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Magnetocaloric effect in La1−xKxMnO3 (x = 0.11, 0.13, 0.15) composite structures in magnetic fields up to 80 kOe
Accepted Manuscript Magnetocaloric effect in La1−xKxMnO3 (x = 0.11, 0.13, 0.15) composite structures in magnetic fields up to 80 kOe A.G. Gamzatov, A.M. Aliev, A.R. Kaul PII:
S0925-8388(17)31103-9
DOI:
10.1016/j.jallcom.2017.03.300
Reference:
JALCOM 41338
To appear in:
Journal of Alloys and Compounds
Received Date: 20 December 2016 Revised Date:
22 March 2017
Accepted Date: 25 March 2017
Please cite this article as: A.G. Gamzatov, A.M. Aliev, A.R. Kaul, Magnetocaloric effect in La1−xKxMnO3 (x = 0.11, 0.13, 0.15) composite structures in magnetic fields up to 80 kOe, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.300. 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.
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Magnetocaloric effect in La1-xKxMnO3 (x=0.11, 0.13, 0.15) composite structures in magnetic fields up to 80 kOe A.G. Gamzatov1,*, A.M. Aliev1, A.R. Kaul2 1
Amirkhanov Institute of Physics of Dagestan Scientific Center, RAS, Makhachkala 367003, Russia. Department of Chemistry, Moscow State University, Moscow 119899, Russia.
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2
* e-mail: [email protected] Abstract
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Magnetocaloric properties of La1-xKxMnO3 (x=0.11, 0.13, 0.15) manganites and their two and three layer composite structures are measured under low and high magnetic fields by the direct method. The relative cooling power (RCP) in composites is revealed to increase by 20-40
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% in moderate magnetic fields of 20 kOe compared with parent materials. The value of the magnetic field, at which the greatest increase in RCP of the composite takes place, depends on the proximity of Curie temperatures of the materials composing the structure; for the studied materials a maximum increase in the RCP is observed in the magnetic field range from 40 to 50
Introduction
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kOe.
The search for materials with giant magnetocaloric effect (MCE) is one of challenging and promising problems both from fundamental and application standpoints. Manganites have
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been considered to be advanced magnetocaloric materials for a long time [1-6], although MCE and RCP values in manganites are rather small in comparison with the best magnetocaloric materials. And yet, a stream of works devoted to the study of magnetocaloric properties of
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manganites continues unabated (according to the data of Web of Science about 300 works per annum). Apart from measurements of the magnetocaloric properties of new materials, a great deal of research is focused on the improvement of the relative cooling power (RCP) of manganites [7-11]. The relative cooling power has been considered to be one of the most important factors for assessing the usefulness of a magnetic refrigerant material and its values are known for a wide class of MCE materials [7-14]. In general, the RCP can be defined as the product of the maximum value of the magnetic entropy change and the full width at half maximum of the magnetic entropy change curve. More precisely it can be estimated (calculated)
as the integral = |∆| . The RCP can be improved by the extension of the temperature
range, where the magnetic entropy considerably changes due to the formation of multiphase or
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composite materials, whereas the MCE value should not decrease significantly [7]. As it was reported in Refs [9, 10], the RCP of a composite prepared from two magnetic materials with close Curie points could be increased by 30%. Ln1-xAx-yByMn1-zCzO3 (Ln=La, Pr, Nd,…; A,B=Sr, Ca, K, Na…; C=Fe, Cr,...) manganites with substitution both in Ln- and in Mnsublattices are poorly studied and are of particular interest for research. In spite of the moderately low values of the MCE, the manganites are suitable objects to study the
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magnetocaloric properties of composites structures because a mixed substitution initiates the shift of TC, while the magnetocaloric properties change only slightly, which is favorable for the composite or layered structure preparation. This paper presents experimental data on magnetocaloric
properties
of
La0.89K0.11MnO3
(LKM11),
La0.87K0.13MnO3
(LKM13),
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La0.85K0.15MnO3 (LKM15) compounds and their composites in magnetic fields up to 80 kOe. La1-xKxMnO3 manganites are the brightest representatives of the materials with noticeable
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magnetocaloric effect among the manganites [15-24]. The choice of these manganites was determined by rather large values of MCE and the peaks of the effect at room temperature [1517].
Samples and experiment
The La1-xKxMnO3 samples were synthesized from Mn(NO3)2, La(NO3)3, and K2CO3
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using a Pechini route (“Reakhim”, “pure for analysis” grade). A stoichiometric amount of K2CO3 together with citric acid and ethylene glycol were added to a solution of Mn and La nitrates. The solution was evaporated at 60 °C until gelatinous state, and the gel was decomposed at 600 °C for 5 hours in air. The obtained powder was pressed into pellets which were placed into the
30 hours in air.
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powder of the same composition to prevent potassium evaporation and annealed at 1000 °C for
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All solid solutions of La1-xKxMnO3 (LKMO) have rhombohedral distorted perovskitetype structure. The XRD patterns of LKMO powder show that the degree of such distortion decreased with an increase in the potassium concentration (x). as can be seen from the decrease of splitting in the pseudocubic reflections (Fig. 1) and the reduction of a rhombohedral angle α (Table. 1) with increasing the potassium concentration. The value of the rhombohedral angle α close to 60° indicates the decrease of the rhombohedral distortion. The structure of LKMO can be represented both in hexagonal and in rhombohedral unit cells. The cell parameters of LKMO are determined from the XRD patterns of powders with addition of high purity silicon as internal standard. The unit cell parameters are presented in Table 1. The XRD patterns for all powdered LKMO solid solutions were collected at room temperature. In this case, because of different temperatures of the ferromagnetic ordering, XRD
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patterns for the sample with x=0.11 were measured in the paramagnetic state and for the samples with x=0.13 and 0.15 in the ferromagnetic state. It is possible that lattice of solid solutions with x=0.13 and 0.15 was distorted due to spontaneous magnetostriction, that causes the feature of potassium content depending on the lattice parameters of LKMO solid solutions. The cation composition of the prepared samples was verified by an atomic-emission spectrometry and massspectrometry of induced coupled plasma. Table 2 summarizes the results of the chemical
is practically equal to the nominal one.
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analysis of samples sintered at 1000 °C. As can be seen, the defined composition for all samples Direct measurements of ∆Tad were performed by the modulation method [25]. Two sources of magnetic fields were used to measure MCE in various ranges of fields. The AC
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magnetic field with the amplitude of up to 4 kOe and frequency of 0.3 Hz was generated by an electromagnet [25]. A cryogen-free superconducting magnet system with a maximum field of 8
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T served as the source of high magnetic fields. In this case of the cyclic effect the field was achieved by moving/removing a temperature insert into/out the magnet bore using a linear actuator with a frequency of 0.13 Hz. To measure the temperature oscillations due to MCE in cyclic magnetic fields, thermocouples made from constantan and chromel wires flattened to a thickness of about 3 µm were used. The signal from the thermocouple passed through the SR554 transformer preamplifier was measured by the SR830 Lock-in-Amplifier [26]. The samples for
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measuring the MCE were prepared in plate forms approximately of 3x3x0.3 mm3 in sizes. In order to study a probable change in the magnetocaloric efficiency of these materials, composite structures were prepared from two samples of the same sizes glued to each other by the butvarphenolic adhesive (see inset in Fig.2a). A thermocouple junction was placed between samples.
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Two configurations of the composite structures LKM11/LKM13 and LKM13/LKM15 were
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prepared in such a manner.
Results and Discussion Detailed investigations on magnetocaloric properties of the La1-xKxMnO3 manganites in
low magnetic fields were reported in Refs. [20, 21]. Magnetocaloric properties of LKM11/LKM13 and LKM13/LKM15 composites in low magnetic fields are shown in Fig. 2 (a, b). As one can see, there are two peaks at temperatures corresponded to those of MCE peaks for each of samples. The MCE magnitude increases at peaks with a rise in the magnetic field. A deep minimum between these peaks is observed on the ∆Tad(T) curve. Evidently, the depth of the minimum will decrease as the magnetic field is increased. The reason is that the MCE above and below the peaks increases faster with the filed than at the peak itself. It stems from that above and below peaks the MCE increases faster with field than at the peaks itself. Fig. 2 (a, b) shows
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the field dependence of MCE for two composites to be slightly different at minimum points. The depth of the observed minima and the dynamics of their change in the magnetic field are governed by the MCE maximum values, the width of MCE and TC proximity of the samples. The experimental results of the MCE for LKM11, LKM13 andLKM15 samples in the magnetic field of 18 kOe are depicted in Fig.3. As is evident from the figure, ∆Tad values at ∆H=18 kOe are roughly equivalent for all samples and involve a wide temperature interval from
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280 to 340 K. Using relationship ∆T=(T/CH)∆S and experimental results on the heat capacity [19, 21] we evaluated relative cooling power RCP(S)=-∆Smax(T, H)·δTFWHM. The RCP values are equal to 68, 61, and 53 J/kg at 18 kOe for LKM11, LKM13, and LKM15 respectively. These values are far less in comparison with those of advanced magnetocaloric materials
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so these materials could not be regarded as promising for the magnetic cooling technology. So we decided to check a change of the magnetocaloric properties of the materials by stacking those
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to create multilayered MCE materials. The temperature dependences of MCE for LKM11/LKM13 and LKM13/LKM15 double and LKM11/LKM13/LKM15 triple structures in 18 kOe are shown in Fig.4. The MCE value for the triple structure was not measured experimentally, it was estimated as a sum of the MCE values obtained experimentally for per sample divided by 3:
(∆ ( , ) = (∆ ( , ) + ∆ ( , ) + ∆ ( , ) )⁄3).
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It was shown earlier in Ref. [9] that the values of ∆T computed for double structures in this way and measured experimentally are in good agreement. The specific heats and masses of butted samples appear to be rather similar. It should be noted here that the demagnetizing fields of the composite and the separate samples can differ drastically from each other. In fact, it can be
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argued that for the considered case the influence of the composite demagnetizing field is weak, otherwise the ∆T of the composite wouldn`t be a simple sum of the ∆T values of the separate
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samples (divided by two or three). The weak influence of the demagnetizing field on ∆T can occur due to the geometry of the experiment (the plane of the plates is parallel to the magnetic field), the small thickness of the sample plates, and high magnetic fields. Even under low magnetic fields, as it is evident from our measurements, the influence of the difference in the demagnetizing fields of the separate samples and the composites is negligible. Fig. 4 depicts that the maximum values of ∆T for composites are smaller than the ∆T values for separate samples (Fig. 3), but the temperature interval of MCE is larger. The calculations evidence that at the magnetic field change of 18 kOe, the RCP values are 79 J/kg (LKM11/LKM13), 66 J/kg (LKM13/LKM15), and 84 J/kg (LKM11/LKM13/LKM15), which are by 20-50 % greater than for LKM11, LKM11, and LKM15 compounds. It is obvious that the LKM11/LKM13/LKM15 triple structure exhibits the best magnetocaloric properties among studied structures. In double
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structures the minima between temperatures of the MCE peaks are observed, while in triple structure a minimum is absent. It is obvious that the increase of number of components in multilayer structures (of samples with concentrations between 0.11 and 0.15) results in formation of a table-like MCE-temperature dependence at a given field. And vice versa, the less number of components of the structure requires higher fields to form a table-like MCE curve. The field dependences of the MCE at temperatures corresponded to maxima and minima
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on ∆T vs. T curves for LKM11/LKM13 and LKM13/LKM15 structures are plotted in Fig. 5. As can be seen, field dependences of the MCE differ notably in low magnetic fields (up to 3 kOe), in high fields they behaves virtually alike. The figure indicates that to attain the highest amplification of the effect one need optimal values of the magnetic field, and the value of the
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field also depends on the TC proximity of separate samples. Lower magnetic fields lead to the formation of an intermediate minimum (Fig. 2). Large difference of critical temperatures
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requires higher magnetic fields to inhibit totally the intermediate minimum on the ∆T(T) dependence. Overly close values of TC results in a slight increase in the temperature interval width, where the substantial change of ∆T occurs. Thus, both of these parameters, TC and ∆Tmax, must be taken into account for practical realization of such composite structures as a working body in magnetic cooling systems.
Farther, we shall consider the MCE behavior of LKM13 sample and composite structures
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in high magnetic fields. Fig. 6(a, b) presents the results of MCE measurements for LKM13 and LKM11+LKM13 structure in 18, 40, 60, and 80 kOe magnetic fields. In LKM13 the increase of magnetic field is accompanied by a broadening of the effect width as well as an increase of its maximum value that reaches ∆Tmax=4.19 K and ∆Smax=6.3 J/kg K (see inset in Fig.6(a)) at
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∆H=80 kOe. A MCE maximum value for the LKM11+LKM13 structure under 80 kOe equals to ∆Tmax=2.98 K (∆Smax=4.6 J/kg K). The figure shows that MCE peaks come close and will merge
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into one maximum at higher magnetic fields. The field dependences of ∆Smax(H) and δTFWMH(H) for LKM13, and the RCP for LKM13
and LKM11/LKM13 structures are plotted in Fig. 7. The field dependence of the MCE at various temperatures obeys a power law, namely ∆S~Hn, where n=1 at T<>TC, and n=2/3 near the TC [27]. One can suppose that ∆Tmax=f(H) dependence for LKM13 can be fitted by ∆T=bHn expression, where b is the magnetic-field-independent parameter. If the field dependence of the specific heat is not taken into account, the values of the n parameter can disagree at the transition from ∆S to ∆T, especially in low fields. Actually, in the magnetic field interval from 18 to 80 kOe the ∆T~H0.64dependence is observed and this dependence agrees well with the predictions of the mean field theory [28] providing the ∆S~H2/3 dependence holds, while ∆Tad~H0.83 is applicable in low magnetic fields (up to 2 kOe) [21]. The RCP is the function of ∆T
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и δTWFHM. The MCE value, ∆Tmax, grows more rapidly at H<40 kOe, and the effect width, δTWFHM, broadens faster at H>40 kOe. In this case, it results in that the field dependence of the RCP gains an exponent n>1, thus, the RCP rises more rapidly than ∆T due to the faster increase in the effect width δTWFHM. The comparison of RCP(H) dependences for LKM13 and LKM11/LKM13 demonstrates that the relative cooling power for the composite is higher than for the separate samples in the magnetic field interval from 18 to 80 kOe. The difference in the
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RCP for LKM13 and LKM11/LKM13 increases with a rise in the magnetic field, after 50 kOe the difference begins to decrease, and in the field of 80 kOe the RCP values virtually coincide (~280 J/kg). The largest increase in the RCP of the composite in comparison with the initial samples is observed in the magnetic fields interval from 40 to 50 kOe. It is evident that the field
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intensity, where the RCP gains the largest value, depends on the proximity of Curie points of the initial materials. Hence, when preparing composites, materials must be selected in such a way
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that the RCP increases maximally at relatively low fields (10-20 kOe).
Conclusion
In practical applications in, a magnetocaloric material will be used as a layered structure made ofthin plates, between which a heat-transport fluid will flow [29]. Such layered structure is believed to be prepared from several compositions of magnetocaloric
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materials. The magnetocaloric properties of the layered structures are rather difficult to study; the closest analog of such structure is a composite prepared from several samples glued to each other. The advantage of this system is that the magnetocaloric properties of the structure may be measured quite well, as well as by the direct method. The
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investigations on the magnetocaloric properties for the composite structures based on the series of La1-xKxMnO3 manganites reveal that the use of the composites prepared from
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materials with similar magnetocaloric properties and related TC enhances the RCP by about 20-40 %. The maximum efficiency of the double structures of La0.11K0.89MnO3 and La0.13K0.87MnO3 occurs at 40-50 kOe. The increase in the temperature differences of the MCE peaks of the samples composing the studied structures results in the growth of the RCP and the rise of the magnetic field, at which the highest difference of the RCP is observed between separate samples and composite structures.
Acknowledgements The reported study was partially supported by RFBR, research project № 17-0201195. The part of the research was carried out on the equipment of the Analytical Center for collective use of the Dagestan Scientific Center of the Russian Academy of Sciences.
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Figure caption:
Fig. 1. Powder XRD pattern of solid solutions La1-xKxMnO3. Fig.2. Temperature dependence of the MCE for composite structures: a) LKM11/LKM13 and b) LKM13/LKM15 at magnetic fields of 1, 2 and 3 kOe. Fig. 3. Temperature dependence of the MCE for samples LKM11, LKM13 and LKM15 at magnetic field of 18 kOe.
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Fig.4. Temperature dependence of the MCE for composite structures LKM11/LKM13, LKM13/LKM15, and LKM11/LKM13/LKM15 at magnetic field of 18 kOe.
Fig. 5. Dependence of ∆T on magnetic field maximum and minimum points of the MCE in composites (see Fig.2).
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Fig. 6. a) Temperature dependence of the MCE for sample LKM13 at magnetic field of 18, 40, 60, and 80 kOe. In the insert ∆S(T,H); b) Temperature dependence of the MCE of
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LKM11+LKM13 composite at magnetic fields of 18, 40, and 80 kOe. Fig.7. Dependence of ∆Smax, δTFWMH and RCP on magnetic field for LKM13 sample and
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LKM11/LKM13 structure.
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Figure 1.
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Figure 2.
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a)
b)
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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b)
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Figure 7.
Table 1. Unit cell parameters of La1-xKxMnO3 solid solutions at room temperature. hexagonal unit cell
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x
rhombohedral unit cell
c, Å
V, Å3
a, Å
α, degrees
0,000
5,5256(3)
13,3384(9)
352,69(3)
5,4722(3)
60,646(6)
0,110
5,5191(3)
13,3788(9)
352,93(3)
5,4810(3)
60,460(6)
5,5171(3)
13,3857(9)
352,86(3)
5,4822(3)
60,422(6)
5,5102(3)
13,3831(9)
351,91(3)
5,4792(3)
60,375(6)
0,130
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0,150
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a, Å
Table 2. Chemical analysis for samples of nominal composition La1-xKxMnO3 Nominal composition La/Mn K/Mn
Determinated composition La/Mn K/Mn
1.000
0.000
0,995(8)
—
0.890
0.110
0,891(7)
0,112(8)
0.870
0.130
0,870(8)
0,124(9)
0.850
0.150
0,850(6)
0,146(9)
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1. Presents the results direct measurements of MCE for composite structures. 2. Maximum value MCE for the structure LKM11+LKM13 under 80 kOe is equal to ∆T=2.98 K. 3. Obtained the values of RCP(T,H) for composite structures.
S0925-8388(17)31103-9
DOI:
10.1016/j.jallcom.2017.03.300
Reference:
JALCOM 41338
To appear in:
Journal of Alloys and Compounds
Received Date: 20 December 2016 Revised Date:
22 March 2017
Accepted Date: 25 March 2017
Please cite this article as: A.G. Gamzatov, A.M. Aliev, A.R. Kaul, Magnetocaloric effect in La1−xKxMnO3 (x = 0.11, 0.13, 0.15) composite structures in magnetic fields up to 80 kOe, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.300. 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.
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Magnetocaloric effect in La1-xKxMnO3 (x=0.11, 0.13, 0.15) composite structures in magnetic fields up to 80 kOe A.G. Gamzatov1,*, A.M. Aliev1, A.R. Kaul2 1
Amirkhanov Institute of Physics of Dagestan Scientific Center, RAS, Makhachkala 367003, Russia. Department of Chemistry, Moscow State University, Moscow 119899, Russia.
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2
* e-mail: [email protected] Abstract
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Magnetocaloric properties of La1-xKxMnO3 (x=0.11, 0.13, 0.15) manganites and their two and three layer composite structures are measured under low and high magnetic fields by the direct method. The relative cooling power (RCP) in composites is revealed to increase by 20-40
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% in moderate magnetic fields of 20 kOe compared with parent materials. The value of the magnetic field, at which the greatest increase in RCP of the composite takes place, depends on the proximity of Curie temperatures of the materials composing the structure; for the studied materials a maximum increase in the RCP is observed in the magnetic field range from 40 to 50
Introduction
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kOe.
The search for materials with giant magnetocaloric effect (MCE) is one of challenging and promising problems both from fundamental and application standpoints. Manganites have
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been considered to be advanced magnetocaloric materials for a long time [1-6], although MCE and RCP values in manganites are rather small in comparison with the best magnetocaloric materials. And yet, a stream of works devoted to the study of magnetocaloric properties of
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manganites continues unabated (according to the data of Web of Science about 300 works per annum). Apart from measurements of the magnetocaloric properties of new materials, a great deal of research is focused on the improvement of the relative cooling power (RCP) of manganites [7-11]. The relative cooling power has been considered to be one of the most important factors for assessing the usefulness of a magnetic refrigerant material and its values are known for a wide class of MCE materials [7-14]. In general, the RCP can be defined as the product of the maximum value of the magnetic entropy change and the full width at half maximum of the magnetic entropy change curve. More precisely it can be estimated (calculated)
as the integral = |∆| . The RCP can be improved by the extension of the temperature
range, where the magnetic entropy considerably changes due to the formation of multiphase or
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composite materials, whereas the MCE value should not decrease significantly [7]. As it was reported in Refs [9, 10], the RCP of a composite prepared from two magnetic materials with close Curie points could be increased by 30%. Ln1-xAx-yByMn1-zCzO3 (Ln=La, Pr, Nd,…; A,B=Sr, Ca, K, Na…; C=Fe, Cr,...) manganites with substitution both in Ln- and in Mnsublattices are poorly studied and are of particular interest for research. In spite of the moderately low values of the MCE, the manganites are suitable objects to study the
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magnetocaloric properties of composites structures because a mixed substitution initiates the shift of TC, while the magnetocaloric properties change only slightly, which is favorable for the composite or layered structure preparation. This paper presents experimental data on magnetocaloric
properties
of
La0.89K0.11MnO3
(LKM11),
La0.87K0.13MnO3
(LKM13),
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La0.85K0.15MnO3 (LKM15) compounds and their composites in magnetic fields up to 80 kOe. La1-xKxMnO3 manganites are the brightest representatives of the materials with noticeable
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magnetocaloric effect among the manganites [15-24]. The choice of these manganites was determined by rather large values of MCE and the peaks of the effect at room temperature [1517].
Samples and experiment
The La1-xKxMnO3 samples were synthesized from Mn(NO3)2, La(NO3)3, and K2CO3
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using a Pechini route (“Reakhim”, “pure for analysis” grade). A stoichiometric amount of K2CO3 together with citric acid and ethylene glycol were added to a solution of Mn and La nitrates. The solution was evaporated at 60 °C until gelatinous state, and the gel was decomposed at 600 °C for 5 hours in air. The obtained powder was pressed into pellets which were placed into the
30 hours in air.
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powder of the same composition to prevent potassium evaporation and annealed at 1000 °C for
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All solid solutions of La1-xKxMnO3 (LKMO) have rhombohedral distorted perovskitetype structure. The XRD patterns of LKMO powder show that the degree of such distortion decreased with an increase in the potassium concentration (x). as can be seen from the decrease of splitting in the pseudocubic reflections (Fig. 1) and the reduction of a rhombohedral angle α (Table. 1) with increasing the potassium concentration. The value of the rhombohedral angle α close to 60° indicates the decrease of the rhombohedral distortion. The structure of LKMO can be represented both in hexagonal and in rhombohedral unit cells. The cell parameters of LKMO are determined from the XRD patterns of powders with addition of high purity silicon as internal standard. The unit cell parameters are presented in Table 1. The XRD patterns for all powdered LKMO solid solutions were collected at room temperature. In this case, because of different temperatures of the ferromagnetic ordering, XRD
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patterns for the sample with x=0.11 were measured in the paramagnetic state and for the samples with x=0.13 and 0.15 in the ferromagnetic state. It is possible that lattice of solid solutions with x=0.13 and 0.15 was distorted due to spontaneous magnetostriction, that causes the feature of potassium content depending on the lattice parameters of LKMO solid solutions. The cation composition of the prepared samples was verified by an atomic-emission spectrometry and massspectrometry of induced coupled plasma. Table 2 summarizes the results of the chemical
is practically equal to the nominal one.
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analysis of samples sintered at 1000 °C. As can be seen, the defined composition for all samples Direct measurements of ∆Tad were performed by the modulation method [25]. Two sources of magnetic fields were used to measure MCE in various ranges of fields. The AC
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magnetic field with the amplitude of up to 4 kOe and frequency of 0.3 Hz was generated by an electromagnet [25]. A cryogen-free superconducting magnet system with a maximum field of 8
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T served as the source of high magnetic fields. In this case of the cyclic effect the field was achieved by moving/removing a temperature insert into/out the magnet bore using a linear actuator with a frequency of 0.13 Hz. To measure the temperature oscillations due to MCE in cyclic magnetic fields, thermocouples made from constantan and chromel wires flattened to a thickness of about 3 µm were used. The signal from the thermocouple passed through the SR554 transformer preamplifier was measured by the SR830 Lock-in-Amplifier [26]. The samples for
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measuring the MCE were prepared in plate forms approximately of 3x3x0.3 mm3 in sizes. In order to study a probable change in the magnetocaloric efficiency of these materials, composite structures were prepared from two samples of the same sizes glued to each other by the butvarphenolic adhesive (see inset in Fig.2a). A thermocouple junction was placed between samples.
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Two configurations of the composite structures LKM11/LKM13 and LKM13/LKM15 were
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prepared in such a manner.
Results and Discussion Detailed investigations on magnetocaloric properties of the La1-xKxMnO3 manganites in
low magnetic fields were reported in Refs. [20, 21]. Magnetocaloric properties of LKM11/LKM13 and LKM13/LKM15 composites in low magnetic fields are shown in Fig. 2 (a, b). As one can see, there are two peaks at temperatures corresponded to those of MCE peaks for each of samples. The MCE magnitude increases at peaks with a rise in the magnetic field. A deep minimum between these peaks is observed on the ∆Tad(T) curve. Evidently, the depth of the minimum will decrease as the magnetic field is increased. The reason is that the MCE above and below the peaks increases faster with the filed than at the peak itself. It stems from that above and below peaks the MCE increases faster with field than at the peaks itself. Fig. 2 (a, b) shows
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the field dependence of MCE for two composites to be slightly different at minimum points. The depth of the observed minima and the dynamics of their change in the magnetic field are governed by the MCE maximum values, the width of MCE and TC proximity of the samples. The experimental results of the MCE for LKM11, LKM13 andLKM15 samples in the magnetic field of 18 kOe are depicted in Fig.3. As is evident from the figure, ∆Tad values at ∆H=18 kOe are roughly equivalent for all samples and involve a wide temperature interval from
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280 to 340 K. Using relationship ∆T=(T/CH)∆S and experimental results on the heat capacity [19, 21] we evaluated relative cooling power RCP(S)=-∆Smax(T, H)·δTFWHM. The RCP values are equal to 68, 61, and 53 J/kg at 18 kOe for LKM11, LKM13, and LKM15 respectively. These values are far less in comparison with those of advanced magnetocaloric materials
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so these materials could not be regarded as promising for the magnetic cooling technology. So we decided to check a change of the magnetocaloric properties of the materials by stacking those
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to create multilayered MCE materials. The temperature dependences of MCE for LKM11/LKM13 and LKM13/LKM15 double and LKM11/LKM13/LKM15 triple structures in 18 kOe are shown in Fig.4. The MCE value for the triple structure was not measured experimentally, it was estimated as a sum of the MCE values obtained experimentally for per sample divided by 3:
(∆ ( , ) = (∆ ( , ) + ∆ ( , ) + ∆ ( , ) )⁄3).
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It was shown earlier in Ref. [9] that the values of ∆T computed for double structures in this way and measured experimentally are in good agreement. The specific heats and masses of butted samples appear to be rather similar. It should be noted here that the demagnetizing fields of the composite and the separate samples can differ drastically from each other. In fact, it can be
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argued that for the considered case the influence of the composite demagnetizing field is weak, otherwise the ∆T of the composite wouldn`t be a simple sum of the ∆T values of the separate
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samples (divided by two or three). The weak influence of the demagnetizing field on ∆T can occur due to the geometry of the experiment (the plane of the plates is parallel to the magnetic field), the small thickness of the sample plates, and high magnetic fields. Even under low magnetic fields, as it is evident from our measurements, the influence of the difference in the demagnetizing fields of the separate samples and the composites is negligible. Fig. 4 depicts that the maximum values of ∆T for composites are smaller than the ∆T values for separate samples (Fig. 3), but the temperature interval of MCE is larger. The calculations evidence that at the magnetic field change of 18 kOe, the RCP values are 79 J/kg (LKM11/LKM13), 66 J/kg (LKM13/LKM15), and 84 J/kg (LKM11/LKM13/LKM15), which are by 20-50 % greater than for LKM11, LKM11, and LKM15 compounds. It is obvious that the LKM11/LKM13/LKM15 triple structure exhibits the best magnetocaloric properties among studied structures. In double
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structures the minima between temperatures of the MCE peaks are observed, while in triple structure a minimum is absent. It is obvious that the increase of number of components in multilayer structures (of samples with concentrations between 0.11 and 0.15) results in formation of a table-like MCE-temperature dependence at a given field. And vice versa, the less number of components of the structure requires higher fields to form a table-like MCE curve. The field dependences of the MCE at temperatures corresponded to maxima and minima
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on ∆T vs. T curves for LKM11/LKM13 and LKM13/LKM15 structures are plotted in Fig. 5. As can be seen, field dependences of the MCE differ notably in low magnetic fields (up to 3 kOe), in high fields they behaves virtually alike. The figure indicates that to attain the highest amplification of the effect one need optimal values of the magnetic field, and the value of the
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field also depends on the TC proximity of separate samples. Lower magnetic fields lead to the formation of an intermediate minimum (Fig. 2). Large difference of critical temperatures
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requires higher magnetic fields to inhibit totally the intermediate minimum on the ∆T(T) dependence. Overly close values of TC results in a slight increase in the temperature interval width, where the substantial change of ∆T occurs. Thus, both of these parameters, TC and ∆Tmax, must be taken into account for practical realization of such composite structures as a working body in magnetic cooling systems.
Farther, we shall consider the MCE behavior of LKM13 sample and composite structures
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in high magnetic fields. Fig. 6(a, b) presents the results of MCE measurements for LKM13 and LKM11+LKM13 structure in 18, 40, 60, and 80 kOe magnetic fields. In LKM13 the increase of magnetic field is accompanied by a broadening of the effect width as well as an increase of its maximum value that reaches ∆Tmax=4.19 K and ∆Smax=6.3 J/kg K (see inset in Fig.6(a)) at
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∆H=80 kOe. A MCE maximum value for the LKM11+LKM13 structure under 80 kOe equals to ∆Tmax=2.98 K (∆Smax=4.6 J/kg K). The figure shows that MCE peaks come close and will merge
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into one maximum at higher magnetic fields. The field dependences of ∆Smax(H) and δTFWMH(H) for LKM13, and the RCP for LKM13
and LKM11/LKM13 structures are plotted in Fig. 7. The field dependence of the MCE at various temperatures obeys a power law, namely ∆S~Hn, where n=1 at T<
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и δTWFHM. The MCE value, ∆Tmax, grows more rapidly at H<40 kOe, and the effect width, δTWFHM, broadens faster at H>40 kOe. In this case, it results in that the field dependence of the RCP gains an exponent n>1, thus, the RCP rises more rapidly than ∆T due to the faster increase in the effect width δTWFHM. The comparison of RCP(H) dependences for LKM13 and LKM11/LKM13 demonstrates that the relative cooling power for the composite is higher than for the separate samples in the magnetic field interval from 18 to 80 kOe. The difference in the
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RCP for LKM13 and LKM11/LKM13 increases with a rise in the magnetic field, after 50 kOe the difference begins to decrease, and in the field of 80 kOe the RCP values virtually coincide (~280 J/kg). The largest increase in the RCP of the composite in comparison with the initial samples is observed in the magnetic fields interval from 40 to 50 kOe. It is evident that the field
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intensity, where the RCP gains the largest value, depends on the proximity of Curie points of the initial materials. Hence, when preparing composites, materials must be selected in such a way
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that the RCP increases maximally at relatively low fields (10-20 kOe).
Conclusion
In practical applications in, a magnetocaloric material will be used as a layered structure made ofthin plates, between which a heat-transport fluid will flow [29]. Such layered structure is believed to be prepared from several compositions of magnetocaloric
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materials. The magnetocaloric properties of the layered structures are rather difficult to study; the closest analog of such structure is a composite prepared from several samples glued to each other. The advantage of this system is that the magnetocaloric properties of the structure may be measured quite well, as well as by the direct method. The
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investigations on the magnetocaloric properties for the composite structures based on the series of La1-xKxMnO3 manganites reveal that the use of the composites prepared from
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materials with similar magnetocaloric properties and related TC enhances the RCP by about 20-40 %. The maximum efficiency of the double structures of La0.11K0.89MnO3 and La0.13K0.87MnO3 occurs at 40-50 kOe. The increase in the temperature differences of the MCE peaks of the samples composing the studied structures results in the growth of the RCP and the rise of the magnetic field, at which the highest difference of the RCP is observed between separate samples and composite structures.
Acknowledgements The reported study was partially supported by RFBR, research project № 17-0201195. The part of the research was carried out on the equipment of the Analytical Center for collective use of the Dagestan Scientific Center of the Russian Academy of Sciences.
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Figure caption:
Fig. 1. Powder XRD pattern of solid solutions La1-xKxMnO3. Fig.2. Temperature dependence of the MCE for composite structures: a) LKM11/LKM13 and b) LKM13/LKM15 at magnetic fields of 1, 2 and 3 kOe. Fig. 3. Temperature dependence of the MCE for samples LKM11, LKM13 and LKM15 at magnetic field of 18 kOe.
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Fig.4. Temperature dependence of the MCE for composite structures LKM11/LKM13, LKM13/LKM15, and LKM11/LKM13/LKM15 at magnetic field of 18 kOe.
Fig. 5. Dependence of ∆T on magnetic field maximum and minimum points of the MCE in composites (see Fig.2).
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Fig. 6. a) Temperature dependence of the MCE for sample LKM13 at magnetic field of 18, 40, 60, and 80 kOe. In the insert ∆S(T,H); b) Temperature dependence of the MCE of
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LKM11+LKM13 composite at magnetic fields of 18, 40, and 80 kOe. Fig.7. Dependence of ∆Smax, δTFWMH and RCP on magnetic field for LKM13 sample and
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LKM11/LKM13 structure.
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Figure 1.
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Figure 2.
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b)
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
Table 1. Unit cell parameters of La1-xKxMnO3 solid solutions at room temperature. hexagonal unit cell
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x
rhombohedral unit cell
c, Å
V, Å3
a, Å
α, degrees
0,000
5,5256(3)
13,3384(9)
352,69(3)
5,4722(3)
60,646(6)
0,110
5,5191(3)
13,3788(9)
352,93(3)
5,4810(3)
60,460(6)
5,5171(3)
13,3857(9)
352,86(3)
5,4822(3)
60,422(6)
5,5102(3)
13,3831(9)
351,91(3)
5,4792(3)
60,375(6)
0,130
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0,150
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a, Å
Table 2. Chemical analysis for samples of nominal composition La1-xKxMnO3 Nominal composition La/Mn K/Mn
Determinated composition La/Mn K/Mn
1.000
0.000
0,995(8)
—
0.890
0.110
0,891(7)
0,112(8)
0.870
0.130
0,870(8)
0,124(9)
0.850
0.150
0,850(6)
0,146(9)
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1. Presents the results direct measurements of MCE for composite structures. 2. Maximum value MCE for the structure LKM11+LKM13 under 80 kOe is equal to ∆T=2.98 K. 3. Obtained the values of RCP(T,H) for composite structures.