Abstract
We report the effect of the simultaneous inclusion of Ag1+ and Sr2+ cations on the crystal structure and the magnetocaloric effect (MCE) of La0.80(Ag1−x Sr x )0.20MnO3 manganites (x = 0.0, 0.25, 0.50, 0.75, 1.0) synthesized by the solid-state reaction method. X-ray photoelectron spectroscopy was used to verify chemical composition and the amount of Mn3+–Mn4+ cations, while the magnetic performance was evaluated by means of a Physical Property Measuring System. The progressive substitution of Ag1+ by Sr2+ causes the increase of the number of Mn4+ cations, together with increasing ionic radii for the A-site contents up to x = 0.75, both favoring the enhancement of the double-exchange interaction and hence, the MCE. Excellent values of magnetic entropy change (−4.6 J/kg K, μ oΔH = 5.0 T) were observed for the x = 0.25 manganite, while for the x = 0.50 sample interesting refrigerant capacity (129 J/kg, μ oΔH = 2.0 T) and a wide interval of temperature at the full-width at half maximum δT FWHM of the magnetic entropy change curve were recorded (101 K, μ oΔH = 2.0 T) caused by two successive magnetic transitions. Magnetic performance was explained in terms of the effect of the cation substitution on the double-exchange interaction and the tolerance factor.
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Introduction
The magnetocaloric effect (MCE) refers to the isothermal magnetic entropy change (or the temperature change under adiabatic conditions) of a magnetic material under the application of an external magnetic field change μ o ΔH. This phenomenon is known since 1881, however, it was not up to the 1930s, that it was used for reaching ultra-low temperatures (i.e., ≪1 K) by means of the adiabatic demagnetization of paramagnetic salts like Gd2(SO4)3·8H2O [1]. The MCE has recently elicited a renewed interest due to the possibility of developing magnetic refrigeration technology applicable at room temperature, with the expectation of achieving higher cooling efficiencies (over 50 % of a Carnot cycle, [2–4]), compared with conventional refrigeration based on gas compression of pollutant compounds like CFC. One of the figures of merit for materials characterization aiming to MCE applications is the variation of the magnetic entropy ΔS M for a given applied magnetic field change μ oΔH = μ o (H f − H i) (where μ o H i is usually zero). In adiabatic conditions, the decrease (increase) of the magnetic entropy due to the application (removal) of the magnetic field leads to the increase (decrease) of the lattice entropy giving rise to a temperature increment (decrement) of the material. An additional parameter for MCE characterization is the refrigerant capacity (RC), which measures the ability of the magnetic refrigerant to transfer thermal energy between the cold (T cold) and hot (T hot) sinks when an ideal thermodynamic cycle is considered. To date, the reference material for MCE applications at room temperature is pure Gd, which possesses the most promising combination of properties for magnetic refrigeration applications at room temperature: ΔS M of −10.2 J/kg K (μ oΔH = 5 T), RC of 410 J/kg, and T C = 294 K [4] alongside an adiabatic temperature change ΔT ad of 6.0 K for μ oΔH = 2 T [5]. Unfortunately, the considerable high price and cost of production (USD $4000/kg [4]), makes Gd a disadvantageous choice as magnetic refrigerant. Because of this, a wide variety of materials have been explored for MCE applications, including MnFePAs alloys (with ΔS M as high as −18 J/kg K μ oΔH = 5 T and T C = 308 K [6]), MnFePAsGe alloys (with ΔS M of up to −40 J/kg K, μ oΔH = 3 T, T C = 270 K [7]) Heusler alloys like NiMnGa (with ΔS M of up to −10.4 J/kg K, μ oΔH = 2 T, and RC = 47 J/kg [8], NiMnCoSn (with ΔS M = −30 J/kg K, μ oΔH = 1 T, T C = 271 K and RC ~ 44 J/kg [9]), or NiMnIn with structural and magnetic entropy changes between −2.6 and −7.2 J/kg K, μ oΔH = 3 T and RC between 60 and 95 J/kg [10], rare earth-based bulk metallic glasses, like GdAlCo-type alloys characterized by ΔS M values between −7.6 and −9.4 J/kg K, μ oΔH = 5 T, and RC 413–679 J/kg, but low Curie temperatures, typically well below 100 K (which constrains refrigerant applications to such temperature intervals) [11] or HoDyAlCo systems with ΔS M of up to −11.7 J/kg K, μ oΔH = 5 T, RC = 365 J/kg and T C = 17 K [12].
An ample group of materials which has been extensively studied for MCE applications are La-based manganites with perovskite structure, because of their competitive cost of production, chemical stability and easy tailoring of magnetic properties [2, 4]. In fact, a significant number of compositional variations have been explored for LaMnO3-type manganites, including cations with 2+ or 1+ valence states, such as LaSr, LaCa, LaBa, LaPb, LaNd, LaY, LaLi, and La(Na,K,Ag), among others [4, 13–16]. However, reports concerning the effect of concomitant cations with mixed-valence 2+ and 1+ on La-based manganites are much less numerous [2, 4]. In this work, we present a systematic study on the effect of the progressive replacement of Ag1+ and Sr2+ cations on the MCE of La0.80(Ag1−x Sr x )0.20MnO3 manganites.
Experimental techniques
Mixed-valence polycrystalline La0.80(Ag1−x Sr x )0.20MnO3 manganites (x = 0.0, 0.25, 0.50, 0.75, 1.0) were synthesized by means of the solid-state reaction method. Details on the synthetic procedure are described elsewhere [17]. Chemical composition across the compositional series was verified by means of X-ray photoelectron spectroscopy in a VG Microtech Multilab ESCA2000 coupled with a CLAM4 MCD detector with Al-Kα radiation. The elemental chemical analysis was carried out by means of deconvolution of the core-level signals of the XPS spectra. This deconvolution process involves the removal of the background by Shirley method [18, 19]. Then, we calibrate the binding energy (BE) with reference materials (La2O3, SrCO3, Mn3O4, and AgO) for which we propose Gaussian–Lorentzian curves with spectroscopic criteria and the sensitivity relative factors reported by Scofield [20]. The fitting of the XPS spectra was performed with the program SDP v4.1 [21]. Magnetization measurements were performed by means of a Physical Property Measuring System equipped with the vibrating sample magnetometer module (PPMS-9T Evercool I, Quantum Design platform). The isothermal magnetic entropy change as a function of temperature ΔS M(T) was calculated from a set of magnetization isotherms measured each 5 K over the temperature interval 200–400 K. The maximum applied magnetic field change μ oΔH max was of 5.0 T. The magnetic entropy variation ΔS M was calculated through the numerical integration of the Maxwell relation [2, 4]:
Results
Details on phase distribution analysis by powder X-ray technique are given in [17], for which the solid solution La0.80(Ag1−x Sr x )0.20MnO3 across the compositional series was identified together with minor secondary phases Mn3O4 and metallic Ag for the sample with x = 0.0. Decreasing peak intensities of the secondary phases was observed as the Sr concentration increases, with no detectable presence of Ag or Mn3O4 for the La0.80Sr0.20MnO3 sample. Very similar phase distribution has been previously described in equivalent La1−x Ag x MnO3 manganites [22–24].
A central issue for the correlation between chemical composition variation and magnetic properties of La-based manganites is the relative amount of Mn4+ and Mn3+ ions present in each sample, as will be described within the “Discussion” section. Firstly, we confirm the Mn divalent state in our samples by the splitting of the Mn 3s core-level signal of the XPS spectra corresponding to the La0.80(Ag0.25Sr0.75)0.20MnO3 manganite as example (Fig. 1a). The linear relation between the Mn valence and the exchange splitting energy has been described in [25, 26]. As a second step, we quantify the content of Mn4+–Mn3+ cations by using the XPS high-resolution spectra of the core-level energies for the Mn 2p1/2 and Mn 2p3/2, for which Fig. 1b displays such core levels for the La0.80(Ag0.50Sr0.50)0.20MnO3 manganite as example, together with the same Mn 2p transition for all the compositions (inset). These core-level signals were used to calculate the Mn 4+–Mn 3+ content by means of a deconvolution process using the peaks located at the following BE: (a) For the Mn 2p3/2 core-level, peaks at BE = 639.43 eV and its satellite at 642.84 eV, both corresponding to Mn3+, while peaks at 640.56 eV and its satellite at 646.13 eV correspond to Mn4+, (b) For the Mn 2p1/2 core-level, peaks at 651.05 eV and its satellite at 655.00 eV, both corresponding to Mn3+, while peaks at 652.25 eV and its satellite at 657.88 eV correspond to Mn4+. This assignment of peaks and satellites for Mn3+ and Mn4+ cations based on the same Mn 2p1/2 and Mn 2p3/2 core levels has been proposed in [27] for equivalent kind of perovskites, studied for catalysis applications. The Mn4+ content shown in Table 1 exhibits an increasing tendency with increasing Sr content x as a result of the oxidation of Mn3+ atoms to compensate the charge of the incorporated Sr2+ ions. These values are consistent with those obtained by other experimental techniques such as redox titrations reported for equivalent La0.80Sr0.20MnO3 manganites (with Mn4+ content of 34.0 at% for the La0.80Sr0.20MnO3 manganite [28]) as well as for equivalent La0.7Sr0.3−x Ag x MnO3 (x values upto 0.2) with Mn4+ content between 29.5 and 34.5 at% for similar Ag concentrations [24]. Complementary, the Mn concentration of the Mn3O4 phase was evaluated by means of the Mn 2p3/2 and Mn 2p1/2 core levels and the peaks located at BE = 641.40 eV and 653.10 eV, respectively. For this phase, the following concentrations (% elemental) were determined: 6.8, 5.2, 4.6, 4.4, 2.4 for the samples x = 0.0, 0.25, 0.50, 0.75, and 1.0, respectively. This decreasing amount of Mn3O4 with increasing Sr content confirms the tendency observed by XRD results [17], except of the x = 1.0 sample, for which XPS indicates that Mn3O4 is still forming at a small fraction of 2.4 %. Such fraction is below the XRD detection limit, and thus it was not recorded in the corresponding X-ray diffractogram. For the silver concentration we found two oxidation states for Ag atoms, one associated to Ag0 (metallic) with BE = 368.20 eV corresponding to the Ag 3d5/2 core-level, and the other to Ag1+ for the BE = 367.03 eV for the same core-level. The presence of Ag1+ confirms the incorporation of Ag atoms within the crystal unit cell of the La(AgSr)MnO3 manganites, while the vanishing content of metallic silver (Table 1) is also consistent with XRD results [17]. No silver atoms were detected for x = 1.0. Details for the Ag composition procedure from the deconvolution of the Ag 3d core-level signal are described in [17, 27, 29].
Concerning magnetic properties, thermomagnetization M(T) curves under field cooled and field heating regimes for an applied low magnetic field of 5 mT are displayed in Fig. 2a for the La0.80(Ag1−x Sr x )0.20MnO3 manganite series. All the curves display a decreasing behavior with increasing temperature as the magnetic order–disorder transition approaches. A small thermal hysteresis (of up to ΔT = 3 K) is manifested for the manganites with x > 0.0. For the x = 0.50 sample, a step is observed on its corresponding M(T) plot between 250 and 260 K, which can be attributed to a second phase present in a volume fraction below the detection limit of the XRD technique (~4 %). This phase probably corresponds to SrMnO3, with T C = 260 K [30], which can remain as an intermediate product of the reaction during the formation of La1−x Sr x MnO3 manganites [31]. A subtle, much less defined step in the M(T) curve within the interval 364–374 K is also visible for the x = 0.75 and 1.0 samples [see Fig. 2a], likely corresponding to the marginal secondary phase La2/3Sr1/3MnO3 (T C = 368 K [2, 4]), which could be segregated during the formation of La1−x Sr x MnO3 manganites [31]. According to the thermomagnetization data of Fig. 2a, this phase should have a volume fraction around 4.0 %, which is at the threshold detection of the XRD technique. The Curie temperatures for all the La0.80(Ag1−x Sr x )0.20MnO3 compounds were taken at the inflexion point of each curve [Fig. 2b]. T C determined from Fig. 2b showed an increasing tendency with increasing Sr content x up to x = 0.75 (from 225 K at x = 0.0 upto 336 K at x = 0.75), followed by a noticeable reduction at x = 1.0 (T C = 324 K). The well defined peak at the magnetic transition observed for all manganites with x > 0.0 reflects a good chemical homogeneity, in contrast with the La0.80Ag0.20MnO3 sample. The magnetic transition temperature of the secondary Mn3O4 phase is not visible in Fig. 2 because such temperature is far below 200 K (~43 K [32]). The tendency observed for T c is in agreement with previous reports concerning equivalent La0.7Sr0.3−x Ag x MnO3 manganites [24].
Isothermal magnetization curves up to μ o H max = 5 T at variable temperature for selected compositions of the manganites series La0.80(Ag1−x Sr x )0.20MnO3 (x = 0.0,0.50,1.0) are displayed in Fig. 3. The transition from ferromagnetic to paramagnetic ordering (i.e., nearly linear-like M(μ o H) is visible for decreasing temperature for all the samples. No marked deviations were observed for the low-field (<1.0 T) M(μ o H) curves across the compositional series, which is indicative of the absence of metamagnetic-like transitions [33]. Very similar behavior of the isothermal magnetization was reported for equivalent LaAgMnO3 manganites prepared by the same method [23, 24]. Arrott plots M 2 versus μ o H/M for selected compositions are shown in Fig. 4 (x = 0.25,0.75). The positive slope observed for these plots is indicative of a ferromagnetic–paramagnetic transition of second-order character [34] and is in agreement with previous reports on equivalent La0.7Sr0.3−x Ag x MnO3 manganites [24].
The magnetic entropy variations |ΔS M| as a function of temperature, calculated at variable μoΔH max from 1 to 5 T, are displayed in Fig. 5a for the La0.80(Ag0.75Sr0.25)0.20MnO3 manganite. These magnetic entropy plots show single-peak behavior across the temperature interval, with |ΔS peakM | values having an increasing tendency with increasing μ o ΔH. For μ o ΔH = 5.0 T, an interesting maximum of 4.6 J/kgK is observed, which is comparable with the |ΔS peakM | = 5.6 J/kg K of pure Gd at μ o ΔH = 2.0 T [35]. Additionally, an interval of temperature at the full-width at half maximum of the ΔS M(T) curve (denoted as δT FWHM = T hot −T cold) of 35 K is observed for μ o ΔH = 2.0 T with |ΔS peakM | = 2.4 J/kg K, which is important for establishing wide thermodynamic cycles for the magnetic cooling process. Complementary, the RC has been calculated by means of the three established criteria used in literature: (a) |ΔS peakM | x δT FWHM (denoted hereafter as RC-1). Here, δT FWHM coincides with the temperature interval of the thermodynamic cycle; (b) calculating the integral, under the ΔS M(T) curves between T hot and T cold (hereafter denoted as RC-2, i.e., \( RC - 2 = \int_{{T_{\text{hot}} }}^{{T_{\text{cold}} }} {[\Updelta S_{\text{M}} (T)]_{\Updelta B} dT} \)), and (c) maximum |ΔS M| x δT RC−3 product under the ΔS M(T) curve (labeled as RC-3; in this case, the temperature interval define different hot and cold temperatures which are been labeled as T RC−3hot and T RC−3cold ) [36]. The field dependence of the RC values for the La0.80(Ag0.75Sr0.25)0.20MnO3 manganite are shown in Fig. 5b. A maximum RC-1 of 215 J/kg was observed at μ oΔH = 5.0 T. For the manganites corresponding to x = 0.75 and 1.0, equivalent behavior was observed for both |ΔS M| and refrigerant capacities, with maximum values of 3.4 J/kg K and 275 J/kg and 3.1 J/kg K and 304 J/kg, respectively. Results of the magnetocaloric parameters of the series studied are summarized in Table 2. Similar magnetic entropy variation has been reported in equivalent La1−x Ag x MnO3 manganites by means of isothermal magnetization curves [22, 23] and by heat capacity measurements [37] and for equivalent La0.7Sr0.3−x Ag x MnO3 (x values upto 0.2) [24].
In addition, the magnetic entropy variation calculated up to μ oΔH = 5.0 T for the La0.80(Ag0.50Sr0.50)0.20MnO3 manganite is shown in Fig. 6, for which a double-peak behavior is manifested due to the contribution of the SrMnO3 secondary phase. For this sample, very larger RC-2 values of 239 and 94 J/kg were recorded for μ oΔH = 5.0 and 2.0 T, respectively (Table 2), together with a large δT FWHM as high as 101 K at μ oΔH = 2.0 T and 110 K at μ oΔH = 5.0 T, respectively. This wide δT FWHM has the additional advantage of embracing temperature intervals around 300 K (265 ≤ T ≤ 312 K), which renders this specific composition as potential candidate for room temperature magnetic refrigeration devices. A similar behavior of the |ΔS M| curve was observed for the manganite corresponding to x = 0.0, with a remarkable δT FWHM of 113 K at μ oΔH = 5.0 T. Unlike the x = 0.50 sample, both T hot and T cold temperatures for the x = 0.0 manganite are well below 300 K. Results are summarized in Table 2.
Discussion
From a fundamental viewpoint, the electronic properties of mixed-valence perovskites relies to a significant extent on the double-exchange interaction, which is afforded by the conversion of Mn3+ cations into Mn4+ ions upon doping the original LaMnO3 compound with divalent or monovalent cations [38]. On the basis of the electronic configuration of Mn3+ (t 32g e 1g ) and Mn4+ (t 32g e 0g ), it can be elucidated that the presence of Mn4+ ions allow the hopping of eg electrons of Mn3+ to neighboring Mn4+ through double-exchange interaction, which in turn mediates ferromagnetic ordering and electronic conduction [38]. Hence, the increasing tendency of the Curie temperature observed for manganites with Sr contents upto x = 0.75 is consistent with the increasing Mn3+ to Mn4+ relative proportions which is reported in Table 1.
A complementary aspect that influences the electronic properties of these kinds of perovskites is the lattice effects caused by doping with 2+ or 1+ cations of different ionic radius, in particular, the variation of the Mn–O–Mn bond angle [38–40]. In this sense, an important criterion for analyzing the crystal structure of perovskite ABO3 compounds (where A corresponds to a trivalent or divalent cation and B corresponds to Mn for the case of manganites) is the tolerance factor t, which represents the capacity of the perovskite structure to accept A and B ions that allows dodecahedral and octahedral coordination, respectively [38, 40]. Stable crystal structures have representative t values as follows: for the ideal cubic perovskite t = 1.0, while for the interval 0.9 < t < 1.0, the unit cell presents a rhombohedral distortion. For t < 0.9 the structure becomes orthorhombic [40]. In order to quantify t, the following Equation has been established [38, 40]:
where r A, r O, r Mn stand for the ionic radii of the cations A, O, and Mn, respectively.
For the present case, and bearing in mind the size and oxidation states of the cations involved (r La3+ = 1.50 Å, r Ag+ = 1.42 Å r Sr2+ = 1.58 Å, r Mn3+ = 0.72 Å, r Mn4+= 0.67 Å, and r O2− = 1.26 Å [41]), we propose the following expressions for the mean radii 〈r A〉 and 〈r B〉 for our La0.80(Ag1−x Sr x )0.20MnO3 phases:
By inserting the experimental Mn3+, Mn4+, and Ag1+ atomic contents of Table 1 into Eqs. 2, 3, and 4, the resulting plots are shown in Fig. 7a. The r A increases up to x = 0.75 as a consequence of the progressive incorporation of large Sr cations into the crystal structure containing Ag atoms. For x = 1.0, r A decreases due to the absence of Ag atoms. The same r A tendency was described by Rietveld analysis in equivalent La0.7Sr0.3−x Ag x MnO3 manganites [24]. In contrast, r B remains constant across the compositional series. In addition, the tolerance factor t (Fig. 7b) exhibits an increasing behavior up to the Sr content x = 0.75, reflecting the tendency to a more cubic-like structure with Mn–O–Mn angles (Θ) becoming closer to 180°. The augmenting Θ angles favors DE interaction, which leads to the progressive improvement of T C observed in Fig. 2 [38, 39]. The reduced t observed at x = 1.0 implies a more distorted crystal structure compared with x = 0.75, which affects the DE yielding reduced Curie transitions. This correlation between structural variation and magnetic properties has been described with profusion in similar La-based manganites [38, 39].
The enhancement of the DE interaction also benefits the MCE performance of the La0.80(Ag1−x Sr x )0.20MnO3 manganites series, since improved DE favors higher magnetizations and steeper M(T) curves at the Curie transition, both of which have a beneficial effect on the magnetocaloric parameter |ΔS peakM |. According to Eq. (1), |ΔS M| is directly influenced by dM/dT. Thus, following the derivatives shown in Fig. 2b, a maximum in |ΔS M| should manifest for the x = 0.25 sample having the highest slope with well defined shape at the Curie transition, together with a marked reduction for x = 1.00 with a significantly reduced steep at T C and intermediate values for the remaining compositions. This tendency is confirmed in Fig. 8 for peak magnetic entropy values |ΔS peakM | as a function of the Sr content x for μoΔH = 5 T, for which the noticeable enhancement of |ΔS peakM | = 4.6 J/kg K observed at x = 0.25 is consistent with the pronounced steep of its corresponding M(T) curve at the Curie point.
Conclusion
Mixed-valence La0.80(Ag1−x Sr x )0.20MnO3 manganites with concomitant Ag1+ and Sr2+ ions were successfully synthesized by the solid-state reaction method. The mixed-valence and content of Mn cations were verified by means of XPS technique, which indicated an increasing number of Mn4+ ions with increasing Sr content up to x = 0.75. The MCE parameters (|ΔS peakM |, RC, δT FWHM) showed a clear dependence with the Sr content as a consequence of the progressive improvement of the double-exchange interaction promoted by the increasing Mn4+ cations and the tolerance factor. Maximum |ΔS M| of 4.6 J/kgK was obtained for the La0.80(Ag0.75Sr0.25)0.20MnO3 manganite at μoΔH = 5.0 T.
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Acknowledgments
I. Betancourt acknowledges financial support from research project UNAM-PAPIIT IN104310. M.E. Amano is grateful for the scholarship received from UNAM-PAPIIT IN104310 and CONACYT-82358 research grants. J.L. Sanchez Llamazares acknowledges the financial support received from CONACYT, Mexico, under project CB-2010-01-156932. Support received from Laboratorio Nacional de Investigaciones en Nanociencias y Nanotecnología (LINAN, IPICYT) is kindly acknowledged. C.F.S.V. thanks CSIC, Spain, for the Ph.D. grant received (JAEPRE-08-00508).
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Amano, M.E., Betancourt, I., Sánchez Llamazares, J.L. et al. Mixed-valence La0.80(Ag1−x Sr x )0.20MnO3 manganites with magnetocaloric effect. J Mater Sci 49, 633–641 (2014). https://doi.org/10.1007/s10853-013-7743-5
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DOI: https://doi.org/10.1007/s10853-013-7743-5