Magnetic and Magnetocaloric Properties of La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} Compounds
- First Online:
- 4 Citations
- 523 Downloads
Abstract
In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganites La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} (0≤x≤0.15), which show a sharp paramagnetic-ferromagnetic phase transition over a wide temperature range (T=250–401 K). The Curie temperature has been analyzed by two methods: using the numerical derivative dM/dT and the thermodynamic model. The experimental results indicate that T_{C} decreases from 368 to 288 K with increasing Ag substitution independently of the method used to obtain T_{C}. Upon 10 KOe applied magnetic field, the large magnetic-entropy change (|ΔS_{M}|) reaches values of 2.75, 3, 3.25 and 3.5 J/kg K for x=0, 0.05, 0.10 and 0.15 compositions, respectively, which are comparable to that of Gd. The relative cooling power (RCP) increases with increasing Ag content from 68.75 (x=0) to 156.27 J/kg (x=0.15) for ΔH=10 KOe. Through these results, La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} materials are strongly suggested for the use of active refrigerants for magnetic refrigeration technology near room temperature.
Keywords
Magnetic refrigerator Magnetic entropy Manganites Relative cooling power1 Introduction
Magnetic refrigeration based on the magnetocaloric effect (MCE) is a viable and competitive cooling technology in the near room-temperature region and it has recently attracted much research interest due to its potential advantage of environmental friendliness over conventional gas refrigeration [1, 2], such as high energy efficiency, less environmental stress, and small volume [3, 4, 5]. When a magnetic field is applied to a magnetic material, which is thermally isolated from its surroundings, a change of the temperature (heating or cooling) of the solid may be observed. This phenomenon is known as the magnetocaloric effect or, more precisely, the adiabatic temperature change due to a change of the applied magnetic field. The magnetocaloric effect was first observed by Warburg [6] in 1881: when iron was placed in a magnetic field, it warmed, and removing that field, the iron specimen cooled down and this was explained physically by Debye in 1926 [7] and Giauque in 1927 [8]. The magnetocaloric effect is the result of entropy changes (|ΔS_{M}|) or temperature (ΔT_{ad}) in a magnetic material with the adiabatic application or removal of a magnetic field. Until recently, the rare earth element gadolinium (Gd) with a large MCE has been considered as the most active magnetic refrigerant in room magnetic refrigeration (MR) [9] since it exhibits a maximum magnetic-entropy change, |ΔS_{M}|, of 10.2 J/kg K at T=294 K under a magnetic applied field change of 5 T [9]. However, the high price of Gd (∼$4000/kg) prevents it from the actual application. Therefore, the search for cheaper working substances and large MCE becomes a main research topic in this field. In 1997, Pecharsky and Gschneidner [10] discovered that the giant magnetocaloric effect MCE in the pseudo-binary alloy Gd_{5}Si_{2}Ge_{2} was twice larger than in Gd. More importantly, this alloy could not only improve the efficiency of large-scale magnetic refrigerators but also open the door to new small-scale applications, such as home and automotive air conditioning. Nonetheless, the Curie temperature of Gd_{5}Si_{2}Ge_{2} is about 276 K, which is much lower than that of Gd of 294 K, making this alloy difficult to be used in room-temperature magnetic refrigerators [11]. For this reason, there is an extensive search of new materials suited for solid-state cooling machines working in this temperature range, such as Ni–Mn–Ga alloys [12], Mn–As–Sb alloys [13], La–Fe–Co–S alloys [14], Mn–Fe–P–As alloys [15] and various compounds of manganites [5].
In the present study, we investigate the magnetic and magnetocaloric effect related to the effects of Ag doping in La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3}, which can be a suitable candidate as a working substance in magnetic refrigeration near room temperature.
2 Experimental
Polycrystalline La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} (0≤x≤0.15) samples were prepared by standard solid-state reaction method. Details of the sample structure characterization are described in Ref. [16].
Magnetization (M) versus temperature (T) and magnetization versus magnetic field (H) curves were carried out by using a Foner magnetometer equipped with a super-conducting coil. The magnetization isotherms were measured with a field step of 10 KOe and with a temperature interval of 5 and 10 K over a temperature range of 250–401 K.
3 Results and Discussion
3.1 Magnetic Measurements
- (1)The first one is a determination of inflection point of the transition by using the numerical derivative dM/dT as indicated in the a-inset of Fig. 1.
Figure 1 displays the temperature dependence of the magnetization for the sample La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} (0≤x≤0.15) under a magnetic field ΔH=500 Oe. The obtained values of T_{C} are 361, 332, 311 and 290 K, respectively, for x=0, 0.05, 0.1 and 0.15 [16].
- (2)The second one is based on the thermodynamic model: Amaral et al. [17] discussed the magnetic properties of manganites in terms of the Landau theory of phase transitions. Here, the magnetic energy MH has been included in the expression of Gibb’s free energy as given byThe coefficients A and B are temperature-dependent parameters containing the magnetoelastic coupling and electron condensation energy [18].$$ G(T,M) = G_{0} + \frac{1}{2}AM^{2} + \frac{1}{4}BM^{4} - MH$$(1)
By plotting the experimental data in the form \(\frac{H}{M} = A +BM^{2}\), the temperature dependence of parameters A and B can be extracted.
Since a sharp PM–FM transition occurs around 290 K, which possibly implies a large magnetic-entropy change near room temperature, we performed a measurement of MCE of the present material.
The temperature dependence of parameters A and B can be obtained from the linear fitting of the Arrott plot of \(\frac{H}{M}\) vs. M^{2}.
Summary of the magnetocaloric properties for La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3}. The present results of La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} (x=0, 0.05, 0.1 and 0.15) are compared with some of the divalent and the monovalent hole-doped manganites as well as pure Gd
Composition | T_{C} (K) | H (KOe) | \(| \Delta S_{M}^{\max} |\) (J/kg K) | |ΔQ_{M}(T_{C}±10 K,10 KOe)| (J/kg) | RCP (J/kg) | Refs. |
---|---|---|---|---|---|---|
Gd | 292 | 10 | 3.25 | 122 (T_{C}±25 K) | – | [22] |
Gd | 294 | 15 | ∼4 | – | 120 | [23] |
Gd_{5}Si_{2}Ge_{2} | 278 | 15 | ∼12 | – | 90 | [23] |
MnFePAs | 197 | 15 | ∼9 | – | 110 | [24] |
La(Fe_{0.88}Si_{0.12})_{13} | 305 | 15 | ∼12 | – | 100 | [25] |
La_{0.67}Pb_{0.33}MnO_{3} | 368 | 10 | 2.75 | 43.05 | 68.75 | This work |
La_{0.67}Pb_{0.28}Ag_{0.05}MnO_{3} | 341 | 10 | 3 | 48.55 | 84.24 | This work |
La_{0.67}Pb_{0.23}Ag_{0.1}MnO_{3} | 316 | 10 | 3.25 | 57 | 130 | This work |
La_{0.67}Pb_{0.18}Ag_{0.15}MnO_{3} | 288 | 10 | 3.5 | 61 | 156.27 | This work |
La_{0.7}Ca_{0.3}MnO_{3} | 256 | 10 | 1.38 | – | 41 | [26] |
La_{0.835}Na_{0.165}MnO_{3} | 342 | 10 | 2.11 | – | 63 | [27] |
La_{2/3}Sr_{1/3}MnO_{3} | 370 | 10 | 1.5 | – | 41 | [19] |
La_{0.80}Ag_{0.2}MnO_{3} | 278 | 10 | 3.4 | – | 41 | [28] |
La_{0.70}Ag_{0.3}MnO_{3} | 306 | 10 | 1.25 | – | 33 | [28] |
La_{2/3}Ba_{1/3}MnO_{3} | 370 | 10 | 1.7 | – | 68 | [5] |
La_{0.7}Ca_{0.18}Ba_{0.12}MnO_{3} | 298 | 10 | 1.85 | – | 45 | [5] |
La_{0.65}Sr _{0.35}MnO_{3} | 305 | 10 | 2.12 | – | 106 | [5] |
La_{0.7}Ba_{0.3}MnO_{3} | 336 | 10 | 1.60 | – | 36 | [29] |
La_{0.6}Sr_{0.2}Ba_{0.2}MnO_{3} | 354 | 10 | 2.26 | – | 67 | [29] |
La_{0.78}Ag_{0.22}MnO_{3} | 307 | 10 | 2.90 | – | 38 | [30] |
La_{0.67}Ba_{0.33}Mn_{0.9}Ti_{0.02}O_{3} | 314 | 10 | 0.93 | – | 45 | [31] |
3.2 Magnetocaloric Effect
Using Eq. (6) and experimental M–H curves at various temperatures, the magnetic-entropy change vs. temperature |ΔS_{M}| can be calculated.
These values are clearly comparable to the pure Gd [22] (|ΔS_{M}|∼3.25 J/kg K at ΔH=10 KOe) and other selected manganites [23, 24, 25, 26, 27, 28, 29, 30, 31] (Table 1).
- (1)
a well-defined transition temperature due to sharp shape of |ΔS_{M}| (T) curve,
- (2)
a modest magnetic-entropy change up on application/removal of a low magnetic field and easily controllable magnetic entropy,
- (3)
good chemical stability with quite high efficiency, and
- (4)
the possibility of being manufactured at a low price.
The large magnetic-entropy change in perovskite manganites is believed to be originated from the role played by spin–lattice coupling in the magnetic ordering process [32]. A significant change accompanying the magnetic transition due to strong coupling between spin and lattice in perovskite manganites has been observed [33] resulting in lattice structure changes e.g. in the 〈Mn–O〉 bond distances as well as in the 〈Mn–O–Mn〉 bond angle. The significant coupling of spin and lattice degrees of freedom provides a more abrupt variation of magnetization near the magnetic transition occurs and results in a large magnetic-entropy change.
To assess the applicability of our samples for magnetic refrigeration, the maximum \(| \Delta S_{\mathrm{M}}^{\max} |\) values determined in the present studies are compared in Table 1 with those reported in literature for several materials having close T_{C} and considered promising for such application. La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} (x=0, 0.05, 0.1 and 0.15) compounds could be a potential candidate for room-temperature magnetic refrigeration because of its advantages in terms of easy preparation method, nearly comparable |ΔS_{M}|, high chemical stability, tenability of T_{C} over a wide temperature range, high resistivity, cost effectiveness, etc.
The corresponding results for x=0.1 of ΔH=1 T was presented in the inset (c) of Fig. 6(a). The material with a larger RCP value usually represents a better magnetocaloric substance due to its high cooling efficiency. As shown in the inset (b) of Fig. 6, \(| \Delta S_{\mathrm{M}}^{\max}|\) and RCP of La_{0.67}Pb_{0.23}A_{0.1}MnO_{3} sample exhibit an almost linear rise with increasing ΔH. For ΔH=1 T, the value of RCP reaches 130 J/kg. The RCP is much larger than that of other manganites and is high enough for magnetic refrigeration. The temperature range δT_{FWHM} is found to be 40 K. This wide temperature range is very beneficial for Ericsson refrigeration cycle as well [35].
Pure Gd, which is considered a good material, exhibits an RCP value of 250 J/kg at 2 T [10], while Gd_{5}(Si_{2}Ge_{2}), which is considered the most conspicuous magnetocaloric material at room temperature, presents an RCP value of 130 J/kg at 2 T [36]. For the samples studied here, the RCP values are found to be 68.75, 84.24, 130 and 156.27 J/kg under the magnetic field variation of 1 T for x=0, 0.05, 0.1 and 0.15, respectively. Compared to other materials considered as good for applications in magnetic refrigerators, our results are interesting enough to pave the way for investigations of materials useful for magnetic refrigeration.
So we can conclude that magnetocaloric materials based on silver-doped manganites can be promising candidates for MR because they show considerable RCP at room temperatures.
4 Conclusion
In conclusion, we have investigated the MCE of perovskite manganite La_{0.67}Pb_{0.33−x}Ag_{x}MnO_{3} (0≤x≤0.15). The magnetic-entropy change (|ΔS_{M}|) of all the samples showed a maximum around their respective T_{C} and its magnitude increases from 2.75 J/kg K (x=0) to 3.5 J/kg K (x=0.15) with increase of Ag content, under 10 KOe field, and the corresponding RCP are 156.27 J/kg (x=0.15) to 68.75 J/kg (x=0). It indicates that such a series of samples can be suitable candidates as room-temperature working substances in magnetic refrigeration technology. The observed temperature dependence of |ΔS_{M}| is in accordance with Landau theory.
Open Access
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.