SM) and, in turn, a large magneto-caloric effect [7]. On the other hand, M-H curves
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did not reach a saturation value below TC for an applied magnetic field up to 5 T. This is a result of the competition between ferromagnetic and antiferromagnetic interactions, leading to a frustrated magnetic system below TC. The ferromagnetic interactions are suppressed below TC, and the formation of antiferromagnetically correlated Mn spins causes unsaturation of the
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magnetization of the compound in ferromagnetic regions. Similar mixed magnetic states were seen from the difference between ZFC and FC (M-T curve) of the compound. The magnetic entropy changes (∆SM) are computed using the values in M-H curves,
change with magnetization; ∂M ∆SM (T, H) = ∫ dH ∂ T H 0 H
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utilizing one of the thermo-dynamical equations of Maxwell relating the magnetic entropy
(1)
For practical reasons, the above integral is approximated with a summation and the derivative
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in the integrand with the finite difference form, with discrete magnetization values at discrete temperatures and applied fields, as the following;
(∆SM )i = ∑
M(Ti +1, H j ) − M(Ti , H j )
j
Ti +1 − Ti
(H j+1 − H j )
(2)
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where (∆SM)i is the magnetic entropy change at a temperature Ti and Mi and Mi+1 are the experimental values of the magnetizations obtained at the temperatures Ti and Ti+1 under the
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magnetic field Hj.
The temperature dependence of |∆SM|, the change in magnetic entropies, is plotted in
Figure 10 at applied magnetic field change µ oH = 1, 2, 3, 4 and 5 T. The maxima of magnetic entropy changes was calculated to be 4.8, 7.0, 8.2, 9.6, 9.8 J/kgK for applied field changes of 1, 2, 3, 4 and 5 T, respectively. The temperatures at which the maximum of |∆SM| curves occurs almost coincide with TC of the sample. We think that the high value of the maximum magnetic entropy change relies on a strong spin–lattice coupling in the magnetic phase transition process which would lead to an additional magnetic entropy change near TC. There are two types of interactions mechanisms competing in the perovskite manganite compounds: the Mn3+-O-Mn4+ double exchange interactions, which are feromagnetically coupled, and the
ACCEPTED MANUSCRIPT coupling betweens Mn3+-O-Mn3+ and Mn4+-O-Mn4+, which are anti-ferromagnetic super exchange interactions. The ZFC and FC behavior in the M-T curve of La0.70Ca0.21Ag0.09MnO3 shows a short-range ferromagnetic order which is a consequence of both ferromagnetic and antiferromagnetic interactions below the TC. Due to the increase in the applied magnetic field up to 5 T, the maximum magnetic entropy changes of the compound do not yield a
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proportional increase. The maxima of |∆SM| at 4 T and 5 T are nearly same. This result can be attributed to the enhancement of antiferromagnetic super exchange interaction between the Mn ions with the increase of applied magnetic field. The antiferromagnetic coupling between Mn ions causes weakening of the ferromagnetic behavior and reduces of the strength of coupling between spin and lattice occurring near the magnetic phase transition. However, the
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corresponding relative cooling power (RCP) values computed from |∆SM|max × δTFWHM increase proportionally (58, 120, 180, 240, 295 J/K) with respective to increasing applied
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magnetic field changes. Here, δTFWHM is the full width at half maximum of the ∆SM curve. Another feature possessed by the ∆SM curves is an asymmetric broadening toward higher temperatures as the applied field is increased. This behavior was also observed for polycrystalline La0.70Ca0.30MnO3 sample by Ulyanov et al. [7]. Tian et al. [8] manufactured single crystal La0.70Ca0.30MnO3 and observed uniform and symmetrical ∆SM curves, which are
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claimed to be more desirable for Ericson-cycle magnetic refrigerators [19]. There are intensive studies on the LaCaMnO3 and LaAgMnO3 based compound which are reported in the literature, some of which are listed in Table 1 for comparison with our results. As can be seen from Table 1, TC of La0.70Ca0.21Ag0.09MnO3 is higher than that of all
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La0.70Ca0.30MnO3 and La0.65Ca0.30Ag0.05MnO3. Pal et al. argued that the enhancement of TC in an Ag-doped La0.70Pb0.30MnO3 compound would be due to the suppression of magnetic
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scattering at grain boundaries, increasing the connectivity between ferromagnetic domains [20]. In summary, La0.70Ca0.21Ag0.09MnO3 has superior magnetic properties, such that TC and |∆SM| values are simultaneously higher in comparison to the other compounds. It is believed that a large magnetocaloric effect is accompanied or caused by a first
order phase transition (FOPT) occurring at a temperature around ferromagnetic transition. The first order magneto-structural transition (FOMST) and magneto-elastic transition (FOMET) are known to produce larger MCE than second order phase transition (SOPT) [14, 23]. Unfortunately, we did not have opportunity to carry out temperature dependent XRD measurements in order to determine whether the observed high ∆SM stems from first order magneto-structural, magneto-elastic transformation, or SOPT.
ACCEPTED MANUSCRIPT 4. Conclusion We
have
simultaneously
doped
Ca
and
Ag
ions
in
the
La-site
of
La0.70Ca0.21Ag0.09MnO3 and studied the effect on its magnetic, magneto-caloric, and electrical properties. From both AFM and SEM investigations, the surface morphology of the compound was found to be granular and porous. By using EDS analysis, it was verified that
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the compound has a stoichiometry of La0.70Ca0.21Ag0.088Mn0.99O3+x which is very close to the targeted compound (La0.70Ca0.21Ag0.09MnO3). Neither XRD nor EDS analyses revealed a secondary phase such as manganite oxides or metallic Ag. X-ray diffraction data were refined by using the Rietveld method. It was found that the sample shows an orthorhombic structure
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in the Pnma space group. It is clear from the diffraction pattern that the synthesized compound consisted of a single phase. The La0.70Ca0.21Ag0.09MnO3 compound shows an insulator-paramagnetic properties above 263 K and metal-ferromagnetic properties below this
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temperature. Large differences between ZFC and FC magnetization curves were evident at low temperatures, which can be related to a spin glass-like behavior occurring in the sample. The ZFC and FC magnetization measurements reveal sharp and narrow PM-FM transition which favors a large MCE. Isothermal magnetizations which are taken different temperatures with respect to varying applied magnetic fields display an S-like shape around the Curie
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temperature of the sample. This behavior is evidence of large magnetic entropy change within the material [7].
Magnetic entropy changes of the compound are as large as the ones
exhibited by the pure Gd and Gd based compounds. In a moderately low field change of 1 T, maximum magnetic entropy change of the compound was found to be 4.8 J/kgK. The results
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reveal that the La0.70Ca0.21Ag0.09MnO3 compound is a suitable candidate for magnetic cooling applications because of its very high entropy change value at low magnetic field, although unfortunately the TC of the material is still considerably lower than room temperature. In
TC.
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conclusion, 9% Ca replacement in La0.70Ca0.30MnO3 with Ag helps improving both ∆SM and
Acknowledgments
We gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) for the support of this study (Grant No: 110T637).
ACCEPTED MANUSCRIPT 5. References
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Vol.78 Num. 6 (1197) 1142-1145.
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[3] Z. B. Guo, Y. W. Du, J. S. Zhu, H. Huang, W. P. Ding, and D. Feng, Physical Rev.Lett.
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[5] Y. Sun, X. Xu, Y. Zhang, Journal of Magnetism and Magnetic Materials 219 (2000) 183-185.
[6] Y. Samancıoglu, A. Coskun, Journal of Alloys and Compounds 507 (2010) 380-385.
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[7] A.N.Ulyanov, J.S.Kim, G.M.Shin,Y.M.Kang, S.I.Yoo, J.Phys.D: App.Phys.40 (2007) 123-126.
[8] S. B. Tian, M. H. Phan, S. C. Yu, N.H.Hur, Physica B 327 (2003) 221-224. [9] M. Khlifi, E. Dhahri, E.K. Hlil, Journal of Alloys and Compounds 587 (2014) 771–777. [10] P. G. Radaelli, G. Iannone, M. Marezio, H. Y. Hwang, S-W. Cheong,J. D. Jorgensen and
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D. N. Argyriou, Physical Review B Vol.56 Num.13 (1197) 8265-8276. [11] Y. Ding, D. Haskel, Y.C. Tseng, E. Kaneshita, M. van Veenendaal, J. F. Mitchell,S. V. Sinogeikin, V. Prakapenka, Ho-kwang Mao, Physical Rev.Lett. 102 (2009) 237201-237205. [12] S. Begum, Y. Ono, H. Fujishiro, T. Kajitani, Physica B 385–386 (2006) 53–56.
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[13] Z. M. Wang, G. Ni, Q.Y.Xu, H.Sang, Y.W.Du, J. Appl. Phys. 90 (2001) 5689-5691. [14] M. H. Phan, S C Yu, N H Hur, Y H Jeong, J. Applied Physics, 96-2 (2004) 1154-1158.
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[15] A.E.Irmak, A.Coskun, E.Tasarkuyu, S.Akturk, G.Unlu, Y. Samancioglu, C.Sarikurkcu, B.M.Kaynar, A.Yucel, Journal of Magnetism and Magnetic Materials 322 (2010) 945–951 [16] E.Tasarkuyu, A.Coskun, A.E.Irmak, S.Akturk, G.Unlu, Y.Samancioglu, A.Yucel, C.Sarikurkcu, S.Aksoy, M. Acet, Journal of Alloys and Compounds 509 (2011) 3717–3722. [17] T. Tang, K.M. Gu, Q.Q. Cao, D.H. Wang, S.Y. Zhang, Y.W. Du, J. Magn. Magn. Mater. 222 (2000) 110–114. [18] N. Zhang, T. Geng, H.X. Cao, J.C. Boa, Chin. Phys. B 17 (2008) 317–322. [19] V. K. Pecharsky, K. A. Gschneidner Jr, Journal of Magnetism and Magnetic Materials 200 (1999) 44-56. [20] S. Pal, A. Banerjee, B. K. Choudhuri, J. Phys. Chem. Solids. 64, (2003), 2063-2067.
ACCEPTED MANUSCRIPT [21] M.Koubaa, W.Cheikhrouhou, A. Cheikhrouhou, J. Alloys and Compounds 473 (2009) 510 [22] J.B.Goodenough, Physical Rev. 100 (1955) 564
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[23] H. Terashita, J. J. Garble, J. J. Neumeier, Phys. Rev. B, 70 094403 (2004) 1-5
ACCEPTED MANUSCRIPT Table 1. The list of the results on similar compounds reported in literature TC (K) 300 256 255 227 227 293 263 216
1.35 1.38 1.65 1.95 1.95 3.25 4.80 6.25
Method of production
17 13 21 8 14 22 sol-gel in this study solid state reaction 7
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solid state reaction sol-gel solid state reaction single crystal single crystal
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La0.70Ag0.3MnO3 La0.70Ca0.30MnO3 La0.65Ca0.30Ag0.05MnO3 La0.70Ca0.30MnO3 La0.70Ca0.30MnO3 Pure Gd La0.70Ca0.21Ag0.09MnO3 La0.70Ca0.30MnO3
|∆SM|max(J/kgK) at 1 T
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Compound
ACCEPTED MANUSCRIPT Figure 1a. 10 µm x 10 µm AFM image (2-D) of the sample surface Figure 1b. 10 µm x 10 µm AFM image (3-D) of the sample surface Figure 1c. The line profile along the diagonal line in Figure1a Figure 2. SEM (BSE) image of the sample surface
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Figure 3. EDS profile taken from the sample surface Figure 4. XRD pattern of the sample
Figure 5a. HRTEM image of the sample
Figure 5b. An image obtained by two successive FFT from the region marked in Figure 5a
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Figure 6a and 6b. The line profiles obtained from the FFT image given in Figure 5b Figure 7. Variation of resistivity with respect to temperature Figure 8. ZFC and FC magnetizations w.r.t. temperature
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Figure 9. Field dependent magnetization at various temperatures
Figure 10. Magnetic entropy changes as a function of temperature at various applied magnetic
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ACCEPTED MANUSCRIPT In this work we have studied the structural, magnetic and electrical properties of La0.70Ca0.21Ag0.09MnO3 . And, the following points were underlined: 1. XRD analysis reveals that the La0.70Ca0.21Ag0.09MnO3 has orthorhombic structure
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belonging to the Pnma space group.
2. Average grain size distribution was found between 0.5 to 1.5 µm.
3. The maximum magnetic entropy change of La0.70Ca0.21Ag0.09MnO3 is higher than that
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of pure Gd (2.8 J/kgK in a magnetic field change of 1 T).
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4. The Curie temperature of the compound was found to be 263 K.
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5. XRD and TEM results were found to coinciding each other.
S0925-8388(16)30231-6
DOI:
10.1016/j.jallcom.2016.01.230
Reference:
JALCOM 36581
To appear in:
Journal of Alloys and Compounds
Received Date: 13 December 2015 Revised Date:
29 January 2016
Accepted Date: 29 January 2016
Please cite this article as: A. Coşkun, E. Taşarkuyu, A.E. Irmak, S. Aktürk, High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.230. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT High magnetic entropy change in La0.70Ca0.21Ag0.09MnO3 compound A. Coşkun, E. Taşarkuyu, A.E. Irmak and S. Aktürk Department of Physics, Faculty of Sciences, Mugla Sitki Kocman University, 48000 Mugla, Turkey Magnetic Materials Laboratory, Research Laboratories Center, Mugla Sitki Kocman University, 48000 Mugla, Turkey
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Corresponding author e-mail:[email protected]
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Abstract
In this work, the structural, electrical, magnetic and magneto-caloric properties of the La0.70Ca0.21Ag0.09MnO3 compound were investigated. The compound was prepared by the sol gel method and sintered at 1000 oC for 24
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hours. Powder X-ray diffraction pattern (XRD) analyzes reveals a single phase orthorhombic structure with the Pnma space group as shown by the Rietveld analysis. The grainy morphology was confirmed with a grain size variation of 0.5-1.5 µm by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. High resolution transmission electron microscopy (HR-TEM) results strongly support an orthorhombic structure with the parameters calculated from the XRD data. Ferromagnetic-paramagnetic phase transitions occur at ~263 K (Curie temperature, TC) and ~265 K (metal-insulator, TMI). The sample undergoes a sharp metal– insulator transition at TMI following the sharp paramagnetic–ferromagnetic transition TC. From the isothermal
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magnetization measurements (M-H), the maximum magnetic entropy change |∆SM| was calculated to be ~4.8 J/kgK for an applied field change of 1 T, which is greater than that of pure Gd.
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1. Introduction
Due to the intrinsic problems of conventional vapor compression in refrigeration
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technology (such as environmental pollution and low efficiency), there is intense ongoing research on materials that have a potential use in magnetic cooling or magnetic refrigeration technology. Manganite-based compounds (generally in the form of Ln1-xTmxMnO3, where usually Ln is a lanthanide and Tm is a transition metal) which exhibit magnetic phase transitions near room temperature with a large magneto-caloric effect (MCE) are good candidates for use in magnetic refrigeration technology because of their relative low cost compared to counterparts containing expensive rare earth elements [1-6]. Lanthanum manganite doped with a monovalent or divalent element are known to have a perovskite structure, and exhibit a transition in their magnetic behavior, such as paramagnetic (PM) to ferromagnetic (FM) phase transition at the Curie temperature, TC. In
ACCEPTED MANUSCRIPT order for a material to be useful in magnetic cooling technologies (for a wide commercial use, such as in household appliances), it has to have a magnetic phase transition around room temperature and also high magnetic MCE in response to a moderate applied magnetic field change, µoH. To achieve this goal, manganite-based perovskite compounds are currently being investigated with monovalent and divalent element substitution.
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In the literature, many reports can be found on monovalent and divalent element doped LaMnO3 (La1-xAxMnO3) compounds. The Ca-doped LaMnO3 (La1-xCaxMnO3) series is especially attractive because of its high magnetic entropy change around room temperature [7-9] and for understanding the underlying physics behind it.[10-12] It is known that the
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structural, electrical and magnetic properties of doped LaMnO3 compound depend on concentration, oxidation state, and ionic radius of dopant elements and resulting Mn+4/Mn+3 ratio in the Mn-site. Ionic radius differences between dopant elements and La in the A-site
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lead a change in average ionic radius, known as the mismatch effect. Additionally, monovalent or divalent element doping in the A-site also changes the number of Mn3+ and Mn4+ ions, resulting a change in B-site average ionic radius because of the differences in ionic radii of Mn3+ and Mn4+. Even small variations in the crystal structure causes distortion MnO6 octahedra, leading a variation in Mn-O-Mn bond angle and Mn-O bond length, in turn,
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affecting almost all physical properties of these compounds.
Notably, La1-xCaxMnO3 compounds prepared by different techniques and applied different heat treatment procedure have diversity in their physical properties even for the same composition. Hence the crystal structures, magnetic phase transition temperatures, and
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maximum magnetic entropy change values are reported differently of this compound [7, 1214]. In the literature, the recorded MCE value of La0.70Ca0.30MnO3 compound is found to be
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6.25 J/kgK at TC=216 K for a magnetic field change of 1 T [7]. This value is nearly two times larger than MCE value of pure gadolinium. In our previous work, we found an increased maximum magnetic entropy change (4.1 J/kg K) at 273 K at a low applied magnetic field for the La0.65Ca0.35MnO3 compound [6]. In another study, we found maximum magnetic entropy change (2.1 J/kg K) at 290 K in a low applied magnetic field for the La0.75Ag0.25MnO3 compound [15]. It is well known that the magnetic properties of the compounds are mostly influenced by the average ionic radius of the A and B-sites and the Mn3+/Mn4+ ratio. The substitution of the monovalent metal ions (K+, Na+, Ag+) for Ca2+ changes the Mn3+ and Mn4+ ratio. It should be emphasized that as a result of Ag doping in A-site, while the average ionic radius of the A-site increases, the average ionic radius of the B-site decreases. This
ACCEPTED MANUSCRIPT happens by altering the valance state of Mn ions from Mn3+ to Mn4+, since the ionic radius of Mn4+ is smaller than Mn3+. In previous works, we had showed that the La0.65Ca0.35MnO3 compound has high MCE value at low temperature and La0.75Ag0.25MnO3 has low MCE value at room temperature. Therefore, in order to achieve the optimization of TC and ∆SM parameters of the LaCaMnO3
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and LaAgMnO3 based compounds, we have studied the structural, electrical, magnetic and magnetocaloric properties of La0.70Ca0.21Ag0.09MnO3 compound in which divalent (Ca2+) and monovalent (Ag+) element substitutions for La3+ were made simultaneously. The nature of the double-exchange and the super-exchange interactions and the strength of these interactions
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depend on Mn–O bond distance and the Mn–O–Mn bond angle. The Mn+4/Mn+3 ratio of the studied compound is 0.39/0.61=0.6393 that is different from La0.70Ca0.30MnO3 compound for 0.33/0.67=0.4285. It is expected that the possible changes in the Mn–O bond length and in the
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Mn–O–Mn bond angle due to the change in average ionic radius of B-site and Mn+4/Mn+3 ratio give rises to a local distortion (the Jahn–Teller distortion) of the MnO6 octahedra which affects the magnetic properties of the materials. 2. Experimental Procedure
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La0.70Ca0.21Ag0.09MnO3 compound was prepared by the sol-gel method, sintered at 1000 oC for 24 hours in air and cooled down to room temperature in the furnace. The details of the sol-gel preparation procedure can be found elsewhere [15, 16]. Powder X-ray diffraction at room temperature (XRD) was performed by a Bruker D8
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advanced x-ray diffractometer using CuKα1. Scanning electron microscopy (SEM) investigations were performed using a JEOL SEM 7700F, equipped with an energy dispersive
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spectrometer (EDS) and back scatter electron (BSE). Atomic force microscopy (AFM) studies were carried out by Solver Pro model AFM from NT-MDT. The high resolution transmission electron microscopy (HR-TEM) images of the sample were taken using a JEOL JEM 2100F. The magnetic properties of the sample were investigated using a quantum design PPMS with a closed cycle helium cryostat from 5 K to 320 K with magnetic fields up to 5 T. The TC value of the compound was determined from the temperature dependent magnetization measurements (M-T) at an applied field of 100 Oe. In the measurement sequence, the sample was first cooled down to 5 K under zero fields, and magnetization was measured while warming up the sample to 320 K with an applied field of 100 Oe (ZFC-FW). Then, under the same magnetic field, the magnetization was measured again while cooling down to 5 K (FC).
ACCEPTED MANUSCRIPT For magnetic entropy change determination, field dependent magnetization (M-H) measurements were performed by changing µ oH from 0 to 5 T around TC value exhibited by the sample with constant temperature intervals of 4 K between 260 K and 320 K. The temperature dependent resistivity (R-T) measurement was performed from 320 K down to 10 K with a standard four probe technique using a closed cycle helium cryostat from Cryo
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Industries. 3. Results and Discussions
AFM and SEM studies were used to investigate the surface morphology and grain size
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distribution of the compound. AFM images (2-D and 3-D) of the sample surface with a size of 10x10 µm2 which taken by contact mode are given in Figure 1a (2-D) and Figure 1b (3-D). In
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Figure 1c, a line profile spectrum of the surface shows the grain size distribution corresponding to the diagonal line in the Figure 1a. The grain size varies between 0.5 to 1.5 µm and these grains constitute grain clusters with a size of nearly 10 µm. A backscattered electron micrograph (BSE) with a magnification of 5000 is showing in Figure 2 to emphasize the compositional differences due to the contrast discrepancy in the
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image. The sample surface has almost identical contrast except the dark gray areas indicating the depth. In many works in which Ag is used as a dopant element, it has been observed that Ag dissociates from the perovskite phase and accumulates at inter-grain vacancies as metallic Ag, resulting in formation of impurity phases [16-18]. In this work, we did not observe
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metallic Ag impurity phases, as can be seen from the image. This is an indicator that the chosen heat treatment temperature was well suited for the Ag-doped compound.
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In Figure 3, the EDS profile and corresponding weight fractions of the elements are given. The actual chemical composition was calculated assuming that the La content remains unchanged (that is, the same as the targeted La content), as La0.70Ca0.21Ag0.088Mn0.99O3+x, which is almost the same as the targeted composition. The room-temperature XRD pattern is shown in Figure 4. The crystal structure of the
sample was refined by the Rietveld profile matching method, using the freely available FullProf Suit software. It was found that the sample has an orthorhombic crystal structure with the Pnma space group, and contains no impurity phase, neither manganese oxides nor metallic Ag, in agreement with BSE results mentioned above. The lattice parameters of the sample were calculated to be a=5.469280 Å, b=7.727343 Å and c=5.468515 Å.
ACCEPTED MANUSCRIPT The microstructure of the sample was investigated using HR-TEM. An image of the sample is given in Figure 5a. In Figure 5b, it was acquired by two successive Fourier Transforms of the real image (Figure 5a) in order to filter out unwanted noise on the images. The line profiles
are also given in Figure 6a and 6b to emphasize inter-atomic distances. The average distances between the most apparent atoms (electron clouds) are found to be 2.80 ± 0.02 Å and 3.90 ±
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0.02 Å. It can also be found that the planes (lines in 2D image) with separations of 5.60 ± 0.02 and 7.80 ± 0.02 Å (see Figure 6a and 6b) and the obliquity between the planes strongly support the orthorhombic structure with the parameters calculated from the XRD data. Figure 7 shows the zero magnetic field electrical resistivity,
R(T), of
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La0.70Ca0.21Ag0.09MnO3 as a function of temperature (320 K to 10 K). As the temperature falls, the resistivity of the sample increases (dR/dT < 0) until it reaches a maximum value at the TMI transition temperature, 265 K. Here it begins changing from an insulator or
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semiconductor state to a more metallic state. Further cooling decreased the resistivity (dR/dT > 0) of the sample to metallic conductivity.
In Figure 8, the graph of temperature-dependent magnetization measurement is given both zero field cooled (ZFC) and field cooled processes (FC). The main purpose of low field ZFC and FC measurements is characterizing the long-range or short-range spin order behavior
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in the compound. The Curie temperature, TC, of the La0.70Ca0.21Ag0.09MnO3 compound was determined from dM/dT and it was found to be 263 K, which is very close to 265 K, the TMI temperature of the sample. For every Ag+ ion substitution for Ca2+ in La-site, two Mn3+ ions oxidized to Mn4+ ions, resulting in the formation of rich Mn4+ and Mn3+ regions in the
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compound. This causes electronic inhomogeneity and possible phase separation into antiferromagnetic insulator (AFI), ferromagnetic insulator (FMI) and ferromagnetic metal (FMM) phases. It can be seen from Figure 8 that the FC and ZFC curves separate from each
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other at the irreversibility temperature. Below TC, the ZFC magnetization is much lower than the FC magnetization. This large difference between ZFC and FC curve below TC implies mixed magnetic states (ferromagnetic, antiferromagnetic, spin-glass) after the magnetic ordering
temperature.
The
remarkable
feature
of
magnetic
properties
of
La0.70Ca0.21Ag0.09MnO3 is the occurrence of short-range ferromagnetic interaction. According to the double-exchange model (DE), ferromagnetism is accompanied by metallic conduction and vice versa. Therefore, the temperature corresponding to ferromagnetic-paramagnetic transition, TC, coincides with that of metallic-insulating transition, TMI. The sharpness or narrowness of PM-FM transition region and the smoothness of the curve in FM region indicates, once again, that the prepared compound is pure and has a single phase.
ACCEPTED MANUSCRIPT The field-dependent magnetization curves (M-H) were taken at various fixed temperatures up to 5 T and are presented in Figure 9. The family of curves consists of characteristic paramagnetic and ferromagnetic behavior well above and below TC. Near TC, the curves gain an S-like character, which is a desirable indicator for a large magnetic entropy change (∆SM) and, in turn, a large magneto-caloric effect [7]. On the other hand, M-H curves
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did not reach a saturation value below TC for an applied magnetic field up to 5 T. This is a result of the competition between ferromagnetic and antiferromagnetic interactions, leading to a frustrated magnetic system below TC. The ferromagnetic interactions are suppressed below TC, and the formation of antiferromagnetically correlated Mn spins causes unsaturation of the
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magnetization of the compound in ferromagnetic regions. Similar mixed magnetic states were seen from the difference between ZFC and FC (M-T curve) of the compound. The magnetic entropy changes (∆SM) are computed using the values in M-H curves,
change with magnetization; ∂M ∆SM (T, H) = ∫ dH ∂ T H 0 H
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utilizing one of the thermo-dynamical equations of Maxwell relating the magnetic entropy
(1)
For practical reasons, the above integral is approximated with a summation and the derivative
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in the integrand with the finite difference form, with discrete magnetization values at discrete temperatures and applied fields, as the following;
(∆SM )i = ∑
M(Ti +1, H j ) − M(Ti , H j )
j
Ti +1 − Ti
(H j+1 − H j )
(2)
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where (∆SM)i is the magnetic entropy change at a temperature Ti and Mi and Mi+1 are the experimental values of the magnetizations obtained at the temperatures Ti and Ti+1 under the
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magnetic field Hj.
The temperature dependence of |∆SM|, the change in magnetic entropies, is plotted in
Figure 10 at applied magnetic field change µ oH = 1, 2, 3, 4 and 5 T. The maxima of magnetic entropy changes was calculated to be 4.8, 7.0, 8.2, 9.6, 9.8 J/kgK for applied field changes of 1, 2, 3, 4 and 5 T, respectively. The temperatures at which the maximum of |∆SM| curves occurs almost coincide with TC of the sample. We think that the high value of the maximum magnetic entropy change relies on a strong spin–lattice coupling in the magnetic phase transition process which would lead to an additional magnetic entropy change near TC. There are two types of interactions mechanisms competing in the perovskite manganite compounds: the Mn3+-O-Mn4+ double exchange interactions, which are feromagnetically coupled, and the
ACCEPTED MANUSCRIPT coupling betweens Mn3+-O-Mn3+ and Mn4+-O-Mn4+, which are anti-ferromagnetic super exchange interactions. The ZFC and FC behavior in the M-T curve of La0.70Ca0.21Ag0.09MnO3 shows a short-range ferromagnetic order which is a consequence of both ferromagnetic and antiferromagnetic interactions below the TC. Due to the increase in the applied magnetic field up to 5 T, the maximum magnetic entropy changes of the compound do not yield a
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proportional increase. The maxima of |∆SM| at 4 T and 5 T are nearly same. This result can be attributed to the enhancement of antiferromagnetic super exchange interaction between the Mn ions with the increase of applied magnetic field. The antiferromagnetic coupling between Mn ions causes weakening of the ferromagnetic behavior and reduces of the strength of coupling between spin and lattice occurring near the magnetic phase transition. However, the
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corresponding relative cooling power (RCP) values computed from |∆SM|max × δTFWHM increase proportionally (58, 120, 180, 240, 295 J/K) with respective to increasing applied
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magnetic field changes. Here, δTFWHM is the full width at half maximum of the ∆SM curve. Another feature possessed by the ∆SM curves is an asymmetric broadening toward higher temperatures as the applied field is increased. This behavior was also observed for polycrystalline La0.70Ca0.30MnO3 sample by Ulyanov et al. [7]. Tian et al. [8] manufactured single crystal La0.70Ca0.30MnO3 and observed uniform and symmetrical ∆SM curves, which are
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claimed to be more desirable for Ericson-cycle magnetic refrigerators [19]. There are intensive studies on the LaCaMnO3 and LaAgMnO3 based compound which are reported in the literature, some of which are listed in Table 1 for comparison with our results. As can be seen from Table 1, TC of La0.70Ca0.21Ag0.09MnO3 is higher than that of all
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La0.70Ca0.30MnO3 and La0.65Ca0.30Ag0.05MnO3. Pal et al. argued that the enhancement of TC in an Ag-doped La0.70Pb0.30MnO3 compound would be due to the suppression of magnetic
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scattering at grain boundaries, increasing the connectivity between ferromagnetic domains [20]. In summary, La0.70Ca0.21Ag0.09MnO3 has superior magnetic properties, such that TC and |∆SM| values are simultaneously higher in comparison to the other compounds. It is believed that a large magnetocaloric effect is accompanied or caused by a first
order phase transition (FOPT) occurring at a temperature around ferromagnetic transition. The first order magneto-structural transition (FOMST) and magneto-elastic transition (FOMET) are known to produce larger MCE than second order phase transition (SOPT) [14, 23]. Unfortunately, we did not have opportunity to carry out temperature dependent XRD measurements in order to determine whether the observed high ∆SM stems from first order magneto-structural, magneto-elastic transformation, or SOPT.
ACCEPTED MANUSCRIPT 4. Conclusion We
have
simultaneously
doped
Ca
and
Ag
ions
in
the
La-site
of
La0.70Ca0.21Ag0.09MnO3 and studied the effect on its magnetic, magneto-caloric, and electrical properties. From both AFM and SEM investigations, the surface morphology of the compound was found to be granular and porous. By using EDS analysis, it was verified that
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the compound has a stoichiometry of La0.70Ca0.21Ag0.088Mn0.99O3+x which is very close to the targeted compound (La0.70Ca0.21Ag0.09MnO3). Neither XRD nor EDS analyses revealed a secondary phase such as manganite oxides or metallic Ag. X-ray diffraction data were refined by using the Rietveld method. It was found that the sample shows an orthorhombic structure
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in the Pnma space group. It is clear from the diffraction pattern that the synthesized compound consisted of a single phase. The La0.70Ca0.21Ag0.09MnO3 compound shows an insulator-paramagnetic properties above 263 K and metal-ferromagnetic properties below this
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temperature. Large differences between ZFC and FC magnetization curves were evident at low temperatures, which can be related to a spin glass-like behavior occurring in the sample. The ZFC and FC magnetization measurements reveal sharp and narrow PM-FM transition which favors a large MCE. Isothermal magnetizations which are taken different temperatures with respect to varying applied magnetic fields display an S-like shape around the Curie
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temperature of the sample. This behavior is evidence of large magnetic entropy change within the material [7].
Magnetic entropy changes of the compound are as large as the ones
exhibited by the pure Gd and Gd based compounds. In a moderately low field change of 1 T, maximum magnetic entropy change of the compound was found to be 4.8 J/kgK. The results
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reveal that the La0.70Ca0.21Ag0.09MnO3 compound is a suitable candidate for magnetic cooling applications because of its very high entropy change value at low magnetic field, although unfortunately the TC of the material is still considerably lower than room temperature. In
TC.
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conclusion, 9% Ca replacement in La0.70Ca0.30MnO3 with Ag helps improving both ∆SM and
Acknowledgments
We gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) for the support of this study (Grant No: 110T637).
ACCEPTED MANUSCRIPT 5. References
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[5] Y. Sun, X. Xu, Y. Zhang, Journal of Magnetism and Magnetic Materials 219 (2000) 183-185.
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[7] A.N.Ulyanov, J.S.Kim, G.M.Shin,Y.M.Kang, S.I.Yoo, J.Phys.D: App.Phys.40 (2007) 123-126.
[8] S. B. Tian, M. H. Phan, S. C. Yu, N.H.Hur, Physica B 327 (2003) 221-224. [9] M. Khlifi, E. Dhahri, E.K. Hlil, Journal of Alloys and Compounds 587 (2014) 771–777. [10] P. G. Radaelli, G. Iannone, M. Marezio, H. Y. Hwang, S-W. Cheong,J. D. Jorgensen and
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D. N. Argyriou, Physical Review B Vol.56 Num.13 (1197) 8265-8276. [11] Y. Ding, D. Haskel, Y.C. Tseng, E. Kaneshita, M. van Veenendaal, J. F. Mitchell,S. V. Sinogeikin, V. Prakapenka, Ho-kwang Mao, Physical Rev.Lett. 102 (2009) 237201-237205. [12] S. Begum, Y. Ono, H. Fujishiro, T. Kajitani, Physica B 385–386 (2006) 53–56.
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[13] Z. M. Wang, G. Ni, Q.Y.Xu, H.Sang, Y.W.Du, J. Appl. Phys. 90 (2001) 5689-5691. [14] M. H. Phan, S C Yu, N H Hur, Y H Jeong, J. Applied Physics, 96-2 (2004) 1154-1158.
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[15] A.E.Irmak, A.Coskun, E.Tasarkuyu, S.Akturk, G.Unlu, Y. Samancioglu, C.Sarikurkcu, B.M.Kaynar, A.Yucel, Journal of Magnetism and Magnetic Materials 322 (2010) 945–951 [16] E.Tasarkuyu, A.Coskun, A.E.Irmak, S.Akturk, G.Unlu, Y.Samancioglu, A.Yucel, C.Sarikurkcu, S.Aksoy, M. Acet, Journal of Alloys and Compounds 509 (2011) 3717–3722. [17] T. Tang, K.M. Gu, Q.Q. Cao, D.H. Wang, S.Y. Zhang, Y.W. Du, J. Magn. Magn. Mater. 222 (2000) 110–114. [18] N. Zhang, T. Geng, H.X. Cao, J.C. Boa, Chin. Phys. B 17 (2008) 317–322. [19] V. K. Pecharsky, K. A. Gschneidner Jr, Journal of Magnetism and Magnetic Materials 200 (1999) 44-56. [20] S. Pal, A. Banerjee, B. K. Choudhuri, J. Phys. Chem. Solids. 64, (2003), 2063-2067.
ACCEPTED MANUSCRIPT [21] M.Koubaa, W.Cheikhrouhou, A. Cheikhrouhou, J. Alloys and Compounds 473 (2009) 510 [22] J.B.Goodenough, Physical Rev. 100 (1955) 564
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[23] H. Terashita, J. J. Garble, J. J. Neumeier, Phys. Rev. B, 70 094403 (2004) 1-5
ACCEPTED MANUSCRIPT Table 1. The list of the results on similar compounds reported in literature TC (K) 300 256 255 227 227 293 263 216
1.35 1.38 1.65 1.95 1.95 3.25 4.80 6.25
Method of production
17 13 21 8 14 22 sol-gel in this study solid state reaction 7
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solid state reaction sol-gel solid state reaction single crystal single crystal
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La0.70Ag0.3MnO3 La0.70Ca0.30MnO3 La0.65Ca0.30Ag0.05MnO3 La0.70Ca0.30MnO3 La0.70Ca0.30MnO3 Pure Gd La0.70Ca0.21Ag0.09MnO3 La0.70Ca0.30MnO3
|∆SM|max(J/kgK) at 1 T
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Compound
ACCEPTED MANUSCRIPT Figure 1a. 10 µm x 10 µm AFM image (2-D) of the sample surface Figure 1b. 10 µm x 10 µm AFM image (3-D) of the sample surface Figure 1c. The line profile along the diagonal line in Figure1a Figure 2. SEM (BSE) image of the sample surface
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Figure 3. EDS profile taken from the sample surface Figure 4. XRD pattern of the sample
Figure 5a. HRTEM image of the sample
Figure 5b. An image obtained by two successive FFT from the region marked in Figure 5a
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Figure 6a and 6b. The line profiles obtained from the FFT image given in Figure 5b Figure 7. Variation of resistivity with respect to temperature Figure 8. ZFC and FC magnetizations w.r.t. temperature
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Figure 9. Field dependent magnetization at various temperatures
Figure 10. Magnetic entropy changes as a function of temperature at various applied magnetic
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ACCEPTED MANUSCRIPT In this work we have studied the structural, magnetic and electrical properties of La0.70Ca0.21Ag0.09MnO3 . And, the following points were underlined: 1. XRD analysis reveals that the La0.70Ca0.21Ag0.09MnO3 has orthorhombic structure
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belonging to the Pnma space group.
2. Average grain size distribution was found between 0.5 to 1.5 µm.
3. The maximum magnetic entropy change of La0.70Ca0.21Ag0.09MnO3 is higher than that
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of pure Gd (2.8 J/kgK in a magnetic field change of 1 T).
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4. The Curie temperature of the compound was found to be 263 K.
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5. XRD and TEM results were found to coinciding each other.